开环聚合-固相缩聚法制备立体嵌段聚乳酸

首先,采用乳酸为引发剂,辛酸亚锡为催化剂,引发丙交酯开环聚合制得具有缩聚活性的L-聚乳酸和D-聚乳酸;然后,将两者熔融共混后进行固相缩聚,合成了一系列立体嵌段聚乳酸。采用核磁共振(NMR)、凝胶渗透色谱(GPC)及差示扫描量热仪(DSC)分析了产物的链结构、重均分子量、热性能,并探讨了均相晶体和立体复合晶体共存情况下的固相缩聚机理。结果表明,固相缩聚产物分子量增长的适宜反应条件为:反应时间30h,较低的催化剂含量,L-聚乳酸质量分数为80%。L-聚乳酸和D-聚乳酸共混物较低的初始立体复合晶体结晶度有利于后续固相缩聚过程中产物分子量的增长;固相缩聚不仅发生在异链之间,而且也发生在同链之间。     

Linear and four‐armed poly(l‐lactide)‐block‐poly(d‐lactide) copolymers and their stereocomplexation with poly(lactide)s

Linear and four‐armed poly(l ‐lactide)‐block‐poly(d ‐lactide) (PLLA‐b‐PDLA) block copolymers are synthesized by ring‐opening polymerization of d ‐lactide on the end hydroxyl of linear and four‐armed PLLA prepolymers. DSC results indicate that the melting temperature and melting enthalpies of poly (lactide) stereocomplex in the copolymers are obviously lower than corresponding linear and four‐armed PLLA/PDLA blends. Compared with the four‐armed PLLA‐b‐PDLA copolymer, the similar linear PLLA‐b‐PDLA shows higher melting temperature (212.3 °C) and larger melting enthalpy (70.6 J g^?1). After these copolymers blend with additional neat PLAs, DSC, and WAXD results show that the stereocomplex formation between free PLA molecular chain and enantiomeric PLA block is the major stereocomplex formation. In the linear copolymer/linear PLA blends, the stereocomplex crystallites (sc) as well as homochiral crystallites (hc) form in the copolymer/PLA cast films. However, in the four‐armed copolymer/linear PLA blends, both sc and hc develop in the four‐armed PLLA‐b‐PDLA/PDLA specimen, which means that the stereocomplexation mainly forms between free PDLA molecule and the inside PLLA block, and the outside PDLA block could form some microcrystallites. Although the melting enthalpies of stereocomplexes in the blends are smaller than that of neat copolymers, only two‐thirds of the molecular chains participate in the stereocomplex formation, and the crystallization efficiency strengthens. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1560–1567     

Synthesis and properties of high‐molecular‐weight stereo di‐block polylactides with nonequivalent D/L ratios

Di‐stereoblock polylactides (di‐sb‐PLA: PLLA‐b‐PDLA) having high molecular weight (M_n > 100 kDa) were successfully synthesized by two‐step ring‐opening polymerization (ROP) of L ‐ and D ‐lactides using tin(2‐ethylhexanoate) as a catalyst. By optimizing the polymerization conditions, the block sequences were well regulated at non‐equivalent feed ratios of PLLA and PDLA. This synthetic method consisted of three stages: (1) polymerization of either L ‐ or D ‐lactide to obtain a PLLA or PDLA prepolymer with a molecular weight less than 50 kDa, (2) purification of the obtained prepolymer to remove residual lactide, and (3) polymerization of the enantiomeric lactide in the presence of the purified prepolymer. Their ¹³C and ³¹P NMR spectra of the resultant di‐sb‐PLAs strongly supported their di‐stereo block structure. These di‐sb‐PLAs, having weight‐average molecular weights higher than 150 kDa, were fabricated into polymer films by solution casting and showed exclusive stereocomplexation. The thermomechanical analysis of the films revealed that their heat deformation temperature was limited probably because of their low crystallinity owing to the non‐equivalent PLLA/PDLA ratio. The blend systems of the di‐sb‐PLAs having complementary stereo‐sequences (the one with a long PLLA block and the other with long PDLA block) were also prepared and characterized to enhance the sc crystallinity. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 794–801, 2010     

Immiscibility–miscibility phase transformation in blends of poly(ethylene succinate) with poly(L‐lactic acid)s of different molecular weights

