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elt–solid polycondensation of lactic acid and its biodegradability
. Maharanaa, B. Mohantyb, Y.S. Negia,∗
Polymer Science and Technology Program, Department of Paper Technology, Indian Institute of Technology Roorkee, Saharanpur Campus,aharanpur 247001, Uttar Pradesh, IndiaDepartment of Chemical Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India
r t i c l e i n f o
rticle history:eceived 1 September 2007eceived in revised form 8 October 2008ccepted 8 October 2008vailable online 6 November 2008
a b s t r a c t
Sequential melt–solid polycondensation of lactic acid (LA) produces high molecular weight(MW) poly(lactic acid) (PLA), which is cost effective and can be put to a variety of uses,including packaging, biomedical and electrical appliances. This paper deals with the tech-nology available for melt as well as solid state polycondensation of LA, the selection ofcatalysts, different synthetic methods, the effects of operating parameters on the MW of theproduct, and various properties of PLAs such as their physical, thermophysical, mechanical,
eywords:
hermophysical propertiesechanical propertiesegradationesign of experimentolid-state polycondensation
electrical properties, degradation behavior and the dependence of these on the molecu-lar weight. The kinetics and mechanisms involved in the different synthetic methods anddegradation of the polymer are also discussed. Finally, a response optimization analysisused in the design of experiments (DOE), suggests a group of parameters under which onecan achieve higher MW PLA, and provides mathematical relationships between synthesis
Poly(lactic acid), PLA, is a biodegradable, biocompat-ible and compostable polyester derived from renewableresources such as corn, potato, cane molasses and beetsugar. It has a bright future as an environmentally friendlythermoplastic. With the help of this green polymer indus-tries will be able to close the carbon cycle, and theirdependence on non-renewable fossil resources will bereduced considerably.
Aliphatic polyesters such as PLA, derived from LAproduced from renewable resources, has promisingapplications in packaging, consumer goods, fibers andin biomedicine because of its excellent mechanicalproperties, transparency, compostability and bio-safety.Earlier, polymers such as poly (�-caprolactone) (PCL),polyhydroxybutyrate-valerate (PHBV), polyurethane (PU),polyethylene (PE), poly(vinyl chloride) (PVC), polystyrene(PS) and cellulosics were used for similar applications.Whereas, PLA was introduced in the market for medicalsutures around 1968, its counterpart PHBV was introducedmuch later, in 1983.
Commercially, PLA is synthesized using ring open-ing polymerization (ROP) of lactide monomer, obtainedthrough dimerization of LA, as well as by polycondensation(PC) of LA as shown in Fig. 1. These methods of synthesisdue to their inherent weaknesses [1–7] increase the pro-duction cost substantially. Table 1 lists some merits anddemerits of these methods of synthesis. To avoid of thedrawbacks, many investigators [1–7] have suggested alter-nate synthesis routes such as melt polycondensation andsequential melt–solid polycondensation. Of these routes,melt polycondensation followed by solid-state polycon-densation with a suitable catalyst, offers high molecularweight PLA with high yield in comparison to the ROProute. Many investigators [2,3,7–9] have synthesized highmolecular weight PLA by adopting a post polycondensationmethod. Of various post polycondensation methods, solid-state polycondensation (SSP) is preferred, as it does notrequire any external agent during polymerization and thus
yields pure PLA. In addition, it provides higher MW as wellas higher yield in comparison with melt polycondensation.However, some investigators have used food grade cata-lysts like tin and organotin compounds such as stannousoctanoate, as well as water tolerant catalysts and molecu-
ical analysisTNBT Titanium(IV) butoxide
lar sieves to adsorb water produced by polycondensation,thereby enhancing the rate of polymerization.
The number of steps suggested, in the melt–solid poly-condensation process for the synthesis of PLA, by different
T. Maharana et al. / Progress in Polym
it5miifaom
2
a
TP
P
R
P
P
M
Fig. 1. Production scheme of PLA through PC and ROP.
nvestigators varies appreciably, being as high as 7. It mayake about 60 h for the production of PLA having a MW of× 105 Da. The biodegradability of PLA has been studied byany investigators [10–16] and it has been concluded that
t mainly depends on the MW. Based on the above backdrop,t appears that a holistic approach should be undertakenor the synthesis of PLA by integrating the knowledge baselready available for this purpose and a plan for synthesisf high MW PLA should be developed. The present papereets this objective.
. Raw material
Potential feedstock for the production of PLA is lacticcid (2-hydroxy propionic acid), which contains a chiral
able 1ros and cons of different methods employed for the synthesis of PLLA.
rinciples Methods Advantages
OP Solution polymerization (i) Chemistry of the reaction ccontrolled thus the propertiepolymer can be varied in a mmanner
Bulk polymerization (ii) Mw varies from 2 × 104 toSuspension polymerization
C Solution PC (i) High Mw can be achieved
Bulk PC Provides low production costMelt PC
ost PC Melt modification (i) High Mw can be achieved
Solid state PC (ii) Simple processRadiation (iii) Low investment
(iv) Low cost of the operation
(v) Easy handling(vi) Suppression of undesiraband hence produces higher p(vii) Cross linking increases h(viii) Less energy requiremen
w: wt. average MW.
er Science 34 (2009) 99–124 101
centre. LA can be produced by fermentation from renew-able resources such as milk and carbohydrates such as corn,potato, cane molasses and beet sugar. The LA producedcontains both D and L forms, though the l-form is predom-inant. l-lactide monomer, a cyclic dimer of LA, is currentlybeing distilled from corn biomass, which naturally containsl-lactic acid, oligo(l-lactide) and other precursors whichare driven to cyclic l-lactide during the distillation pro-cess. As a rough estimate, in the distilled product, l-lacticacid can be present as high as 90 to 99%. The 90% purecommercial grade lactic acid contains impurities such asarsenic (<1 ppm), iron (<5 ppm), heavy metals (<5 ppm),chloride (<10 ppm), sulfates (<10 ppm), sulfated ash (max-imum 0.05%), reducing sugar, methanol and methyl ester.Stereochemical purity of the raw materials (lactic acid) isan important factor in deciding the properties of PLA.
3. Structure of PLA
The structural formula of PLA is shown in Fig. 2. It is achiral polymer in which molecules containing asymmet-ric carbon atoms have a helical orientation. Two opticalisomers, PLLA and PDLA, exist in PLA. In 1968, de Santisand Kovacs [17] reported the pseudo-orthorhombic crys-
tal structure of PLLA [18]. The crystal structure (Fig. 3) wasfound to be a left-handed helix conformation for the �-form. While drawing fibers of high molar mass PLLA, athigh draw ratio, a partial modification of the �-form to astable ß-form takes place [19]. Recently, a �-form has also
Disadvantages
an be accuratelys of the resultantore controlled
(i) Higher cost of production due to thecomplicated purification process of the lactide
6.8 × 105 Da (ii) Azeotropic distillation of solvent used(iii) High cost prevents commodity applications
(i) Hard to remove solvent completely from theend product(ii) Substandard mechanical properties(iii) Competitive reaction of lactide formationand simultaneous degradation process at hightemperature(iv) Difficulty in driving the dehydrationequilibrium to the direction of esterification(v) Use of solvents results in complex processcontrol, leading expensive PLLA(vi) Inability of formation of PLLA with asufficiently high molecular weight(vii) Severe increase in melt viscosity(viii) Byproducts are formed
(i) May require external agent (example watertolerant catalyst and molecular sieve)(ii) Purification of lactide(iii) Low reaction rate(iv) Solid particle processability problemsarising from sintering
le side reactionsurity polymerseat resistancet
102 T. Maharana et al. / Progress in Polym
Fig. 2. Structural formula of PLA, chiral molecule.
been obtained by epitaxial crystallization of PLA in hex-amethylbenzene [20]. The crystal structure of PLA is basedon an orthorhombic-base-centered unit cell that containstwo 10/3 helical chains arranged along the c axis [21,22] as
Fig. 3. PLLA crystal structure projected onto (1 1 0) and (0 0 1);orthorhombic system; point group D2; 10/3 helical chain; a = 1.06 nm;b = 0.61 nm; c = 2.88 nm [21].
er Science 34 (2009) 99–124
shown in Fig. 3. PLA shows hexagonal packing. The pointgroup of PLLA crystal is D2. However, the structure of PLAhas not been completely established. Different investiga-tors [17,20–22] suggest different parameters of the unitcell.
4. Synthesis routes for PLA
Raw LA is purified by removing impurities, dehydrated,and then polymerized. The presence of an asymmetric cen-ter in LA helps in the formation of different enantiomerssuch as PLLA, PDLA or PDLLA or a combination of these.Polymers derived from LA by PC are referred to as poly-lactic acid whereas; those from lactide by ROP are termedpolylactide. However, in common terminology both theseproducts are called PLA. At present there are two estab-lished routes for the synthesis of PLA from monomer, LA.These are: ring opening polymerization (ROP) and poly-condensation as shown in Fig. 1. A further subdivision ofabove synthesis routes is shown in Fig. 4. In ROP, the poly-merization can be through cationic, anionic, coordinationor free radical polymerization whereas, in PC, condensa-tion polymerization takes place by the elimination of smallmolecules such as water.
4.1. Ring opening polymerization (ROP)
Using ROP, it is possible to control the chemistry ofpolymerization accurately, and thus, the properties ofresulting polymer can be varied to suit the application.High molecular weight PLLA is produced commercially,by ROP of l-lactide obtained by decomposition of lowmolecular weight PLLA. The ROP route includes PC of LA fol-lowed by depolymerization to the dehydrated cyclic dimer,lactide (3,6-dimethyl-1,4-dioxane-2,5-dione), which thenundergoes ROP to produce high molar mass PLA. Thedepolymerization is done conventionally by simultane-ously lowering the pressure to ≤266.6 Pa, maintaining thetemperature between 150 ◦C and 220 ◦C, and then distillingoff the produced lactide.
Because there are two stereo forms d-LA and l-LA, theoptically active lactide can be found in three differentstereoisomers: i.e. d,d-lactide, l,l-lactide and d,l-lactide(meso-lactide) [23]. The composition of the polymer, interms of these stereoisomers, affects the properties of thepolymer [24,25].
Some of the leading producers of PLA like Cargill Dow(Nature Works LLC), Shimadzu and Dupont are producingPLA of varying molecular weight by ROP. The ROP of lac-tide with Sn(Oct)2 catalyst with catalyst concentration of100–1000 ppm gives PLA having molecular weights up to106 Da at 140–180 ◦C in 2–5 h [26].
