Investigation of poly(lactide) stereocomplexation between linear poly(L‐lactide) and PDLA‐PEG‐PDLA tri‐block copolymer

In this study, stereocomplexed poly(lactide) (PLA) was investigated by blending linear poly(l ‐lactide) (PLLA) and tri‐block copolymer poly(d ‐lactide) ? (polyethylene glycol) ? poly(d ‐lactide) (PDLA‐PEG‐PDLA). Synthesized PDLA‐PEG‐PDLA tri‐block copolymers with different PEG and PDLA segment lengths were studied and their influences on the degree of sterecomplexation and non‐isothermal crystallization behaviour of the PLLA/PDLA‐PEG‐PDLA blend were examined in detail by DSC, XRD and polarized optical microscopy. A full stereocomplexation between PLLA and PDLA‐PEG_4k‐PDLA200 could be formed when the L/D ratio ranged from 7/3 to 5/5 without the presence of PLA homocrystals. The segmental mobility and length of both PEG and PDLA are the dominating factors in the critical D/L ratio to achieve full stereocomplexation and also for nucleation and spherulite growth during the non‐isothermal crystallization process. For fixed PEG segmental length, the stereocomplexed PLA formed showed first an increasing and then a decreasing melting temperature with increasing PDLA segments due to their intrinsic stiff mobility. Furthermore, the effect of PEG segmental mobility on PLA stereocomplexation was investigated. The results clearly showed that the crystallization temperature and melting temperature of stereocomplexed‐PLA kept increasing with increasing PEG segmental length, which was due to PEG soft mobility in the tri‐block copolymers. However, PEG was not favourable for nucleation but could facilitate the spherulite growth rate. Both the PDLA and PEG segmental lengths in the tri‐block copolymers affect the crystallinity of stereocomplexed‐PLA and the stereocomplexation formation process; they have a different influence on blends prepared by solution casting or the melting method. © 2015 Society of Chemical Industry     

Enhanced crystallization rate and mechanical properties of poly(l‐lactic acid) by stereocomplexation with four‐armed poly(ϵ‐caprolactone)‐block‐poly(d‐lactic acid) diblock copolymer

Poly(l ‐lactic acid) (PLLA) was blended with a series of four‐armed poly(? ‐caprolactone)‐block ‐poly(d ‐lactic acid) (4a‐PCL‐b ‐PDLA) copolymers in order to improve its crystallization rate and mechanical properties. It is found that a higher content of 4a‐PCL‐b ‐PDLA copolymer or longer PDLA block in the copolymer lead to faster crystallization of the blend, which is attributed to the formation of stereocomplex crystallites between PLLA matrix and PDLA blocks of the 4a‐PCL‐b ‐PDLA copolymers. Meanwhile, the PDLA block can improve the miscibility between flexible PCL phase and PLLA phase, which is beneficial for improving mechanical properties. The tensile results indicate that the 10% 4a‐PCL_5k‐b ‐PDLA_5k/PLLA blend has the largest elongation at break of about 72% because of the synergistic effects of stereocomplexation between enantiomeric PLAs, multi‐arm structure and plasticization of PCL blocks. It is concluded that well‐controlled composition and content of 4a‐PCL‐b ‐PDLA copolymer in PLLA blends can significantly improve the crystallization rate and mechanical properties of the PLLA matrix. © 2017 Society of Chemical Industry     

Non‐isothermal crystallization behavior of stereo diblock polylactides with relatively short poly(d‐lactide) segments from the melt

