Shape memory polymer.pdf

7/22/2019 Shape memory polymer.pdf

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SHAPE MEMORY POLYMER

1. INTRODUCTION:Materials that respond dynamically to environmental stimuli can be called intelligent or smart materials. They

have significant potential applications in various fields. Whether smart materials are responding in an adaptiveway is questionable A very smart adaptive response is exhibited ifthe materials/material systems are able torespond dynamically to a number of input stimuli and if this response is repeatable.

The term very smart also refers to materials that can(1) Respond reversibly to the changes in the surrounding environment and

(2) Contribute an optimal or useful response by either changing its physical properties, geometry, mechanical

properties, or electromagnetic properties

Shape memory materials (SMM) are able to sense a change in temperature and react by changing into a

prescribed shape [1]. SMM have additional properties, which include pseudo elasticity or recoverable stroke(strain), high damping capacity and adaptive properties, which are due to the ability to reverse the

transformation during phase transitions. The materials deform into a temporary shape and returns to its originalshape by external environmental stimuli such as chemicals, temperature, or pH. Shape memory materials are

stimuli-responsive materials. Shape memory materials (SMM) are able to sense a change in temperature and

react by changing into a prescribed shape. There are a variety of physical changes that SMM can sense in theirenvironment, including thermal, mechanical, magnetic or electric. These physical factors are able to simulate

the shape memory effect (SME) enabling them to respond and transform into a prescribed shape, position,

strain, stiffness, natural frequency, damping, friction and other static and dynamic characteristics of material

systems.

1.1FEATURES OF THE SHAPE MEMORY POLYMER:A sharp transition that can be used to promptly fix the secondary shape low temperatures and trigger shape

recovery at high temperature.

Super elasticity (high deformability) above the transition temperature to avoid residual

Strain (permanent deformation).

Rapid fixing of temporary shape by immobilizing the polymeric chains without creep. SMPs possess two material phases. The glass and the rubber phases. In the glass phase, the material is

rigid and cannot be easily deformed.

When the temperature is greater than "Glass transition temperature", the material enters the soft rubber

phase and becomes easily definable


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1.3CLASSIFICATION OF THE MATERIALS SHOWING SHAPE MEMORY EFFECTSMM can be seen in a variety of materials such as alloys, ceramics, polymers and gels. These have been

explored and developed as they demonstrate SME behavior [1-3]. These materials have also been commercially

developed as they are a part of some consumer products [4].

1.3.1 Shape memory alloys (SMA)1.3.2 Shape memory Ceramics

1.3.3 Shape memory polymers1.3.4 Shape memory gels

1.3.1 Shape memory alloys (SMA) :

The most prominent and widely used shape-memory materials currently are shape-memory alloys (SMAs)

Their shape-memory effect stems from the existence in such materials of two stable crystal structures: a hightemperature favored austenitic phase and a low temperature-favored (and yield-able) martensitic phaseDeformations of the low temperature phase, occurring above a critical stress, are recovered completely during

the solidsolid transformation to the high temperature phase.This shape-memory effect witnessed by SMAs is

considered to have been first observed in a AuCd alloy by Chang and Read in 1951[5]. The discovery of theshape memory effect in the equi atomic nickeltitanium alloy (NiTi, Nitinol1) in 1963 led to greatly enhancedinterest towards commercial applications due to the combination of a desirable transition temperature close to

body temperature, super elasticity, biocompatibility, and a so-called two-way shape-memory capability.[6-9]

1.3.2 Shape memory Ceramics:

Certain Zro2 ceramics undergo a transition from tetragonal to monoclinic structure like a martensitic transition thermally

off by the application of stresses. These ceramics are called Martensitic Ceramics .