ROP has been carried out by solution polymerization,bulk polymerization, melt polymerization and suspensionpolymerization [27]. The mechanism of polymerizationinvolved in ROP can be of ionic-, coordination- or free
radical-type depending on the catalyst used [28,29]. TheROP of lactide is catalyzed by compounds of transition andnon-transition metals such as tin [30–34], lead [35], zinc[36,37], bismuth [35], yttrium [38], iron [39], aluminum[40,41] and magnesium [42]. Of these, tin(II) compounds
T. Maharana et al. / Progress in Polymer Science 34 (2009) 99–124 103
ethods f
a[aailcboch
vmrmfcht
4
ytasl
TO
M
LLLLrr
H(
Fig. 4. Different m
re frequently used and are considered to be most efficient1,2]. The mechanism, of tin(II) 2-ethylhexanoate, involves
pre-initiation step in which it is converted to a tin(II)lkoxide by reacting with an alcohol. Then the polymer-zation proceeds on the tin–oxygen bond of the alkoxideigand [33]. The highly active catalysts prepared using tinompounds are toxic [43]. However, other tin compoundased catalysts such as organotin compounds and stannousctanoate exhibit low toxicity. Low environmental impactatalyst such as oxides of titanium (titanium(IV) butoxide)ave also been developed for lactide polymerization [44].
The presence of catalyst in the synthesized polymeraries from as low as 9 ppm to a few hundred ppm. Further-ore, synthesized polymers also contain some unwanted
esidual monomer which should be removed from the poly-er. The process of purification and isolation of l-lactide
rom PLA makes it high-priced and thus prevents its appli-ations as a commodity material. Although ROP producesigh MW PLA with high yield, it has also some disadvan-ages, as shown in Table 1.
.2. Polycondensation (PC)
Polycondensation of LA in the presence of catalysts
ields PLA and water as a byproduct. It has been observedhat binary catalysts, comprising of metal compoundsctivated with proton acids, are more effective thaningle-metal-compound based catalysts [1]. Binary cata-ysts produce PLA of the order of ca. 105 Da with yield as
able 2verview on different LA based polymers prepared by direct PC or by PC and chai
onomer Linking agent Macromolecular ar
LA – Linear homopolymLA – Linear homopolymLA HMDI Linear homopolymLA Dipentaerythritol Star-shaped homopac-LA – Random 50/50 sterac-LA Dipentaerythritol Random 50/50 ster
high as ca. 98%. PLA formed by PC of lactic acid consistmainly of lactyl units. Such polymer is either composed ofone stereoisoform of two or a combination of both in var-ious ratios [45]. One of the disadvantages, for the directPC, is that a low molar mass polymer showing substan-dard mechanical properties is usually obtained, owing tosevere increase of melt viscosity and higher operating tem-perature. Until 1995, it was believed that a high MW PLLAcould not be achieved by the direct polycondensation ofLA owing to inherent difficulty in driving the dehydrationequilibrium in the direction of esterification—which is arequirement for the formation of sufficiently high molecu-lar weight PLLA.
To overcome this difficulty, direct polycondensation ofLA to a high molar mass is obtained by manipulating theequilibrium between LA, H2O and PLA by using eitheran organic solvent [46,47] or a multifunctional branchingagent (e.g. dipentaerythritol) [4,6,48]. A multifunctionalbranching agent leads to a star-shaped polymer [49]. How-ever, use of solvents such as diphenyl ether demandscomplex process control leading to an expensive PLLA. Itis also difficult to remove solvent completely from the endproduct. In an effort to avoid these difficulties, a new pro-cess, called melt polycondensation has been developed for
obtaining a high molecular weight PLLA at reduced cost[1,50]. Polycondensation of LA can also be done in thepresence of difunctional monomers (e.g. diols or diacids),giving rise to telechelic prepolymers [51]. The telechelicprepolymers thus produced, are used to yield a high molar
n extension [6].
chitecture Molecular size Reference
er A, B [53,54]er B, C [1–4,55]er C [46,56]olymer C [34]eocopolymer A [53,57]eocopolymer C [58]
edium molar mass polymer (20–70 kDa); C: high molar mass polymer
104 T. Maharana et al. / Progress in Polymer Science 34 (2009) 99–124
by the a
Fig. 5. Hypothetical condensation mechanism
mass polymer through a second reaction step, involv-ing a linking molecule such as diisocyanate [46,48] orbis(amino-ether) [52]. These polymers show similar behav-ior to poly(lactide) homopolymers prepared by ROP. Table 2presents an overview of the different lactic acid basedpolymers prepared by polycondensation and also providesreferences on different aspects of structure and propertiesof these polymers [6].
4.2.1. Mechanism of polycondensationThough the effect of TSA on the polycondensation mech-
anism is not yet well understood, a plausible mechanismhas been proposed by Moon et al. [1]. The mechanisticaspects are shown in Fig. 5.
1. Terminal groups of PLLA form coordinate bonds with thecatalyst center of Sn(II), present on tin(II)-oxide clus-ter(2), formed by hydrolysis of SnCl2·2H2O. Hydroxyl andcarboxylate ligands present in PLLA are responsible fordehydration with the formation of Sn–OH (3).
2. The amounts of both terminal hydroxyl and carboxylgroup decrease with the increasing molecular weight ofPLLA. When the molecular weight of the PLLA becomeshigh enough, the coordination sites of the catalyst centerare not filled with the terminal groups (3). The catalystsite with this ligand vacancy induces side reactions, suchas the decomposition of l-lactide, causing discolorationand racemization of PLLA. In fact, the reaction of l-lactidewith the catalyst at high reaction temperature causesserious discoloration.
3. The proton acid added to the catalyst works as a ligandfor the catalyst site (4). As the proton acid is not involvedin the esterification, it fills the open coordination sites ofthe catalyst to prevent side reactions. Addition of a strongproton acid like TSA also stimulates the dehydration of
Compound 3 to 5 via 4, as shown in Fig. 5, and therebyincreases the rate of reaction.
A detailed analysis is needed for confirmation of thesemechanistic features of the melt polycondensation in
ction of the Sn(II) and Sn(II)–TSA systems [1].
which a high molecular weight PLLA (ca. 105 Da) canbe obtained by the catalysis of tin(II) chloride dihydratewith an equimolar amount of TSA within 35 h under0.13–2.66 kPa pressure and within a temperature range of180–200 ◦C with an average yield of 67%.
If the mole ratio of TSA w.r.t. catalyst is too high, thecatalyst activity is hindered. This may be attributed to thedecrease in the number of vacant sites that are available forthe coordination of the polymer tails. During the polycon-densation reaction, under the rigorous reaction conditions,it appears that TSA evaporates from the reaction mixtureto induce side reactions such as racemization and discol-oration. Discoloration of the product has been a seriousproblem in the polycondensation of PLLA. During polycon-densation, the color of PLA first changes to yellow, thento brown, and finally it became black. This discolorationmay be due to various factors, such as, high reaction tem-peratures, long reaction times, catalyst used, solvents andbyproducts. It has been observed that with the additionof TSA, the product discoloration is effectively preventedand the growth rate of the molecular weight is greatlyenhanced.
Polycondensation of LLA in bulk has been known to pro-duce PLLA with molecular weight of order of only 104 Dadue to the unfavorable reaction equilibrium constant [59].As the low molecular weight PLLA, thus produced, is toobrittle to be used as a useful material, increase of themolecular weight is imperative. Solution PC of LLA alsoyields high molecular weight PLLA, as obtained by ROP,by using a large volume of solvent compatible with PLLA[60,61]. Solvents with high boiling point such as p-xyleneand diphenyl ether, o-dichlorobenzene, o-chlorotolueneare used for the removal of the dissociated water byazeotropic distillation [59,62]. Polycondensation of LAinvolves two reaction equilibria, namely, dehydration equi-
librium for esterification and ring-chain equilibrium fordepolymerization of PLLA into l-lactide as shown in Fig. 6.Multiple reactors and complex facilities are needed forthese processes and thus invariably increase the produc-tion cost of PLLA. Moreover, flammability and toxicity
T. Maharana et al. / Progress in Polym
ot
4
cmlPopmomita
4pb[(aidcpitiiocdaobtp
4dpctpda(
Fig. 6. Two reaction equilibria involved in polycondensation [1].
f the solvents make the solution process less attrac-ive.
.2.2. Post-polymerizationThe low molecular weight PLA obtained by PC or ROP
an be processed further by various post-polycondensationethods, e.g. melt modification, radiation induced cross-
inking and solid-state PC, to obtain high molecular weightLA. Melt modification has also been applied to PLAbtained from ROP. Grafting and blending are two otherost-processing methods for PLA to produce heteropoly-ers [6,45]. The PLA, obtained after polycondensation
r ROP, forms a homogeneous supercooled state with aonomer ratio more than 5 wt%. During the post polymer-
zation process, crystallization of PLA occurs. In addition,he monomer consumption reaches 100% as the monomernd catalyst are concentrated in the amorphous part.
.2.2.1. Melt modification. Structural melt modifications ofolymers are often related to radical reactions, which cane generated by peroxides [63] or by high-energy radiation64]. Peroxide melt-modification of poly(�-caprolactone)PCL) has been investigated in a number of studies [65–67]nd it has been found to undergo reactions resultingn branching and cross-linking by use of 0.05–3 wt%icumylperoxide [66]. Peroxide melt-modification of PLAauses drastic changes in a number of properties of theolymer. Branching has been suggested to be the dom-
nating structural change in PLA at peroxide contents inhe range of 0.1–0.25 wt% and also cross-linking at perox-de additions above 0.25 wt% [68]. The peroxide reactionncreases the melt strength [69]. Morphological changesccurred in the peroxide modified PLA because of reducedrystallization rate and thus result in a hydrolytic degra-ation [11]. Further, the tensile modulus is reduced andmore flexible material is obtained [68,69]. Benzoyl per-
xide stabilizes the polymer against thermal degradationy deactivating the catalyst residues. The melting point ofhe stabilized polymer decreases with increasing benzoyleroxide concentration.
.2.2.2. Radiation induced cross-linking. PLLA was irra-iated using an electron beam (EB) in the presence ofolyfunctional monomers (PFMs) such as triallyl iso-yanurate (TAIC), trimethallyl isocyanurate (TMAIC),
rimethylolpropane triacrylate (TMPTA), trimethylol-ropane trimethacrylate (TMPTMA), 1,6-hexanedioliacrylate (HDDA), ethylene glycol, bis[pentakis(glycidylllyl ether)]ether, and hydroxyl terminated ethylene glycolEG), as cross-linking agents. The PFM, triallyl isocyanurate
er Science 34 (2009) 99–124 105
(TAIC) at 3% concentration was found to be most effectivefor cross-linking of PLLA by irradiation. Cross-linking ofPLA, by high-energy radiation, increases the degree ofcross-linking as a function of radiation dose and manip-ulates the mechanical properties of PLA, affects the heatresistance and reduces the solubility [8]. Cross-linkedPLLA is transparent and has heat resistance above 200 ◦C.It is used for heat-shrinkable tubes, cup and plates. Anunirradiated cup has been found to deform and becametranslucent whereas, the cross-linked PLLA cup retains itsoriginal shape and transparency even after boiling wateris poured into it. Because of the cross-linked structure ofPLLA the process of crystallization is inhibited and heatresistance is improved. Thus, cross-linking is beneficial inexpanding the applications of PLLA.