Using differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and Fourier transformed infrared spectroscopy (FTIR), upper critical solution temperature (UCST) phase behavior with immiscibility–miscibility transformation in blends of poly(ethylene succinate) (PESu) with poly(lactic acid)s (PLAs), such as poly(D ,L ‐lactic acid) (PDLLA), poly(L ‐lactic acid) (PLLA), poly(D ‐lactic acid) (PDLA), differing in D/L configurations and molecular weights were investigated. All three binary blends of PDLLA/PESu, PLLA/PESu, and PESu/PDLA exhibit UCST behavior, which means they are immiscible at ambient temperature but can become miscible upon heating to higher temperatures at 240–268 °C depending on molecular weights. The PLLAs/PESu blends at UCST could be reverted back to the original phase‐separated morphology, as proven by solvent redissolution. The blends upon quenching from above UCST could be frozen into a quasi‐miscible state, where the Flory‐Huggins interaction parameter (χ₁₂) was determined to be a negative value (by melting point depression technique). The interaction between PDLLA and PESu in blend resulted in significant reduction in spherulite growth rate of PESu. Furthermore, blends of PESu with lower molecular weight PLLA or PDLA (M_w of PLLA and PDLA are 152,000 and 124,000 g/mol, respectively), instead of the higher M_w of PDLLA (M_w of PDLLA = 157,000 g/mol), are immiscible with UCST phase behavior, which are affected by molecular weights rather than the ratio of L/D monomer in the chemical structure of PLAs. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1135–1147, 2010     

Effects of posttreatment on the properties of modified PLLA/PDLA fibers

Poly(L ‐lactic acid)/poly(D ‐lactic acid) (PLLA/PDLA) blended with plasticizer poly(ethylene glycol) and nucleation agent TMC‐306 as‐spun fibers were prepared by melt spinning. The posttreatment was applied by hot drawing at 70°C and then heat‐treating at different temperatures for 30 minutes. In the process of hot drawing, orientation induced the further formation of the sc crystals and increased the degree of crystallinity of drawn fibers. When the hot drawing ratio reached 3 times, the properties of the fibers were relatively better. The highly oriented fibers containing pure sc crystals with high crystallinity were obtained by heat‐treating at a temperature above the melting point of α crystals. The posttreated PLLA/PDLA fibers with poly(ethylene glycol) and TMC‐306 (LDTP) obtained by hot drawing to 3 times at 70°C and then annealing at 170°C for 30 minutes exhibited the best antioxidative degradation and heat resistance properties. The initial decomposition temperature (T_5%) and heat resistance of posttreated LDTP fiber were about 94°C and 20°C higher than those of the commercial PLLA fiber, respectively.     

Hydrogen bonding assists stereocomplexation in poly(l‐lactic acid)/poly(d‐lactic acid) racemic blends

In this communication, we reported the sequence variation of stereocomplex crystals (SC) and homocrystals (HC) in poly(l ‐lactic acid)/poly(d ‐lactic acid) (PLLA/PDLA) racemic blends melts. It was evidenced that the emerging sequence of the SC and HC depends on the hydrogen bond formation in the melt, and the hydrogen bond is required for the stereocomplexation in PLLA/PDLA racemic blend. First, by combining a commercial fast‐scan chip‐calorimeter (Flash DSC 1) and micro‐FTIR, we found that hydrogen bonds were formed in the melt during cooling at 2.5 K/s, but not at 3000 K/s. Second, annealing the melt without hydrogen bonds at 100 °C led to HC emerging first, while annealing the melt with hydrogen bonds resulted in SC emerging at first. Third, the crystallization kinetics of the racemic blends after cooling to predefined T_c at 2.5 or 3000 K/s further verified that the hydrogen bonding can be inhibited effectively by cooling the racemic blends isotropic melt at fast enough rate. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 83–88     

Enhanced stereocomplex formation of poly(L-lactic acid) and poly(D-lactic acid) in the presence of stereoblock poly(lactic acid)

Stereoblock poly(lactic acid) (sb-PLA) is incorporated into a 1:1 polymer blend system of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) that has a high molecular weight to study its addition effect on the stereocomplex (sc) formation of PLLA and PDLA. The ternary polymer blend films are first prepared by casting polymer solutions of sb-PLA, PLLA, and PDLA with different compositions. Upon increasing the content of sb-PLA in the blend films the sc crystallization is driven to a higher degree, while the formation of homo-chiral (hc) crystals is decreased. Lowering the molecular weight of the incorporated sb-PLA effectively increases the sc formation. Consequently, it is revealed that sb-PLA can work as a compatibilizer to improve the poor sc formation in the polymer blend of PLLA and PDLA.     

