66
0,,11- Chapter I ((Investigations on Shape rpoCymers",}In Overview
0,,11-
Chapter I
((Investigations on 'Tfi,ermo-~sponsive Shape
~emory rpoCymers",}In Overview
Cfzapter 1
1.1 INTRODUCTION
'Polymer' refers to a molecule of extraordinarily large size. Polymers are
complex and giant molecules and are different from low molecular weight compounds
like NaCl. To contrast the difference, the molecular weight ofNaCI is only 58.5, while
that of a polymer molecule can be as high as several hundred thousands [1]. The
molecular weight of most natural and synthetic polymers ranges from 104 to 107
daltons. These big molecules or macromolecules are made up of much smaller
molecules. The individual small molecules from which the polymer is formed are
known as monomers and the process by which the monomer molecules are linked to
form big molecule is called polymerization. The overall properties of a given polymeric
material depend on (i) the constituent monomer units and the way they are arranged in
the macromolecule and (ii) the spatial arrangement of the constituent macromolecular
chains and the nature of the intermolecular interactions that hold them together. These
interesting structure-property relationships provide a broad basis to allow for the
development of various polymeric materials (e.g. polymer fibers, films, powders) with
different properties from the same macromolecule. The overall insight into polymer
science and technology is so deep that a scientist can create an almost limitless range of
new materials.
Linear polymers have a linear skeletal structure, which may be represented by a
chain with two ends. Non-linear skeletal structure of polymers are of branched or
network polymers. Branched polymers have side chains or branches of significant
length, which are bonded to the main chain at branch points. Network polymers have
three-dimensional structures in which each chain is connected to all other chains. If
only one kind of monomer is involved in a polymer chain it is called a homopolymer.
Polymers consisting of two or more different monomer units with many different ways
in which they can be arranged in the macromolecule is called a copolymer.
Polymers have become widely used in all aspects of our daily life. After all, our
clothes are made from synthetic fibers, car tyres from rubber and computer chips from
plastics. It is now difficult to imagine what our life would be if there were no polymers.
With recent advances in nanoscience and nanotechnology, various nanostructured
polymers have been devised for wide range of advanced applications. Examples of
these advances include the use of polymer nanoparticles as drug delivery devices,
polymer nanofibers as conducting wires, polymer thin films and periodically structured
Cliapter 1 2
polymeric structures for optoelectronic devices [2, 3]. The development of
nanostructured polymers has opened up novel fundamental and applied frontiers, which
have attracted tremendous interest in recent years.
The quest for the ability to manipulate and control the structure of
macromolecules in three dimensions has long fascinated scientists, as it would provide
additional freedom for tailoring molecular size, shape and local microenvironments.
Unlike most conventional linear macromolecules, three-dimensional macromolecules
could posses highly specific molecular architectures and show significantly improved
new properties. The best examples of three-dimensional macromolecules include the
tree-like molecules known as dendrimers and the soccerball-like fullerenes such as C60.
Dendritic macromolecules with their molecular cores sterically shielded from the
surrounding environment can be potentially used for various applications, including the
attenuation of luminescence quenching of encapsulated dye molecules, delivering drugs
or genes into cells, chemical/biosensors, regulating electron transfer to and from a
redox-active core moiety, and molecular antennae for absorbing light energy [4, 5].
Advance polymeric materials find tremendous applications in today's world.
The Nobel-prize-winning discovery of buckminsterfullerene (C60) is a conjugated
molecule having a soccer ball-like structure comprised of 12 pentagons and 20
hexagons facing symmetrically which has created an entirely new branch of carbon
chemistry [6, 7]. Subsequent discovery of CNT opened up new era in material science
and nanotechnology [8, 9]. The elongated nanotubes consists of carbon hexagons
arranged in a concentric manner with both ends normally capped by fullerens like
structures containing pentagons. Having a conjugated carbon structure with unusual
molecular symmetries, fullerenes and carbon nanotubes show interesting electronic,
photonic, magnetic and mechanical properties attractive for various applications
including optical limiters, photovoltaic cells and field emitting displays.
Polymers have been traditionally used as electrically insulating materials. Over
the last three decades, various polymers have been synthesized with electronic,
photonic and magnetic properties. The visit of MacDiarmid to Shirakawa at Tokyo
Institute of Technology in 1974 and later Shirakawa to MacDiarmid and Heeger at the
University of Pennsylvania led to the discovery of conducting polyacetylene. This
finding opened up the important new field of polymers for electronic applications and
was recognized by the 2000 Nobel Prize in chemistry.
Cliapter 1 3
Recently, the self-assembling of macromolecules to create functional materials
and smart devices have attracted great deal of interest [10]. Tools originally developed
by biochemists and biotechnologies to deal with biomolecular machines found in nature
can be redirected to make new functional structures and smart devices. Self-assembling
at the liquid Iso lid interface has also been exploited for the development of various
smart surfaces.
The development of "smart polymers" opened up new advances in polymer
science and technology. 'Stimuli Responsive (smart) Polymers' are that one changes its
conformations in a controllable, reproducible and reversible manner in response to the
external stimuli such as temperature, pH, light, solvent, electric and magnetic field etc
[11]. In broad, smart polymers are one that responds to external stimuli. In a most
general sense, a smart structure consists of three parts a sensor, an actuator and a
processor [12].
• Sensor corresponds to a nervous system reacting to stimulus (e.g. heat,
pH, electricity etc.).
• Actuator analogous to a muscle, which appropriately modifies the
characteristics of a structure.
• Processor performs brain-like control function that the structure
responds in the prescribed way.
1.2. DIFFERENT CLASSES OF STIMULI RESPONSIVE POLYMERS
1.2.1 Electro Active Polymers
Certain types of polymers can change shape or size in response to electrical
stimulus called electro active polymers (EAP) [13]. EAP can be deformed repetitively
by applying external voltage across the EAP, and they can rapidly recover their original
configuration upon reversing the polarity of the applied voltage. Electro active
polymers are developed to enable effective, miniature, inexpensive, light and low
power actuators for planetary applications. The characteristics of inducing large
displacements and the capability to emulate biological muscles are making EAP materials
attractive for consideration in an increasing number of fields. Main attractive
Cfiapter 1 4
characteristics of electro active polymer are their operational similarity to biological
muscles, and ability to induce large actuation strains [14]. The artificial muscles are
generally electrically activated by external electrical power and the associated
electronics and robotic control [15]. Other potential applications include acoustic,
impact and strain sensors, microphones and hydrophones, optical/mechanical switching
devices, tunable membrane structures, and electromechanical transducerslMEMS
devices. A potential niche application for EAP material is the development of Braille
displays. Perfluorsulfonate polymers can be used to form electro active polymer
composites that deform in response to an applied electrical potential across an EAP
composite. Poly(vinylidene fluoridetrifluoroethylene) copolymers[13], a giant
electrostriction (strain >5%) can be induced when the polymer is treated with properly
high energy electron irradiation as shown in figure-I. 1.
Figure-I.1 Left - The bimorph with no voltage applied to it. Right - The same bimorph
after the voltage was turned on.
1.2.2 Piezoelectric Polymers
Materials, which exhibit piezoelectric behavior, generate a charge in response to
a mechanical deformation or alternatively undergo a mechanical deformation in
response to an applied electric field. This capability facilitates the use of these smart
materials for either sensors or actuators in intelligent material systems and structures
[16]. The applications of piezoelectric materials ranges over many fields, including
ultrasonic transducers, actuators and ultrasonic motors; electric components such as
resonators, wave filters, delay lines; surface acoustic wave devices and transformers
Cfiapter 1 5
and high voltage applications; and gas igniters and ultrasonic cleaning and machining
[17]. Examples for piezoelectric materials; lead-zirconate-titanate, lithium niobate etc.
The most widely used commercial piezoelectric material is the various phases of lead
zirconate titanate. Polymer piezoelectric materials are known to exist since the end of
twenties. Piezoelectric polymers are associated with a low noise and inherent damping
that makes them very effective receivers as well as broadband transmitters for high
frequency tasks [18].
Pioneering work in the area of piezoelectric polymers by Kawai [19] has led to
the development of strong piezoelectric activity in polyvinylidene fluoride and its
copolymers with trifluoroethylene and tetraflouoroethylene. These semicrystalline
fluoro polymers represent the state of the art in piezoelectric polymers and are currently
the only commercially available piezoelectric polymers. Odd numbered nylons, the
next most widely investigated semicrystalline piezoelectric polymers, have excellent
piezoelectric properties at elevated temperatures but have not yet been embraced in
practical application. Other semicrystalline polymers including polyureas, liquid
crystalline polymers, biopolymers and an array of blend combinations have been
studied for their piezoelectric potential.
1.2.3 pH Sensitive Polymers
Smart polymers consist of polymers whose transition between the soluble and
insoluble state is created by decreasing the net charge of the polymer molecule. The net
charge can be decreased by changing the pH to neutralize the charges on the
macromolecules and hence to reduce the hydrophilicity of the macromolecules [20].
Polyelectrolyte complexes that have pH dependant solubility were successfully used in
different bioseperation procedure. pH responsive behaviour are utilized for the
preparation of so-called 'smart' drug delivery systems, which mimic biological
response behaviour to a certain extent. The possible environmental conditions to use for
this purpose are limited due to the biomedical setting of drug delivery as application.
Different organs, tissues and cellular compartments may have large differences in pH,
which makes the pH a suitable stimulus [21].
Work has been done on a commercially available enteric polymer Eudragit S
100 for its application in bioseparation and biocatalysis [22, 23]. This non-toxic
Cfiapter 1 6
methylmethacrylate polymer is a reversibly soluble polymer, which precipitates at
slightly acidic pH (~4.8) and redissolves around pH 6.0 (The exact transition point for
solubility depends upon medium properties such as ionic strength). This pH-responsive
behavior is the result of the presence of both free carboxyl groups and hydrophobic
moieties. At lower pH, carboxyl groups are protonated and hydrophobic interactions
dominate, this leads to precipitation of the polymer. The presence of hydrophobic
moieties enables application of the polymer as protein folding.
1.2.4 Temperature Sensitive Materials
These are the materials that respond to temperature. The first materials known
to have thermo-responsive shape memory properties are shape memory alloys (SMA)
(eg. TiNi, CuZnAl and FeNiAI alloys) [24]. The structure phase transformation of these
materials is known as martensitic transformation. Shape memory alloys have been
studied extensively for over 40 years and they are now utilized in a variety of
applications including flexible eyeglass frames, self-adjusting orthodontic wires, stents,
actuators and temperature responsive valves [25-27].
The investigation of SMA promoted the study of shape memory characteristics
in polymeric materials [28-32]. Thermo-responsive shape memory polymers (SMPs)
have attracted considerable attention due to their unique recovery phenomenon upon
heating [33-35]. SMP comprises a new type of 'intelligent' polymers.
Advantages of SMP over SMA
Compared to SMA, SMP have attained more attention due to the advantages
like [36-40]
(i) Lighter weight
(ii) Larger deformation
(iii) Economical to produce
(iv) Easier molding and processing
(v) Freedom from rust
(vi) Ability to impart color to products
Cliapter 1 7
(vii) Amenability for variations in transition temp & mechanical properties
(viii) Normal molding processes, permit complex shapes
Moreover, the shape recovery in alloys is enthalpy driven and it is the entropy
driven process in polymers. SMP materials have the capacity to recover large strains
imposed by mechanical loading. The unconstrained recoverable strain limits in SMP
materials are on the order of 100%, but in SMA it is 10% [41].
1.3. THERMO-RESPONSIVE SHAPE MEMORY POLYMERS
The shape memory effect is not related to a specific material property of a
single polymer but results from a combination of the polymer structure and the polymer
morphology together with the applied processing and programming technology [42].
The process of programming and recovery of a shape is schematically shown in Figure
1. 2. First, the polymer is conventionally processed to receive its permanent shape.
Then, the polymer is deformed at a temperature above its transition temperature and the
temporary shape is fixed by lowering the temperature below its transition temperature.
This process is called programming. The programming process either consists of
heating, deforming and cooling the sample or drawing the sample at a lower
temperature. The permanent shape is now stored while the sample shows the temporary
shape. Heating up the SMP above a transition temperature induces the shape memory
effect, which results the recovery ofpermanent shape ofa SMP.
Permanent shape deformed shape recovered shape
(permanent shape)
Figure-I. 2 Schematic ofprocessing ofSMP
SMPs consists of two components, one of which is the cross-links that
determine the permanent shape and the other is the switching segments phase
(thermally reversible phase) that fix the temporary shape [43]. Cross linkages can be
Cliapter 1 8
achieved either by physical interactions or by chemical bond. Thermally reversible
phase is designed to have a large drop in elastic modulus on heating above the shape
recovery temperature. Therefore, either the glass transition temperature or the melting
temperature of a polymer can be used as the recovery temperature [44]. Almost all
SMPs are rubber elastic to a certain extent. SMP have to be either covalently or
physically cross-linked. Polymer networks are less susceptible to creep, thus ensuring
excellent reproducibility of the material even after many cycles [45-48].
The thermally stimulated shape memory process of SMP can be briefly
described as follows [35, 31]. In SMPs rubber elasticity is observed at the temperature
range between Ttrans and the melting temperature of hard segment (Tmh) because of
micro Brownian motion and the restricted molecular motion of the soft segment due to
the crystalline hard segment phase. When they are deformed in the temperature range
below Ttrans and subsequently cooled below Ttrans under a constant strain, the deformed
shape is fixed because their micro-Brownian movements are frozen. When they are
reheated to a temperature above Ttrans and original shape is recovered as a result of
shape memory effect. That is the original shape is recovered by the elastic force stored
during the deformation [34]. Shape recovering process is schematically shown in
Figure-I.3.
Original shape
Deformed shape
Heat
T>Ttrans
Heat
T>Ttrans
Rubbery stage
Original shaperecovered
Apply stress
•
Figure-t.3 Thermally stimulated shape-recovering process ofSMP
Cfiapter 1 9
SMP materials have the ability to recover large strains imposed by mechanical
loading. They are designed to have a large change in elastic modulus above and below
the glass transition temperature (Tg) of the amorphous phase. Characteristics such as
shape recoverability and shape fixity exist due to the difference in mechanical
properties of the material above and below the phase transition temperature. The ability
of the shape memory polymer to fix the deformed shape by cooling below the transition
temperature is called the shape fixity. Shape recovery can be defined as the ability of
the material to recover its original shape.