1.3.3 Shape memory polymers (SMPs):

The polymers which exhibit shape memory effect is called shape memory polymers. The mechanism of shape

memory polymers is entirely different from that of shape memory alloys. The shape memory polymers depend

largely on the glass transition temperature for the shape memory effect. They can be stimulated by temperature,pH, chemicals, and light, and are defined as polymer materials with the ability to sense and respond to externa

stimuli in a predetermined way. [10]

1.3.4 Shape memory gels:Another class of polymers possessing shape-memory properties are shape-memory gels. These materials are

usually more flexible than shape-memory rubber and are best represented by y. Osadas work from hokkaidouniversity in japan.[1114] Analogous to shape memory rubber, a typical shape-memory gel is a cross-linked

material having a hydrophilic fraction that can be swelled in water and hydrophobic sections with reversibleorderdisorder structures controlled by temperature. While cross-linking sets the permanent (high temperature)shape, the ordered structure that forms at temperatures, T , Tcritical, can be used to fix secondary shapesestablished by deformations at higher temperature, T . Tcritical. Heating above Tcritical then triggers quite

complete shape recovery


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1.4 ADVANTAGES OF SHAPE-MEMORY POLYMERS:Compared with shape-memory alloys, polymeric shape memory materials possess the advantages of high elastic

deformation (strain up to more than 200% for most of the materials), low cost, low density, and potential

biocompatibility and biodegradability. They also have a broad range of application temperatures that can be

tailored, tunable stiffness, and are easily processed. These two materials (polymers and metal alloys) alsopossess distinct applications due to their intrinsic differences in mechanical, viscoelastic, and optical properties.

A comparison of the different characteristics of SMPs and SMAs is summarized in Table 1

Table no 1: Properties of SMP and SMA

Property Shape memory polymer Shape memory Alloy

Density in g/cm

0.9-1.1 6-8

Extent of the deformation (%) Up to 800% T(trans) GPa (0.1-10) 10- 28.41

Biocompatibility ad biodegradability Can be biocompatible/biodegradable

Some are bio compatible notbiodegradable


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2. SHAPE MEMORY POLYMER

2.1 PRINCIPLE OF SHAPE MEMORY POLYMER:

The polymer materials have various elasticity from hard one like a glass to soft one like a rubber. The shape

memory polymers, however, have the characteristics of both of them, and its elasticity modulus showsreversible change with the glass transition temperature (here after called tg) as the border. The dependence ofthe elasticity modulus to the temperature is shown in figure1. The shape memory polymers, when heated above

Tg, get as soft as rubber and are easy to change the shape, and when cooled below Tg, it retains the shape intact

(shape fixing characteristic). When heated up again above Tg, the material autonomously returns to the originalshape (shape recovery characteristic). The material property which is repeatedly returning back to the original

shape is called shape memory." [16]

Fig. 1 Principle of shape memory polymer

2.2 WAYS OF ACTIVATING SHAPE MEMORY POLYMER:

2.2.1 Thermo responsive shape memory polymers

2.2.2 Electrical heating induced shape memory effect

2.2.3 Light induced shape memory polymers

2.2.4 Magnetically induced shape memory effect

2.2.5 Water activated shape memory effect


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2.2.1 Thermo responsive shape memory polymers

The shape memory polymers which change in shape with the change of temperature are called thermo

responsive shape memory polymers. By far these are the most common shape memory polymers. [17]

2.2.2 Electrical heating induced shape memory effect

The shape memory polymers are generally not conducting. So they are made Conductive by blending with

carbon Nano powders. The electric current is converted into heat. They recover the original shape when an

electric current is passed through the shape memory polymers.[17]

2.2.3 Light induced shape memory polymers

The shape memory polymers which are to be activated by light should have some photo sensitive groups which

act as molecular switches. The shape memory polymers are stretched and illuminated by a light of wavelength

greater than a fixed wavelength and the photosensitive groups form cross links .The polymer is locked in the

new shape and retains the temporary shape even when the stress is released. When this is illuminated by a light

of lower frequency, the cross linking cleaves allowing the material to go back to its original state [18]