Irradiation effects on aliphatic polyesters were studiedby D’Alelio et al. [70] in the late 1960s. Accounts of radicalinduced modification of copolymers of lactide and otherring-form monomers have not been found in the litera-ture. It has also been seen that radiation has no effect onTg, Tm and rate of hydrolytic degradation. Tensile modulusand tensile strength of PLA decrease upon irradiation [6].
Irradiation of PLA causes mainly chain-scission at dosesbelow 250 kGy [71,72]. At higher doses, cross-linking ofpolymer chains increases as a function of the irradia-tion dose, both in air as well as in an inert atmosphere[72]. In air and nitrogen, and on irradiation with �-rays at room temperature, PLA undergoes chain scission,cross-linking and decrease in crystallinity simultaneously.For PLA homopolymers, irradiation causes a substantialdecrease in the tensile strength and thus makes PLA brittle[73,74]. Similar observations were recorded for copoly-mers of lactide, glycolide [75] and copolymers of lactideand �-caprolactone [76]. As radiation induced reactions,mainly take place in the amorphous phase of the poly-mer, the degree of crystallinity which indirectly defines theamount of amorphous phase present in the polymer, is animportant parameter [71]. Irradiation does not acceleratethe hydrolytic degradation of poly(l-lactide) [77], poly(d,l-lactide) [73] and copolymers of glycolide and lactide [75].
Peng et al. [78] suggested a new route for the synthe-sis of PLA by microwave radiation melt polycondensation:i.e. placing the LLA in an auxiliary reaction medium(SiC), increasing the system vacuum from 0.050–0.070 MPato 0.080–0.099 MPa and applying microwave irradiationfor 30–50 min. With this process they obtained a pre-polymer of 3000–5000 Da determined on the basis ofviscosity-average molecular weight. They further mixedthe prepolymer with catalyst (SnCl2·2H2O (0.3–0.50 wt% ofprepolymer) and p-TSA at a mole ratio of 1:0.5–2.0, and irra-diated with microwaves for 2–5 min under normal pressurefollowed by further irradiation for 25–50 min by microwavepower (500–550 W) under vacuum (0.085–0.1 MPa) toobtain PLA with Mv of 40,000–60,000 Da. They did not pro-vide structure–property correlation data for the PLA thusproduced.
4.2.2.3. Solid-state polycondensation (SSP). AlthoughVouyiouka et al. [79] reviewed and discussed kineticsand simulation of SSP of polyamides and PET they didnot discuss it for PLA. A limited amount of literature,
in Polym
106 T. Maharana et al. / Progress
including patents, is available on post-polycondensationof PLA [9,80–100]. However, a substantial amount ofwork is available on SSP of PBT, PET, polyamides andpolycarbonates.
SSP appears to be an effective route for PLA synthesiswhen compared with ROP and simple polycondensation.The process comes under Green Chemistry: it is simple,easy to handle, and because of the lower reaction tempera-ture compared with melt polymerization, does not promoteundesirable side reactions. Further, high molecular weightPLA can be synthesized by melt polycondensation fol-lowed by SSP [81]. SSP involves both chemical and physicalsteps. Since it is controlled by reaction kinetics, reactivechain-end mobility in the amorphous phase and conden-sate removal through diffusion are important issues. SSPincreases the degree of polymerization considerably, andthus can increase the molecular weight of a polycondensateup to 20 times [86].
In SSP, a semicrystalline solid prepolymer, of relativelylow molecular weight, in powder, pellet, chip or fiberform is heated to a temperature below Tm but above Tg
(to improve mobility and subsequent reaction of the endgroups) in the presence of a suitable catalyst. Simultane-ous removal of the byproduct of condensation from thesurface of the material is necessary, after it diffuses outfrom the bulk, either by evaporation under reduced pres-sure or by driving it away by a carrier gas. An optimumamount of crystallinity is required to prevent agglomera-tion of particles in the reactor [12,101,102]. SSP essentiallytakes place in the amorphous region of the polymer, whereall the reactive end groups reside. Since SSP actually startsat much lower temperatures, compared to polymerizationin the molten state or in solution [103], the reaction tem-perature can range from sufficiently below Tm [104] to just5–15 ◦C above Tm [105]. The Tm of PLLA is ca. 175 ◦C. Thisis, therefore, also a high temperature reaction. Because ofthe restricted and slow mobility of end groups, the timeneeded to reach a particular molecular weight is generallymuch longer than in the melt or solution [104]. SSP reducesdiscoloration and degradation associated with high tem-perature melt polymerization, thereby, making it useful inpolyester and polyamide synthesis.
Although, SSP takes a considerably longer time, veryhigh molecular weight PLLA can be obtained, which can-
not be accomplished in melt or solution polymerization,owing to viscosity restrictions and hydrolytic, thermal andoxidative degradation [106–108]. Almost all catalysts formelt and solution phase polycondensation can be used ascatalysts for SSP [109,110].
Moon et al. [3] obtained PLLA of 6 × 105 Da by melt–solidpolycondensation of LLA catalyzed by a SnCl2·2H2O/p-TSAbinary system. They first prepared a polycondensate witha molecular wt. of 2 × 104 Da by melt-polycondensation.This product was heat treated around 105 ◦C to promotecrystallization; and then it was heated at a temperaturesomewhere within 140–150 ◦C for 10–30 h for further poly-condensation to produce PLLA of 6 × 105 Da. A high-qualityPLLA, with high yield of about 90% having a molecularweight exceeding 5 × 105 Da, can be obtained within a rel-atively short reaction time of about 40 h. The molecular wt.of PLLA thus achieved is comparable with that obtainedby the lactide method using ROP. They further observedthat, the polycondensate obtained after heat treatmentdoes not show a crystallization exotherm in DSC, but doesshow a clear melting endotherm at 158 ◦C. The heat-treatedpolycondensate did not melt up to 150 ◦C, thus SSP was con-ducted at 150 ◦C for different lengths of time (10–30 h) toobtain high polymer without discoloration. An optimumamount of crystallinity is required to prevent agglomera-tion of particles in the reactor [5].
Plasticizers can be used to increase the mobility of theend groups in the amorphous region and consequently toincrease the rate of the SSP reaction. Polyethylene glycols(PEG) of two different number average molecular weightshave been selected as plasticizers [111]. Although, citrateesters, dimethyladipate, tributylcitrate, acetyltributylci-trate and PCL are reported as biodegradable plasticizers(Table 3) for PLA polymers [112–116], these should not beused because they have aliphatic alcohol and ester groupswhich can undergo transesterification with the PLA chainends. One of the crucial properties of a plasticizer is that itshould be miscible with PLA, so that a homogeneous blendcan be created by polar interactions. Further, the plasti-cizer should not be too volatile because this could causeevaporation to occur at the elevated temperature used dur-ing processing. Furthermore, the plasticizer should not beprone to migration because this could cause contaminationof materials in contact with the plasticized PLA. It couldalso cause the blended materials to regain the brittlenessof pure PLA. Some problems faced during synthesis or withsynthesized polymer and their remedies are indicated inTable 4. M/s Cargill uses low amounts of an epoxidizednatural oil to introduce branching into the polymer chain
during polymerization [113]. Branching achieved duringpolymerization has the additional benefit of reducing meltviscosity, which further assists in processing. Melt elasticityand subsequent effects can be controlled by various tech-niques such as controlling base PLA, comonomer, reactor
Effects Reference
tributylrtial
Tg/Tm decreases and elongation at breakincreases
[114]
urate Flexibility and impact resistance improves;gains deformation and resilience
[115]
in (TAC)l
Tg/Tm decreases and elongation at breakincreases
[116]
T. Maharana et al. / Progress in Polymer Science 34 (2009) 99–124 107
Table 4Solutions to problems faced during melt–solid polycondensation.
Problem Solution Remark
Discoloration Addition of protonic acid like TSA as cocatalystwhich also enhances the rate of growth ofmolecular weight
May be caused by various factors like high reactiontemperature, long reaction time, catalysts, solventsand byproducts
Low MW Polycondensation of lactic acid in presence ofwater tolerant catalyst followed by solid-statepolycondensation in presence of molecular sievesas water absorbents
Molecular sieve is represented by a chemicalformula [M(I),M(II)]O·Al2O3·nSiO2·mH2O, whereM(I) and M(II) are monovalent metal and bivalentmetal, respectively, which include Na, K, Ca, Sr, Ba,n = 2–10, m = 0–9
Use of binary catalyst also enhances molecularweight
Poor melt elasticity leading toproblems during extrusion processes
The most promising method of increasing the levelof entanglement is by introducing branching intothe polymer
Poor elasticity results from the low degree ofmolecular chain entanglement
Branching agent like epoxidized natural oil andcross linking agent like peroxides, can be used toattain this
Lactide formation at high temperature The reaction can be carried out at high temperatureand low pressure to overcome this problem
Low pressure increases the boiling point
Backward reaction occurs due toformation of H2O molecules duringcondensation
Water tolerant catalysts can be used. During solidstate polycondensation molecular sieves can beused as water absorbents to enhance the reactionrate
Reaction rate is enhanced and the reactionproceeds to formation of high MW PLA
Residual monomer (a) Crystallization followed by SSP (1) Polymer crystallization was induced duringpolymerization to promote monomer consumption
ture ∼ cr
csi
c
TV
R
[[
[[
[
[[
[
[[
Bmp
(b) Polymerization temperatemperature (Tc)
onfiguration, initiator and cross-linker. As the polydisper-
ity index (PDI) of the polymer increases, the melt elasticityncreases.
Table 5 gives some input parameters of signifi-ance, involved in melt polycondensation. Some progress
able 5ariation of operating parameters on output variables during synthesis of PLA by
eference Input parameters
Amount ofmonomer
Time ofRxn (h)
Temp (◦C) Catalyst P
44] NA 40 180 Ti(OBu)4 150] NA 12 180 SnCl2/p-TSA 1
Sn(II)Oct N
117] NA 10 170–180 SnCl2 (0.5 wt%) 7118] NA 23 160 Sn(II)Oct N
119] 10 g OLLA <15 180 SnCl2 with 0.2% 1Al(OiPr)3
Ti(OiPr)4
Y(OiPr)3
Si(OEt)4
Ge(OEt)4-Best
120] 30 g 24 100–220 NA N1] 200 g <15 180 SnCl2/p-TSA 1
121] 1000 ml 30 120–200 Sn(II)Oct 6Maximum at 0.05 wt%
57] NA 1–7 200 No catalyst used N55] 36.2 g 20–40 140 Sn powder (0.14 g) N
olycondensation; Rxn: reaction; NA: not available.
to reach 100%ystallization (2) Monomer consumption was found to reach
100% because the monomer and catalyst could beconcentrated in the amorphous part
has recently been achieved in obtaining high molar
mass polymer by sequential melt–solid polyconden-sation [2,7,44]. The advantages and disadvantages ofthe different methods of synthesis are summarized inTable 1.
melt polycondensation method.
