Effect of the addition of poly(d-lactic acid) on the thermal property of poly(l-lactic acid)

Thermal property and crystallization behavior of PLLA blended with a small amount of PDLA (1-5 wt%) were studied. PDLA molecules added in PLLA formed stereocomplex crystallites in the PLLA matrix. When the blend was cooled to a temperature below T_m of PLLA, stereocomplex crystallites acted as nucleation sites of PLLA and enhanced the crystallization of PLLA significantly (heterogeneous nucleation). Such crystallization enhancement was not observed when the blend with lower PDLA content was cooled from 240 °C at which both PLLA crystal and the stereocomplex disappeared. Low molecular weight PDLA isolated in the matrix of PLLA did not form a stereocomplex crystallite with a large surface area enough to act as a nucleation site. On the other hand, high molecular weight PDLA chains formed a large stereocomplex crystallite. With increasing PDLA content, stereocomplex crystallites were more easily formed and they acted as nucleation sites. PLLA crystal near the stereocomplex crystallites has an incomplete structure and showed a melting peak at a lower temperature than pure PLLA crystal.     

Melt preparation and nucleation efficiency of polylactide stereocomplex crystallites

A melt blending procedure was developed for the preparation of poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) stereocomplex crystallites dispersed in a PLLA matrix. All PLLA/PDLA blends were prepared in a batch melt mixer with ≥95% PLLA. Three PDLA homopolymers with a range of molecular weights were used as the minority (≤5%) component. The presence of the stereocomplex in the PLLA matrix was verified by differential scanning calorimetry (DSC) and optical microscopy. The effectiveness of the in situ formed stereocomplex crystallites for nucleating PLLA crystallization was evaluated using self-nucleation and non-isothermal DSC methods. With only 3 wt% of the 14 kg mol⁻¹ PDLA, nucleation efficiencies near 100% could be obtained. In addition, fast crystallization kinetics were observed in isothermal crystallization experiments at 140 °C. The stereocomplex crystallites were much more effective at enhancing the crystallization rate of PLLA compared to talc, a common nucleating agent.     

Controlled crystal nucleation in the melt-crystallization of poly(l-lactide) and poly(l-lactide)/poly(d-lactide) stereocomplex

Among the various inorganic nucleators examined, Talc and an aluminum complex of a phosphoric ester combined with hydrotalcite (NA) were found to be effective for the melt-crystallization of poly(l-lactide) (PLLA) and PLLA/poly(d-lactide) (PDLA) stereo mixture, respectively. NA (1.0 phr (per one hundred resin)) can exclusively nucleate the stereocomplex crystals, while Talc cannot suppress the homo crystallization of PLLA and PDLA in the stereo mixture. Double use of Talc and NA (in 1.0 phr each) is highly effective for enhancing the crystallization temperature of the stereo complex without forming the homo crystals. The stereocomplex crystals nucleated by NA show a significantly lower melting temperature (207 °C) than the single crystal of the stereocomplex (230 °C) in spite of recording a large heat of crystallization ΔH_c (54 J/g). Photomicrographic study suggests that the spherulites with a symmetric morphology are formed in the stereo mixture added with NA while the spherulites do not grow in size in the mixture added with Talc. The exclusive growth of the stereocomplex crystals by the melt-crystallization process will open a processing window for the PLLA/PDLA.     

Synchronous and separate homo-crystallization of enantiomeric poly(l-lactic acid)/poly(d-lactic acid) blends

High-molecular-weight poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) are blended at different ratios and their crystallization behavior was investigated. Solely homo-crystallites mixtures of PLLA and PDLA were synchronously and separately formed during isothermal crystallization in the temperature (T_c) range of 90–130 °C, irrespective of blending ratio, whereas in addition to homo-crystallites, stereocomplex crystallites were formed in the equimolar blends at T_c above 150 and 160 °C. Interestingly, in isothermal crystallization at T_c = 130 °C, the spherulite morphology of blends became disordered, the periodical extinction (periodical twisting of lamellae) in spherulites disappeared, and the radial growth rate of spherulite (G) of the blends was reduced by the synchronous and separate crystallization of PLLA and PDLA and the coexistence of PLLA and PDLA homo-crystallites. However, the interplane distance (d), the crystallinity (X_c), the transition crystallization temperature (T_c) from α′-form to α-form, the alternately stacked structure of the crystalline and amorphous layers, and the nucleation mechanism were not altered by the synchronous and separate crystallization of PLLA and PDLA and the coexistence of PLLA and PDLA homo-crystallites. The unchanged d, X_c, transition T_c, long period of stacked lamellae, and nucleation mechanism strongly suggest that the chiral selection of PLLA or PDLA segments on the growth sites of PLLA or PDLA homo-crystallites to some extent was performed during solvent evaporation and this effect remained even after melting.     