2.2.4 Magnetically induced shape memory effect

Non-contact triggering of shape changes in polymers has been realized by incorporating magnetic nanoparticles

in shape memory polymers and inductive heating of these compounds in alternating magnetic fields. Magnetic

nanoparticles having an iron (III) oxide core in silica matrix could be incorporated into the shapememory

polymers. [17-18]

Fig. 2: Sequence of photographs showing the shapememory polymer (black strip) changing to its permanent shape

in the presence of an alternating magnetic field


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2.2.5 Water activated shape memory effect

The Actuation of the polymer can be achieved by immersion in water. A shape memory polymer which has the

permanent shape of a straight rod is programmed into a Z shape . The left part of the polymer is dipped into

water and the right part is not dipped . There is a reduction of the glass transition temperature for the left part

and it gets actuated ie gets back to its original form with the help of the room temperature water itself. [19]

Fig.3 Recovery of functionally gradient SMP actuated by water in a sequence

Fig.4 Appearance recovery in magic mirrors (the appearance of an old woman recovers to her young

appearance in a magic mirror)


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2.3 Structure and mechanism of SMPs

The common conventional SMP systems include cross-linked PE [20] and PE/nylon6 graft copolymers [21],

trans-polyisoprene (TPI) [22], cross-linked ethylenevinyl acetate copolymer [23], styrene-based polymers[24,25], acrylate-based polymers [26], polynorbornene [27], cross-linked polycyclooctene [28], epoxy-basedpolymers [29], thio-ene-based polymers [30], segmented polyurethane (PU) [31], and segmented PU ionomers

Additionally, some new biopolymers, such aspoly(3-hydroxyalkanoate)s (PHAs) copolymers composed ofdodecanedioic acid or sebacic acid monomers[5557] and bile acid-based polyesters [58], have been developedto exhibit SME, but their shapememory or mechanical properties are not highly desirable and require furtheroptimization.

Fig.5 The molecular models of thermally induced SMEs for cross-linked PE and different systems.[20]

Fig.5 demonstrates the progress of representative models developed for thermal-sensitive SMPs. For example,

in chemically cross-linked semi crystalline PE, the crystalline phase with a crystal melting temperature (Tm) isused as a switch unit to provide the shape fixity capacity [20]. The chemically cross-linked PE network

memorizes the permanent shape after deformation upon heating. Subsequently, the general molecularmechanism of thermally induced SMPs was proposed (Fig. 5b) in which the network structure is eitherchemically or physically cross-linked and the switch units are made from a semi crystalline or amorphous soft

phase. If the increase in temperature is higher than Ttrans of the switching segments, these segments are flexible


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(shown in red) and the polymer can be deformed elastically. The temporary shape is fixed by cooling down

below Ttrans (shown in blue). If the polymer is heated up again, the permanent shape is recovered.An elastomer /polymer will exhibit shape-memory functionality if the material can be stabilized in the

deformed state in a temperature range that is relevant for the particular application. This can be reached by

using the network chains as a kind of molecular switch. For this purpose the flexibility of the segments should

be a function of the temperature. One possibility for a switch function is a thermal transition ( Ttrans) of thenetwork chains in the temperature range of interest for the particular application. At temperatures above Ttrans

the chain segments are flexible, whereas the flexibility of the chains below this thermal transition is at least

partly limited. In the case of a transition from the rubber -elastic or viscous state of the glassy state, theflexibility of the entire segment is limited. If the thermal transition chosen for the fixation of the temporary

shape is a melting point, strain -induced crystallization of the switching segment can be initiated by cooling the

material which has been stretched above the Ttrans value. The crystallization achieved is always incompletewhich means that a certain amount of the chains remains amorphous. The crystallites formed to prevent the

segments from immediately reforming the coil -like structure and from spontaneously recovering the permanent

shape that is defined by the net points. The permanent shape of shape -memory networks is stabilized by

covalent net points, whereas the permanent shape of shape -memory thermoplasts is fixed by the phase with thehighest thermal transition at Tperm. The molecular mechanism of programming the temporary form and