Crystallization and stereocomplexation behavior of poly(D‐ and L‐lactide)‐b‐poly(N,N‐dimethylamino‐2‐ethyl methacrylate) block copolymers

The thermal properties, crystallization, and morphology of amphiphilic poly(D ‐lactide)‐b‐poly(N,N‐dimethylamino‐2‐ethyl methacrylate) (PDLA‐b‐PDMAEMA) and poly (L ‐lactide)‐b‐poly(N,N‐dimethylamino‐2‐ethyl methacrylate) (PLLA‐b‐PDMAEMA) copolymers were studied and compared to those of the corresponding poly(lactide) homopolymers. Additionally, stereocomplexation of these copolymers was studied. The crystallization kinetics of the PLA blocks was retarded by the presence of the PDMAEMA block. The studied copolymers were found to be miscible in the melt and the glassy state. The Avrami theory was able to predict the entire crystallization range of the PLA isothermal overall crystallization. The melting points of PLDA/PLLA and PLA/PLA‐b‐PDMAEMA stereocomplexes were higher than those formed by copolymer mixtures. This indicates that the PDMAEMA block is influencing the stability of the stereocomplex structures. For the low molecular weight samples, the stereocomplexes particles exhibited a conventional disk‐shape structure and, for high molecular weight samples, the particles displayed unusual star‐like shape morphology. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1397–1409, 2011     

A Novel Synthetic Approach to Stereo-Block Poly(lactic acid)

Stereoblock poly(lactic acid) (sb-PLA), consisting of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) in a blocky sequence, can successfully be synthesized by solid-state polycondensation of a stereocomplexed mixture of PLLA and PDLA. First, the melt polyconden-sation of L- and D-lactic acids is conducted to obtain PLLA and PDLA with medium molecular weights. Then, both polymers are melt-blended to easily form the stereocomplex. The resulting stereocomplexed mixture (melt-blend) is subjected to solid-state polycondensation for chain extension. The molecular weight (M_w) of the resultant sb-PLA is strongly affected by the lactide/oligomer content in the melt-blend, which is determined by the melt-blending conditions, because it is directly correlated with the polymer crystallinity of the polycondensation products.     

Novel poly(l‐lactide)/poly(d‐lactide)/poly(tetrahydrofuran) multiblock copolymers with a controlled architecture: Synthesis and characterization

Novel poly(l ‐lactide) (PLLA)/poly(d ‐lactide) (PDLA)/poly(tetrahydrofuran) (PTHF) multiblock copolymers with designed molecular structure were synthesized by a two‐stage procedure. Well‐defined PDLA‐PLLA‐PTHF‐PLLA‐PDLA pentablock copolymers were prepared by sequential ring opening polymerization of l ‐ and d ‐lactides starting from PTHF glycol, with the length of the (equimolar) PLLA and PDLA blocks being varied. Then, these dihydroxyl‐terminated pentamers were transformed into multiblock copolymers by melt chain‐extension with hexamethylene diisocyanate–being the first time that the coupling of pentablock units is reported. The successful formation of macromolecular chains with a multiblock and well‐defined architecture was demonstrated by ¹H NMR spectroscopy. The thermal properties and structuring of the resulting materials were investigated by means of DSC and WAXD measurements and DMA analysis. Stereocomplexation was found to be promoted during solution and melt crystallization. This approach affords materials combining the high rigidity and strength (other than improved thermal resistance) of the hard stereocomplex crystallites with the flexibility imparted by the soft block, whereby their properties can be finely tailored through the composition of the basic pentablock units without limitations on the final molecular weight. The adopted reaction conditions make this process highly appealing in view of the possibility to perform it in extruder. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 3269–3282     

Synthesis of high‐molecular‐weight aliphatic–aromatic copolyesters from poly(ethylene‐co‐1,6‐hexene terephthalate) and poly(L‐lactic acid) by chain extension