1.3.1 Prime Requirement for Shape Memory Property
Polymeric materials having shape memory properties can be designed by taking
the polymer networks in which the polymer chains are able to fix a given deformation
by cooling below a certain transition temperature [49-51]. This transition temperature
can be a glass transition temperature or a melting temperature. A reasonable way to
produce polymers with shape memory properties is the preparation of networks
containing crystallizable polymer chains. The crystallinity can be induced by polymer
chains or to a side chain [52, 53]. Thus the prime condition to have shape memory
property is that the polymer should crystallize. This can be achieved only if the
crystallizable polymer can aggregate in domains, which means that the morphology of
the network plays an important role in the final material properties [49].
SMPs show mechanical behaviour including fixing the deformation as the
plastic and also recovering the deformation as the rubber. They behave as plastic and as
rubber under a series of thermo-mechanical treatments. In addition, the fixed
deformation function of the materials required the higher phase-transition temperature
of the reversible phase in the operating temperature range [54]. Shape memory PU
consists of two segments, soft segments and hard segments. The hard segment domains,
which dispersed in the soft matrix, act as physical junction points. The soft segment is
responsible for a thermal shape memory effect that occurs when the polymer is heated
above the shape memory transition temperature (Ttrans). Soft segments are glassy or
semi-crystalline below the transition temperature. The purpose of hard segments is to
Cliapter 1 10
fix the permanent shape through physical cross-links, which must be thermally stable
above the Ttrans.
1.3.2 Polyurethane Based Systems
PUs are segmented copolymers consisting of soft domains derived from a
polyol and hard segments domains derived from a diisocyanate and a chain extender as
shown in Scheme-1.1 [55].
OH-R'-OHOH-- _
OH + NCO-- R --NCO
o 0
II 1\--0----______ O-C-NI+--R-NH-C-O-R'--O--
L..-I --JI I Isoft segment hard segment
Scheme-1.1 Urethane reaction forming soft and hard segments
A vast amount of research has been done on the morphology [56-61], crystal
structure [61-64], thermal behaviour [65-67], deformation behaviour [68, 69] and
rheological properties [70] of different kinds of PUs in the past decade. However this
polymer has been investigated mainly on its practical property as a rubber material.
Recently, it has been noticed that certain PUs exhibited interesting shape memory
properties. PU constitutes most important thermoplastic among SMPs [42].
Compared to other SMPs, apart from the advantage of easier processing, PU
shape memory polymers have a wide range of shape recovery temperature (from 30
70°C), higher recoverable strain (maximum recoverable strain>400%) and better
biocompatibility [31, 71]. The structure property relationships of PUs are extremely
diverse and easily controlled.
Cliapter 1 11
1.3.2.1 Poly (&-caprolactone) Based Systems
o NCO
(1) Poly (e-caprolactone) (2) 4,4'-methylenebis(phenylisocyanate)
. ./"'..... ./"'..... /OHOH' ~ ~
(3) 1,4-Butane diol
Scheme-I. 2 Components of PCI-based PU with shape memory properties
In the mid 1990 Kim and coworkers reported structurally simple and well
defined segmented polyurethanes with (PCL) soft segments that showed excellent
shape memory properties [72-74]. The components of PCI-based PU are shown in
scheme-1.2. These shapes memory polymers were derived from PCI (1) of varying
lengths (Mn, 2000-8000g/mol), 4,4'-methylenebis(phenyl isocyanate) (MDI) (2) and
1,4-butane diol (3) as chain extender via a prepolymer method common to many
polyurethane synthesis. The highest thermal transition corresponding to the melting
temperature of the hard segment determining blocks was found in the range between
200 and 240°C. PCI with a number average molecular weight between 1600 and
8000g/mol formed the switching segments. Switching temperatures of the shape
memory effect can be varying between 44 and 55°C depending on the molecular weight
of the soft segment (PCI) and the weight fraction of the switching segments (variation
from 50 to 90wt%). The crystallinity of the PCL segments increased with increasing
Mn, whereas the polymers made from a PCL with a Mn of only 2000g/mols were
unsuitable because they failed to crystallize, presumably because of the low degree of
phase separation. The optimum hard segment was 30-45% whereas below 10wt%, the
hard domains were unable to provide the required physical cross-links and led to
insufficient mechanical strength. In cyclic thermo-mechanical measurements, an
increase of the initial slope with the number of cycles is observed. A behaviour, which
is named 'cyclic hardening', can be observed during the initial four to five cycles. This
effect is caused by a relaxation of the material in the stretched state, which results in an
Cliapter 1 12
increasing orientation, and crystallization of the chains. As a consequence, the
resistance of the material against the strain grows with the number of cycles. The
materials reached constant strain recovery rates after the third cycle. The best shape
memory properties were obtained for polymers containing (a) long PCI segments (Mn
4000 or 8000g/mol) and (b) high fraction (30-45wt %) of hard segments. Strain
recovery rates approached 98% in these cases provided that the applied strain was kept
low. Besides, the crystallinity of the switching segments, the decisive factors that
influence the recovery properties and the stability of the hard segment forming domains
especially in the temperature of the switching segment crystallites are important
parameters. Above a lower limit of the hard segment determining blocks of IOwt%, the
hard segments domains are no longer sufficiently pronounced to function efficiently as
physical cross-links. Table-I.I and table-I.2 shows the values determined for shape
recovery and shape fixity rates for different Mn of PCl.
Table-I.!
Dependence of the strain recovery rate (Rr) after the first cycle (1) in %) at Em 80% onthe switching segment content SC [wt%] for different Mn of PCI [74]
M n SC Rr (1) M n SC Rr (1)
4000 81 98 7000 92 60
5000 89 50 7000 88 93
5000 84 96 7000 85 95
5000 80 98 7000 81 98
Table-I. 2
Dependence of the strain recovery rate (Rr) and strain fixity rate (Rr) after the first (1)and fourth (4) cycle on the switching segment content (SC) [wt%] and the strain Em fordifferent Mn of PCI [73]
M n SC Em Rr (1)[%] Rr (4)[%] Rr(1) [%] Rr(4)[%]
2000 70 200 48 20 95 98
2000 55 200 73 65 58 60
4000 75 600 75 60 85 90
4000 70 200 82 73 92 95
8000 55 200 62 52 88 90
Chapter 1 13
1.3.2.2 Polyether Based Systems
~o
(4)Poly(tetramethyleneoxide)
Scheme-1.3 Structure ofPTMO
Lin and Chen [32, 75] have succeeded in developing polyether based PUs
derived from poly (tetramethyleneoxide) (4) (PTMO), MDI (2) and 1,4-butanediol (3)
as the cross-linker that exhibited shape memory behaviour. Scheme-I.3 shows the
structure of PTMO. A two-step polymerization method was adopted for the PU
synthesis. In the first part of their studies, they varied the molar ratios of MDI and 1,4
butane diol, which constitute the hard segment phase varying from 57 to 95% in the
case of PTMO 250 series and from 31 to 86% in the case of PTMO of molecular
weight 650. Thermal studies of PTMO 250 series exhibited glass transition
temperature, recrystallisation and crystal melting step by step. But PTMO 650 series
exhibited only crystal melting behaviour. Tg of the PU shifted to high temperature with
increase in mole ratio of MDI and l,4-butane diol. The compatibility of the soft
segment and hard segment phase was also better on increasing the mole ratio of MDI
and l,4-butane diol. The modulus ratio of the PUs decreased with increase in MDI and
BD. The shape memory behaviours of the PU were influenced by the modulus ratio. In
the second part of their studies, polyol (PTMO) was varied in order to study the shape
memory behaviour. Molecular weight of PTMO was varied from 250 to 2900g/mol.
With increase in molecular weight of PTMO, Tg shifted to lower temperature. The
shape memory behavior ofPU depended on its morphology.
Research on thermoplastic shape memory PU has been of great interest, which
could find potential applications in the development of highly vapour permeable thin
film. Its high vapor permeability at warm temperature and low permeability at cold
temperature has been of great interest. Breathable fabrics [76, 77] impermeable to
liquid water and permeable to water in the vapour phase were obtained by coating
shape memory PU [78] on to polyester, nylon, cotton or silks fabrics.
Cfiapter 1 14
1.3.2.3 Incorporation of Ionic Components or Mesogenic Moities in the
Hard Segment-forming Phase of PU
Several investigations have dealt with question as to whether the incorporation
of ionic or mesogenic moieties into the hard segment forming phase can influence the
mechanical and shape memory properties through the introduction of additional
intermolecular interactions by possibly enhancing the degree of phase separation.
1.3.2.3.1 Incorporation of ionic components
(5) 2,2-bis(hydroxymethyl)propionic acid
Scheme-1.4 Structure of ionic moiety
To study the incorporation of ionic components, PUs were synthesized from
PCI (1), MDI (2), 1,4-butane diol (3) as chain extender and 2,2
bis(hydroxymethyl)propionic acid (5) (structure is shown in scheme-lA) as IOmc
component [79]. Additional interactions were introduced by substituting half of the
chain extending l,4-butane diol which forms the hard segment phase with 2,2-bis
(hydroxymethyl)propionic acid. PCI with molecular weight varying from 2000 to
8000g/mol were used as the switching segment phase and the switching soft segment
content was varied from 55 to 90 wt% with PCI of Mn 4000g/mol. The switching
temperatures for the shape memory effect were between 44 and 55°C. These multiblock
copolymers having polyelectrolyte character are so called 'ionomers'. Due to the
presence of additional columbic interactions, these polymers exhibited higher elastic
modulus and higher mechanical strength than the systems containing uncharged hard
segment blocks. The transition from the uncharged multiblock copolymers to ionomers
Chapter 1 15
result in an increase in the elastic moduli between 24 and 34% at 25°C, depending on
the molecular weight and weight fraction of the PCl used. At 65°C, increase in elastic
moduli was between 38 and 156%. Increase in hard segment content increased the
mechanical stability of the materials. The charged systems showed approximately
higher strain recovery rates and equal strain fixity rates compared to the uncharged
systems. Table-1.3 shows the strain recovery and strain fixity rate for the ionomer and
the nonionomer systems, both based on PCl, with Mn 4000g/mol and 70wt% PCl
content after the first cycle and fourth cycle.
Table-1.3
Comparison of strain recovery rate (Rr) and strain fixity rate (Rr) at cm=200% after first
cycle (1) and fourth cycle (4) for 70wt% PCl content [79]
Ionomer
Nonionomer
1.3.2.3.2
Rr (1)% Rr (4)% Rr(1)% Rr(4)%
66 45 95 95
62 35 95 95
Incorporation of mesogenic moieties
OH� 0 O
�OH
(6) 4,4'-bis(2-hydroxyethoxy) biphenyl
HO� 0 O
�OH
(7) 4,4'-(6-hydroxyhexoxy) biphenyl
Scheme-1.5 Components of mesogenic moities
Chapter 1 16
Incorporation of mesogenic diols such as 4,4' -bis(2-hydroxyethoxy) biphenyl
(6) (BEBP) or 4,4'-(2-hydroxyhexoxy)biphenyl (7) (BHBP) (structures are shown in
scheme-1.5) into the hard segment determining blocks based on MDI (2) and 1,4
butanediol (3) resulted in an increased solubility of these blocks in the switching
segment made of PCI (1) of molecular weight 4000g/mol [80]. As a result, a mixed
glass transition (Tg) value of the hard segment and the switching segment forming
phase is observed. An increase in strain fixity rate of more than 20% is observed in the
case of polymers with addition of BEBP with a soft segment content increasing from
60 to 80wt% and a maximium strain of 100%. Addition of BHBP on polymers showed
an increase in the strain fixity rate of 5 to 10% with a strain of 100 to 300%.
1.3.2.4 Polylactide Based Polyurethane
Polylactide based PUs exhibiting excellent shape memory property were
synthesized from poly(L-lactide)diols (PLLA), hexamethylene diisocyanate (HDI) and
l,4-butane diol (3) [36]. Polylactide and its copolymers have been used in applications
such as bio-assimilable sutures and drug-loading devices because of their
biodegradability, biocompatibility and good mechanical strength [81-83]. A series of
segmented PUs with PLLA as soft segment, and HDI and l,4-butane diol together
constituting the hard segments were synthesized and their Tg was in the range of 33
53°C which is influenced by the molecular weight of PLLA and the soft segment-hard
segment ratio. By changing the hard to soft segment ratio and molecular weight of
PLLA, Tg and shape recovery temperature could be adjusted close to body temperature.
Hence, these type polymers find potential applications in medical field. '
A series of segmented poly (L-Iactide)-polyurethanes (PLLA-PU) were
synthesized by Peng et al [84] by a two-step method, with oligo-poly (L-Iactide) as the
soft segments and the reaction product of TDI and ethylene glycol as the hard
segments. The processed PLLA-PUs recovered almost 100% to their original shape
within lOoC from the lowest recovery temperature. In the recovery process, the PLLA
PUs showed a maximum contracting stress of shape change in the range of 1.5-4 MPa
depending on the PLLA segmental length and the hard segmental content. This kind of
polyurethane can be used as implanted medical devices with a shape memory property.
Cliapter 1
1.3.3 Block Copolymers with Shape Memory Property
17
1.3.3.1 Triblock Copolymers
ABA triblock copolymers with a central PTMO (4) block (B block) with number
average molecular weights between 4100 and 18800g/mol with terminal poly (2
methyl-2-oxazoline) blocks (A block) with molecular weights of 1500g/mol were
developed by cationic ring-opening polymerization [85]. The A blocks which
represents the hard-segment forming phase exhibited glass transition temperatures
around 80°C. The PTMO blocks are semicrystalline and exhibit a melting temperature
between 20 and 40°C, depending on their molecular weight. These melting
temperatures are used as switching temperatures for a thermally induced shape-memory
effect. The materials with PTMO blocks having a molecular weight greater than
13000g/mol exhibited very good mechanical strength at room temperature If a sample
with a number-average molecular weight of the central PTMO block of 19000g/mol is
stretched to 250% elongation at 22°C, exhibited nearly complete recovery upon heating
the sample to 40°C.
1.3.3.2 Block Copolymers of Polyethyleneoxide and Polyethylene Terephthalate
o
(8) Polyethyleneterephthalate (9) Polyethylene oxide
Scheme-1.6 Components of block copolymers
Shape memory polymers with hard segment forming phase based on
polyethyleneterephthalate (8) (PET) and switching segment blocks of polyethylene
oxide (9) (PEO) were reported [86-89]. The structures are shown in scheme-1.6. The
thermally induced shape memory effect was triggered by the melting temperature of the
Cliapter 1 18
PEO crystallites, which varied from 40 and 60°C depending on the molecular mass of
the PEO blocks or on the PET content. The crystallinity of the switching segment
increased as the molecular weight of PEO increased. The highest thermal transition is
the melting point of the PET at 260°C. The crystallization of the PEO block is highly
hindered as the weight fraction of PET increases. With an increase in content of hard
segment determining PET blocks, the melting temperature of the switching segment
with the same molecular weight decreased. For a stretched sample, at a temperature
near the melting temperature of the PEO segments, crystallites of PEO segments
resulted.