Stereo diblock polylactides (SDB‐PLAs) composed of relatively short poly(d ‐lactide) (PDLA) segments and relatively long poly(l ‐lactide) (PLLA) segments were synthesized to have a wide number‐average molecular weight (M_n) range of 2.5 × 10⁴–2.0 × 10⁵ g mol^?1 and d ‐lactyl unit content of 0.9–38.6%. The effects of incorporated short PDLA segments (M_n = 2.0 × 10³–7.7 × 10³ g mol^?1) on crystallization behavior of the SDB‐PLAs were first investigated during heating after complete melting and quenching or during slow cooling after complete melting. Stereocomplex (SC) crystallites can be formed at d ‐lactyl unit content as low as 4.3 and 5.8% for heating and slow cooling, respectively, and for M_n of PDLA segments as low as 2.0 × 10³ and 3.5 × 10³ g mol^?1, respectively. With decreasing M_n and increasing d ‐lactyl unit content, the cold crystallization temperature during heating decreased and the crystallization temperature during slow cooling increased. With increasing d ‐lactyl unit content, the melting enthalpy (ΔH_m) of SC crystallites during heating and the crystallinity (X_c) of SC crystallites after slow cooling increased, whereas ΔH_m of PLLA homo‐crystallites during heating and X_c of PLLA homo‐crystallites after slow cooling decreased. The total ΔH_m of SC crystallites and PLLA homo‐crystallites during heating and the total X_c after slow cooling became a minimum at d ‐lactyl unit content of 10–15% and gave a maximum at d ‐lactyl unit content of 0%. Despite the accelerated crystallization of some of SDB‐PLAs, the low values of total ΔH_m and X_c at d ‐lactyl unit content of 10–15% are attributable to the formation of two crystalline species of SC crystallites and PLLA homo‐crystallites.     

Crystallization behavior of biodegradable amphiphilic poly(ethylene glycol)‐poly(L‐lactide) block copolymers

Poly(ethylene glycol)‐poly(L ‐lactide) diblock and triblock copolymers were prepared by ring‐opening polymerization of L ‐lactide with poly(ethylene glycol) methyl ether or with poly(ethylene glycol) in the presence of stannous octoate. Molecular weight, thermal properties, and crystalline structure of block copolymers were analyzed by ¹H‐NMR, FTIR, GPC, DSC, and wide‐angle X‐ray diffraction (WAXD). The composition of the block copolymer was found to be comparable to those of the reactants. Each block of the PEG–PLLA copolymer was phase separated at room temperature, as determined by DSC and WAXD. For the asymmetric block copolymers, the crystallization of one block influenced much the crystalline structure of the other block that was chemically connected to it. Time‐resolved WAXD analyses also showed the crystallization of the PLLA block became retarded due to the presence of the PEG block. According to the biodegradability test using the activated sludge, PEG–PLLA block copolymer degraded much faster than PLLA homopolymers of the same molecular weight. © 1999 John Wiley amp; Sons, Inc. J Appl Polym Sci 72: 341–348, 1999     

Isothermal crystallization and spherulite growth behavior of stereo multiblock poly(lactic acid)s: Effects of block length

Stereo multiblock poly(lactic acid)s (PLA)s and stereo diblock poly(lactic acid) (DB) with a wide variety of block length of 15.4–61.9 lactyl units are synthesized, and the effects of block length sequence on crystallization and spherulite growth behavior are investigated at different crystallization temperatures, in comparison with neat poly(L ‐lactide) (PLLA), poly(D ‐lactide) (PDLA), and PLLA/PDLA blend. Only stereocomplex crystallites as crystalline species are formed in the stereo multiblock PLAs and DB, irrespective of block length and crystallization temperature. The maximum crystallinities (33–61%), maximum radial growth rate of spherulites (0.7–56.7 μm min^?1), and equilibrium melting temperatures (182.0–216.5°C) increased with increasing block length but are less than those of PLLA/PDLA blend (67 %, 122.5 μm min^?1, and 246.0°C). The spherulite growth rates and overall crystallization rates of the stereo multiblock PLAs and DB increased with increasing block length and are lower than that of PLLA/PDLA blend. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Enhanced toughness and strength of poly (d‐lactide) by stereocomplexation with 5‐arm poly (l‐lactide)

The enhancement of mechanical properties were achieved by solution blending of poly(d ‐lactide) (PDLA) and 5‐arm poly(l ‐lactide) (5‐arm PLLA). Differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD) results indicated almost complete stereocomplex could be obtained when 5‐arm PLLA exceeded 30wt %. Tensile test results showed that the addition of 5‐arm PLLA in linear PDLA gave dramatically improvement both on tensile strength and elongation at break, which generally could not be increased simultaneously. Furthermore, this work transformed PDLA from brittle polymer into tough and flexible materials. The mechanism was proposed based on the TEM results: the stereocomplex crystallites formed during solvent evaporation on the blends were small enough (100–200 nm), which played the role of physical crosslinking points and increased the interaction strength between PDLA and 5‐arm PLLA molecules, giving the blends high tensile strength and elongation at break. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 132, 42857.     