A series of aliphatic–aromatic multiblock copolyesters consisting of poly(ethylene‐co‐1,6‐hexene terephthalate) (PEHT) and poly(L ‐lactic acid) (PLLA) were synthesized successfully by chain‐extension reaction of dihydroxyl terminated PEHT‐OH prepolymer and dihydroxyl terminated PLLA‐OH prepolymer using toluene‐2,4‐diisoyanate as a chain extender. PEHT‐OH prepolymers were prepared by two step reactions using dimethyl terephthalate, ethylene glycol, and 1,6‐hexanediol as raw materials. PLLA‐OH prepolymers were prepared by direct polycondensation of L ‐lactic acid in the presence of 1,4‐butanediol. The chemical structures, the molecular weights and the thermal properties of PEHT‐OH, PLLA‐OH prepolymers, and PEHT‐PLLA copolymers were characterized by FTIR, ¹H NMR, GPC, TG, and DSC. This synthetic method has been proved to be very efficient for the synthesis of high‐molecular‐weight copolyesters (say, higher than M_w = 3 × 10⁵ g/mol). Only one glass transition temperature was found in the DSC curves of PEHT‐PLLA copolymers, indicating that the PLLA and PEHT segments had good miscibility. TG curves showed that all the copolyesters had good thermal stabilities. The resulting novel aromatic–aliphatic copolyesters are expected to find a potential application in the area of biodegradable polymer materials. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 5898–5907, 2009     

Stereoselective Interaction between Isotactic and Optically Active Poly(lactic acid) and Phenyl‐Substituted Poly(lactic acid)

Isotactic and optically active poly(D ‐lactic acid) (PDLA) and phenyl‐substituted poly(lactic acid)s (Ph‐PLAs), i.e., poly(D ‐phenyllactic acid) (Ph‐PDLA) and poly(L ‐phenyllactic acid) (Ph‐PLLA), were synthesized and stereospecific interactions between the synthesized polymers were investigated by their thermal properties and crystallization behavior using differential scanning calorimetry (DSC). The DSC measurements indicated that PDLA is miscible with Ph‐PLAs and that the attractive interaction between PDLA and L ‐configured Ph‐PLA is higher than that between PDLA and D ‐configured Ph‐PDLA. In other words, the latter result means that poly(lactic acid) (PLA) has a higher stereoselective attractive interaction with Ph‐PLA with the reverse configuration than with Ph‐PLA of the same configuration. These results strongly suggest that PLA‐based materials with a wide variety of physical properties and biodegradability can be fabricated by blending them with substituted PLAs with the reverse and same configurations.

     

Enhancing crystallization behavior for optimized performances of poly(TMC‐b‐(LLA‐ran‐GA)) by PDLA/PLLA stereocomplex crystallization

The blends of poly(1,3‐trimethylene carbonate‐b‐(l ‐lactide‐ran‐glycolide)) (PTLG) with poly(d ‐lactide) (PDLA) were prepared via solution‐casting method using CH₂Cl₂ as solvent. The poly(l ‐lactide) (PLLA) segments of PTLG with PDLA chain constructed as stereocomplex structures and growth stereocomplex crystals of PLA (sc‐PLA). The effects of sc‐PLA crystals on thermal behavior, mechanical properties, thermal decomposition of the PTLG/PDLA blends were investigated, respectively. The differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD) results showed that the total crystallinity of the PTLG/PDLA blends was increased with the PDLA content increasing. Heterogeneous nucleation of sc‐PLA crystals induced crystallization of the PLLA segments in PTLG. The crystallization temperature of samples shifted to 107.5°C for the PTLG/PDLA‐20 blends compared with that of the PTLG matrix, and decreased the half‐time of crystallization. The mechanical measurement results indicated that the tensile strength of the PTLG/PDLA blends was improved from 21.1 MPa of the PTLG matrix to 39.5 MPa of PTLG/PDLA‐20 blends. The results of kinetics of thermal decomposition of the PTLG/PDLA blends by TGA showed that the apparent activation energy of the PTLG/PDLA blends was increased from 59.1 to 72.1 kJ/mol with the increasing of the PDLA content from 3 wt% to 20 wt%, which indicated the enhancement of thermal stability of the PTLG/PDLA blends by addition of PDLA. Furthermore, the biocompatibility of the PTLG/PDLA blends cultured with human adipose‐derived stem cells was evaluated by CCK‐8 and live/dead staining. The experiment results proved the PTLG/PDLA blends were a kind of biomaterial with excellent physical performances with very low cytotoxicity.     

Different enantioselectivity of two types of poly(lactic acid) depolymerases toward poly(l-lactic acid) and poly(d-lactic acid)

Poly(lactic acid) (PLA) depolymerases are categorized into protease-type and lipase-type. Protease-types can hydrolyze poly(l-lactic acid) (PLLA) but not poly(d-lactic acid) (PDLA). Lipase-types, including cutinase-like enzyme (CLE) from Cryptococcus sp. strain S-2 preferentially hydrolyze PDLA. Both enzymes degraded not only PLA emulsion but also PLA film, in which amorphous region is preferentially attacked, but crystalline region can be also attacked. Stereocomplex PLA (sc-PLA) formed by 50:50 blending of PLLA and PDLA included no homo crystals, but a tiny homo crystallization peak appeared and crystallinity increased by 5% when attacked by CLE, although no significant change of molecular weight and crystalline size was found. Enantioselective degradation must occur in amorphous region of PLLA/PDLA film and preferentially hydrolyzed PDLA, resulting in a slightly excess amount of PLLA remained, which must be crystallized.     