Output variables Remarks
ressure (Pa) Monomerused
Yield % MW (Da)
33.3 l-LA NA 1.3 ×105 DP used,333.3 l-LA 42 30,000 BC more
efficient. MPused
A 45 19,000
0 (d,l)-LA NA 4,100 MP usedA (d,l)-LA 80 8.2 × 103 MP used
,333.2 l-LA 40,000 BC & MP used13 2.7 × 104
38 5.7 × 104
37 1 × 104
78 2.2 × 104
65 3.7 × 104
A (d,l)-LA NA 1 × 105 MP used,333.2 LLA 67 1 × 105 BC and MP used
66.6–101,325 LA NA 1.6 × 104 MP used
A (d,l)-LA NA Mn 3800 DP usedA LLA NA 1.4 × 105 DP used
ensation method; DP: direct polycondensation method; BP: bulk poly-d solution; Da: Dalton; Sn(II)Oct: stannous octanoate; SSP: solid-state
108 T. Maharana et al. / Progress in Polymer Science 34 (2009) 99–124
Table 6Industrial production of PLA.
Company name/location Trade name Production capacity(metric tons/year)
Purac plc./Netherlands Purasorb® PL NA 3.5 × 105 1.5 × 105 ROP/Solution PC
Mitsui Chemicals Co./Japanc LACEA 500 NA NA Solution PCBirmingham Polymers/AL, USAd LACTEL NA NA NA NAToyobo/Japan Vyloecol NA 43 × 103 NA NAHisun Biomaterials Co. Ltd./Chinae REVODE 5,000 NA 70 × 103 NA
NA: not available; Mw: weight average molecular weight; Mn: number average molecular weight.a http://www.natureworksllc.com/corporate/nw pack home.asp.b http://www.lakeshorebiomaterials.com/about.html.c http://www.mitsui-chem.co.jp/e/.d http://www.birminghampolymers.com/.e www.plaweb.com.
properties of PLLA become almost constant when its molec-ular weight is above a threshold value of 70,000 Da [119]. Ingeneral, for a particular use, the mechanical, physical andbiodegradability properties of PLLA must be considered.
Table 8Crystallinity benefits [113].
Products Benefits
Fibers/non-wovens Improved heat set; chemical resistance; higherstrength
A number of industries are involved in the synthesisof PLA by various methods. Before discussing the newenhanced methods, employed for the production of PLA,it will be interesting to analyze the commercial prod-ucts, their methods of synthesis and product specifications.The properties of the PLA, produced by these industries,have been collected and analyzed, and are reported inTables 6 and 7.
There are also some other producers of PLA, such asKanebo Gohsen Limited/Australia (Lactron), Kaneka Cor-poration/Japan/USA (Kanepearl PLA foam), Toyota MotorCorporation/USA (Toyota Ecoplastics U’z series), etc., otherthan those listed, which produce PLA in small amounts.
6. Properties of PLA
The thermal, mechanical and biodegradation propertiesof PLA are largely dependent on the ratio and distribution of
the two stereoisomers of LA within the polymer chains [18].Polymers with high l-isomer produce crystalline productswhereas, the higher d-levels (>15%) result in an amorphousproduct. Thus, commercial PLLA products are semicrys-talline polymers with a high melting point ca. 180 ◦C and
3250 1.8 2.0n/a n/a n/a1950 4.0 6.02350 3.5 5.0
a glass transition temperature in the range 55–60 ◦C. It isdesirable that PLA should have some crystalline content asit benefits the finished product, as shown in Table 8.
The degree of crystallinity depends on many factors,such as molecular weight, thermal and processing history,and the temperature and time of annealing treatments. Themeso- and d,l-lactide form atactic PDLLA which is amor-phous. The mechanical properties and degradation kineticsof the semicrystalline PLLA are quite different from thoseof completely amorphous PDLA. Mechanical and thermal
he properties of PLA and its copolymers, synthesizedy the polycondensation, are not different from those ofolymers obtained by the conventional lactide process [4].
.1. Physical properties
Physical properties of polymeric materials depend onheir molecular arrangement as well as ordered struc-ures such as crystalline thickness, crystallinity, spheruliteize, morphology and degree of chain orientation. Forany products, crystallinity is a desirable property. For
SP of PLLA or PDLA single polymer, molecular weightncreases rapidly in comparison with block copolymer ofLLA and PDLA. Physical properties are very important ashey reflect the highly ordered structure of the polymer andnfluence mechanical properties and their change duringydrolysis. Some significant properties of PLA are given inables 9 and 10 [122].
The concept of solubility parameters was developed bycatchard and Hildebrand [123] on the basis of the theory ofegular solutions. The Hildebrand solubility parameter ı atgiven temperature can be expressed as the square root of
he cohesive energy �Ev divided by the molecular volume, i.e. the square root of the cohesive energy density [123]:
= �Ev
V(1)
The Hildebrand concept, in its most simplified form,s used in terms of a progression, attaching one solu-ility parameter to each solvent or solute. Substancesith ı values of similar magnitude should be miscible
r soluble. Solubility, of lactic acid based polymers, isighly dependent on the molar mass, degree of crys-allinity and other comonomer units present in the polymer6]. Chlorinated or fluorinated organic solvents, dioxane,
ioxolane and furane are found to be good solvents fornantiomerically pure PLA. However, PDLLA is soluble inrganic solvents such as acetone, pyridine, ethyl lactate,etrahydrofuran, xylene, ethyl acetate, dimethylsulfoxide,,N-dimethylformamide and methylethyl ketone in addi-
able 10hysical properties of PLLA [122].
v Tma (◦C) �Hm
a (J g−1) Tgb (◦C) �Cp
b (J g−1 K−1)
5,300 156 61 55 0.6020,000 174 68 59 0.54
691,000 186 59 64 0.54
a By DSC, 10 K min−1.b By DSC, 20 K min−1.
er Science 34 (2009) 99–124 109
tion to those listed above. Non-solvents for lactic acid basedpolymers are water, and unsubstituted hydrocarbons.
6.2. Thermophysical properties
Thermophysical properties of PLA have been studied byvarious investigators using thermal analyzers such as TGA,DTA, DSC and TDMA. The thermal analyzer can be coupledwith a mass spectrometer, NMR, HPLC, etc. to study thedegradation products and kinetics of degradation.
Enantiomerically pure PLA, is a semicrystalline polymerwith Tg of about 55 ◦C and Tm of about 180 ◦C, whereas,polymers prepared from meso- or rac-lactide are in generalamorphous and do not have a sharp melting point. Poly-mers having tacticity high enough for crystallization havebeen obtained by using stereoselective catalysts [6]. Themelt enthalpy estimated for enantiopure PLA of 100% crys-tallinity is 93 J/g [124], the value most often referred to inthe literature, although higher values (up to 135 J/g) havealso been reported [125,126]. The Tm and degree of crys-tallinity are dependent on the molar mass, thermal historyand purity of the polymer [127–129]. Crystallization kinet-ics and melting behavior of PLAs of different optical purityhave been investigated in several studies [25,130–135]. Ithas been observed that an optical purity of at least 72–75%,corresponding to about 30 isotactic lactyl units, is requiredfor crystallization to take place [24,136]. However, Sarasuaet al. [137] have been able to crystallize a poly(lactide) of aslow as 43% optical purity, using salen–Al–OCH3 (a complexresulting by using a Schiff base on AlEt2Cl) as an initiator forpolymerization. This is possible because of the formationof long isotactic sequences. They reported that a sample, of47% optical purity, has Tm of 99 ◦C (�Hm = 18 J/g). Monodis-perse oligomers of 22 isotactic lactyl units prepared byfractionation have also been reported to crystallize, show-ing a melting temperature of 59 ◦C (�Hm = 39 J/g) [138].Enantiomeric oligomers of a few lactyl units show a molarmass dependent glass transition temperature. Values ofTm, Tg and �H are dependent mainly on the structureof PLA and molecular weight [139,140]. Thermal proper-ties of PLA can be changed by copolymerization of PLAwith monomers such as glycolide, some lactone deriva-tives, trimethylene carbonate, etc.—and also by addition ofcross-linkers [141–143] and plasticizers.
6.3. Electrical properties
In order to apply PLA as an insulating material, forexample, in electric wires and cables, it is necessary tostudy the basic electrical insulation properties: volumeresistivity, dielectric constant, dielectric loss tangent,impulse breakdown strength and storage of space charge,at room temperature. The basic electrical insulation prop-erties of biodegradable PLA were measured by Nakagawaet al. [21]. They measured the volume resistivity, dielectricconstant, and dielectric loss tangent at room temperature
as given in Table 11. These were found to be comparable tothose of crosslinked polyethylene (XLPE), currently used asinsulating material for cables and electric wires [21]. Thedielectric constant of PLLA is higher than that of XLPE. Onereason for this might be the existence of a carbonyl group
c-PLLA: degrees of crystallinity higher than 50%.a-PLLA: degrees of crystallinity lower than 10%.
in the polymer chain of PLLA. The dielectric constant ofPVC, currently used for insulating electric wires, is 3.4, andthe dielectric constant of oil-immersed insulating paperused for insulating high-electrical-field cables, is about3.5. The dielectric constant of PLLA is smaller at about 3.0.
The current waveform of PLLA was recorded with anoscilloscope by Nakagawa et al. [21]. They observed thatsamples were destroyed when current was passed throughthem. Further, they observed that the applied impulsevoltage increased when current did not flow through thesample. They found the mean impulse breakdown strengthof PLA to be about 1.3 times higher than that of XLPE. Theymeasured the storage of space charge in XLPE and PLLAby the pulsed electroacoustic method, with a dc electricfield applied to the sample and found that homochargemainly accumulates in the neighborhood of the anode afterelectric charging although the amount of accumulation isless than in XLPE [21]. The electric charge induced in theanode of PLLA is compared with that of XLPE, with theelectrode grounded, after the applied field is switched off.In this situation, the electric charge of PLLA is about 60%that of XLPE. Although, the electrical properties of PLA havebeen characterized, it requires thorough characterizationsfor day-to-day use. PLA composites have been developedrecently for use in electronic products.