1.3.3.3 Block Copolymers of Polystyrene and Poly (l,4-butadiene)
(10) Poly (l,4-butadiene) (11) Polystyrene
Scheme-1.7 Structure of block copolymers
It is not always mandatory to use crystallisable switching segments to exhibit
shape memory behaviour. This can be achieved by designing copolymers with
appropriate hard and soft segments. Thus, block copolymers of poly (l ,4-butadiene)
(10) and polystyrene (11) have been found to exhibit shape memory properties [90,91].
Scheme-1.7 shows the structure of copolymers. The melting temperature of the poly
(1,4-butadiene) crystallites represents the switching temperature for the thermally
induced shape-memory effect. Polystyrene supplies the hard-segment-determining
blocks. For a material with a poly (l,4-butadiene) content of 86 mol% and a weight
average molecular weight of 70,000g/mol, the formation oftwo crystal modifications in
the polybutadiene blocks was reported. A high strain recovery rate in the range of 80%
was observed (Ttrans = 80°C) leading the application of a maximum strain in the range
Cliapter 1 19
of 100%. Recovery rate reached values of around 60% at a maximum strain of more
than 400%. The appearance of the shape-memory effect was explained by the
formation of oriented crystallites upon application of high elongations of up to 600%.
Shape memory polymer comprising of styrene and vinyl compounds cross
linked by a multifunctional cross-linking agent as divinyl benzene, bis(4
vinyloxy)butyl)terephthalate or bis(4-vinyloxy)methyl)cyclohexyl)methylterephthalate
polymerized by free radical or cationic initiator exhibiting shape memory property have
also been reported [92]. This SMP finds particular application as contact lens mold.
1.3.3.4 Polyethylene-poly (Vinylacetate) Copolymers
(12) Poly[ethylene-co-(vinyl acetate)]
Scheme-1.8 Structure of poly [ethylene-co-(vinyl acetate)]
Cross-linked poly [ethylene-co- (vinyl acetate)] (12) (EVA) (Scheme-1.8 shows
the structure of EVA) with linear EVA is synthesized by using dicumyl peroxide as
radical initiator in a thermally induced cross-linking process [93]. Depending on the
initiator concentration, the cross-linking temperature, and the curing time, materials
with different degrees of cross-linking are obtained. A copolymer with 28-wt% vinyl
acetate content showed a melting temperature of 70°C for the polyethylene crystallites.
Melting temperature is independent of the cross-linking. Strain fixity and strain
recovery rates for materials which have been cross-linked at 170°C at a maximum
elongation of 100%, exhibited an increased strain recovery rate and decreased strain
fixity rate with increase in gel content.
Cliapter 1 20
1.3.3.5 PolyethylenelNylon-6-Graft Copolymer
a
(13) Polyethylene (14) Nylon-6
Scheme-1.9 Structures ofPE and nylon-6
Polyethylene (13) (PE) grafted with nylon-6 (14) that has been produced in a
reactive blending process of PE with nylon-6 by adding maleic anhydride and dicumyl
peroxide showed shape-memory properties [94]. Structure of PE and nylon-6 are
shown in Scheme-1.9. The nylon-6 content of these materials is between 5 and 20-wt%.
Nylon-6, which has a high melting temperature of around 220°C that forms the stable
physical net points, comprised the hard-segment-forming domains in a matrix of
semicrystalline PE. The melting point of the PE crystallites at 120°C determined the
switching temperature for the thermally induced shape-memory effect. Strain fixity
rates of around 99% and strain recovery rates between 95 and 97% have been obtained
for these materials for an elongation of 100%. Table-IA shows the dependence of
strain fixity rate and strain recovery rate of the copolymers.
Table-l,4
Dependence of the strain recovery rate (Rr) and strain fixity rate (Rr) on nylon-6 content
Material Rr [%] Rr[%]
PE with 5 wt% nylon-6 95.0 99.8
PE with 10 wt% nylon-6 94.9 - 96.9 99.0 - 99.5
PE with 15 wt% nylon-6 96.0 98.9
PE with 20 wt% nylon-6 96.6 98.6
Cliapter 1 21
1.3.4 Shape Memory Polymers with Semi-inter Penetrating Networks
Liu et al [95] have succeeded in developing semi-IPN shape memory polymers.
These semi-IPN polymers were synthesized by radical polymerization and cross
linking of methylmethacrylate and linear polyethylene glycol (PEG) (9) using AIBN as
initiator (0.5wt%) and ethylene glycol dimethacrylate as cross-linker (0.6- 5.8wt%).
This semi-IPN polymer possesses two independent shape memory effects at two
transition temperatures. Two-shape memory effect occurred at melting temperature of
PEG crystals and glass transition temperature of semi-IPN (PMMA-PEG) respectively.
When semi-IPNs were introduced, the crystalline PEG served as switching segments
and the PMMA-PEG semi-IPNs show shape memory behaviour based on the reversible
order-disorder transition of crystalline aggregates of PEG [96].
1.3.5 Shape Memory Polymer Containing Short Aramid Hard Segments
Rabani et al [27] developed a SMP by reacting 4-aminobenzoyl, end
functionalized poly (E-caprolactone) (1) with three aromatic diacid dichloride in the
presence of chlorotrimethylsilane. Aramid units possess high elongation at break
(800%) similar to many other PU counterparts. And also, they provided effective
physical cross-links even at a relatively low hard segment content of about 12wt%.
Two aramid containing polymers derived from PCI showed promising characteristics of
shape memory polymers. Stress-strain curves became almost superimpossible from the
third cycle onwards. Although the cold drawing process is not suited for this class of
SMPs, these SMPs were good enough for loading at RT (cold drawing), which has an
important economical advantage. Such shape memory polymers find applications in
actuator, toys and simple heat sensitive devices.
Cfzapter 1
1.3.6
1.3.6.1
OTHER POLYMERIC SYSTEMS
Polynorbornene
22
(15) Polynorbomene
Scheme-1.10 Structure of poly (norbomene)
Norsorex is a linear, amorphous polynorbomene (15) developed by the
companies CdF Chemie/Nippon Zeon in the late 1970s by a ring-opening metathesis
polymerization of norbomene using a tungsten-carbene complex as catalyst. Structure
of poly (norbomene) is shown in scheme-I. 1O. The obtained polynorbomene contains
70 to 80mol% of trans-linked norbomene units and has a glass transition temperature
between 35 and 45°C [97, 98]. The shape memory effect of amorphous material is
based on the formation of a physically cross-linked network due to the entanglements
of the high molecular weight linear chains, and on the transition from the glassy state to
the rubber-elastic state [99]. The norsorex is not purely amorphous. Sakurai and
Takahashi have reported a high molecular weight polynorbomene with a high
proportion of trans-linked norbomene units showing a tendency towards strain-induced
crystallization [93, 94].
Organic or inorganic hybrid polymers consisting of polynorbomene (15) units
that are partially substituted by polyhedral oligomeric silsesquioxanes (POSS, that is,
polycyclic silicon - oxygen compounds of the general formula Si2nH2n03n) also form an
amorphous polynorbomene material [100]. The hydrogen atoms of the POSS units are
either substituted by cyclohexyl or cyclopentyl substituents. A POSS modified
polynorbomene is obtained by a ring-opening metathesis polymerization of hybrid
POSS-norbomene monomers and norbomene using a molybdenum catalyst with a
content of 60 to 73 mol% cis-linked norbomene units. The weight-average molecular
weights obtained are in the range between 75,000 and 7,40,000g/mol. The increase in
Chapter 1 23
the glass transition temperature of the polynorbomene by incorporation of the POSS
modified comonomers results in an improved heat and oxidation resistance. It should
be possible to obtain better shape-memory properties by increasing the glass transition
temperature to additionally slow down the relaxation of the polynorbomyl chains after
stretching above Tg. For a maximum elongation of300%, both the pure polynorbomene
and the POSS-modified polynorbomene with cyclopentyl substituents showed a strain
recovery rate of approximately 75% upon heating to a temperature above Tg. When the
recovery experiment was performed under a stress of 0.8 MPa for the polynorbomene
homopolymer, because of the increased softening at temperatures above Tg after
reaching the maximum recovery of 75% at a temperature of 70°C, a stretching to over
200% at a temperature of 90°C could be observed. However, the polynorbomene with a
content of 50-wt% POSS-modified norbomene with cyclopentyl substituents retained
its shape even at high temperatures (up to BO°C).
1.3.6.2 trans-polyisoprene and Urethane Segments
Ni and Sun [101] have developed trans-polyisoprene segmented urethane
copolymer with good shape recovery behaviour. Copolymer with 70% trans
polyisoprene segment showed a recovery rate of 85% and shape fixity close to 100% at
a maximum strain of 100%. The synthesized copolymer film exhibited a distinct
microphase separation with urethane segments, arranged into spherical domains, and
the driving force for the phase separation originated from the incompatibility of trans
polyisoprene and urethane segments.
1.3.6.3 Polymers Cross-Linked by Ionizing Radiation
The memory effect of PE (13) was first described by Charlesby by ionizing
radiation (y rays or neutrons) [102]. A chemically cross-linked network is formed by
application of low doses of irradiation. Polyethylene crystals are formed on cooling
below its crystallization temperature, and can fix the temporary shape by acting as
physical net points. Heating the material above the melting temperature of the
crystalline domains results in recovery of the permanent shape, which was fixed during
the irradiation process. This technology has reached high economic significance in the
field of heat-shrinkable products. The energy and dose of the radiation have to be
Cfiapter 1 24
adjusted to the geometry of the sample to reach a sufficiently high degree of cross
linking and hence sufficient fixation of the permanent shape. Heat-shrinkable products
made of cross-linked; semi crystalline polymers find applications as semi finished
products, for example, as heat-shrinkable tubing, or as the final products in the
expanded state. These are connected to a substrate by heat treatment.
1.3.6.4
1.3.6.4.1
Acrylate Based Systems
Oligo(E-caprolactone)dimethacrylates
Lendlein et al [103] have studied polymer networks prepared through the cross
linking of oligo(E-caprolactone)dimethacrylates (PCl dimethacrylates) under photo
curing with or without initiator that exhibited thermally induced shape memory effect
With increase in molecular weight of PCI, melting temperature of the polymer
networks increased and it was possible to alter the mechanical properties of the
polymer to a wide range. The polymeric materials are adjustable with respect to Ttrans
from 30 to 50°C through the molecular weight of macro dimethacrylate. They exhibited
excellent shape memory property with a shape recovery rate between 92 and 97% and
strain fixity rate between 86 and 97% after five cycles. These materials found potential
applications in biomedicine including catheters and stents.
Shape memory polymers prepared by copolymerizing, alkylmethacrylate
monomers (methyl methacrylate, MMA and butyl methacrylate, BMA) and the cross
linking agent (tetraethylene glycol dimethacrylate, TEGDMA), in which each
monomers separately produce polymers that were characterized by different glass
transition temperatures in the presence of a difunctional monomer whereby the
copolymer formed was cross-linked during the polymerization to form a thermoset
network [104]. The transition temperature of the final polymers was adjusted by the
ratio of the monomers selected, from about 20°C to about 110°C, while the degree of
cross-linking controlled the rubbery modulus plateau. The shape memory polymers can
be processed as castable formulations in the form of coatings and films. The
copolymers are optically transparent and are useful as medical plastics.
Chapter 1
1.3.7 SHAPE MEMORY POLYMER NANOCOMPOSITES
25
Fabrication and characterization of composite based on a shape memory
polymer matrix and nanoparticulate SiC reinforcements were studied by Gall et al [34].
Four different composition of SiC by weight % (10%, 20% 30% and 40%) were added
to the shape memory polymer epoxy resin (CTD-DP7) to create different nano
particulate composite materials. Electron microscopy showed that the nanoparticulate
reinforcements were well dispersed throughout the shape memory polymer matrix. The
elastic modulus and the microhardness of the nanocomposites increased by
approximately by a factor of 3 with the addition of 40wt% SiC into the SMP resin. The
increase in both of these material properties is a direct consequence of the relatively
high hardness/modulus of the SiC particle relative to the polymer matrix. The increase
in hardness and modulus is directly proportional to the weight fraction of SiC. The
recoverability ofthe composites depends on the presence and the volume fraction of the
SiC reinforcement. Experimental result demonstrates that the addition of SiC lowers
the unconstrained recoverable strain limit and increases the attainable constrained
recovery stress force. The decrease in the recoverable strain limit is due to the inability
of the finite fraction of SiC particle to exhibit shape memory characteristics.
Constrained bending recovery force in the nanocomposite was shown to increase by
50% with the addition of 20wt% SiC.
Liu et al [105] have studied the shape memory property of nanoscale SiC
reinforcements on shape memory polymer epoxy system (DP7AR, supplied by CTD).
In this studied they fabricated a SMP nanocomposite with 20wt% of SiC nano powder
of particle size 700nm. The glass transition of the SMP is about 88°C. With the
addition of 20wt% SiC nanoparticles, Tg increases by ~10°C. Storage moduli for the
SMP nanocomposites at 26°C and 118°C are higher than that of the SMP resin. When
pre-deformed at a temperature above the glass transition temperature, the relationship
between recoverable strain/stress and stress/strain constraint of SMPs was governed by
the stress- strain response of the materials at the pre-deformation temperature.
The bonding mechanism of poly (D,L-Iactide) (PDLLA) and hydroxyapatite
(HA) nanoparticles have achieved great importance because of their increasing
application in medical fields. Hydrogen bonding between PDLLA and HA in
Cliapter 1 26
PDLLA/HA nanocomposites were investigated by SEM, DSC, FTIR, and X-ray
photoelectron spectroscopy (XPS) [106]. Structural morphology and glass transition
temperature (Tg) of the nanocomposites showed that there was a close interaction
between polymer matrix and inorganic nanoparticles. The results from FTIR and XPS
indicated that the hydrogen bonding between the C=O in PDLLA and the surface P-OH
groups ofHA nanocrystalline was formed. Due to the existence of hydrogen bonding in
these nanocomposites, shape memory properties were improved.
Shape memory properties of nanoclay-tethered polyurethane nanocomposites
were reported by Cao et al [107]. Polyurethanes based on PCL diol (1), MDI (2), and
l,4-butane diol (3) and their nanocomposites of reactive nanoclay were prepared by
bulk polymerization in an internal mixer and the values of shape fixity and shape
recovery stress were determined as function of clay content. The melting point of the
crystalline soft segment was used as the transition temperature to actuate the shape
memory actions. It was observed as exfoliated clay particles in the polymer matrix. A
20% increase in the magnitude of shape recovery stress was obtained with the addition
of 1 wt% nanoclay. The room temperature tensile properties were seen to depend on the
competing influence of reduced soft segment crystallinity and the clay content.