Rheology and crystallization behavior of asymmetric PLLA/PDLA blends based on linear PLLA and PDLA with different structures

In this study, several asymmetric poly(L‐lactide)/poly(D‐lactide) (PLLA/PDLA) blends were prepared by adding small amounts of PDLA with different structures into linear PLLA matrix. The effect of PDLA on rheological behavior, crystallization behavior, nucleation efficiency and spherulite growth of PLLA was investigated. Rheological results indicated that PLLA/PDLA blends showed solid‐like viscoelastic behavior at low temperature (<200°C), and the cross‐linking density of PLLA/PDLA melt at 180°C followed the order: PLLA/6PDLA > PLLA/L‐PDLA > PLLA/3PDLA > PLLA/4PDLA. No‐isotherm and isotherm crystallization results indicated that the crystallization capacity of PLLA/PDLA blends was strongly related to the PDLA structure, crystallization temperature and thermal treatment temperature. Furthermore, the dimension of crystal growth during isotherm crystallization presented the obvious dependent on the PDLA structure. The nucleation efficiency of sc‐crystallites in the blends and spherulite density during isothermal crystallization were also studied. Nucleation efficiency of sc‐crystallites in the PLLA/S‐PDLA blends showed the obvious dependent on thermal treatment temperature with respect to PLLA/L‐PDLA, and nucleation efficiency sc‐crystallites in the PLLA/S‐PDLA blends first decreased and then increased as the thermal treatment temperature increased. Spherulite density of PLLA/PDLA blends was also related to thermal treatment temperature and the PDLA structure. This study has discussed the temperature dependence of the stereocomplex networks between PLLA and PDLA with different structure, and then its consequential influence on rheology and crystallization capacity of PLLA, which would provide the theoretical direction for PLA processing. Copyright © 2016 John Wiley & Sons, Ltd.     

Melt polycondensation of L‐lactic acid with Sn(II) catalysts activated by various proton acids: A direct manufacturing route to high molecular weight Poly(L‐lactic acid)

Poly(L ‐lactic acid) (PLLA) was produced by the melt polycondensation of L ‐lactic acid. For the optimization of the reaction conditions, various catalyst systems were examined at different temperature and reaction times. It was discovered that Sn(II) catalysts activated by various proton acids can produce high molecular weight PLLA [weight‐average molecular weight (M_w ) ≥ 100,000] in a relatively short reaction time (≤15 h) compared with simple Sn(II)‐based catalysts (SnO, SnCl₂ · 2H₂O), which produce PLLA with an M_w of less than 30,000 after 20 h. The new catalyst system is also superior to the conventional systems in regard to racemization and discoloration of the resultant polymer. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1673–1679, 2000     

Synthesis of star‐shaped poly(D,L‐lactic acid‐alt‐glycolic acid)‐b‐poly(L‐lactic acid) with the poly(D,L‐lactic acid‐alt‐glycolic acid) macroinitiator and stannous octoate catalyst

Two types of three‐arm and four‐arm, star‐shaped poly(D,L ‐lactic acid‐alt‐glycolic acid)‐b‐poly(L ‐lactic acid) (D,L ‐PLGA50‐b‐PLLA) were successfully synthesized via the sequential ring‐opening polymerization of D,L ‐3‐methylglycolide (MG) and L ‐lactide (L ‐LA) with a multifunctional initiator, such as trimethylolpropane and pentaerythritol, and stannous octoate (SnOct₂) as a catalyst. Star‐shaped, hydroxy‐terminated poly(D,L ‐lactic acid‐alt‐glycolic acid) (D,L ‐PLGA50) obtained from the polymerization of MG was used as a macroinitiator to initiate the block polymerization of L ‐LA with the SnOct₂ catalyst in bulk at 130 °C. For the polymerization of L ‐LA with the three‐arm, star‐shaped D,L ‐PLGA50 macroinitiator (number‐average molecular weight = 6800) and the SnOct₂ catalyst, the molecular weight of the resulting D,L ‐PLGA50‐b‐PLLA polymer linearly increased from 12,600 to 27,400 with the increasing molar ratio (1:1 to 3:1) of L ‐LA to MG, and the molecular weight distribution was rather narrow (weight‐average molecular weight/number‐average molecular weight = 1.09–1.15). The ¹H NMR spectrum of the D,L ‐PLGA50‐b‐PLLA block copolymer showed that the molecular weight and unit composition of the block copolymer were controlled by the molar ratio of L ‐LA to the macroinitiator. The ¹³C NMR spectrum of the block copolymer clearly showed its diblock structures, that is, D,L ‐PLGA50 as the first block and poly(L ‐lactic acid) as the second block. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 40: 409–415, 2002     