6.4. Mechanical properties
The mechanical properties of polymers of similar molarmasses, but prepared by different polymerization pro-cesses, do not much differ. This has been noticed forPLAs, prepared by both polycondensation and ROP [6].The mechanical properties of PLA can be varied to a largeextent, ranging from soft and elastic plastics to stiff andhigh-strength materials. Semicrystalline PLA is preferredto amorphous polymer when better mechanical propertiesare desired. The molar mass of the polymer [144–147] aswell as the degree of crystallinity [148,149] has a significantinfluence on the mechanical properties. SemicrystallinePLA of 30–38% crystallinity has a tensile modulus of approx-imately 9.2 × 103 MPa and tensile strength of 870 MPa[114,150–153]. It has been shown that tensile strength andmodulus of PLLA increase by a factor of 2 when the weight-average molar mass is raised from 50 × 103 to 105 Da [145].A further increase in molar mass to 3 × 105 Da does notinfluence the properties of the polymers in any signifi-cant way. The biomaterials tested, however, have different
degrees of crystallinity, which affects their properties [145].Grijpma and Pennings [148] varied the crystallinity of PLAby preparing stereo-copolymers with small amounts of d-lactide and found maximum impact strength of 37 kJ/m2
er Science 34 (2009) 99–124
when the copolymer had a �Hm of 60 J/g, correspondingto a crystallinity of 65%. In addition, a low degree of chainentanglements occurred in highly crystalline material[154]. When the crystallinity of the PLA was reduced, thedensity of chain entanglements increased, even though thecrystallinity was high enough to give physical cross-linkingand thus the material was not as brittle as amorphousor low-crystalline material. Similar crystallinity dependentimpact strength results were also obtained when PLLA wascross-linked with 2,4,7,9-tetraoxaspiro[5.5]undecane-3,8-dione or 5,5′-bis(oxepane-2-one) [86,94,101,102]. Superiormechanical properties have been achieved by stereocom-plexation of enantiomeric PLAs, which was ascribed toformation of stereocomplex crystallites giving intermolec-ular cross-links [144].
The viscometric molecular weight of PLA decreaseswhen fibers are drawn from it by melt-spinning. Weightloss of 90% occurred during extrusion and 10% during hot-drawing [151]. PLLA of high molar mass has sufficientstrength for use as load bearing material in medical applica-tions, but it degrades slowly because of the reinforcing crys-talline domains [154,155]. The crystallinity can be reducedby copolymerizing withd-lactide, leading to an amorphousdl-PLA with a faster degradation profile [156]. Copolymer-ization, however, will also reduce the toughness and impactstrength of PLA by a factor of 3 [157]. In order to enhance thetoughness of fast degrading amorphous PLA, modificationby blending and by block copolymerization has been sug-gested [143,149,157–161]. A number of scientists have stud-ied blends of PLLA with P(LLA-co-CL) rubber [161], P(TMC-co-CL) or P(CL-co-VL) [157]. Blending improved impactstrength, an effect that can be attributed to phase sepa-ration of the block segments in the copolymer [158,159].Cross-linking also enhances the mechanical properties,especially the impact strength. The influence of differentplasticizers (Table 3), e.g. lactide, on the mechanical proper-ties of PLLA has been reported [114,150,162]. Jacobsen et al.[162] found a linear dependence between residual lactidein the PLLA and tensile strength at monomer conversionsexceeding 90%. When the polymer contained 10% residuallactide the tensile strength of the PLLA was found to be 15%lower than that for a PLLA with 97% conversion [162]. Guptaet al. [163] presented a more detailed critical review of PLAfiber. They discussed the structure-property relationshipof PLA fiber. Degradation of PLA can be studied either byhydrolysis or by thermal methods. The various methods,discussed here, can be classified as in Fig. 7. All of thesemethods follow hydrolytic degradation.
6.5. Degradation
6.5.1. Thermal degradationThe degradation of polymers has been defined as the
number of chain scissions produced during a known periodand can be expressed by Eq. (2) [164]:
1 1
DP DP 0D
where DP0 and DP are, respectively, the initial and finalvalues of the average degree of polymerization, kD is thedegradation rate constant and t is time. This equation
T. Maharana et al. / Progress in Polymer Science 34 (2009) 99–124 111
egradati
ibifmmt
wmmfs
ecmicerAeTongds
ard[
TI
P
PPPPPP
Fig. 7. Different biod
s valid for condensation polymers when the amount ofroken bonds is low, i.e., kDt � 1. The degree of depolymer-
zation can be monitored by the average molar mass as aunction of the degradation time. As the viscosity of a poly-
er solution or a melt can be related to the average molarass, the degree of polymerization can be correlated with
he viscosity changes as given by
1(�0,t)
˛ = 1(�0)˛ + kDt (3)
here the exponential factor ˛ depends on molar mass andelt viscosity and is a constant equal to 0.294 for molarasses above the critical molar mass [165]. Eq. (3) is valid
or polymer melts for degradation by random main-chaincission [166].
For PLA, most of the degradation reactions were consid-red to involve highly concentrated ester bonds on the mainhain. These reactions include thermohydrolysis, depoly-erization, cyclic oligomerization and intermolecular and
ntramolecular transesterification. Low molecular weightompounds associated with the polymer and the hydroxylnd groups of the main chain seem to play an importantole in lowering the molecular weight at high temperatures.n increased amount of polymerization catalyst in thend product also catalyzes the degradation reactions [127].he degradation compounds include water, monomers,ligomers, and polymerization catalysts. Removal of theon-polymeric contents and blocking the hydroxyl end-roups enhanced the thermal stability of PLA. The thermalegradation was found to proceed by random main-chaincission.
Benzoyl peroxide was found to stabilize the polymergainst thermal degradation by deactivating the catalystesidues. The melting point of the stabilized polymerecreased with increasing benzoyl peroxide concentration167]. The presence of oxygen is significant for the stabiliza-
able 12nfluence of the peroxide addition and polymer purification on the degradation ra
olymer Tin content (mg/g of PLLA)
LLA base polymer 0.69LLA + 0.05 mol% tBuO2Bz 0.69a
tion of PLLA with peroxides. However, the degradation rateconstant for the PLLA modified with di-tert-butyl perox-ide was less than that for the unmodified PLLA determinedunder similar conditions. It seems that access to oxygenwas limited but not excluded, even if nitrogen was flushedinto the kneader [166].
The purity of the polymer affects the melt degradationof PLLA significantly [127]. Melt degradation is retardedwhen dissolved PLLA in chloroform was precipitated innonsolvents such as methanol and n-hexane. This proce-dure removes the non-bonded tin(catalyst) and low-molarmass impurities, as shown in Table 12. Acid extraction ofthe dissolved PLLA was also found to remove a part of thebonded tin, which resulted in further reduction of the meltdegradation [166].
Addition of tert-butyl peroxybenzoate to the PLLA dras-tically retards the melt viscosity. The melt viscosity is onlyslightly retarded with addition of the same amount of per-oxides to the purified PLLA—which confirms that the maineffect of peroxides in the PLLA melt is the deactivation of atin compound [166].
6.5.2. Degradation by radiationThe molecular weight decreases rapidly with increas-
ing radiation dose but the molecular weigh distributionof the irradiated copolymer does not change significantlyfor doses up to 250 kGy, perhaps because of the random-ized distribution of the monomer units in the copolymer[71–73,168]. A drastic decrease in the tensile strength andsubstantial embrittlement occur at higher dose levels [73].Radiation-induced reactions take place mainly in the amor-
phous phase of the polymer; and the degree of crystallinityof the polymer, which in fact, decides the extent of amor-phous phase in PLLA, is, therefore, an important parameter.At lower radiation doses, chain-scission mainly occurs inthe amorphous regions and random chain scission pro-
te constants during 10 min of melt mixing at about 190 ◦C [166].
a MW: molecular weight; GPC: gel permeation chromatography.
112 T. Maharana et al. / Progress
ceeds further into crystalline regions at higher doses. ThePLLA samples irradiated at 30–100 kGy showed a markeddepression in mechanical properties due to the oxidativechain-scissions in amorphous regions. Gupta et al. [72]studied the effect of �-irradiation on PLA, synthesized bythe solution polymerization of lactic acid under air and N2.The presence of air causes a decrease in both chain scissionand cross-linking. The melting temperature decreases with�-irradiation dose.
6.5.3. BiodegradationBiodegradation is influenced by solid-state morphol-
ogy, degree of crystallinity, primary chemical structure,such as the presence of functional groups and thehydrophilicity–hydrophobicity balance of PLA [169]. Thedegree of crystallinity is the major rate-determining fac-tor for biodegradation of solid polymers [13]. Consequently,while attempting to investigate the effects of chemicalstructure on biodegradation, it is an important require-ment that the degree of crystallinity of the various samplesshould be almost equal, otherwise, incorrect conclusionsmight be drawn as to the identity of the dominant factorthat affects the rate of biodegradation of solid polymers[13]. In general, chain scission of the PLA main chain takesplace where ester bonds are located, leading to formation ofoligomers. Thus, the number of oligomers after chain scis-sion will depend upon the number of ester bonds presentin the PLA main chain.
Normally, biodegradation occurs in three steps. In thefirst step, depolymerization occurs; and then the depoly-merized PLA produces lactic acid in the second step. Finally,in the third step, the lactic acid is consumed in the cit-ric acid cycle wherein it breaks down to CO2 and H2Oin the presence of an enzyme produced by microorgan-isms. Biodegradation of PLA occurs in two ways: namely,enzymatic degradation, and non-enzymatic degradation,which include chemical methods such as pH degrada-tion. Research on the structure and function of degradingenzymes is advancing rapidly.
The principal mode of degradation for lactic acid-basedhomopolymer and copolymer is hydrolysis [121,167] which
takes place in three important steps:
1. Degradation proceeding by diffusion of water into thematerial (initially into the more amorphous zones) fol-lowed by random hydrolysis.
Table 14Thermal properties of poly(l-lactide) films aged in 0.01 N NaOH (by DSC) [14].
Degradation time (days) A1
Tg (◦C) �Hr (J g−1) Tc (◦C) Tm (◦C)
0 55 – 98.5 179.64 63 4.68 99.1 178.6
42 69 4.89 99.3 178.360 70 2.16 – 177.7
140 72 2.10 – 177.8175 72 0.94 – 178.0
A1 = 300,000 Da; A4 = ∼3,000,000 Da.Tg, Tc and Tm: glass transition, crystallization and melting temperatures, respectivXc(%) = 100 × (�Hm − �Hc)/�H100%.�Hr: excess enthalpy relaxation.
b Tm: melting temperature at the second heating; DSC: differential scan-ning calorimetry.
c Xc (%) = 100 × (�Hm − �Hc)/�H100%.
2. Fragmentation of the material to OLLA.3. Finally, through a more extensive hydrolysis accompa-
nied by phagocytosis, diffusion and metabolism.
The extent of hydrolysis depends on the size,hydrophilicity of the given polymer implant, crystallinityof the polymer and environmental factors such as pHand temperature [170]. Usually, the degradation timeis shorter for low-molecular-weight-, more-hydrophilicand more-amorphous polymers. Polymers can often betailored to meet specific degradation time through copoly-merization, molecular weight and end-group selection.Once the polymer is hydrolyzed, the products of hydrolysisare either metabolized or excreted.
Degradation is a function of (polymer structure, solu-tion pH and temperature. Investigations have been carriedout on film samples of PLA having characteristics given inTable 13 [14]. DSC analysis revealed that the crystallinityof PLLA increased with degradation time, as shown inTable 14. PLLA undergoes enzymatic and non-enzymatichydrolysis when exposed to aqueous environments, result-ing in mass loss through heterogeneous bulk degradationwithout generation of harmful byproducts. After hydrolyticdegradation, molecular weight of the resulting particlesdecreases and crystallinity increases.