However, the tensile modulus measured at temperatures above the melting point of the
soft segment crystals showed continued increase with clay content.
Carbon nanotubes (CNT) have remarkable mechanical properties with very high
elastic modulus and electrical conductivity. CNT reinforced SMP nanocomposites were
developed with the typical CNTs of the vapor growth carbon fibers (VGCFs) [108]. A
fine and homogeneous dispersion of VGCF throughout the SMP matrix was obtained.
The specimens with different VGCF weight fraction, such as SMP bulk, 1.7 wt.%, 3.3
wt.% and 5.0 wt.%, were prepared, and their dynamic mechanical properties and shape
recovery behavior were investigated. It was found that storage elastic modulus was
improved obviously with increment of VGCF weight fraction, and the SMP
nanocomposites showed a good shape memory effect. It was observed that the recovery
stress of CNT/SMP nanocomposites with only 3.3% weight fraction of carbon
nanotubes reached almost twice of that in SMP bulk.
Thermoplastic polyurethanes having an alternating sequence of hard and soft
segments in which a nanostructured polyhedral oligomeric silsesquioxane diol (POSS)
was used as a chain extender to form a crystalline hard segment, exhibiting SMPs has
Cfiapter 1 27
been reported [109]. The polyurethanes were synthesised by reacting a polyol (PEG),
chain extender dihydroxyl-terminated POSS and a diisocyanate in a one step reaction.
The hybrid polyurethanes exhibited excellent shape recovery effect at the recovery
temperature and a retracting force, which was adjustable according to the composition
of the POSS. The developed polyurethanes have multiple applications including,
implants for human health care, drug delivery matrices, superabsorbant hydrogels,
coatings, adhesives, temperature and moisture sensors, etc.
1.3.8 SHAPE MEMORY POLYMER COMPOSITES
1.3.8.1 Fiber Reinforced Shape Memory Composites
Glass fiber reinforced shape memory polyurethane (DIARY, MM4510:
DIAPLEX Co. Ltd) exhibiting shape memory behaviour was developed by Ohki et al
[110]. Chopped strand glass fibers with fiber length of 3mm were used as the
reinforcement in the developed composite material. Weight fractions of the composite
were varied from 0 to 30%. The tensile strength of the developed composites became
higher with the increment of fiber weight fraction. The resistance to cycle loading and
crack propagation in the developed composite material increased due to fiber
reinforcement. There exists an optimum fiber weight fraction between 10 and 20% to
have an extremely low residual strain during cyclic loading. In O-wt% fiber content,
strain recovery ratio increased gradually with the number of cycles. However, for the
developed composites, the strain recovery ratio reached 70% or more and become
constant after second cycle with the increment of fiber fraction. It was confirmed that
the developed composite material kept the shape memory effect.
SMP based films and carbon fiber fabrics were developed by Zhang et al [111].
Polyesterpo1yol series of polyurethane SMP was used and its Tg was about 45°C. The
sequence and position of the SMP film and carbon fiber fabric in a laminate were
changed. Four kinds of specimens with different laminations of carbon fiber fabric and
SMP sheet were prepared for the shape memory behavior. The developed composites
have a large storage modulus compared with a SMP sheet. Composites showed
excellent shape recoverability. The bending recovery ratio of the SMP based laminates
was larger than that of SMP sheet. Recovery time greatly influence on the bending
recoverability.
Cliapter 1 28
1.3.8.2 Sandwich Type Shape Memory Composites
Sandwich type layered composites were developed by Yang et al [112] from
epoxy beam (epoxy was proprietary) and PU copolymers. PU was derived from PTMO
(4), MDI (2) and l,4-butane diol (3) as the chain extender, by a two-step process.
PTMO of Mn 2000g/mol was used for the studies. The hard segment content was varied
from 30-45wt%. Higher shape recovery was observed for PU with increased hard
segment content. The composite materials made of shape memory PU attained 5 times
impact strength and 4 times better damping effect than the epoxy beam alone. Hence it
is quite feasible for the shape memory PU to be utilized as a layered composites
material in the fields of high vibration control ability.
1.3.8.3 Shape Memory Composites Based on Copolymers
Copolymers of PET (8) and poly(ethylene glycol) (9) (PEG) cross-linked with
glycerol and sulfoisophthalate were synthesized to investigate the feasibility of
fabricating smart vibration controlling composite laminates [113]. The composition of
glycerol and sulfoisophthalate was varied to get copolymers with best shape memory
properties. Sandwich-type laminate composite was prepared by compressing two 1mm
epoxy laminate beams with PET/PEG copolymer sheet in the middle layer at 210°C in
a heating press. Shape recovery at low mol% of glycerol showed an initial decrease and
a sharp increase with sulfoisophthalate content, whereas with 2.5mol% glycerol
copolymer series showed a gradual increase with sulfoisophthalate content. Composite
laminates prepared with copolymers showed high impact strength and tan delta.
Poly(D,L-lactide) (PDLLA) and Hydroxyapatite (HA) composites were
prepared and studied for their biodegradation, biocompatibility and shape memory
properties [114]. Weight ratios of PDLLA and HA were varied. Transition
temperatures of the composites slightly increased with the addition of HA. The
composites showed desirable shape recovery effect; their recovery ratios were all above
95%. These composites have advantages, e.g. excellent shape memory effect,
biodegradation, biocompatibility, and osteoconductivity of HA, which are potential for
application in minimally invasive surgery, and bone and tissue repair.
Cliapter 1
1.3.9 BIODEGRADABLE SHAPE MEMORY POLYMERS
29
Fully biodegradable polymeric stent that can self-expand at body
temperatures (~3rC), using the concept of elastic memory was developed by Subbu et
al [115] by using poly L-Iactide-glycolide 80/20 copolymer, and poly D-L-Iactide
glycolide 53/47 copolymer, and polyethylene glycol of molecular weight 4000g/mol
were used as plasticizer. A solution-casting method was used to make the polymer
films. Bi-layered biodegradable stent prototypes were produced from poly-L-Iactic acid
and poly glycolic acid polymers. Elastic memory was imparted to the stents by
temperature conditioning. The thickness and composition of each layer in the stents are
critical parameters that affect the rate of self-expansion at 37°C, as well as the collapse
strengths of the stents. The rate of self-expansion of the stents, as measured at 37°C,
exhibited a maximum with layer thickness of about 6mm diameter. The Tg of the outer
layer is another significant parameter that affected the overall rate of expansion. The
application (coronary or peripheral vascular), of these stents are projected to be
endothelialized in 1-2 months, and hence the degradation of any layer is expected to
occur only when embedded in tissue.
Biodegradable shape-memory polymers-polylactide-co-poly(glycolide-co
caprolactone) multiblock (PLAGC) copolymers-were synthesized by the coupling
reaction of both macrodiols of polylactide and poly(glycolide-co-caprolactone) (PGC
diol) in the presence of 1,6-hexanediisocyanate as coupling agent [116]. The
copolymers formed were found to be thermoplastic and easily soluble m common
solvents. The mechanical properties and the shape-memory properties of the PLAGC
copolymers increased with the increase of PLLA segment contents and molecular
weights of both the PLLA and PGC segments. The shape-memory transition
temperatures of the PLAGC copolymers were about 45°C, which makes them possible
for medical use.
1.3.10 ELASTIC MEMORY COMPOSITE
Elastic Memory Composite (EMC) materials are a newly developed class of
composite materials based on shape memory polymer matrix hosting an ideal
combination of properties for deployable space structures. EMC materials retain the
structural properties of traditional fiber-reinforced composites while also functioning as
Cliapter 1 30
shape memory materials. These properties enable the creation of structurally efficient,
controllable, deployable space structures and deployment mechanisms with no moving
parts.
EMC materials differ from traditional composites in their ability to "freeze"
good levels of strain (i.e., 2-5% strain) without damage, and recover this "frozen" strain
upon subsequent heating. This high-strain and shape-memory behavior is inherent in
the shape memory matrix that is used to fabricate the EMC structure. When heated
above its glass transition temperature, an EMC resin becomes compliant and the fiber
reinforced EMC structure becomes capable of handling a very large amount of
recoverable strain without degradation [117]. This high-strain capacity allows the EMC
structure to be deformed or packaged into a compact geometry, and subsequently
cooled when the packaging is complete. Once cooled, the material becomes rigid again
and naturally holds or "freezes" its packaged shape indefinitely. When heated the
material elastically returns to its original 'remembered' shape, hence the use of the term
elastic memory. The material is highly damped, by its viscoelastic nature, when
recovering its original shape and therefore returns slowly and without shock.
The design of the EMC hinge/actuator allows for variation of fiber orientation
and type, laminate thickness, sectional depth and overall size. These design parameters
can be easily scaled to target varying design requirements for a wide range of
spacecraft applications. Processes have been developed for fabrication of high-quality
EMC hinge/actuators, and numerous prototypes have been built for testing. Finally, a
series of tests has been initiated to verify the analytical models and the space flight
readiness of the EMC hinge/actuator. Results from preliminary testing confirm the
robustness and predictability of the EMC hinge/actuator's performance.
A hinge/actuator for space flight deployable structures has been developed by
Francis et al [118] using EMC materials. They have succeeded in the development and
testing of a hinge/actuator, which utilizes EMC materials. The EMC hinge/actuator was
designed to replace traditional mechanical hinge/actuators for the deployment of a
variety of space flight structures such as reflectors, radiators, and solar arrays.
Michael et al [119] reviewed new developments in EMC materials technology
including material properties, analytical and designs tools, testing and evaluation
protocols, and new applications. A component using EMC materials was fabricated in
its deployed, on orbit shape using conventional composite manufacturing processes.
Cfiapter 1 31
Then by heating the material and applying force this fully cured composite material
could be folded or deformed for packaging. When cooled, it retained the packaged
shape indefinitely. When reheated the structure, the original shape was recovered with
little or no external force. This packaging/deployment cycle is reversible.
DARPA and NASA have recognized ultra-lightweight shape memory
Rigidizable Inflatable (RI) structures as an enabling technology for future space and
interplanetary missions requiring large space systems [120]. The enabling feature is the
ability of the material to be compacted very tightly; extremely large structures can be
stowed into existing launch vehicles or smaller launchers.
Gossamer inflatable structures are evaluated by many government and industrial
organizations for application in space systems. This class of structures offers many
potential advantages over conventional mechanical structures in the areas of mass;
launch volume and configurability of the package, as well as in cost.
Rigidizable materials, as described in terms of gossamer structures, can be defined
as materials that are initially flexible to facilitate inflation or deployment, and become
rigid when exposed to an external irifluence.
The SMP composite material has also been used to fabricate and test deployable
parabolic dishes. Currently, the use of this technology is tested for JHU/APL's Hybrid
Inflatable Antenna [121]. Two 0.5-m reflectors, which were manufactured from the
SMP composite, have demonstrated the feasibility of this applicatio:p. and identified
potential folding schemes. Figure-1.4 shows both the deployed and packed
configuration of SMP composite. Currently, further research and development work is
on going to modify the properties of the SMP composite to better fit this application.
Figure-l.4 A 0.5-m Diameter SMP Reflector for the JHU/APL HybridInflatable Antenna in both deployed and packed configurations
Cfiapter 1
Another recent innovation has been the introduction of shape memory
materials in the development of elastically folded self-locking hinge joints [120].
These shape memory hinges can have the mechanical simplicity of elastically folded
hinges, along with deployment control afforded by the introduction of shape
memory material. The effort described in this paper focused on the adaptation of an
elastic memory composite (EMC) self-locking actuator as shown in Figure-1.S for
optical precision deployment. The EMC hinge/actuator is a relatively simple device
and is self-locking, similar to the elastically folded hinge. At the end of deployment,
the EMC hinge/actuator automatically locks and provides a micro dynamically
stable member for supporting the deployed mirror panel. This actuator concept is
unique and novel in the deployable optics world, as it eliminates the need for a
mechanical latch ofany kind.
Figure-1.5 EMC hinge/actuator with embedded heater
32
A patent [122] has been filed for a boom structure deployed by inflating the
structure ton a desired shape and rigidizing the structure through an external influence.
The frame structure consists of a series of frame members, which was made up of a
fibrous material and a resin material. The resin material was one of thermosetting resin,
shape memory resin, thermoplastic resin, UV curable resin and a solvent based resin.
ATK space system in collaboration with NASA Glenn Research Center [123]
developed lightweight deployable solar array, which is an innovative technology
advance that provides leapfrog performance over the state-of-the-art. Current state-of
the-art solar arrays systems are based on rigid composite honeycomb panel
construction. These existing systems are heavy, provide low deployed first mode
natural frequencies, and occupy a large stowage volume. Contrary to the existing
system, lightweight deployable solar array provides structural performance and the
highest available specific power with a very low stowed volume and footprint.
Cliapter 1 33
Figure-1.6 depicts the stowed and the deployed state of the solar array. When deploying
in a rotational "fan" fashion, each interconnected triangular shaped substrate unfolds;
upon full deployment the structure becomes tensioned into a rigid shallow umbrella
shaped structure.•
Figure-1.6 Stowed and the deployed state of the solar array
1.3.11 SHAPE MEMORY GELS
Stimuli responsive gels have aroused worldwide research attention due to their
great potential applications as smart materials, including in controlled drug-delivery
systems, culture substrates, chemical valves and gentle actuators [124]. A polymer gel
consists of an elastic cross-linked network and a fluid filling the interstitial spaces of
the network. Polymer gels can be easily deformed by external stimuli and generate
force or execute work on the external environment. The ability of polymer gels to
undergo substantial swelling and collapsing, as a function of their environment is one
ofthe most remarkable properties ofthese materials.
Stimuli responsive polymer gels have generated an explosion of interest since
their relatively recent discovery due to their potential application in extraction,
absorption, actuator and drug delivery systems. These polymers are water-soluble but
undergo a sharp conformational change to become water insoluble and separate out of
solution, when a small change in temperature, pH or solvent composition takes place. A
thermo-sensitive smart polymer is soluble at temperature below its lower critical
solution temperature (LCST) and then undergoes a distinct phase change at the LCST
[125]. When chemically cross-linked such polymers give stimuli responsive gels.
Chapter 1 34
Recently considerable attention has been focused on intelligent hydrogels that
are able to alter their swelling behaviour and other properties in response to
environment stimuli, such as temperature pH, and ionic strength. Because of their
drastic swelling and shrinkage in response to environmental stimuli, these polymeric
hydrogels have been investigated for many biomedical and pharmaceutical applications
including controlled drug delivery, molecular separation, tissue culture substrate and
material for improved biocompatibility.