Relationship between the crystallization behavior of poly(ethylene glycol) and stereocomplex crystallization of poly(L‐lactic acid)/poly(D‐lactic acid)

Poly(l ‐lactic acid) (PLLA) is a good biomedical polymer material with wide applications. The addition of poly(ethylene glycol) (PEG) as a plasticizer and the formation of stereocomplex crystals (SCs) have been proved to be effective methods for improving the crystallization of PLLA, which will promote its heat resistance. In this work, the crystallization behavior of PEG and PLLA/poly(d ‐lactic acid) (PDLA) in PLLA/PDLA/PEG and PEG‐b‐PLLA/PEG‐b‐PDLA blends has been investigated using differential scanning calorimetry, polarized optical microscopy and X‐ray diffraction. Both SCs and homocrystals (HCs) were observed in blends with asymmetric mass ratio of PLLA/PDLA, while exclusively SCs were observed in blends with approximately equal mass ratio of PLLA/PDLA. The crystallization of PEG was only observed for the symmetric blends of PLLA_39k/PDLA_35k/PEG_2k, PLLA_39k/PDLA_35k/PEG_5k, PLLA_69k/PDLA_96k/PEG_5k and PEG‐b‐PLLA_31k/PEG‐b‐PDLA_27k, where the mass ratio of PLLA/PDLA was approximately 1/1. The results demonstrated that the formation of exclusively SCs would facilitate the crystallization of PEG, while the existence of both HCs and SCs could restrict the crystallization of PEG. The crystallization of PEG is related to the crystallinity of PLLA and PDLA, which will be promoted by the formation of SCs. © 2017 Society of Chemical Industry     

Synthesis of star‐shaped poly(ε‐caprolactone)‐b‐poly(L‐lactide) copolymers: From star architectures to crystalline morphologies

Hexa‐armed star‐shaped poly(ε‐caprolactone)‐block‐poly(L ‐lactide) (6sPCL‐b‐PLLA) with dipentaerythritol core were synthesized by a two‐step ring‐opening polymerization. GPC and ¹H NMR data demonstrate that the polymerization courses are under control. The molecular weight of 6sPCLs and 6sPCL‐b‐PLLAs increases with increasing molar ratio of monomer to initiator, and the molecular weight distribution is in the range of 1.03–1.10. The investigation of the melting and crystallization demonstrated that the values of crystallization temperature (T_c), melting temperature (T_m), and the degree of crystallinity (X_c) of PLLA blocks are increased with the chain length increase of PLLA in the 6sPCL‐b‐PLLA copolymers. On the contrary, the crystallization of PCL blocks dominates when the chain length of PLLA is too short. According to the results of polarized optical micrographs, both the spherulitic growth rate (G) and the spherulitic morphology are affected by the macromolecular architecture and the length of the block chains. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Effect of the melting temperature on the crystallization behavior of a poly(l‐lactide)/poly(d‐lactide) equimolar mixture

The effect of the final melting temperature (T_f) on the crystallization of poly(l ‐lactide) (PLLA)/poly(d ‐lactide) (PDLA) was studied via a combination of differential scanning calorimetry, wide‐angle X‐ray scattering, polarized optical microscopy, and Fourier transform infrared (FTIR) spectroscopy. We observed that a residual stereocomplex (SC) crystal induced the formation of SC crystals during cooling from a T_f (230°C) just above the melting peak of the SC crystals. On cooling from a T_f (240°C) just above the endset temperature of SC crystal melting [T_m(S)(E)], the possible order structure and the strong interchain interaction promoted the preferential crystallization of SC crystals; this enhanced the formation of α crystals. During cooling from a T_f (≥250°C) far above T_m(S)(E), the crystallization peaks of α and SC crystals converged. The FTIR results indicated that the residual SC crystals, possible ordered structure, and interchain interactions in the melt might have been the key factors for the different crystallization of PLLA/PDLA. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43015.     