recovering the permanent shape is demonstrated schematically in Figure 4 for a linear multi block copolymer,

as an example of a thermoplastic Shape-memory polymer, as well as for two covalently cross -linked polymer

networks. This expression describes the property of an elastomer one would not expect in an amorphouspolymer chain. The memory effect represents a problem in the processing of non vulcanized naturarubber. In the case of a quick deformation of the amorphous material of a sudden subsequent decrease or

removal ( or reduction) of the external force, the polymer re -forms its original shape. Such polymers will alsoexhibit a shape -memory effect if a suitable programming technique is applied. In this case, temporary

entanglements of the polymer chains which act as physical net points can be used for the fixation of the

permanent shape. This thermal transition can be used as a switching transition if the glass transition of theamorphous material is in the temperature range that is relevant for a specific application.

2.4 Draw backs of shape memory polymers:

A significant drawback of shape memory polymer is because of the relatively low stiffness values.A low

stiffness value indicates a relatively small value of the recovery force under constraint. But addingreinforcements help in overcoming the drawbacks . They allow us to tailor the material stiffness. Fiberglass and

Kevlar reinforcements increased the stiffness of the SMP resins and reduced recoverable strain levels

Moreover, discontinuous fiber reinforced composites showed shape recovery in all directions, while continuous

fiber reinforced composites only showed recoverability under transverse tension or bending

The addition of Nano particulate SiC reinforcements increases both the hardness and elastic modulus of the base

resin material. The increase in both of these material properties is a direct consequence of the relatively high

hardness/modulus of the SiC particles relative to the polymer matrix. The hardness and modulus increases are

directly proportional to the weight fraction of SiC. The hardness and modulus of the composite material can betailored for a given application by altering the weight fraction of the SiC, or using alternative reinforcement

materials/architectures


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3. APPLICATIONS OF SMPSShape memory polymers find its application in various fields due to its special and unique properties

3.1 Shape Memory Fabric :The shirt with long sleeve could be programmed so that the sleeves shorten as room temperature becomes

hotter. The fabric can be rolled up, pleated, creased and returned to its former shape by applying heat. Ex

blowing air through hair dryer.

3.2 Ergonomic:

The violin is made from the combination of shape memory polymer and carbon fibers. The shape memorypolymer used here is "Veriflex". It's designed to help reduce the neck and shoulder pain of the player, as it can

be reshaped as desired by the player.

3.3 SI Suits:

The suit was developed to help the sailors on the oceans and sea. It adapts to the temperature variations and

maintains a person's body temperature constant. The membrane gives optimal breathability in any given

atmospheric condition.

3.4 Morphing aircrafts:

Developing and demonstrating morphing materials and technologies that are necessary to construct deployable

Morphing aircrafts and other innovative adaptive structures critical to air force are taking place.

3.5 For selfTightening suturesShape memory polymers are very useful as selftightening sutures . In general the sutures are to be stitchedvery carefully. They may damage the cells of the skin if the threads are stitched very tight .on the other hand , if

the threads are too loose , it does not do the required function .The shape memory polymers if used , the threadscan stitched loosely and when the threads come into contact with the body , because of the body temperature , itcontracts to tighten it to the just required force

Fig.6 the suture are getting tightened as the temperature reaches the body temperature

3.6 Bard backs smart fibers for surgery:

Shape-memory polymers have the potential to completely revolutionize medical surgery, as well as having a

broad range of other applications, and the first product developed by the name Science was smart suture thatties itself into the perfect knot. This means that potentially, surgeons will be able to seal hard -to-reach wounds

with the aid of a shape -shifting thread that knows how to tie itself and never needs to be removed. The new

smart biodegradable plastic fiber cannot itself when heated to a few degrees above body temperatureResearchers believe the same material could be made to last much longer and one day be used for self-repairing

medical devices and also to shrink otherwise bulky implants such as screws that hold bones together.