Poly (N-isoproplyacrylamide) (PNIPA) hydrogel is one of the most favorable
member of temperature-sensitve hydrogels studied extensively by numerous
researchers as an intelligent polymeric matrix, because of its sharp volume phase
transition at about 35°C in aqueous solution [126-128]. Formation of shape
remembered hydrogel upon heating, which consists of a N-isopropylacrylamide
telomer with a stilbazole terminal group exhibiting shape memory property has been
reported [129].
The comb-type grafted hydrogels based on poly(N-isopropylacrylamide)-g
polyeN-isopropylacrylamide) and poIy(acrylic acid)-g- poIyeN-isopropylacrylamide)
were synthesized by reversible addition-fragmentation chain transfer polymerization
with thermo-responsive property has been reported [130]. The measurements of the
swelling equilibrium, and swelling and deswelling kinetics indicated that the hydrogels
with more chains and longer chain lengths allowed higher equilibrium swelling and
rapid shrinking.
SMP gels based on interpenetrating network exhibiting thermo-responsive
behavior have been reported by Li et al [131]. These gels consist of two parts; a control
element, which is responsive to a designated environmental stimulus and a non
responsive substrate element. By designing the pattern in the gelation process, a variety
of shapes were obtained including spiral square, fish, numbers, alphabets and tubes.
The changes between two different shapes can be controlled by external stimuli such as
temperature and is reversible.
pH and temperature responsive hydrogels based on linear sodium alginate (SAl)
and cross-linked PNIPA with semi-interpenetrating network (semi-IPN) have been
studied by Zhang et al [132]. These hydrogels underwent volume phase transition at
around 33°C irrespective of the pH value of the medium, but their pH sensitivity was
evident only below their volume phase transition. Under basic conditions the swelling
Cfzapter 1 35
ratios of SAlIPNIPA semi-IPN hydrogels were greater than that of pure PNIPA
hydrogel and increased with increased SA content incorporated into the hydrogel, but
inverse effect was observed under acidic conditions. SAlIPNIPA semi-IPN hydrogels
with higher SAl content, the faster was the response rate to both pH and temperature
change.
A new type thermo-responsive hydrogel based on PNIPA has been synthesized
with the sol-gel technology [133]. For the preparation of this type of nano-structured
hydrogels, the inorganic silica phase was synthesized by the sol-gel process in the
presence of an aqueous solution of high molecular weight PNIPA. This combination of
the organic and inorganic phases form hybrid hydrogels with a semi-IPN morphology.
The unique structure of these hydrogels improved the mechanical stability to a great
extent as compared to conventional PNIPA-hydrogels. This was shown by stress-strain
experiments and the capability to absorb and desorbs large amounts of water. Influence
of silica on the transition temperature of the hydrogels was very small but allows the
variation of the thermo-responsive properties of the materials to a great extent.
Cross-linked polymeric gel films prepared by a radical copolymerization of
stearyl acrylate (SA) and acrylic acid (AA) with N, N'-methylenebisacrylamide (MBA)
as a cross-linker exhibiting thermo-responsive property has been reported [134].
Aggregation structure of the (SA/AA/MBA, 24.7/74.3/1.0; molar ratio) gel film was
investigated on the basis of X-ray diffraction study. The gel films in both a dried and a
swollen state in dimethyl sulfoxide formed crystalline lamellar structure at room
temperature. The swelling ratio of the gel film increased with an increase in
temperature up to 320 K (the melting temperature of stearyl acrylate side chain
crystals), whereas it decreased above 320 K. In order to improve the thermo-responsive
behavior, the stretched gel film was prepared by uniaxial stretching. The stretched gel
film abruptly shrinked to the original length upon heating at melting temperature of
stearyl acrylate side chain crystals.
Thermo-responsive copolymers which consist of N-isopropylacrylamide
(NIPA) and 2-hydroxyisopropylacrylamide (HIPAA) showed sensitive phase transition
and/or separation in aqueous media [135]. The cloud point of copolymers increased
with increased HIPAA composition. Moreover, poly(NIPA-co-HIPAA) with high
content of HIPAA unit showed thermo-response behavior with forming coacervate.
Coacervate gels were prepared by cross-linking coacervate droplets with divinyl
Cliapter 1 36
sulfone. Coacervate droplets disappeared when the temperature of aqueous solution
lowered below cloud point. Thermally induced coacervate can have potential
application for biological system, such as drug carrier and bioseparation.
Poly (carboxylic acid) can form intermacromolecular complexes with PEG (9)
due to the H-bonding between the carboxyl group of poly(carboxylic acid) and the
ether O-atom of PEG [136]. Hydrogen bonded complex is highly sensitive to a change
in the concentration or molecular weight of PEG, temperature and other parameter
[137, 138].
Guan et al [139] have developed poly(acrylic acid-eo-methyl
methacrylate)/cetyltrimethylammonium bromide (P(AA-co-MMA)/ C16TAB)
exhibiting shape memory behaviour. The shape memory principle of this kind of
complex is based on a reversilble order-disorder transition due to the formation of
crystalline aggregates among the long alkyl chains of C16TAB in the complex.
Complex of poly (acrylic acid-eo-methyl methacrylate) (p(AA-co-MMA))
network with PEG stabilized by the H-bonding was prepared by Liu et al [44]. PEG of
number average molecular weight varied from 400 to 20,000g/mol. PEG with a low
molecular weight of 400g/mol does not interact with p(AA-co-MMA) gel and in order
to form stable complex PEG of sufficient long chain is necessary. Maximum
interaction was observed for PEG of molecular weight 6000g/mol, above that
interaction decreased due to the steric hindrance. The complexes showed shape
memory properties due to the large difference in storage modulus below and above the
glass transition temperature.
1.4 RECENT DEVELOPMENTS IN SMP
A series of segmented poly (L-Iactide)-polyurethanes (PLA-PU) were
synthesized by Peng et al [140] by a two-step method, with oligo-poly (L-Iactide)
(PLLA) as the soft segments and the reaction product of 2,4-tolylene diisocyanate
(TDI) and ethylene glycol as the hard segments. The processed PLA-PUs recovered
almost 100% to their original shape within 10°C from the lowest recovery temperature.
In the recovery process, the PLA-PUs showed a maximum contracting stress of shape
change in the range of 1.5-4 MPa depending on the PLLA segmental length and the
hard segmental content. The experiments of cell incubation showed that the
Cfiapter 1 37
biocompatibility of PLA-PU was comparable to that of the pure PLLA. This kind of
polyurethane is recommended as implanted medical devices with a shape memory
property.
A senes of cross-linked shape memory polyurethanes of PCl diol (1) (Mn
4000g/mol), 1,4-butane diol (3), dimethylol propionic acid (DMPA), triethylamine,
glycerin, and MDI (2) were found to exhibit shape recovery characteristics [141]. Due
to the chemical cross-linking bonds existed in the hard segment; the cross-linked shape
memory PUs exhibited better mechanical properties, especially above the melting of
soft segment. The analysis of the shape memory effect showed that PU with 70
percentage soft segment exhibited a good shape memory effect and that a suitable
elongation was important to improve cyclic shape memory retention and shape memory
recovery.
Wang et al [142] has reported the synthesis of thermoplastic PU with a shape
memory property by reacting PTMO (4) with MDI (2), and chain extended with
various extenders such as linear aliphatic 1,4-butanediol (3), benzoyl-type 4,4' -bis(4
hydroxyhexoxy)-isopropylane and naphthalate-type bis(2-phenoxyethanol)-sulfone or
naphthoxy diethanol. The resulted PUs showed that the tensile strength, elongation at
break, and initial modulus at 300% of these copolymer films were in the range of 31-64
MPa, 42%-614%, and 8.26-11.5 MPa, respectively. Glass-transition temperatures of
these copolymers was in the range of -73°C to -50°C for the soft segment and 70°C
106°C for the hard segment. The extender with a benzoyl or naphthalate group was
better able to promote its shape memory property than the regular polyurethane.
Degradable shape-memory polymer networks intended for biomedical
applications were synthesized by photopolymerization process from oligo-[(~:;
hydroxycaproate)-co-glycolate]dimethacrylates with glycolate contents between 0 and
30 mol% has been reported by Steffen et al [143]. In addition, AB copolymer networks
were prepared by adding 60 wt% n-butyl acrylate as comonomer. All synthesized
polymer networks are semicrystalline at room temperature. A melting transition (Tm)
between 18 and 53°C was observed, which can be used as switching transition for the
shape-memory effect. Copolymer networks based on macrodimethacrylates with a Mil
of up to 13,500 g/mol and a maximum glycolate content of21 mol % show quantitative
strain recovery rates in stress-controlled cyclic thermo-mechanical experiments.
Hydrolytic degradation of polymer networks performed in phosphate buffer solution at
Cfiapter 1 38
37°C proved that the degradation rate could be accelerated by increasing the glycolate
content and decelerated by the incorporation of n-butyl acrylate.
Due to a large difference in storage modulus below and above the glass
transition temperature, a novel shape-memory poly[(methyl methacrylate)-co-(N
vinyl-2-pyrrolidone)]/poly(ethylene glycol) with semi-interpenetrating polymer
networks structure was synthesized, which was stabilized by hydrogen-bonding
interactions has been developed by Liu et al [144]. The recovery ratio of these
polymers could reach 99%. In such a system the maximum molecular weight of PEG of
1000g/mol required for the formation of semi-IPN structures.
Cornerstone Research Group, Inc. (CRG) has demonstrated the feasibility of
adding nanoparticules into their SMP resin systems [145]. The process oftransitioning
SMP nanocomposites from lab-scale to large-scale production was carried out by CRG.
The influence of the strain-holding conditions on the shape recovery and
secondary-shape forming was investigated for the polyurethane-shape memory polymer
film [146]. It was found that the secondary-shape forming appears distinctly if the
holding temperature is higher than the glass transition temperature and does not appear
if the holding temperature is lower than the glass transition temperature. The
irrecoverable strain increases in proportion to the holding strain.
The shape memory polyurethane cationomers composed of polY(E
caprolactone) (PCL) (1), 4,4'-diphenylmethane diisocyanate (MDI) (2), 1,4-butanediol
(3), and N-methyldiethanolamine (NMDA) or N,N-bis(2-hydroxyethyl)isonicotinamide
(BIN) were developed by Zhu et al [147] to illustrate the importance of cationic groups
within hard segments on shape memory effect in segmented polyurethane (SPU). The
shape memory effect between NMDA series and BIN series was correlated. Stress at
100% elongation was reduced for these two series of PU cationomers with increasing
ionic group content. Especially for NMDA series, the stress reduction was more
significant. The fixity ratio and recovery ratio of the NMDA series was improved
simultaneously by the insertion of cationic groups within hard segments, but not for the
BIN series. Characterizations with DSC and DMA suggested that the crystallisability of
soft segment in shape memory PU cationomers was enhanced by incorporation of ionic
groups into hard segments, leading to a relative high degree of soft segment
crystallization. Compared with the corresponding nonionomers, incorporation of
charged ionic groups within hard segments could enhance the cohesion force among
Cfzapter 1 39
hard segments particularly at high ionic group content. This methodology offers good
control of the shape memory characteristic in thin films and is supposed to be
beneficial to the shape memory textile industries.
Novel polyesterurethane/poly (ethylene glycol) dimethacrylate (PEGDMA)
interpenetrating networks (IPNs) with good shape-memory properties, synthesized
using solvent casting method has been reported [148]. The star-shaped oligo [(rac
lactide)- co-glycolide] was coupled with isophorone diisocyanate to form a
polyesterurethane network, and PEGDMA was photopolymerized to form another
polyetheracrylate network. IPNs were transparent and gel content exceeded 92%. The
values of strain fixity rate and strain recovery rate were above 93%. The hydrophilicity,
transition temperatures, and mechanical properties of IPNs could be conveniently
adjusted through variation of network compositions to match the promising potential
clinical or medical applications.
To illustrate the shape memory properties of shape memory polyurethane fiber
and the difference of thermal/mechanical properties between shape memory PU fiber
and other various man-made fibers, series of shape memory polyurethane having
various hard segment content were synthesized by Zhu et al [149] with the pre
polymerization method and spun with the wet spinning process. DSC, DMA, and
mechanical testing were used to study the particular thermal/mechanical properties of
shape memory polyurethane fiber in comparison with other man-made fibers such as
nylon6, polyester, Lycra and XLA. The thermo-mechanical and cyclic tensile testing,
suggested that the thermal setting temperature has a huge influence on the mechanical
properties and shape memory property due to the elimination of internal stress.
A set of polyurethane fibers and films with varying hard-segment content
exhibiting shape memory property were synthesized by Ji et al [150]. The shape
memory effect of the thin films and the fibers was investigated through a series of
thermo-mechanical cyclic tensile tests and it was found that the fibers showed less
shape fixity but more shape recovery compared with the thin films.
The shape-memory composite belt with a TiNi-SMA WIre fiber and a
polyurethane-SMP sheet matrix was fabricated [151]. The bending actuation
characteristics of the belt were investigated by the thermo-mechanical tests. Both the
rate of shape fixity and the rate of shape recovery were close to 100%. Recovery force
appeared by heating under constant residual deflection. The recovery force was 93-94%
Chapter 1 40
of the maximum force. Thus, the development of high functionality of shape-memory
composite elements is expected by various combinations of SMAs and SMPs.
The arrangements whether block or random type, of the soft segments of
polyurethane block copolymers prepared with MDI (2) and two kinds of PTMO (4) in
various ratios were compared for possible effects on the physical properties of the
copolymers [152]. A long soft segment, PTMO-2000g/mol, was superior in all
mechanical properties (strain, stress, and modulus) because a long chain length could
provide more motional freedom than a short one (PTMO-I000g/mol) could and
therefore was helpful in forming strong interchain attractions among hard segments.
These copolymers exhibited good shape recovery properties.
Monodomain chiral smectic e elastomer exhibiting a biaxial-shape-memory
effect, demonstrated its spontaneous and reversible deformation on heating and cooling
where successive phase transitions took place have been reported [153]. Not only
shrinkage due to the smectic-isotropic (isotropic-smectic) transition but also
spontaneous shear-deformation associated with molecular tilting in the temperature
range of the smectic phases was observed.
Temperature-dependent recovery of atomic force mIcroscope tip-formed
indentations in a thermoset shape memory polymer have been reported [154]. The
indentations are made both at room temperature and 69°e, and then recovered at
temperatures between 400 e and 70oe. The shape recovery was more complete for
higher anneal temperatures, and was relatively independent of time for 102_104 s.