Synthesis and characterization of AB‐type copolymers poly(L‐lactide)‐block‐poly(methyl methacrylate) via a convenient route combining ROP and ATRP from a dual initiator

Diblock copolymers of poly(L ‐lactide)‐block‐poly(methyl methacrylate) (PLLA‐b‐PMMA) were synthesized through a sequential two‐step strategy, which combines ring‐opening polymerization (ROP) and atom transfer radical polymerization (ATRP), using a bifunctional initiator, 2,2,2‐trichloroethanol. The trichloro‐terminated poly(L ‐lactide) (PLLA‐Cl) with high molecular weight (M_n,GPC = 1–12 × 10⁴ g/mol) was presynthesized through bulk ROP of L ‐lactide (L ‐LA), initiated by the hydroxyl group of the double‐headed initiator, with tin(II) octoate (Sn(Oct)₂) as catalyst. The second segment of the block copolymer was synthesized by the ATRP of methyl methacrylate (MMA), with PLLA‐Cl as macroinitiator and CuCl/N,N,N′,N″,N″‐pentamethyldiethylenetriamine (PMDETA) as catalyst, and dimethyl sulfoxide (DMSO) was chosen as reaction medium due to the poor solubility of the macroinitiator in conventional solvents at the reaction temperature. The trichloroethoxyl terminal group of the macroinitiator was confirmed by Fourier transform infrared spectroscopy (FTIR) and ¹H‐NMR spectroscopy. The comprehensive results from GPC, FTIR, ¹H‐NMR analysis indicate that diblock copolymers PLLA‐b‐PMMA (M_n,GPC = 5–13 × 10⁴ g/mol) with desired molecular composition were obtained by changing the molar ratio of monomer/initiator. DSC, XRD, and TG analyses establish that the crystallization of copolymers is inhibited with the introduction of PMMA segment, which will be beneficial to ameliorating the brittleness, and furthermore, to improving the thermal performance. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Investigations on the morphology and melt crystallization of poly(L‐lactide)‐poly(ethylene glycol) diblock copolymers

Ring opening polymerization of L ‐lactide was realized in the presence of monomethoxy poly(ethylene glycol), using zinc lactate as catalyst. The resulting PLLA‐PEG diblock copolymers were characterized by using ¹H‐NMR, SEC, WAXD, and DSC. All the copolymers were semicrystalline, one or two melting peaks being detected depending on the composition. Equilibrium melting temperature (T_m⁰) of PLLA blocks was determined for three copolymers with different EO/LA molar ratios. T_m⁰ decreased with decreasing PLLA block length. A copolymer with equivalent PLLA and PEG block lengths was selected for melt crystallization studies and the resulting data were analyzed with Avrami equation. The obtained Avrami exponent is equal to 2.6 ± 0.2 in the crystallization temperature range from 80 to 100°C. In addition, the spherulite growth rate of PLLA‐PEG was analyzed by using Lauritzen‐Hoffmann theory in comparison with PLLA homopolymers. The nucleation constant was found to be 2.39 × 10⁵ K² and the free energy of folding equal to 53.8 erg/cm² in the range of 70–94°C, both higher than those of PLLA homopolymers, while the spherulite growth rate of the diblock copolymer was lower. POLYM. ENG. SCI., 2008. © 2007 Society of Plastics Engineers     

Poly(L‐lactide)/Poly(D‐lactide)/clay nanocomposites: Enhanced dispersion,crystallization, mechanical properties,and hydrolytic degradation