Synthesis and properties of multiblock copolymers consisting of poly(L‐lactic acid) and poly(oxypropylene‐ co‐oxyethylene) prepared by direct polycondensation

A multiblock copoly(ester–ether) consisting of poly(l ‐lactic acid) (PLLA) and poly(oxypropylene‐co‐oxyethylene) (PN) was prepared and characterized. Preparation was done via the solution polycondensation of a thermal oligocondensate of l ‐lactic acid, a commercially available telechelic polyether (PN: Pluronic‐F68), and dodecanedioic acid as a carboxyl/hydroxyl adjusting agent. When stannous oxide was used as the catalyst, the molecular weight of the resultant PLLA/PN block copolymers became very high (even with a high PN content) under optimized reaction conditions. The refluxing of diphenyl ether (solvent) at reduced pressure allowed the efficient removal of the condensed water from the reaction system and the feed‐back of the intermediately formed l ‐lactide at the same time in order to successfully bring about a high degree of condensation. The copolymer films obtained by solution casting became more flexible with the increasing PN content as soft segments. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 1513–1521, 1999     

Polylactide stereocomplex crystallites as nucleating agents for isotactic polylactide

A nucleation efficiency scale for isotactic poly(L ‐lactide) (PLLA) was obtained with self‐nucleation and nonisothermal differential scanning calorimetry experiments. The maximum nucleation efficiency occurred at the highest concentration of self‐nucleating sites, and the minimum efficiency occurred in the absence of these sites (pure PLLA polymer melt). Blends of PLLA and isotactic poly(D ‐lactide) (PDLA) led to the formation of a 1/1 stereocomplex. In comparison with the homopolymer (PLLA), the stereocomplex had a higher melting temperature and crystallized at higher temperatures from the melt. Small stereocomplex crystallites were formed in PLLA/PDLA blends containing low concentrations of PDLA. These crystallites acted as heterogeneous nucleation sites for subsequent PLLA crystallization. Using the PLLA nucleation efficiency scale, we evaluated a series of PLLA/PDLA blends (0.25–15 wt % PDLA). A maximum nucleation efficiency of 66% was observed at 15 wt % PDLA. The nucleation efficiency was largely dependent on the thermal treatment of the sample. The nucleating ability of the stereocomplex was most efficient when it was formed well before PLLA crystallization. According to the efficiency scale, the stereocomplex was far superior to talc, a common nucleating agent for PLLA, in its ability to enhance the rate of PLLA crystallization. In comparison with the PLLA homopolymer, the addition of PDLA led to reduced spherulite sizes and a reduction in the overall extent of PLLA crystallization. The decreased extent of crystallization was attributed to the hindered mobility of the PLLA chains due to tethering by the stereocomplex. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 300–313, 2001     

The role of cold crystallization of homochiral crystallites in the superb heat resistant poly(lactic acid)

The superb heat resistance poly(lactic acid) (PLA) were prepared by blending PLA and poly(d ‐lactic acid) (PDLA) with various molecular weight (M_n). Formation of the stereocomplex in the blends was confirmed by differential scanning calorimetry and wide‐angle X‐ray diffraction. The results of the heat resistance implied it is possible that elevating the Vicat penetration temperature of PLA up to 150°C by blending with PDLA. The cold crystallization of homochiral crystallites is proven to be the critical factor affecting the heat resistance of PLA. While the PLA or PLA/PDLA blends were heated to cold crystallization temperature of samples, both the crystal content and the rigid amorphous region content are increased due to the cold crystallization and tethering effect, and the stiffness and heat resistance of the sample are improved. The cold crystallization homochiral crystallites kinetics of PLA and PLA/PDLA blends was also studied. The results showed the activation energy (?E) of cold crystallization increased from 120.30 kJ/mol to 144.66 kJ/mol with the increasing of PDLA content from 2% to 10%.