Thermo-mechanical and shape memory behavior on thermo plastic segmented
polyurethane copolymers based on PTMO (4) (Mn, 2900 g/ml), l,4-butane diol (3),
MDI (2) and with/without polyethylene glycol (9) (Mn, 3400 g/mol) were reported
[155]. PU with 5-wt% of PEG showed the highest tensile storage modulus at all
temperatures due to the increased interaction between the polymer chains. On the other
hand, SPU containing 15-wt% of PEG has a broad loss modulus due to increasing chain
flexibility. The recovery process in shape memory behavior has noticed to be the
orientation of disoriented chains due to the thermodynamic entropy effect. The shape
recovery properties were improved by increasing the percent crystallinity and/or
physical cross-linking between the polymer chains.
Cliapter 1 41
Shape memory poly (lactic acid) (PLA) based polymer with cross-linked
structure having thermo-reversible bonding based on Diels-Alder reaction (furan and
maleimide) has been reported [156]. In order to improve the mechanical properties of
the cross-linked PLA, flexible methylene segment were introduced into maleimide
linker, hexamethylene dimaleimide (HDM) and dodecamethylene dimaleimide (DDM).
The flexure strength of the cross-linked PLA with DDM was higher than that of HDM.
This shows that the flexible methylene segment relaxed the internal strain of the cross
linked PLA because of bulky furan and maleimide bonding.
The thermo-mechanical behaviour of shape memory polymers can be tailored
by modifying the molecular structure of the polymer or by using the polymer as a
matrix of multiphase composites. Shape memory behaviour of functionalized multi
wall carbon nano tube (MWNT) reinforced segmented polyurethane (SPU) copolymer
exhibiting shape recovery properties have been reported [157]. Shape memory effect of
MWNT reinforced SPU composites were influenced by the weight fraction of MWNT
in the polymer matrix. Low concentration of MWNT in the SPU does not have much
influence on shape memory behaviour. However, the shape recovery properties were
improved for the SPU having 2.5 wt% of MWNT. All the SPU/MWNT-SPU samples
showed excellent shape fixity.
Shape-memory materials have been proposed in biomedical device design due to
their ability to facilitate minimally invasive surgery and recover to a predetermined
shape in vivo. Use of the shape-memory effect in polymers is proposed for
cardiovascular stent interventions to reduce the catheter size for delivery and offer
highly controlled and tailored deployment at body temperature. Shape memory polymer
networks were synthesized via photopolymerization of t-butyl acrylate and poly
(ethylene glycol) dimethacrylate to provide precise control over the thermo-mechanical
response of the system. The free recovery response of the polymer stents at body
temperature as a function of glass transition temperature, cross-link density,
geometrical perforation, and deformation temperature, all of which can be
independently controlled [158].
Basic characterization of a SMP as a suitable structural material for morphing
aircraft applications has been reported [159]. Tests were performed for monotonic
loading in high shear at constant temperature, well below, or just above the glass
transition temperature. The SMP properties were time-and temperature-dependent.
Cfiapter 1 42
Based on the testing SMPs appear to be an attractive and promising component in the
solution for a skin material of a morphing aircraft. Their multiple state abilities allow
them to easily change shape and, once cooled, resist large loads.
The shape memory polyurethanes based on liquidated-MDI and l,4-butanediol
(3) as the hard segment, and poly(butylenes adipate) glycol, poly(hexaylene adipate),
poly(ethylene adipate) glycol, polyethylene glycol and PCL (1) as soft-segment
materials were reported [160]. It was found that the shape memory polyurethane
required the soft-segments of good crystalline ability. In these materials, the PHAG has
a good crystalline ability shape memory properties. It is the best soft-segment material
among the soft segments.
The sensitizing effects of polyfunctional poly (ester acrylate) on the radiation
crosslinking of PCL (1) were studied by Zhu et al [161]. The influences of the use of
the polyfunctional material, the number of functional groups, and the radiation dose on
the radiation crosslinking, dynamic mechanical properties, and shape-memory
behaviors of PCL were investigated. The shape-memory results revealed that
sensitizing crosslinked PCL exhibited 100% recoverable deformation and a quicker
recovery rate.
Polyurethane block copolymer was synthesized and was followed by a sol-gel
reaction with tetraethoxysilane to prepare high performance polyurethane-silica hybrids
with shape memory function [162]. PUs showed good shape retention and shape
recovery of more than 80% for all samples. Consequently, by silica hybridization, an
improvement in the mechanical properties and shape recovery force of PU could be
achieved without any decrease in their shape recovery effect.
1.5 APPLICATIONS OF SHAPE MEMORY POLYMERS
1.5.1 General Applications
Shape memory polymers belong to an emerging group of "smart materials" that
could radically change product design. Apart made with shape memory polymer reverts
to its original form after being manipulated to a different shape. This dynamic process
of change and recovery can occur numerous times, making parts fabricated of SMP
perfect for applications that require multiple levels of functionality. SMP finds
applications both in space and earth environment. SMPs have recently been receiving a
Cliapter 1 43
great deal of interest in the scientific community for their use in applications ranging
from light weight structures in space [163] to micro-actuators in biomedical devices
[164, 165].
Applications of SMP based on thermoplastic polyurethane developed by
Mitsubishi Heavy Industries Ltd include handles on household items that change shape
for easy handling by arthritics, and rigid IV tubes that are softened by body heat for
comfort after insertion.
Cornerstone Research Group (CRG) has developed an innovative use for SMP
in composite tooling-as a filament-winding mandrel for 3-D parts like ducts. The
polymer is fabricated in a flexible cylindrical shape, which is its memory stage. The
shape is placed in a preform mold, where it is heat-activated to form the shape of the
finished part. Once cooled, the mandrel is demolded and used to filament-wind and
cure the part. Demolding occurs by reheating the mandrel so it reverts to its original
shape, which is easily removed from the duct. The tooling and fabrication costs can be
reduced by as much as 50%.
Potential applications include large implants introduced into the body in a
compacted state through tiny incisions, which in contact with body heat would recall
their original expanded shape. A benefit of using SMPs is that if the medical structure
is not intended to be permanent, it is possible to include biodegradability in the
polymer. Scaffolding devices for assisting in bone and tissue repair might be one
application of such a biodegradable SMP, while prosthetic implants could be made
permanent.
The medical possibilities of shape-memory polymer were recently demonstrated
in the form of a self-tightening knot [33]. A challenge in endoscopic surgery is the
tying of a knot with instruments and sutures to close an incision or open lumen. It is
especially difficult to manipulate the suture so that the wound lips are pressed together
under the right stress. When the knot is fixed with a force that is too strong, necrosis of
the surrounding tissue can occur. If the force is too weak, scar tissue, which has poorer
mechanical properties, forms and may lead to the formation of hernias. A possible
solution is the design of a smart surgical suture, whose temporary shape would be
obtained by elongating the fiber with controlled stress. This suture could be applied
loosely in its temporary shape; when the temperature was raised above Ttrans, the suture
would shrink and tighten the knot, applying the optimum force.
Cliapter 1 44
Lendlein and Langer [33] have developed biodegradable elastic memory
polymers for potential biomedical applications from oligo (c-caprolactone)diol as the
precursor for the switching segments and oligo(p-dioxanone)diol as a hard segment that
provide physical cross-links. This material has the potential to influence how implants
are designed and could enable new surgical devices as smart sutures.
Stroke is the third leading cause of death in the US, with approximately 600,000
ischemic strokes occurring each year [166]. RougWy two thirds of those are caused by
thrombotic vascular occlusion (formation or lodging of a blood clot) in the arterial
network supplying the brain [167], depriving parts of the brain of oxygen often
resulting in permanent disability [167, 168]. Traditionally, treatment has been limited
to administration of recombinant tissue plasminogen activator, a thrombolytic (clot
dissolving) drug that is infused over a I-hour period into the systemic circulation.
Clinical protocol requires that treatment may only be initiated within 3 hours of the
onset of symptoms due to the possibility of intracerebral hemorrhage. Using SMP
micro actuator in its secondary straight rod form through a catheter distal to the
vascular occlusion, which is mounted on the end of an optical fiber, is then transformed
into its primary corkscrew shape by laser heating. Once deployed, the micro actuator is
retracted and the captured thrombus is removed from the body to restore blood flow.
Figure-I.7 depicts this. Description of figure-I.7 is (a) In its secondary straight rod
form, the microactuator is delivered through a catheter distal to the thrombotic vascular
occlusion (clot). (b) The microactuator is then transformed into its primary corkscrew
form by laser heating. (c) The deployed rnicroactuator is retracted to capture the
thrombus.
(a)
(b)
(c)
Figure-I.7 Depiction of endovascular thrombectomy using the laser-activated SMP
rnicroactuator coupled to an optical fiber.
Cliapter 1 45
The water vapor permeability of Temperature-sensitive polyurethane (TS-PU)
membrane could undergo a significant increase as temperature increases within a
predetermined temperature range [169-175]. This property can be tailored for
applications in textile industry, medicine, environmental fields and so on.
The wide range of potential applications has generated a great deal of research
interest in the preparation of TS-PU for smart textiles. In 1992, Horii et al. claimed, in
their patent, to have prepared a urethane polymer that can be used to develop such
smart textiles [176]. In 1993, Hayashi and colleagues reported their investigation into
the microstructure and water vapor permeability of the high moisture permeability
polyurethane. Water vapor permeability of the PU film increased by three times when
temperature rose from 10 to 40°C. Until 1998, this kind of polyurethane was applied to
develop a smart textile, 1 namely Diaplex, by Mitsubishi International Corporation. In
2001, Graham- Rowe reported that the US Army's Soldier and Biological Chemical
Command Laboratory had used Diaplex fabrics in the design of an amphibious diving
suit, which enabled wearers to be comfortable out of the water [177]. Ding et al [178]
developed thermoplastic PU, which could be applied to the development of smart
textiles.
SMP applications are diverse as appliance, toys, implantable medical devices,
building and construction products, recreational goods, and industrial components.
SMP is even being projected for tooling, because its shape-change property can
expedite setup and demolding.
1.5.2 Space Applications
Applications of SMP ranges from the exotic to the mundane. At one level is the
potential for aircraft to change shape while in flight. The implications for aircraft
performance are immense. For example, the Defense Advanced Research Projects
Agency, the main R&D group for the U.S. Dept. of Defense, has initiated a Morphing
Aircraft Structures program utilizing skins of SMP. SMPs are being investigated by the
US Army for stiffening fabric wings, which can be packaged in a standard bag and
deployed from a conventional aircraft.
Cliapter 1 46
Composite Technology Development (CTD) targets for SMP include high-heat
applications, automotive, and deployable satellite components like booms for space
flight.
Fiber reinforced plastics (FRP) where the SMP is used as matrix resin, when
apply on inflatable structure in space, finds applications as compact folding of the FRP
molded to final shape to transport to the space before expanding it to the original shape
in space [179].
The concept called cold Hibernated Elastic Memory (CHEM) utilizes PU based
SMP in open cellular (foam) structures, which are self deployable and are using the
foams elastic recovery plus their shape memory effect to erect a structure. In practice
CHEM foams are compacted to small volume above their softening temperature (Tg)
and can be stored below their Tg without constraint. Heating above their Tg restores
their original shape. The advantage of this exciting new technology is that structures
when compressed and stored below Tg are a small fraction of their original size and are
lightweight. CHEM foam concept performs robustly in the Earth environment as well
as in space. At NASA's Jet Propulsion Laboratory, find a use for polyurethane based
SMP developed by Mitsubishi Heavy Industries in Nagoya, Japan. They reckon open
cellular foam of the SMP could be used to make flat-packed wheels, which take up
little space in their carrier spacecraft, but on exposure to the sun's heat at touchdown,
shapeform robust wheels.
Some of many potential CHEM space applications require a high precision
deployment and surface accuracy during operation. However, a CHEM structure could
be slightly distorted by the thermo-mechanical processing as well as by thermal space
environment. Therefore, the sensor system is desirable to monitor and correct the
potential surface imperfection. The surface control of CHEM smart structures was
demonstrated using a Macro-Fiber Composite (MFC) actuator developed by the NASA
LaRC and US Army ARL [180]. The MFC actuator performed well before and after
processing cycles.
Lightweight, deployable antenna for a variety of outer space and terrestrial
applications would be designed and fabricated according to the concept of cold
hibernated elastic memory structures. Mechanically deployable antennas now in use are
heavy, complex, and unreliable, and they utilize packaging volume inefficiently. The
CHEM antenna structures are simple and would deploy themselves without need for
Cliapter 1 47
any mechanisms and, therefore, would be more reliable. The proposed CHEM antenna
structures also weigh less, could be packaged in smaller volumes, and would cost less,
relative to mechanically deployable antennas. Currently the CHEM foam concept is
well formulated with clear space and commercial applications. One of examples is the
demonstration of CHEM wheels developed for the subscale nano-rover as shown in
figure-1.8 (very small planetary-exploration robotic vehicles). Other potential
applications such as hom antenna, camera mast, and a sensor delivery system, were
studies under various programmes at JPL. Besides space structural use, the impact
energy absorption applications such as self-deployable soft-landing systems and
micrometeoroid shielding systems.
Figure-1.8 CHEM nano rover wheel
Composite Technology Development (CTD; Lafayette, CO) developed an
elastic memory composite that is lighter than shape memory alloy but delivers more
force than shape memory plastic, which expands more slowly than shape memory alloy
and provides less risk of damage to delicate equipment. This can replace the shape
memory alloys currently used for deployment of spacecraft solar array hinges, thereby
effecting a weight and cost savings
The need for deployable structures m space application required when the
physical dimensions of the spacecraft's operational configuration are larger than the
launch vehicle fairing dimensions thus requiring reconfiguration of structural systems
for stowage and on-orbit deployment. Deployable instrument masts, antennas, solar
arrays, radiators and sunshields are some examples of deployable space structures
currently in operation.
Cliapter 1 48
ILC Dover has developed, tested and matured numerous SMP composite
systems for large space structural applications [181]. The final deployed or operational
shape of the SMP composite structure was fixed during its initial cure cycle. Once the
SMP material is completely cured, it can be heated to the folding temperature, above its
Tg where it becomes flexible and can be tightly packed. The flexibility of the SMP
composite material at the folding temperature depends strongly on both the resin and
fiber properties. DARPA and NASA have recognized ultra-lightweight shape memory
Rigidizable Inflatable (RI) structures as an enabling technology for future space and
interplanetary missions requiring large space systems [120]. The enabling feature is the
ability of the material to be compacted very tightly; extremely large structures can be
stowed into existing launch vehicles or smaller launchers.