Poly(L ‐lactide) (PLLA)/poly(D ‐lactide) (PDLA)/clay nanocomposites are prepared via simple melt blending method at PDLA loadings from 5 to 20 wt%. Formation of the stereocomplex crystals in the nanocomposites is confirmed by differential scanning calorimetry and wide‐angle X‐ray diffraction (WAXD). The internal structure of the nanocomposites has been established by using WAXD and transmission electron microscope analyses. The dispersion of clay in the PLLA/PDLA/clay nanocomposites can be improved as a result of increased intensity of shear during melt blending. The overall crystallization rates are faster in the PLLA/PDLA/clay nanocomposites than in PLLA/clay nanocomposite and increase with an increase in the PDLA loading up to 10 wt%; however, the crystallization mechanism and crystal structure of these nanocomposites remain unchanged despite the presence of PDLA. The storage modulus has been apparently improved in the PLLA/PDLA/clay nanocomposites with respect to PLLA/clay nanocomposite. Moreover, it is found that the hydrolytic degradation rates have been enhanced obviously in the PLLA/PDLA/clay nanocomposites than in PLLA/clay nanocomposite. POLYM. ENG. SCI., 54:914–924, 2014. © 2013 Society of Plastics Engineers     

Fully bio‐based poly(ɛ‐capolactone)/poly(lactide) alternating multiblock supramolecular polymers: Synthesis,crystallization behavior,and properties

Supramolecular poly(?‐capolactone)/poly(lactide) alternating multiblock copolymers were prepared by UPy‐functionalized poly(lactide)‐b‐ poly(?‐capolactone)‐b‐ poly(lactide) copolymers. The prepared supramolecular polymers (SMPs) exhibit the characteristic properties of thermoplastic elastomers. The stereo multiblock SMPs (sc‐SMPs) were formed by blending UPy‐functionalized poly(l ‐lactide)‐b‐ PCL‐b‐ poly(l ‐lactide) (l ‐SMPs) and UPy‐functionalized poly(d ‐lactide)‐b‐ PCL‐b‐ poly(d ‐lactide) (d ‐SMPs) due to stereocomplexation of the PLLA and PDLA blocks. Sc‐SMPs with low content of d ‐SMPs (≤20%) are transparent, elastic solids, while those having high d ‐SMPs content are opaque, brittle solids. The effects of l ‐SMPs/d ‐SMPs mixing ratios on thermal, crystallization behaviors, crystal structure, mechanical and hydrophilic properties of sc‐SMPs were deeply investigated. The incorporation of UPy groups depresses the crystallization of polymer, and the stereocomplex formation accelerates the crystallization rate. The used initiator functionalized polyhedral oligomeric silsesquioxanes causes a different effect on the crystallization of PLA and PCL blocks. The tensile strength and elongation at break of l _d /d _d ‐SMPs (d represents the initiator diethylene glycol) are significantly larger than that of l _p /d _p ‐SMPs (p represents the initiator polyhedral oligomeric silsesquioxanes), and their heat resistance and hydrophilicity can be also modulated by the l ‐SMPs/d ‐SMPs mixing ratios and the different initiators. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45575.     

Confined crystallization morphology of poly(ϵ‐caprolactone) block within poly(ϵ‐caprolactone)–poly(l‐lactide) copolymers

The confined crystallization of poly(?‐caprolactone) (PCL) block in poly(?‐caprolactone)–poly(l ‐lactide) (PCL‐PLLA) copolymers was investigated using differential scanning calorimetry, polarized optical microscopy, scanning electronic microscopy and atomic force microscopy. To study the effect of crystallization and molecular chain motion state of PLLA blocks in PCL‐PLLA copolymers on PCL crystallization morphology, high‐temperature annealing (180 °C) and low‐temperature annealing (80 °C) were applied to treat the samples. It was found that the crystallization morphology of PCL block in PCL‐PLLA copolymers is not only related to the ratio of block components, but also related to the thermal history. After annealing PCL‐PLLA copolymers at 180 °C, the molten PCL blocks are rejected from the front of PLLA crystal growth into the amorphous regions, which will lead to PCL and PLLA blocks exhibiting obvious fractionated crystallization and forming various morphologies depending on the length of PLLA segment. On the contrary, PCL blocks more easily form banded spherulites after PCL‐PLLA copolymers are annealed at 80 °C because the preexisting PLLA crystal template and the dangling amorphous PLLA chains on PCL segments more easily cause unequal stresses at opposite fold surfaces of PCL lamellae during the growth process. Also, it was found that the growth rate of banded spherulites is less than that of classical spherulites and the growth rate of banded spherulites decreases with decreasing band spacing. © 2019 Society of Chemical Industry     