Enzymes such as lipases and PHA depolymerases cleavethe ester bond of aliphatic polyesters including PLA. Thehydrophobic domains of enzymes adhere to solid sub-strates by hydrophobic interactions before hydrolysis by
catalytic domains. The activity of enzymes like lipases isstrongly dependent on the microorganism source, probablyfor the reason that the sequence structure of a bind-ing domain is different, while enzymes have a common
mino acid sequence around the active center regardlessf microorganism species [13].
The rate of enzymatic degradation decreases withncrease in crystallinity. In larger size devices of PLA, theate of hydrolytic degradation is higher inside than at theurface of the material because of the autocatalytic effectsf the carboxylic acid groups trapped inside the device5,156,171,172]. The rate of this hydrolytic degradation isrimarily dependent on temperature and humidity. The
ntrinsic viscosities of all polymers decrease continuouslyhen exposed to phosphate buffered saline and a very
harp mass loss occurs [173]. Pranamuda et al. [10] showedhat out of 25 strains of the genus Amycolatopsis, 15 formedlear zones on agar plates emulsified with PLA, suggest-ng a large distribution of PLA degraders within this genus.he clear zones were also observed with other polyestersnd silk fibroin plates. In liquid cultures of PLA degraders,here were strains with and without the ability to assimilateegradation products like LLA.
There are two approaches for gaining further insightsnto environmental microorganisms which could degradeplastic polymer. The first is to screen and isolate microor-anisms degrading the polymer, followed by identificationf their phylogenetic affiliation. The second is to exam-ne the ability of a given type of strain to degrade theolymer. A few reports have been published on microbialegradation of PLA. Torres et al. [16] reported that fun-al strains can utilize lactic acid and oligomers generatedrom degradation of PLA through abiotic degradation butere unable to degrade PLA. Ikura et al. [174] reported
solation of a PLA degrading actinomycete which was tax-nomically similar to the Amycolatopsis strain. An isolatedtrain of Amycolatopsis used in degradation was confirmedo form a clear zone on a PLA plate [175]. The main degra-ation mechanism of PLA is hydrolysis of the hydrolysable
Fig. 8. Hydrolysis o
er Science 34 (2009) 99–124 113
ester linkages under different conditions of pH, enzyme andtemperature.
6.5.3.1. Biodegradation mechanism. During the first stageof degradation, the molecular weight decreases rapidlywith little weight loss. By contrast, in the second stage, thedecrease in molecular weight slows and severe weight lossas well as initiation of monomer formation is observed.During the third and final stage, when total weight lossis experienced, about 50% of the polymer is convertedto monomer. The hydrolysis of the soluble oligomerscontinues further, until all are converted to lactic acid[176]. Recently, a number of published reports haveprovided strong evidence that the degradation kineticscannot only be described by random chain scissions, butthe role of the end groups as an important contributorcannot be ignored [177]. For example, the phenomenonof auto-catalysis by the carboxylic chain ends that areformed during degradation of PLA is well documented[156]. Shih [178,179] suggested that chain-end scission isfaster than random chain scission in acidic conditions. Anunambiguous mechanistic explanation, however, cannotbe given for this phenomenon, since the rate constants areinfluenced by many factors, including water accessibility,molecular mobility, local dielectric constant, local pH, etc.It can be anticipated that the local environment of the chainends is much different from that of the bulk chains in thesolid state [180]. Obviously, the contribution of chain-endscission increases with decreasing molecular weight of thepolymer, since the fraction of chain ends increases as a con-
sequence of the degradation process. In recent applications,low molecular weight PLA is used instead of relatively highmolecular weight polymer. For example, these are used asdegradable crosslinks in hydrogels, as tablet coating and asdrug release agents [181–187]. Moreover, oligomers of lac-
f PLA [113].
in Polym
114 T. Maharana et al. / Progress
tic acid are water-soluble to some extent, which is expectedto have a dramatic influence on the degradation kinetics,as compared to solid polymers. The degradation rate, ofwater-soluble OLLAs (degree of polymerization <10), hasbeen studied by capillary electrophoresis [188], whichindicated the preferential formation of lactoyllactate,subsequently degrading slowly to lactic acid. However, adetailed kinetic and mechanistic analysis of the hydrolysisprocess of lactic acid oligomers has not been reported so far.
As illustrated in Fig. 8, cleavage of the ester linkagesby absorption of water produces a successive reduction inmolecular weight. PLA undergoes fragmentation and thenbiodegradation when hydrolyzed, and finally produces H2Oand CO2 as end products. Thus by measuring the amount ofCO2 evolved one can calculate the percentage of biodegra-dation. In the initial stage, no microorganism is involvedin the degradation but as PLA is fragmented to about10,000 Da, soil microorganisms can digest the fragments,producing CO2 and H2O as shown in Eq. (4)
PLA of high MWH2O−→PLA of low MW of about 10, 000
H2O−→Lactic acid → CO2 + H2O (4)
A new PLA-degrading actinomycete, Kibdelosporangiumaridum, degraded more than 97 mg out of 100 mg highmolecular weight (Mn = 3.4 × 105) PLA film within 14 daysin liquid culture. l-Lactic acid, the monomeric degradationproduct of PLA, was totally assimilated by the actinomycete[189].
6.5.3.2. Enzymatic degradation. Enzymatic degradationproceeds only on the surface of a solid substrate both bysurface erosion and weight loss, because enzymes cannotpenetrate solid polymer substrate. The enzymes degradeselectively amorphous or less ordered regions which allowenzymes to diffuse into the substrate and subsequentlythe crystalline regions are also eventually degraded. Inthis process, the molecular weight and molecular weightdistribution of undegraded solid substrate do not changeduring the enzymatic degradation because only the poly-mer on the surface of substrate is degraded and the lowmolecular weight degradation products are removed fromthe substrate by solubilization in the surrounding aqueousmedium. There are two types of degradation processbased on the point of cleavage. Cleavage can occur eitherat random points along the polymer chain (endo-typedegradation) or at the ends of the polymer chain (exo-typedegradation). The degradation processes of lipases orPHA depolymerases are primarily based on the endo-typescissions, and thus are not dependent on the molecularweight and molecular weight distribution [57,180].
6.5.3.3. Non-enzymatic degradation. Degradation due to achange of pH of the medium [180] is one type of non-enzymatic degradation. Small amounts of non-enzymatic
catalysts and reagents can diffuse into polymer systems andcause in-depth degradation. Crystallinity, crosslinking andother morphological properties of polymers affect the dif-fusion of catalysts into the system. The shape of the massloss vs. time curve supports the occurrence of a hetero-
er Science 34 (2009) 99–124
geneous bulk degradation process [15]. After hydrolyticdegradation the residual particles showed a molecularweight decrease and a crystallinity increase. It is inter-esting to note that non-enzymatic degradation proceedsby an apparently inert first stage of degradation withoutweight loss but it results in random cleavage of polymerchain backbone (endo-type degradation) with a concomi-tant substantial decrease in molecular weight, leading to adecrease in mechanical properties such as tensile strength,ultimate elongation and impact strength [15].
Depending on the above discussions and cited literature,Table 15 shows effects of various parameters on propertiesof PLA. 13C NMR spectra show that PLA is composed of land d lactic acid units. The amount of d-lactic acid in PLLAis 13–16%. The mechanical and thermal properties of PLLAbecome constant when its MW is above a threshold MW of70,000 Da [119].
7. Melt–solid polycondensation
Synthesis, properties and degradation behavior of PLAhave been discussed above. Criteria leading to synthesis,selection of catalyst, and effect of operating parameters onMW of the product are discussed in succeeding sections.The kinetics and mechanism involved in the different stepsleading to the synthesis of the polymer are also discussed.The mathematical relationships between parameters ofsynthesis and MW are presented.
Melt–solid polycondensation is an approach to increasethe molecular weight of PLLA by increasing the degree ofpolymerization. A few investigators [2,3,5,7,9] have workedon sequential melt–solid polycondensation of LA. Xu etal. [7] studied the effect of crystallization time of PLLApre-polymer on the molecular weight of PLLA. They firstprepared PLLA pre-polymer with a molecular weight of18,000 by the ordinary melt polycondensation process.The prepolymer was then crystallized at 105 ◦C for differ-ent time periods, and then heated at 135 ◦C for 15–50 hfor further solid-state polycondensation (SSP). Differentialscanning calorimetry (DSC) and viscosity measurementswere used to characterize the crystalline properties andmolecular weight of the resulting PLLA polymers. Theresults showed that the molecular weight reached a maxi-mum value for a crystallization period of 30 min and SSP of35 h.
7.1. Key steps leading to PLA synthesis
The polycondensation of LA takes place in several stepsas discussed below. The associated mechanism for eachstep is also discussed.
7.1.1. DehydrationCommercial grade LA, used for PLA synthesis by poly-
condensation reaction, contains about 80–95 wt% LLAalong with 10–15% water, DLA and other impurities. Dur-
ing PC water is also produced and thus, for the forwardreaction to proceed, the water molecules must be removedfrom the reaction product mixture as quickly as possible.This necessitates the removal of water from the raw mate-rial, LA, before the commencement of the reaction and is
T. Maharana et al. / Progress in Polym
Tab
le15
Effe
ctof
vari
ous
par
amet
ers
onth
ep
rop
erti
esof
PLA
.