Thermoplastic rigidizable materials exhibit many promising features that make
them appealing for a variety ofapplications. [122].
SMP is used as matrix resin to the inflatable structure in space. This aims at
compact folding of the FRP molded to final shape to transport to the space before
expanding it to the original shape in space as shown in Figure-l.9.
Fol eo
pa "lded intooriq inal shapein t e space
Figure-1.9 Inflatable material for space structure
Chapter 1
Gossamer inflatable structures are evaluated by many government and industrial
organizations for application in space systems. Applications including solar arrays,
antennas, solar sails, and sunshields (Figure-I. 10). This class of structures offers
many potential advantages over conventional mechanical structures in the areas of
mass; launch volume and configurability of the package, as well as in cost.
Figure-l.l0 The 25m diameter ARlSE Antenna Concept for VLBI (NASA
JPL)
Recent shape memory composite developmental work performed at ILC
Dover (Figure-I.I!) has greatly advanced the state of the art ofmaterials that can be
used to construct rigidizable gossamer space inflatable structures [122]. The shape
memory function allows the fabrication of structures that consist of small diameter
tubes where it would be inefficient to inflate all elements individually. The
individual tubes return to shape by the shape memory effect of the resin, while an
outer polymeric :film shell is inflated to deploy the structure.
49
Cliapter 1
Figure-l.ll ILC Dover SMP Inflatable Space Frame
A truss structure assembled from cylindrical boom has recently been fabricated
and tested at ILC Dover [181]. The truss structure was assembled from 45.7-mm
diameter by 1.5- m long booms. The booms were fabricated using 2 plies of IM7
(trade name) and shape memory polymer. To demonstrate the packing and
deployment of this technology, heaters were integrated onto the outside of the
booms and covered by multi-layer insulation. Figure-l.12 shows the truss in both the
deployed and packed positions. This SMP truss can be designed to a compaction
ratio (deployed-to-packed length ratio) of 100:1.
50
Figure-1.12 SMP truss in both packed and deployed configurations
Cliapter 1
The EMC deployment system test-bed was created by modifying an existing
precision deployable telescope test bed at the University of Colorado (CU) by
replacing the original mechanical hinge/actuator with an EMC hinge/actuator like
that shown in figure-1.5 [120]. The test article represents one deployable mirror
segment from a multi-panel deployable reflector structure.
51
Chapter 1
1.6 REFERENCES
52
1. ' VR Gowarikar, NV Viswanathan, S Jayadev. Polymer Science, New Age
International (P) (1986)
2. Hanna SN. Handbook of Nanophase Materials, Goldstein AN (ed) Marcel
Dekker, Inc. New York. (1997)
3. T Liu, J Tang, HQ Zhao, YP Deng, L Jiang. Langmuir. 18 (2002) 5624.
4. A Adronov, SL Gilat, JMJ Frechet, K Ohta, FVR Neuwahl, GR Fleming. J
Am Chern Soc. 122 (2000) 1175.
5. M Kawa, JMJ Frechet. Chern Mater 10 (1998) 286.
6. A Hirsh. The Chemistry of Fullerens, Thieme Stuttgart (1994)
7. R Taylor. (ed) The Chemistry of Fullerenes, World Scientific, Singapore.
(1995)
8. MS Dresselhaus, G Dresselhaus, P Ekllund. Science of Fullerenes and
Carbon Nanotubes, Academic, San Diego. (1996)
9. PJF Harris. Carbon Nanotubes and related structures, New Materials for the
Twenty First Centuary, Cambridge University press, Cambridge. (2001)
10. L Brunsveld, BJB Folmer, EW Meijer, RP Sijbesma. Chern Rev.101 (2001)
4071.
11. D Liming. Intelligent Macromolecules for Smart Devices, Springer-Verlag
London Berlin Heidelberg. (2004) p. 81.
12. M Taylor. Smart Power. 14 (1999) 158.
13. BC Yoseph. Proceedings of the SPIE Smart Structures and Materials
Symposium, EAPAD Conference, San Diego (2002) March 18-11.
14. BC Yoseph. WW-EAP Newsletter. 1 (1999) (1).
15. EA Kottke, LD Partridge, M Shahinpoor. J Intelig Mater Syst and Struct.
(2006) (Article in press).
16. N Mehrdad, S Pourjalali, M Uyema, Ali Yousefpour. J Thermo Compost
Mater. 19 (2006) 309.
Chapter 1 53
17. M Schwartz. Encyclopedia of Smart Polymers. 1 (2002) 150.
18. B-C Yoseph, T Xue, S-S Lih. NDTnet-Septemper, 1 (1996) (09).
19. JS Harrison, Z Ounaies. ICASE, NASA Langley Research Center,
Hampton. VA 23681-2199 (2001).
20. M Schwartz. Encyclopedia of Smart Polymers. 2 (2002) 835.
21. S Dirk. Advanced Drug Delivery Reviews. 58 (2006) 1655.
22. M Sardar, I Roy, MN Gupta. Enzyme Microb Techno!. 27 (2000) 672.
23. A Sharma, MN Gupta. Biotechnol Bioeng. 80 (2002) 228.
24. B Yang, WM Huang, C Li, L Li. Polymer. 47 (2006) 1348.
25. ZG Wei, R Sandstrom, S Miyazaki. J Matr Sci. 33 (1998) 3743.
26. DJ Maitland, AP Lee, DL Schumann, LD Silva. University of California.
US Patent No.6102917 (2000).
27. G Rabani, H Leftmann, A Kraft. Polymer. 47 (2006) 4251-4260.
28. K Nakyama. Int Polym Sci Techno!. 18 (1991) 43.
29. S Hayashi. Int Progm Urethane. 6 (1993) 90.
30. M Ishii. Plast Age. 15 (1989) 158.
31. S Hayashi, P Kapadia, E Ushioda. Plast Eng. 51 (1995) 29.
32. JR Lin, LW Chen. J Appl Polym Sci. 69 (1998) 1563.
33. A Lendlein, Langer R. Science. 296 (2000) 1673.
34. K Gall, ML Dunn, YP Liu, D Finch, M Lake, NA Munshi. Acta Mater.
50 (2002) 5115.
35. BS Lee, BC Chun, YC Chung, KI SuI, JW Cho. Macromolecules. 34
(2001) 6431.
36. W Wenshow, P Peng, C Xuesi, J Xiabin. Euro Polym J. (2006) (article in
press)
37. BC Chun, SH Cha, C Park, Y-C Chung, MJ Park, JW Cho. J Appl Polym
Sci. 90 (2003) 3141.
Cliapter 1 54
38. C Liang, CA Rogers, E Malafeew. J Intell Mater Syst Struct. 8 (1991) 380.
39. T Takagil. J Intell Mater Sys. 1 (1990) 149.
40. HL Sung, WK, BK Kim. Smart Mater Struct. 13 (2004) 1345.
41. G Ken, LD Martin, L Yiping, F Dudley, L Mark, AM Naseem. Acta
Materialia. 50 (2002) 5115.
42. A Lendlein, K Steffen. Angew Chern Int Ed. 41 (2002) 2034.
43. A Armin, F Yakai, K Steffen, A Lendlein. Angew Chern Int Ed. 44 (2005)
1188.
44. BK Kim, YJ Shin, SM Cho, HM Jeong. J Polym Sci Part: B Polym Phys.
38 (2000) 2652.
45. C Liu, SB Chun, PT Mather, L Zheng, EH Haley, EB Coughlin.
Macromolecules. 35 (2002) 9868.
46. IA Rousseau, PT Mather. J Am Chern Soc. 125 (2003) 15300.
47. G Liu, X Ding, Y Cao, Z Zheng, Y Pengo Macromolecules. 37 (2004) 2228.
48. A Lendlein, AM Schmidt, R Langer. Proc Nat! Acad Sci USA. 98 (2001)
842.
49. WG Reyntjeins, E Filip Du Prez, EJ Goethals. Macromol Rapid Commun.
20 (1999) 250.
50. K Takeda, M Akiyama, T Yamamizu. Angew Makromol Chern. 157 (1998)
123.
51. Y Kagami, JP Gong, Y Osada. Macromo Rapid Commun. 17 (1996) 539.
52. NA Plate, VP Shibaev. J Polym Sci Macromol Review. 8 (1974) 117.
53. EF Jorda, DW Feldeien, AN Wrigley. J Polm Sci Polym Chern Ed. 9
(1971) 1835.
54. JR Lin, LW Chen. J Appl Polym Sci. 69 (1998) 1563.
55. B Bengsten, C Feger, WJ Macknight, NS Schneider. Polymer. 26 (1985)
895.
56. RM Briber, EL Thomas. J Macromol. Sci, Phys. B22 (1983) 509.
Cfzapter 1 55
57. J Blackwell, CD Lee. J Polym Sci Polym Phys Ed. 21 (1983) 2169.
58. J Blackwell, CD Lee. J Polym Sci Polym Phys Ed. 22 (1984) 759.
59. RB Briber, EL Thomas. J Polym Sci Polym Phys Ed. 23 (1985) 1915.
60. JR Quay, Z Sun, J Blacwell, RM Briber, EL Thomas. Polym J. 31 (1990)
1003.
61. W Neumuller, R Bonarat. J Macromol Sci Phys. B21 (1982) 203.
62. J Koberstein, RS Stein. J Polym Sci Polym Phys Ed. 21 (1983) 1439.
63. J Koberstein, TP Russell. Macromol. 19 (1986) 714.
64. PE Gibson, JW van Bonart, SL Cooper. J Polym Sci Part: B Polym Phys.
24 (1986) 885.
65. TR Hesketh, JWC van Bogart, SL Cooper. Polym Eng Sci. 20 (1980) 190.
66. EJ Woo, G Farber, RJ Farris, CP Lillya, JCW Chien. Polym Eng Sci. 25
(1985) 834.
67. JT Koberstein, AF Galambos. Macromolecules. 25 (1992) 5618.
68. H Ishihara, I Kimura, N Yoshihara. J Macromol Sci Phys. B22 (1983-84)
713.
69. H Ishihara. J Macromol Sci Phys. B22 (1983-84) 763.
70. G Spathis, E Kontou, V Kefalas, L Apekis, C Christodoulides, P Pissis. J
Macromol Sci Phys. B29 (1990) 31.
71. DJ Tartakowska, A Hentrich, MH Wagner. J Mater Sci Mater Med. 14
(2003) 109.
72. F Li, J Hou, W Zhu, X Zhang, M Xu, X Lou, D Ma, BK Kim. J Appl
Polym Sci. 62 (1996) 631.
73. BK Kim, SY Lee, M Xu. Polymer. 37 (1996) 5781.
74. F Li, X Zhang, J Hou, X Lou, D Ma, BK Kim. J Appl Polym Sci. 64 (1997)
1511.
75. JR Lin, LW Chen, J. Appl. Polym. Sci. 69 (1998) 1575.
76. MS Yen, KL Chenh. J Coated Fabric. 25 (1996) 87.
Cfiapurl 56
77. K Krishnan. J Coated Fabrics. 25 (1995) 103.
78. JW Cho, YC Chung, BC Chun, Y-C Chung. J Appl Polym Sci. 92 (2004)
2812.
79. BK Kim, SY Lee, SH Beak, YJ Choi, JO Lee, M Xu. Polymer 39 (1998)
2803.
80. HM Jeong, JB Lee, SY Lee, BK Kim. J Mater Sci.35 (2000) 279.
81. P van Caeter, EJ Goethals, V. Gancheva, R Velichkova, Polym. Bull. 39
(1997) 589.
82. M Wang, X Luo, X Zhang, D Ma. Polym Adv Technol. 8 (1997) 136.
83. X Luo, X Zhang, M Wang, D Ma. M Xu, F Li. J Appl Polym Sci. 64 (1997)
2433.
84. M Wang, Z Lide. J Polym Sci PartB. 37 (1999) 101.
85. M Wang, X Lu, D Ma. Eur Polym 1. 34 (1998) 1
86. K Sakurai, Y Shirakawa, T Kashiwagi, T Takahashi, Polymer. 35 (1994)
4238.
87. K Sakurai, H Tanaka, N Ogawa, T Takahashi, J Macromol Sci Phys. B 36
(1997) 703.
88. TH Tong. US Patent. (2004) US Patent No. 6759481 B2.
89. F Li, W. Zhu, X. Zhang, C Zhao, M Xu. J Appl Polym Sci. 71(1999) 1063.
90. F Li, Y Chen, W Zhu, X Zhang, M Xu, Polymer. 39 (1998) 6929.
91. NK Abayasinghe, KPU Perera, C Thomas et al. Biomater Sci Polym Ed. 15
(2004) 595.
92. RA Jain. Biomaterials. 21 (2000) 2475.
93. J Panyam, V Labhasetwar. Adv Drug Delivey Rev. 55 (2003) 329.
94. P Peng, W Wen-Shou, Z Pei-Biao, C, Xue-Si, J Xia-Bin. Gaodeng
Xuexiao Huaxue XuebaolChemical Journal of Chinese Universities, 28
February, (2007) 371.
95. G Liu, X Ding, Y Cao, Z Zheng, Y Pengo Macromolecul Rapid Commun.
26 (2005) 649.
Cliapter 1 57
96. H Okuzaki, Y Osada. Macromolecules. 28 (1995) 380
97. K Sakurai, T Takahashi. J Appl Polym Sci. 38 (1989) 1191.
98. K Sakurai, T Kashiwagi, T Takahashi. J Appl Polym Sci. 47 (1993) 937.
99. P T Mather, H G Jeon, T S Haddad. Polym Prepr Am Chern Soc Div
Polym Chern. 41 (2000) 528.
100. PT Mather, HG Jeon, A Romo-Uribe. Macromolecules. 32 (1999) 1194.
101. X Ni, X Sun. J Appl Polym Sci. 100 (2006) 879.
102. A Charlesby. Atomic Radiation and Polymers, Pergamon Press, Oxford,
(1960) p. 198.
103. A Lendlein, AM Schmidt, M Schroeter, R Langer. J Polym Sci Part:A
Polym Chern. 43 (2005) 1369.
104. MT Patrick, L Changdeng. (2006) US Patent No. 20060041089.
105. Y Liu, K Gall, ML Dunn, P McCluskey. Mechanics of Materials. 36 (2004)
929.
106. Z Shaobing, Z Xiaotong, Y Xiongjun, W Jianxin, W Jie, L Xiaohong, F Bo,
Y Ming. Chemistry of Materials. 19 (2007) 247.
107. F Cao, CJ Sadhan. Polymer. 48 (2007) 3790.
108. Q-Q Ni, C-S Zhang, Y Fu, G Dai, T Kimura. Composite Structure
Composite Structure. 81 (2007) 176.