Enhanced crystallization of poly(lactide) stereocomplex by xylan propionate

The effect of xylan propionate (XylPr) as a novel biomass‐derived nucleating agent on the poly(lactide) sterecomplex was investigated. Addition of XylPr to an enantiomeric blend of poly(l ‐lactide) (PLLA) and poly(d ‐lactide) (PDLA) was performed in either the solution state or molten state. The solution blend of PLLA/PDLA with XylPr was prepared by mixing equal volumes of 1 wt% XylPr/PLLA and 1 wt% XylPr/PDLA solutions in chloroform and precipitating in methanol. The solution blend with XylPr showed shorter half‐time crystallization than the solution blend without XylPr in isothermal crystallization between 80 and 140 °C, although homocrystallization occurred. Enhanced stereocomplex crystallization in the solution blend with XylPr was observed at 180 °C, where no crystallization occurred in the solution blend without XylPr. Addition of XylPr to PLLA/PDLA blend in the molten state was performed at 240 °C. Thereafter, the melt blend of PLLA/PDLA with or without XylPr was either quenched in iced water or isothermally crystallized directly from the melt. Isothermal crystallization of the melt‐quenched blend with XylPr gave a similar result to the solution blend with XylPr. In contrast, the melt‐crystallized blend with XylPr formed only stereocomplex crystals after crystallization above 140 °C. Furthermore, the melt‐crystallized blend with XylPr showed a higher crystallinity index and melting temperature than the melt‐crystallized blend without XylPr. This shows that XylPr promotes stereocomplex crystallization only when the blend of PLLA/PDLA with XylPr is directly crystallized from the molten state just after blending. © 2016 Society of Chemical Industry     

Effect of nucleating agents on the crystallization behavior and heat resistance of poly(l‐lactide)

Poly (l ‐lactide) (PLLA) blends with various nucleators were prepared by melt processing. The effect of different nucleators on the crystallization behavior and heat resistance as well as thermomechanical properties of PLLA was studied systematically by differential scanning calorimetry, X‐ray diffraction, heat deflection temperature tester, and dynamic mechanical analysis. It was found that poly(d ‐lactide), talcum powder (Talc), a multiamide compound (TMC‐328, abbreviated as TMC) can significantly improve the crystallization rate and crystallinity of PLLA, thus improving thermal–resistant property. The heat deflection temperature of nucleated PLLA can be as high as 150°C. The storage modulus of nucleated PLLA is higher than that of PLLA at the temperature above T_g of PLLA. Compared with other nucleating agents, TMC was much more efficient at enhancing the crystallization of PLLA and the PLLA containing TMC showed the best heat resistance. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42999.     

Influence of the synthesis conditions on the structural and thermal properties of poly(l‐lactide)‐b‐poly(ethylene glycol)‐b‐poly(l‐lactide)

The poly(l ‐lactide)‐b‐poly(ethylene glycol)‐b‐poly(l ‐lactide) block copolymers (PLLA‐b‐PEG‐b‐PLLA) were synthesized in a toluene solution by the ring‐opening polymerization of 3,6‐dimethyl‐1,4‐dioxan‐2,5‐dione (LLA) with PEG as a macroinitiator or by transterification from the homopolymers [polylactide and PEG]. Two polymerization conditions were adopted: method A, which used an equimolar catalyst/initiator molar ratio (1–5 wt %), and method B, which used a catalyst content commonly reported in the literature (<0.05 wt %). Method A was more efficient in producing copolymers with a higher yield and monomer conversion, whereas method B resulted in a mixture of the copolymer and homopolymers. The copolymers achieved high molar masses and even presenting similar global compositions, the molar mass distribution and thermal properties depends on the polymerization method. For instance, the suppression of the PEG block crystallization was more noticeable for copolymer A. An experimental design was used to qualify the influence of the catalyst and homopolymer amounts on the transreactions. The catalyst concentration was shown to be the most important factor. Therefore, the effectiveness of method A to produce copolymers was partly due to the transreactions. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40419.     