Ther
mal
pro
per
ties
Phys
ical
pro
per
ties
Mec
han
ical
pro
per
ties
Elec
tric
alp
rop
erti
esD
egra
dat
ion
pro
per
ties
T mT g
�H
m�
C pT c
Vis
co-
sity
Solu
bi-
lity
Den
sity
%C
ryst
alli
-n
ity
Ten
sile
stre
ngt
hTe
nsi
lem
odu
lus
Imp
act
stre
ngt
hFl
exu
ral
mod
ulu
sYo
un
g’s
mod
ulu
sEl
onga
-ti
onM
elt
stre
ngt
hD
iele
ctri
cco
nst
ant
Die
lect
ric
loss
Vol
um
ere
sist
ivit
yH
ydro
-ly
tic
Ther
mal
Rad
ia-
tion
Stru
ctu
ralp
rop
erty
Mol
ecu
lar
wei
ght
↑↓↑↓
↑↓↓
↑↑
↓↑↓
↑↓↑
↑↑↓
↑↓↑
↑↓↓
↓
%C
ryst
alli
nit
y↓
↓↑↓
↑↓
↑↑↓
Rat
ioof
ster
eois
omer
s↓
↓↑
Am
oun
tof
d-L
A↓
↓↑↓
↑↓↑↓
↑↓↑↓
↑↓↑↓
↑↓↑↓
↓↑↓
↑↑
↑
Pres
ence
ofco
mon
omer
↑↓↑↓
↑↓↑↓
↑↓↑↓
↓↓
↓↑↓
↓↑
↑↑
Mel
tch
ain
dim
ensi
onA
dd
itiv
esPl
asti
cize
r↓
↓↑↓
↑↓↑
↓↓
↑↑
Stab
iliz
er↑
Cro
ss-l
inke
rs↑↓
↑↓↑
↑↑
↑↑
Pero
xid
es↓
↓↑
↓↑
↓↓
↑↑
↑↑
↑R
adia
tion
↓↓
↓↑↓
↓↓
↓↑↓
↑↑
↑↓:
dec
reas
ing
tren
d;↑:
incr
easi
ng
tren
d;↑↓
:B
oth
incr
easi
ng
and
dec
reas
ing
tren
ds;
0:U
nch
ange
d.↑
↓ :B
oth
incr
ease
and
dec
reas
eca
noc
cur
dep
end
ing
onth
en
atu
reof
the
fact
oraf
fect
ing
the
said
pro
per
ty.
er Science 34 (2009) 99–124 115
carried out during the dehydration step. This includes heat-ing LA under nitrogen at a temperature from 100 to 150 ◦Cunder a pressure of 1000–2000 Pa to reduce the residualwater content to 1–2% [44,82]. If the dehydration step is car-ried out under continuous flow of N2 gas, removal of watervapor will be comparatively more rapid by helping to driveout water molecules. The complete dehydration process iscarried out in a series of steps involving different tempera-tures, pressures and dehydration time periods [1–3,44,50].For example, LLA is first heated at constant temperature ofca. 150 ◦C at atmospheric pressure for 2 h; then the pressureis reduced to 100 mm Hg(abs) and heating is continued foranother 2 h at the same temperature; and finally the pres-sure is reduced to 30 mm Hg(abs) and the sample is heatedagain for another 4 h without changing the temperature[1–3,50]. Dehydration is done in stepwise manner to expelwater in a controlled way. However, single-step dehydra-tion is not uncommon. Chen et al. [44] carried out one-stepdehydration at 100 ◦C for 1 h at 760 mm Hg(abs). Duringdehydration, oligo(l-lactic acid) (OLLA) is obtained with adegree of polymerization (DP) varying from 8 to severalhundred.
7.1.2. EsterificationIn this step, LA is converted into PLLA along with the for-
mation of water molecules as a byproduct. As the reactionproduct contains water it is imperative that the cata-lyst should be water–tolerant to get a better yield andhigh molecular weight. Some investigators [5,44,190] haveemployed water tolerant catalysts so that the degree ofpolymerization is not affected. Appropriately substituteddistannoxane catalysts are found to be hydrophobic due tothe presence of bulky alkyl groups around the tin atomsand, therefore, can act as water-tolerant catalysts [44]. Theesterification reaction is generally carried out at 180 ◦Cunder a pressure of 30 mm Hg(abs). Higher temperatureincreases the vapor pressure of water and helps watermolecules to escape from the polymer melt and therebyenhances the rate of the forward reaction. The reduction ofpressure also helps the water removal process.
7.1.3. DecompressionThis step appears to be trivial but experimentation
shows that it helps in achieving high molecular weightPLLA. Little information is available on the suitability of thisstep. The decompression time may range from 3 to 7 h inwhich pressure decreases from 30 to 1 mm Hg. It appearsthat the decompression step also removes water formedduring polycondensation and thereby enhances the rate ofreaction. In fact, decompression can increase the molecu-lar weight from 3 × 104 to 13 × 104 Da in the case of a TNBTcatalyzed PC reaction [44].
7.1.4. Melt polycondensationThe polycondensation of LA depends on two thermo-
dynamic equilibria: the dehydration/hydration equilibrium
for ester formation and ring/chain equilibrium for depoly-merization to lactide formation. Melt polycondensation iscarried out above Tm of PLLA, as at this temperature thelactide formed is evaporated to produce a high yield ofpolymer [3]. Racemization is also induced during poly-
in Polymer Science 34 (2009) 99–124
116 T. Maharana et al. / Progress
condensation [119] and is, most likely, due to the esterinterchange reaction between the polymer chains [190].There are two ways, in which, the ester linkages betweensuccessive lactic acid units can cleave and reform. One isacyl oxygen cleavage that does not involve the chiral car-bon of the lactyl unit. The other is alkyl oxygen cleavagein which the covalent bond between the oxygen and thechiral carbon breaks and subsequently reforms resulting inan inversion of configuration. The change from the l-formto d,l-form in the presence of the TNBT catalyst is due tothe racemization reaction, which has been confirmed fromanalysis of DSC and XRD results [191]. The strong protonacid co-catalyst promotes the breaking of the ester bondthrough the typical carbonyl-oxygen bond cleavage. As thereaction temperature increases, the probability of alkyl-oxygen cleavage increases and results in the formation ofan inverted configuration [14,44]. Chen et al. [44] proposedthat the polymerization temperature should be as high as180 ◦C to produce high molecular weight PLLA.
7.1.5. Heat treatmentHeat treatment is done around the crystallization tem-
perature (Tc) of PLLA. In this step the PLLA, which is inthe form of white solid polycondensate, is crushed intogranules and is put into a test tube which is then heatedin vacuum at ca. 105 ◦C for 1–2 h. Since the crystallizationexotherm is known to extend from 100 to 107 ◦C, the meltpolycondensate is heat treated at 105 ◦C to crystallize thePLLA. It has been reported that the extent of crystallinity is29 and 30% after 1 and 2 h, respectively. The product, afterheat treatment, becomes resistant to fusion, even whenheated at a higher temperature. Further, it did not reveala crystallization exotherm in the DSC curve, which showedonly a melting endotherm at 158 ◦C. In the process of crys-tallization, both monomer and catalyst are segregated inthe amorphous region of PLLA. This helps the polymeriza-tion reaction to take place, even in the solid state, to allowthe yield to reach 100%. Moon et al. [3] reported that duringheat treatment, the MW of PLLA increased from 1.3 × 104 Dato 1.5 × 104 Da.
7.1.6. Solid-state polycondensationAlthough the reaction rate is usually slow in solid-
state reactions, increase in crystallinity does not hinderthe dehydration reaction significantly. This is attributednot only to the high activity of binary catalyst, even atlow temperature, but also to the high mobility of the PLLAchains in the amorphous phase [3]. SSP is generally car-ried out above Tg to enhance the molecular translationalmobility within the amorphous regions of semicrystallinepolymers, while the crystalline regions retain the geomet-rical shape of the polymer during polymerization [192].Although the structure of the catalytic site is still unknown,the polymer terminals extended from the existing crystalsurfaces can be brought sufficiently close to other crystalend groups present in the amorphous region to assist the
process of esterification and thereby facilitate the growthof crystals. The polymer chains, thus elongated, can par-ticipate in the process of crystallization over the crystalsurface available at crystal–amorphous borders, as shownin Fig. 9. The polymer chains held inside the crystals expe-
Fig. 9. Plausible mechanism of solid-state post-polycondensation of PLAcatalyzed by binary catalyst [3].
rience difficulty in reacting with the neighboring chainsand thus remain as relatively low molecular weight poly-mer, as can be verified from the bi-modal GPC curves.The increase in crystallinity can continue until the crys-tallinity exceeds 43–45%. The crystal growth during thepost-polymerization is very large in the monomer-freeproducts, and the remaining monomer ratio decreases withincreasing crystal growth [80]. At this stage, the ester-forming rate among polymer terminals becomes too slowand the intramolecular ester exchange reaction may over-come the chain extension to form cyclics together withthe linear fragments. The above supposition can be rea-sonably supported by the observed decrease in molecularweight and crystallinity of the polycondensate obtainedafter heating for more than 30 h. The bimodal GPC curveobtained after solid-state polycondensation suggests thatthe chain elongation had proceeded in a heterogeneousmanner during the crystallization of the polymer. If ahigh vacuum environment is created for an SSP process,subsequent return to atmospheric pressure may result inoxidation and discoloration of the polymer [79]. A numberof patents [82,83] are available in which investigators haveused molecular sieves to adsorb water during solid-statepolycondensation. Molecular sieves can be represented by achemical formula [M(I),M(II)]O·Al2O3·nSiO2·mH2O, where,M(I) and M(II) are respectively monovalent (Na and K) andbivalent (Ca, Sr and Ba) and the range of values for n and mare 2–10 and 0–9, respectively.
It is reported that in moving from the melt PC step tothe heat treatment step MW increases by 1.5 times andagain increases by 44.7 times in proceeding from the heat
treatment step to the SSP step [3]. The experimental condi-tions employed by different investigators are summarizedand analyzed in Table 16. On the basis of the informationin Table 16 the following experimental conditions can beselected for melt–solid polycondensation.
T.Maharana
etal./Progress
inPolym
erScience
34(2009)
99–124117
Table 16In depth analysis of different stages of melt/solid polymerization of LA and their contribution towards molecular weight (MW).
State(S) Pressure (mm Hg)and atmosphere
Temperature(◦C)
Steps of polymerizationreaction
Time range(h)
Optimumtime (h)
Enhancementfactor for MW
Remarks
Liquid 760 N2 100 Dehydration 1 1 1. LA is dehydrated under N2 atmosphere in to Oligo(l-LA) (OLLA) whosedegree of polymerization (DP) is 8 to few hundreds2. In this step excess water in LA gets vaporized
Melt 760 N2 180 Esterification 3–7 7 4 [44] 1. In this step, water is produced and thus water tolerant catalyst is required toenhance the reaction2. This step controls the MW3. Higher time gives higher MW
1–760 N2 180 Decompression 3–7 7 4.3 [44] 1. Esterification time and decompression time are very important forenhancing MW2. High MW of PLLA can be obtained even in the absence of any additionalreaction promoters, if the decompression step and/or the esterification step iswell controlled3. Water gets evaporated in this step, thus enhancing reaction rate4. Higher time gives higher MW
1–10 N2 180 Polycondensation 1–50 20, 40 1. The reaction is done at a temperature (Tr) where Tg < Tr < Tm
2. Long time heating in melt state of PLLA induces lactide formation andrelevant polymer decomposition rather than polycondensation and hence thistime should be optimized2. Time of polycondensation reaction is important for controlling MW and yield4. With time it increases and then decreases. But it has less effect on MW thanesterification and decompression time
Solid 0.5 105 Heat treatment 1–2 2 1.15 times [3] 1. This step is done ∼Tc
2. During this preheating the product crystallizes and became resistive tofusion even when heated at a higher temperature3. MW increases with increase in heat treatment time from 1 to 2 h4. It increases yield
0.5 150 Solid statepost-polycondensation
10–30 20 2.5 times after 2 hheat treatment [3]
1. MW increases and then decreases with increase in SSP time
2. SSP is conducted at ∼Tm for different length of time (10–30 h) to obtain highpolymer. An optimum amount of crystallinity is required to preventagglomeration of particles in the reactor3. SSP reaction essentially takes place in the amorphous region of the polymer,where all the reactive end groups reside4. SSP has to be performed at a temperature > Tg (to allow mobility of the endgroups to react) and < Tm
5. Since solid state reactions actually start at much lower temperatures,compared to molten or solution state, the reaction temperature can range fromsufficiently below Tm to just 5–15 ◦C below Tm
6. SSP reduces discoloration and degradation associated with hightemperature melt polymerization in molten state7. Almost all catalysts for melt and solution phase polycondensation arecatalysts for SSP8. Plasticizers can be used with a view to increase the mobility of the endgroups in the amorphous region, consequently increasing the rate of the SSP
118 T. Maharana et al. / Progress in Polym
Table 17Relationship between Mw of PLA and catalyst used.