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3.7APPLICATION OF THE SHAPE MEMORY POLYMER IN THE TEXTILE

SMPs can be made in the form of fibers (macro, micro and Nano fibers) [3235], solutions [36,37] films [38,39]and foams [40,41] for further textile and apparel applications, such as non-woven materials coatings finishing

lamination weaving and knitting The following provides the current status of SMP applications to textiles.

3.7.1 SMP macro fibers

Significant progress has been made in the manufacture of textile fibers from shapememory polymers throughcommon spinning methods. It is easy to make a polymer fiber, as long as the polymer has adequate molecular

weight, sufficient viscosity or a suitable melting point, but it is difficult to make a fiber for textiles. The

durability, elongation, tactile properties and process ability of fibers are the main factors that influence theirfinal end uses. SMP fibers include micro fibers, Nano fibers and non-woven. [42-45]

3.7.1.1 Properties of SMFs:

Macro scale SMFs can be made by wet, melt and dry spinning Compared with other traditional chemical fibers,

particularly elastic fibers, shapememory fibers (SMFs) are unique because their elastic modulus changeswithin a specified utilization temperature range, and they exhibit a recovery ability with an external stimulus

Their molecular orientation, recovery stress, hard-segment micro domains and post-treatments were also studiedFig. 7 compares the change in modulus under increasing temperature, which shows that the SMF is superior,

with better temperature response performance than spandex, nylon, XLA (olefin-based fiber product of Dow

company), etc.Fig. 8 compares the stressstrain curves of shapememory fibers and other fibers, and the curvesof shapememory fibers are between those of spandex and nylon fibers.[46]

Fig 7 Comparison of the elastic modulus between SMPU fiber and existing man-made fibers.


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Fig.8 Stressstrain curve comparison of different fibers

Up to now, a series of shapememory fibers have been developed with variable mechanical property rangeshapememory fixity, recovery and switch temperature range, which allow for diversified textile designsBecause textile processing includes dying, finishing, and heat-setting, shapememory fibers should have anti-oxidant, anti-thermal, chlorine-resistant and anti-aging properties, which present challenges for the productionof commercially viable SMFs.

3.7.1.2 Novel and functional SMFs

Melt spinning also provides an efficient method to prepare fibers with various profiles and functionalities. A

new SMPU hollow filament with a thermal-sensitive internal diameter via melt spinning, as shown in Fig.9

[47]. Additionally, another electro-active SMFs by incorporating MWCNTs [48] and Temperature-regulating

fibers made of PEG-based smart copolymers [49].

Fig.9 Thermally adjustable internal cavities of hollow SMFs

(deformed by transverse pressing, aoriginal shape; bdeformed shape; crecovered shape).


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3.7.1.3 SMF yarns, fabrics and garment design:

The appeal of the application of SMFs to textiles is their shape fixity and lengthwise recovery along the fiber,

accompanied by changes in their stress. SMFs have been made into yarns by ring and friction spinning

techniques, and the yarns have been applied to the knitting and weaving of fabrics. [50]

3.7.2 SMP solutions for finishing fabrics

Alternatively, an SMP solution has been directly used as a finishing agent to obtain valuable properties in

different types of textile fabrics, such as cotton or wool. Another method for producing shapememory fabricsis shapememory finishing, which is a process that transfers the shapememory properties from polymers tofabrics. Compared with the shapememory fabrics knitted or woven with SMFs, a small content of SMP issufficient to transfer the SME to the fabric during shapememory finishing. This process is highly efficient.[45-47]

Fig.10 Wrinkle recovery mechanism of shape memory fabric finished with SMPs

In Fig.10, wrinkles and damaged creases of a fabric treated through shapememory finishing recover back to

their original flat and creased form when the temperature is raised to or above the transition temperature of theSMP. The typical shapememory effects of a fabric are shown in Fig.11.


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Fig11 Typical shape memory recovery effects of shape memory finished fabrics

3.7.3 Smart Fabrics Based on NiTi SMA:

Figure12(a) illustrates the shape memory recovery of smart textile having the trained two - way NiTi SMA

Spring varied with temperature. The shape of the smart fabric will vary as the trained SMA is deformed andspring back to the original shape.