109. M Patrick, G Qing, L Changdeng. (2006) US Patent No.7,091,297.
110. T Ohki, Q-Q Ni, N Ohsako, M Iwamoto. Composites Part: A. 35 (2004)
1065.
111. C-S Zhang, Q-Q Ni. Composite Structures. 78 (2007) 153.
112. JH Yang, BC Chun, Y-C Chung, JW Cho, BG Cho. J Appl Polym Sci. 94
(2004) 302.
113. C Park, JY Lee, BC Chun, Y-C Chung, JW Cho, BG Cho. J Appl Polym
Sci. 94 (2004) 308.
114. X Zheng, S Zhou, XLi, J Weng. Biomaterials. 27 (2006) 4288.
Cfzapter 1 58
115. SV Subbu, LP Tan, J FD Joso, YC F Boey, X Wang. Biomaterials. 27
(2006) 1573.
116. C Min, W Cui, J Bei, S Wang. Polym Adv Technol. 16 (2005) 608.
117. KH John, FL Carl Knoll, EW Cliff. 47th AIAAIASME/ASCE/AHS/ASC
Structures, Structural Dynamics, and Materials Confere. Newport, Rhode
Island. May 1 - 4 (2006).
118. W Francis, MS Lake, K Mallick. 44th IAA/ASME/ASCE/AHS Structures,
Structural Dynamics, and Materials Conference Norfolk, Virginia. April 7-
10 (2003).
119. M Tupper, K Gall, M Mikulas. Jr. IEEE. 5 (2001) 2541.
120. W Francis, M Lake. Composite Technology Development, Inc, Lafayette,
Colorado, 80026. 45th AIAA/ASMEIASCEIAHSIASC Structures,
Structural Dynamics & Materials Confer. Palm Springs, California. April
19-22 (2004).
121. CE Willey, RS Bokulic, WE Skullney, DP Cadogan, JK Lin. The Hybrid
Inflatable Antenna, AIAA-2001-1258, 42nd IAAIASME/ASCE/AHS/ASC
Structures, Structural Dynamics, and Materials Conference and Exhibit
AIAA Gossamer Spacecraft Forum, Seattle, WA. April 16-19 (2001).
122. DP Cadgon, SE Scarborough, John KH Lin, GH Sapna. (2005) US patent
No. 6910308 B2.
123. S Franklin, J Ku, B Spence, M McEachen, S White, J Samson, R Some, J
Zsoldos IEEEAC paper No.1544, Version 2. Nov (2005).
124. WG Liu, JR Zhang, KD Yao. J Appl Polym Sci. 86 (2002) 259.
125. IY Galeav, MN Gupta, B Mattiason. Use of Smart Polymers For
Bioseparation, Enabling Science. (1996) 19.
126. W-F Lee, R-J Chiu. J Appl Polym Sci. 90 (2003) 1683.
127. SJ Kim, CK Lee, YM Lee, SI Kim. J Appl Polym Sci. 90 (2003) 3032.
128. Y Li, Z Hu, Y Chen. J Appl Polym Sci. 63 (1997) 1173.
129. H Hachisako, R Murakami. Recent Res Develo in Pure and Appl Chern. 2
(1998) 547.
Cliapter 1 59
130. Q Liu, P Zhang, M Lu. JPS Part A Polym Chern. 43 (2005) 2615.
131. Y Li, Z Hu, Y Chen. J Appl Polym Sci. 63 (1997) 1173.
132. GQ Zhang, LS Zha, MH Zhu, IH Ma, BR Liang. J Appl Polym Sci. 97
(2005) 1931.
133. L Wouter, DP Filip. Macromolecular Symposia. 210, (2004) 483.
134. H Xiaowei, 0 Yushi, T Atsushi, K Tisato. Polymer Journal. 28 (1996) 452.
135. M Tomohiro, T Miki, Y Kazuya, A Takao. Polymer Preprints, Japan. 54th
SPSJ Symposium on Macromolecules - Polymer Preprints, Japan. 4700
(2005).
136. E Tsuchida, K Abe. Adv Polym Sci. 45 (1982) 1.
137. T Ikawa, K Abe, E Tsuchida. J Polym Sci Part A. 13 (1975) 1505.
138. Y Osada, M Sato. J Polym Sci Part:C. 14 (1976) 129.
139. Y Guan, YP Cao, YX Peng, ASC Chen. Chern Commun. 17 (2001) 1694.
140. P Peng, W Wen-Shou, Z Pei-Biao, C Xue-Si, J Xia-Bin. G Xuexiao, H
Xuebao. Chemical Journal of Chinese Universities. 28 (2007) 371.
141. Y Zhuohong, H Jinlian, L Yeqiu, Y Lapyan. Materials Chemistry and
Physics. 98 (2006) 368.
142. W Huei-Hsiung, Y Uen-En. J Appl Polym Sci. 102 (2006) 607.
143. K Steffen, S Susi: AM Schmidt, A Lendlein. Biomacromolecules. 8(2007)
1018.
144. G Liu, G Chunlong, X Hesheng, G Fuquan, X Ding, P Yuxing.
Macromolecular Rapid Communications. 27 (2006) 1100.
145. D Fortener. Cornerstone Research Group, Inc., Dayton, OH. Feb (2006) p.
26.
146. H Tobushi, K Hoshio, N Miwa. Smart Materials and Structures. 15 (2006)
1033.
147. Y Zhu, J Hu, K-W Yeung, K-F Choi, Y Liu, H Liem. J Appl Polym Sci.
103 (2007) 545.
Cfzapter 1 60
148. Z Shifeng, F Yakai, Z Li, S Junfeng, X Xu, Y Xu. J Polym Sc Part A:
Polym Chem. 45 (2007) 768.
149. Y Zhu, J Hu, L-Y Yeung, Y Liu, F Ji, K-W Yeung. Smart Mater and Struct.
15 (2006) 13 85.
150. F Ji, Y Zhu, JL Hu, Y Liu, L-Y Yeung, GD Ye. Smart Mater and Struct. 15
(2006) 154 7.
151. H Tobushi, K Hoshio, S Hayashi, N Miwa. Key Engineering Materials.
340-341 II (2007) 1187.
152. BC Chun, CT Keun, Y-C Chung. J Appl Polym Sci. 103 (2007) 1435.
153. H Kazuyuki. Polymer Preprints, Japan - 55th SPSJ Annual Meeting. 55
(2006) 943.
154. BA Nelson, WP King, K Gall. Applied Physics Letters. 86 (2005) 103108-
1-3.
155. M Subrata, JL Hu. J Elastomers and Plastics. 39 (2007) 81.
156. Y Midori, I Kazuhiko, I Masatoshi. Polymer Preprints, Japan. Polymer
Preprints, Japan - 55th SPSJ Annual Meeting. 55 (2006) 2227.
157. M Subrata, JL Hu. Iranian Polymer Journal (English Edition). 15 (2006)
135.
158. MY Christopher, S Robin, L Craig, R Bryan, E Alex, G Ken. Biomaterials.
28 (2007) 2255.
159. MM Keihl, RS Bortolin, B Sanders, S Joshi, Z Tidwell. Proceedings of
SPIE - The International Society for Optical Engineering, Smart Structures
and Materials 2005 - Industrial and Commercial Applications of Smart
Structures Technologies. 5762 (2005) 143.
160. S-J Chen, J-C Su, W-B Zhao, P-S Liu, C Gaofenzi, YG Kexue. Polymeric
Materials Science and Engineering. (21) (2005) 166.
161. GM Zhu, QY Xu, GZ Liang, Zhou HF. Journal of Applied Polymer
Science. 95 (2005) 634.
162. JW Cho, SH Lee. European Polymer Journal. 40 (2004) 1343.
163. SJ Tey, WM Huang, WJ Sokolowski. Smart Mater Struct. 10 (2001) 321.
Cliapter 1 61
164. WJ Benett, PA Krulevitch, AP Lee, MA Northrup, JA Folta. (1998) US
Patent No. 5783130
165. MF Metzger, TS Wilson, D Schumann, DL Matthews, DJ Maitland.
Biomedical Microdevices. 4 (2002) 89.
166. American Heart Association, Heart Disease and Stroke Statistics, Dallas.
(2005).
167. DO Kessel, JV Patel. Clin Radiol. 60 (2005) 413.
168. Center for Disease Control and Prevention, Morbidity and Mortality,
Weekly Report. 50 (2001) 120.
169. GH Chen, AS Hoffman. Nature. 373 (1995) 49.
170. XM Ding, JL Hu, XM Tao. Textile Res 1. 74 (2004) 39.
171. XM Ding, JL Hu, XM Tao, S Hayashi. The 6th Asia Textile
Conference, Hong Kong Institution of Textile and Apparel, Hong Kong
(2001).
172. M Grassi, SH Yuk, SH Cho. J Membrane Sci. 152 (1999) 241.
173. GR Lomax. Textile Asia. Sept 39-50 (20,01).
174. SA Stem. J Membrane Sci. 94 (1994) 1.
175. R Yoshida, K Uchida, Y Kaneko, K Sakai, A Kikuchi, Y Sakurai, T
Okano. Nature 374 (1995) 240..
176. F Horii, H Maruyama, S Hayashi, S Kondo, T Kato. Moisture-permeable
Waterproof Fabric and its Production. Japan, Unitika Ltd. Japan; Mitsubishi
Heavy Industrial Ltd. Bairon KK, Maeda Kasei KK (1992).
177. Graham-Rowe, D Going Commando. New Scientist. 22, March-31 (2001).
178. XM Ding, JL Hu, XM Tao, CP Hu. Textile Research 1. 76 (2006) 406.
179. S Hayashi, Y Tasaka, N Hayashi, Y Akita. Mitsubishi Heavy Industries
Ltd. Technical Review. 41, No.1 Feb. (2004).
180. W Sokolowski, R Ghaffarian, Proceedings of SPIE - The International
Society for Optical Engineering, Smart Structures and Materials, Smart
Sensor Monitoring Systems and Applications. 6167 (2006) 61670Y.
Cliapter 1 62
181. ER Abrahamson, et.a!. 43 rd AIAA/ASME/ASCE/AHS/ASC SDM
Conference, Denver, CO. AIAA Paper No. 02-1562, April 22-25 (2002).
63
Scope and06jectives ofthe Present Wor~
Thermo-responsive shape memory polymer (SMP) belongs to a class of smart
materials, which is recognised to sense thermal changes in its surrounding and respond
effectively by utilizing its discrete properties such as shape memory, shape recovery
and shape retention. Shape memory effect is known to originate from the phase
separated structures between hard and soft polymer segments and the reversible phase
transformations of the soft segments. SMPs have several advantages over alternative
active materials such as shape memory alloys. The low density of unreinforced or
reinforced SMP is an advantage for lightweight applications. Much work has been done
in industries and laboratories for development and application of new smart materials
with shape memory properties.
Literature cites the development of a host of polymers belonging to the class of
thermoplastics and a few thermosets exhibiting shape memory characteristics. From the
practical application point of view, the most important ones among them are
polyurethanes. Polyurethane based shape memory foams have been employed in self
deployable system for space applications. Polyurethanes are, by and large, interesting
from the point of view of amenability for structural modification, compounding,
copolymerisation etc: which opens immense avenues for tuning their shape memory
properties. However, in open literature, one can rarely cite any systematic study
directed towards correlating the shape memory properties to their structural, thermo
mechanical and morphological features. Among the thermosets, epoxy resins are the
most versatile in view of their ease of synthesis, umpteen ways of effecting the curing,
copolymerisation, compounding, composite processing etc. Infact, epoxies are the best
suited for elastic memory composites with good strength characteristics and shape
memory properties. Many reports refer to the use of proprietary shape memory epoxy
formulations, and composites derived thereof, for development of several self
deployable system for potential space applications. However, there are no details on
their synthesis or processing aspects nor on the structural features leading to the
manifestation of shape memory properties. Bigels that are inter-chain assembled
through specific interaction provide alternate polymer systems with shape memory
characteristics. Use of shape memory gels in extraction, absorption, actuator and drug
delivery systems are of immense importance. A few bigels with thermo-responsive
64
shape memory properties have been reported. There is immense scope for developing
shape memory gels based on the combinations of various cross-linked and
thermoplastic polymers designed to have specific interchain secondary interactions.
In this backdrop, the objectives of the present thesis are defined as
(i) Synthesis and detailed characterisation of shape memory polyurethane
and investigations on the effect of structural, compositional and nano
modifications on the macroscopic properties of the resultant matrices.
(ii) Synthesis and characterisation of shape memory epoxy resin and
development of their elastic memory composites.
(iii) Molecular design, characterisation and shape memory property
evaluation of acrylate-polyether gels.
The work carried out in these perspectives has been compiled in eight chapters.
Chapter-l gives a survey of shape memory polymers and the related elastic
memory composites. It highlights the current trend of research with particular reference
to thermo-responsive shape memory polymers.
Chapter-2 explains the materials and characterisation techniques employed for
different investigations.
Chapter-3 has been divided into two- 3A and 3B respectively. First part details,
the synthesis and characterizations of different formulations of
poly(tetramethyleneoxide)-based polyurethanes. The influence of hard/soft segments
on the thermal, mechanical and shape memory properties of the copolymers are
analysed and correlated to their composition and phase morphology. Second part of this
chapter describes the dependencies of nanoclay modification on the thermo-mechanical
and shape memory properties of selected polyurethane matrix.
Chapter-4 deals with synthesis and characterization of oxazolidone-modified
polyurethane derived by the reaction of isocyanate-terminated
poly(tetramethyleneoxide) prepolymer with a diepoxy resin. The polymers of different
soft and hard segment- contents have been analysed for their physical, thermal and
shape memory properties.
65
Chapter-5 describes the synthesis and characterisations of epoxy-based shape
memory polymers derived from different epoxy resins (GY-250, EPN-1179 and
triphenylolmethane triglycidylether) cured by an amine telechelic poly(tetrarnethylene
oxide). This chapter also explains the cure kinetics of epoxy monomer by the polyether
amine to optimize the cure conditions.
Chapter-6 describes the processing and thermo-mechanical properties of elastic
memory composite derived from different shape memory epoxies reinforced by carbon
fabric.
Chapter-7 details the synthesis and characterization of the copolymer gel of
poly(acrylic acid-eo-acrylonitrile) complexed with PTMO. Investigations on the
physical, thermal and shape recovery characteristics of the complexed gels derived
from the acrylic copolymer of varying compositions are explained.
Chapter 8 gives the overall conclusions of the work carried out in the present
study. A performance of the various shape memory polymers investigated in this study
has also been made.