Catalyst Molecular weight of PLA
104 105 106
Protonic acids H3PO4 H2SO4
Nafion-H MSA p-TSA
Metal compoundsMetals Mg Ti Al Zn SnOxides SnIV TiIV ZnII SbII SnII
to the square of the amorphous ratio of PLA, defined as
MSA: methane sulfonic acid; p-TSA: p-toluene sulfonic acid; acac: acetyl-acetonato anion; Nafion-H: perfluorinated resinsulfonic acid (registeredtrademark of the Dupont Company).
7.2. Selection of catalyst
As PLLA is primarily used for biomedical and food-packaging applications, the catalysts selected for itssynthesis should be compatible with these applications.Many investigators [1–5,119] have studied the effect ofsuch catalysts for the synthesis of PLA. They have eitherused catalysts prepared from transition metal compoundsor a combination consisting of transition metal compoundalong with protonic acid in the role of a cocatalyst, as listedin Table 17. Tin compounds [1,4] are found to be effective forhigh molecular weight polymers. Inorganic tin compoundsare less toxic than organotins and tin(IV) compounds areless toxic than tin(II) compounds. Thus inorganic tin willbe a better choice as a catalyst in comparison to organ-otins. Toxicity grew with increasing tin concentrations.Tetraphenyltin [190] is a catalyst approved by FDA andtherefore, can be used safely for the synthesis of polymerstargeted for biomedical applications. Very few investiga-tors have studied the effect of binary catalysts [1,49,95].Their investigations shows that stannous octanoate is one
of the most effective catalysts for the production of highmolecular weight PLA with high yield. However, like manyother catalysts, it is difficult to remove this catalyst fromthe polymer, which can lead to cytotoxicity and thus lim-
Fig. 10. Synthesis of PLA by
er Science 34 (2009) 99–124
its its application. Although, some investigators have usedmetal–salen Schiff base complexes as initiator for ROP[192,193], no such investigation is reported so far on theuse of these catalysts for polycondensation reactions.
It can be seen from Table 17 that, tin metal, tin(II)halide, tin oxide and Ni(II) produce high molecular weightPLA. Appropriately substituted distannoxane catalysts arethemselves hydrophobic because of bulky alkyl groupsaround the tin atoms and, therefore, can act as water tol-erant catalysts [44]. Tin atoms, which work as catalyticcenters, retard hydrolysis of ester linkages to some extent[120]. From a study of variation of the substituents (R = n-Bu, X = Cl, OH, NCS) on the distannoxane ladder structure,it was concluded that the molecular weights of the synthe-sized polymers are relatively insensitive to the nature ofthe substituents [5].
7.3. Kinetics and reaction mechanism of sequentialmelt–solid polycondensation
Homopolycondensation of hydroxycarboxylic acidssuch as lactic acid is a reversible process, and in orderto prepare a high molar mass polymer the equilibriumconstant for condensation KC has to be high enough. Thepolycondensation rate depends on both chemical (chemicalreaction) and physical processes (heat treatment, crys-tallization). The possible rate-determining steps are (i)chemical reaction control (a reversible chemical reaction);(ii) interior diffusion control (diffusion of the volatile reac-tion products in the solid polymer); (iii) surface diffusioncontrol (diffusion of the volatile reaction product fromthe surface of the polymer to the surrounding inert gas).Depending on the process and operating variables, therate of SSP is controlled by one or more of these steps, asdepicted in Fig. 10.
Vouyiouka et al. [79] has also observed that there isno universal agreement on the relevant chemical kineticexpressions for SSP. The kinetic analysis of SSP shows thatthe rate of monomer consumption is inversely proportional
the reciprocal of the crystal ratio, or the fraction of thepolymer which is crystalline [80]. Shinno et al. [80], how-ever, observed that the molecular weight did not increasewith the monomer consumption as various oligomers are
melt–solid PC [3].
T. Maharana et al. / Progress in Polymer Science 34 (2009) 99–124 119
Table 18Experimental and proposed characteristic MW of poly(l-lactic acid).
Time of the esterification at760 mm Hg (h)
Duration of the decompressionfrom 760 to 1 mm Hg (h)
Time of the polymerizationat 1 mm Hg (h)
Mw (Da)
Experiment by Chen (2006) 3 7 40 1.3 × 105
3 3 40 0.3 × 105
7 3 40 1.2 × 105
3 3 50 0.3 × 105
3 3 20 0.15 × 105
P 49.99 1.35 × 105
40 2.232 × 105
50 2.286 × 105
fcToaraofe
8
saaay[tenra(i
M
TmmcThwewtmtc
ficd
Table 19Experimental conditions for melt polycondensation of LLA.
Parameter Range
LLA 20–40 gCatalyst: co-catalyst (equimolar ratio) 0.4–1.0 wt% w.r.t. OLLAES time 2–8 hDC time 7–11 hMPC temperature 180–240 ◦CMPC time 10–30 h
Table 20Fixed operating parameters for melt polycondensation.
Operating parameter Value
DH temperature 150 ◦CDH time 2, 2, 4 hDH pressure 760, 100, 30 mm Hg
The Taguchi method or central composite design methodsfor design of experiment can then be applied to study theeffects of listed parameters within the specified range ofinterest for the synthesis of PLLA. This will lead to a bettercorrelation between input and output parameters. Again
Table 21Operating parameters for heat treatment followed solid-statepolycondensation.
Parameter Range
Amount of PLLA 6–10 g
roposed by MINITAB 3 6.997 77 7
ormed in the post-polymerization stage by the ester inter-hange reaction, in contrast to most observations [3,86].he plausible mechanism of solid-state polycondensationf PLLA catalyzed by the binary catalyst can be explaineds shown in Fig. 9. The rate determining factor in SSP is theate of removal of the condensate water. Greater surfacerea within the polymerizing solid favors faster evolutionf small molecules. The condensate water can be removedrom the solid by using static or dynamic vacuum or byxposure to a stream of inert gas [194].
. Suggestions
Literature review on melt polycondensation and solid-tate polycondensation has been carried out with a view tonalyzing the published data and to configure better oper-ting conditions for the polycondensation process with theim of synthesizing high molecular weight PLA with highield. Under this backdrop, data published by Chen et al.44] was analyzed by using MINITAB software. While tryingo study the main effect plots and the interaction param-ters, it was found that the interaction parameters couldot be studied because of insufficient data points. Whenegression analysis was carried out for molecular weight asfunction of esterification time (ET), decompression time
DCT) and polymerization time (PT), it provided the follow-ng correlation:
hrough the response surface optimization, an attempt wasade to figure out whether higher molecular weight poly-ers, as compared to that obtained by Chen et al. [44],
ould be generated by altering the operating parameters.he study revealed that it was possible to synthesize aigh MW polymer having a MW of about 2.286 × 105 Daithin the range of operating parameters studied by Chen
t al. [44], as reported in Table 18. It appears that, theyould have obtained much higher MW polymer had
hey followed the sequential melt–solid polycondensationethod. Thus, from the above studies, it can be suggested,
hat melt-polycondensation followed by solid-state poly-
ondensation will provide high MW PLA.
It has also been observed from the literature thatfteen parameters: the amount of LLA, amount ofatalyst, amount of co-catalyst, dehydration time, dehy-ration temperature, dehydration pressure, esterification
ES pressure 30 mm HgES/DC temperature 180 ◦CDC pressure 30–10 mm HgMPC pressure 10 mm Hg
time, esterification temperature, esterification pressure,decompression time, decompression temperature, decom-pression pressure, melt polycondensation time, meltpolycondensation temperature and melt polycondensationpressure affect the process of melt polycondensation of PLAdirectly or indirectly. However, it is quite cumbersome toinvestigate such a large number of factors through exper-iments. Therefore, on the basis of reviewed literature forsynthesis of PLLA, six most significant parameters (such asamount of LLA, amount of catalyst, MPC temperature, ES,DC and MPC time) can be selected, keeping the remainingparameters at fixed values, as shown in Tables 19 and 20.
Particle size 150–180 �mHT temperature 100–120 ◦CHT time 1–5 hSSP time 10–30 hSSP temperature 160–200 ◦C
in Polym
120 T. Maharana et al. / Progress
for carrying out SSP, we have sorted six parameters of sig-nificance as shown in Table 21 and fixed HT/SSP pressureto 0.5 mm Hg.
9. Concluding remarks
After reviewing the various routes for the synthesis ofPLA, it can be concluded that production of high molecu-lar weight PLA with high yield (comparable to ROP), canbe achieved by adopting the route of sequential melt–solidpolycondensation catalyzed by water tolerant catalysts.Further, the production cost of PLA will be lower than thatof ROP or solution PC. The suggested route for PLA synthesisis provided, as a flowchart, in Fig. 11.
The salient conclusions of this review are as follows:
1. Ring opening polymerization, solution polyconden-sation, melt polycondensation, post-polymerizationmethods such as melt modification, radiation inducedcross-linking and solid-state polycondensation are dif-ferent processes available for the production of PLA. Allof these processes have their relative advantages anddisadvantages. From the analysis, it can be concludedthat high molecular weight PLA can be synthesized by
adopting the process of polycondensation followed bysolid-state polycondensation. In this method, the resid-ual monomer present in the polymer is substantiallyless because, during the solid-state polycondensationprocess, the monomer and catalyst are concentrated in
Fig. 11. Schematic representation of PLA synthesis.
er Science 34 (2009) 99–124
the amorphous regions of the polymer; and as a resultalmost 100% of the monomer is converted to polymer.During SSP, crystallization takes place in the amorphousregion of the polymer
2. SSP is a slow process. Its deficiencies can be overcome bythe use of specially designed water adsorbent molecularsieves, so that rate of SSP is enhanced.
3. Properties of PLA do not differ much when prepared bydifferent routes. Study of various properties of PLA showsthat it can be an effective polymer for application in var-ious fields: biomedical, packaging, electrical, etc. Amongall these applications, biomedical applications have beenstudied to a great extent while study of other applica-tions is still in the infant stage. Being a biodegradable andbiocompatible polymer it can protect our environmentwithout causing pollution.
Acknowledgements
One of the authors Ms. T. Maharana is thankful to AICTE,India for providing a National Doctoral Fellowship. Theauthors are also grateful to Prof. I.K. Varma for valuablesuggestions during the preparation of this review article.
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