Fig.12 Shape Memory Recovery of Smart Textile Having a Trained SMA spring with a thermo mechanicaltraining process


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In this case, the smart fabrics can display two-way shape memory effect when the spring is trained for two -way

SME with a thermo mechanical training process shown in Figure (b). The effect of annealing temperature onSME is related to the change of internal stress fields and the anisotropic distribution of dislocation, as well as

the states of parent phase due to shape and disperses degree of Ti3Ni4 precipitates. This dislocation structure

creates an anisotropic stress field in the matrix, which guides the formation of marten site phases into variants

of preferential orientations in relation to the deformation adopted in the training procedure, thus resulting in amacroscopic shape change during subsequent thermal transformation cycles.

Fig.13: Examples of shape memory fabrics after shape memory training

Figure13 illustrates some complex examples of the trained shape memory texture structure of SMA, by a set of

designed clamping devices. For SME training of the SMA fabric, initially SMA woven samples, covered with

these clamp devices, are heated in an oven for an appropriate thermo mechanical training process when kept

with the binder, and then quenched in water. The shape of the SMA woven fabric could be transformed from an

initial predestined state at a low temperature, into a trained coil state at a high temperature, with a high rate of

shape recovery rate.

3.7.4 SMP films, foams and laminated textiles

Shapememory films and foams have a number of applications in laminated smart fabrics [51]. The functions ofSMP films applied to textiles include waterproofing, WVP, seam sewing, crease recoverability and crease

fixing. To optimize these effects, various properties of SMP films have been investigated, such as their thermo-

mechanical properties, the effects of different structural factors on their physical and water vapor transportproperties [39,51-52], their molecular weights [53] the effects of crystal melting and the influence of differen

processing temperatures.

3.7.4.1 Water vapor permeability of SMP films

WVP is one of the significant adaptive parameters of thermal-induced SMP films, which enables broadapplications that include textiles. SMPUs have become good candidates for breathable laminated nonporousfabrics due to the sensitivity of their WVP to temperature and humidity. The WVP properties of SMPUs with an

amorphous reversible phase, and water vapor permeable fabrics were prepared by coating the SMPU

membranes on a fabric substrate.


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3.7.4.2 Applications in breathable textiles:

A higher comfort level can be achieved using shape memory polymer coated garments. By coating with shape

memory polymers like shape memory poly urethanes, we can control the permeability to a large extent. we can

have a high water vapour permeability at a higher temperature and a lower water vapour permeability at a lower

temperature, thus making it useful to be used in both the conditions .This allows it to be used for breathable

textiles. The polymer forms a thin layer of nonporous surface on the fibres .so the water vapour permeability

would be a three step mechanism of sorptiondiffusiondesorption mechanism. The water vapour molecules

would be absorbed onto the polymer surface and then the diffusion takes place, finally the water molecules aredesorbed out of the surface because of the concentration differences with the environment. At the crystal

melting point of the soft phase, the micro Brownian motion of the soft phase would occur. This leads todiscontinues changes in the density and provide enough gaps for the water vapor molecules to pass through.

4. FUTURE OUTLOOKSMPs represent a class of smart polymeric materials with a variety of chemical compositions, mechanisms andmechanical responses. SMPs has Accelerated growth in research activities and New SMPs with novelstructures and diversified functionality. No of significant progress in enabling SMP technologies, such as

materials, processes, and techniques; however, these application successes remain limited. Innovating the

integration of all or major functions in practice is worth our concerted effort. As for the distinctive weak pointssuch as low recovery stress, we must look for new technologies to produce more powerful materials. While we

cannot be unreasonable in our expectations for the near future, the existing problems can be overcome by

finding more innovative applications that exploit the existing features and capacity of SMPs. This goal requirestremendous creativity and sharp vision as well as investment


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