Compatibility of poly(butadiene‐co‐acrylonitrile) with poly(L‐lactide) and poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate)

Poly(L ‐lactide) (PLLA) and poly(3‐hydrobutyrate‐co‐3‐hydroxyvalerate) (PHBV) were blended with poly(butadiene‐co‐acrylonitrile) (NBR). Both PLLA/NBR and PHBV/NBR blends exhibited higher tensile properties as the content of acrylonitrile unit (AN) of NBR increased from 22 to 50 wt %. However, two separate glass transition temperatures (T_g) appeared in PLLA/NBR blends irrespective of the content of NBR, revealing that PLLA was incompatible with NBR. In contrast, a single T_g, which shifted along with the blend composition, was observed for PHBV/NBR50 blends. Moreover NBR50 suppressed the crystallization of PHBV, indicating that PHBV was compatible with NBR50. Decrease of both elongation modulus and stress at maximum load was less significant and increase of elongation at break was more pronounced in PHBV/NBR50 blends than in PLLA/NBR50 blends. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3508–3513, 2004     

Stereocomplex formation between poly(L‐lactic acid) and poly(D‐lactic acid) with disproportionately low and high molecular weights from the melt

Poly(L ‐lactic acid) (PLLA) and poly(D ‐lactic acid) (PDLA) with very different weight‐average molecular weights (M_w) of 4.0 × 10³ and 7.0 × 10⁵ g mol^?1 (M_w(PDLA)/M_w(PLLA) = 175) were blended at different PDLA weight ratios (X_D = PDLA weight/blend weight) and their crystallization from the melt was investigated. The presence of low molecular weight PLLA facilitated the stereocomplexation and thereby lowered the cold crystallization temperature (T_cc) for non‐isothermal crystallization during heating and elevated the radial growth rate of spherulites (G) for isothermal crystallization, irrespective of X_D. The orientation of lamellae in the spherulites was higher for the neat PLLA, PDLA and an equimolar blend than for the non‐equimolar blends. It was found that the orientation of lamellae in the blends was maintained by the stereocomplex (SC) crystallites. Although the G values are expected to decrease with an increase in X_D or the content of high‐molecular‐weight PDLA with lower chain mobility compared with that of low‐molecular‐weight PLLA, G was highest at X_D = 0.5 where the maximum amount of SC crystallites was formed and the G values were very similar for X_D = 0.4 and X_D = 0.6 with the same enantiomeric excess. This means that the effect of SC crystallites overwhelmed that of chain mobility. The nucleating mechanisms of SC crystallites were identical for X_D = 0.1–0.5 in the T_c range 130–180 °C. Copyright © 2011 Society of Chemical Industry     

Part 7. Effects of poly(L‐lactide‐co‐ϵ‐caprolactone) on morphology,structure, crystallization,and physical properties of blends of poly(L‐lactide) and poly(ϵ‐caprolactone)

Blended films of poly(L ‐lactide) [ie poly(L ‐lactic acid)] (PLLA) and poly(?‐caprolactone) (PCL) without or mixed with 10 wt% poly(L ‐lactide‐co‐?‐caprolactone) (PLLA‐CL) were prepared by solution‐casting. The effects of PLLA‐CL on the morphology, phase structure, crystallization, and mechanical properties of films have been investigated using polarization optical microscopy, scanning electron microscopy, differential scanning calorimetry and tensile testing. Addition of PLLA‐CL decreased number densities of spherulites in PLLA and PCL films, and improved the observability of spherulites and the smoothness of cross‐section of the PLLA/PCL blend film. The melting temperatures (T_m) of PLLA and PCL in the films remained unchanged upon addition of PLLA‐CL, while the crystallinities of PLLA and PCL increased at PLLA contents [X_PLLA = weight of PLLA/(weight of PLLA and PCL)] of 0.4–0.7 and at most of the X_PLLA values, respectively. The addition of PLLA‐CL improved the tensile strength and the Young modulus of the films at X_PLLA of 0.5–0.8 and of 0–0.1 and 0.5–0.8, respectively, and the elongation at break of the films at all the X_PLLA values. These findings strongly suggest that PLLA‐CL was miscible with PLLA and PCL, and that the dissolved PLLA‐CL in PLLA‐rich and PCL‐rich phases increased the compatibility between these two phases. © 2003 Society of Chemical Industry