Influence of PLA stereocomplex crystals and thermal treatment temperature on the rheology and crystallization behavior of asymmetric poly(L-Lactide)/poly(D-lactide) blends

Asymmetric poly(L-lactide)/poly(D-Lactide) (PLLA/PDLA) blends were prepared by adding small amounts of PDLA into the PLLA matrix with the formation of stereocomplex crystallites (sc-crystallites). Rheological results indicated that the PLLA/PDLA melt at lower temperatures (<T_m,sc, the melting temperature of the formed stereocomplex crystallites) underwent the transition from liquid-like to solid-like viscoelastic behaviors with increasing of the PDLA concentration, which was related to the sc-crystallites reserved in the melt of asymmetric PLLA/PDLA blends. Dissolution experiment indicated the presence of sc-crystallites network structure in the PLLA/PDLA blends, and the size of the sc-crystallite junction particles network increased with increasing of the PDLA concentration. DSC and POM studies indicated that the PDLA concentration and the thermal treatment temperature had a significant influence on the PLLA crystallizability behavior. At low thermal treatment temperature (<T _m,sc ), reserved sc-crystallites showed an obvious promoting effect for PLLA crystallization. With increasing of the thermal treatment temperature, its promoting effect decreased due to melting of the sc-crystallites. This result suggests the sc-crystallites played two roles: nucleation sites and cross-linking points, and the two roles had a competitive relationship with change of the thermal treatment temperature and the PDLA concentration.     

Stereocomplex crystallization and spherulite growth behavior of poly(l-lactide)-b-poly(d-lactide) stereodiblock copolymers

The non-isothermally and isothermally crystallized stereodiblock copolymers of poly(l-lactide) (PLLA) and poly(d-lactide) (PDLA) with equimolar l-lactyl and d-lactyl units and different number-average molecular weights (M_n) of 3.9 × 10³, 9.3 × 10³, and 1.1 × 10⁴ g mol⁻¹, which are abbreviated as PLLA-b-PDLA copolymers, contained only stereocomplex crystallites as crystalline species, causing higher melting temperatures of the PLLA-b-PDLA copolymers compared to those of PLLA homopolymers. In the case of non-isothermal crystallization, the cold crystallization temperatures of the PLLA-b-PDLA copolymers during heating and cooling were respectively lower and higher than those of PLLA homopolymers, indicating accelerated crystallization of PLLA-b-PDLA copolymers. In the case of isothermal crystallization, in the crystallizable temperature range, the crystallinity (X_c) values of the PLLA-b-PDLA copolymers were lower than those of the PLLA homopolymers, and were susceptible to the effect of crystallization temperature in contrast to that of homopolymers. The radial growth rate of the spherulites (G) of the PLLA-b-PDLA copolymers was the highest at the middle M_n of 9.3 × 10³ g mol⁻¹. This trend is different from that of the PLLA homopolymers where the G values increased monotonically with a decrease in M_n, but seems to be caused by the upper critical M_n values of PLLA and PDLA chains as in the case of PLLA/PDLA blends (in other papers), above which homo-crystallites are formed in addition to stereocomplex crystallites. The disturbed crystallization of PLLA-b-PDLA copolymers compared to that of the PLLA/PDLA blend is attributable to the segmental connection between the PLLA and PDLA chains, which interrupted the free movement of those chains of the PLLA-b-PDLA copolymers during crystallization. The crystallite growth mechanism of the PLLA-b-PDLA copolymers was different from that of the PLLA/PDLA blend.     

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     

Non‐Isothermal Crystallization Behavior of Poly(L‐lactic acid) in the Presence of Various Additives

Summary: The effects of various additives: poly(D ‐lactic acid) (PDLA), talc, fullerene C₆₀, montmorillonite, and various polysaccharides, on the non‐isothermal crystallization behavior of poly(L ‐lactic acid) (PLLA), during both the heating of melt‐quenched films from room temperature, and the cooling of as‐cast films from the melt, was investigated. When the melt‐quenched PLLA films were heated from room temperature, the overall PLLA crystallization was accelerated upon addition of PDLA or the stereocomplex crystallites formed between PDLA and PLLA, the mixtures containing PDLA, and the mixture of talc and montmorillonite. No significant effects on the overall PLLA crystallization were observed for talc, C₆₀, montmorillonite, and the mixtures containing C₆₀. Such rapid completion of the overall PLLA crystallization upon addition of the aforementioned additives can be ascribed to the increased density (number per unit volume or area) of PLLA spherulites. When the as‐cast PLLA films were cooled from the melt, the overall PLLA crystallization completed rapidly, upon addition of PDLA, talc, C₆₀, montmorillonite, and their mixtures. Such rapid overall PLLA crystallization is attributable to the increased density of the PLLA spherulites and the higher nucleation temperature for PLLA crystallization. In contrast, the addition of various polysaccharides has no significant effect, or only a very small effect, on the overall PLLA crystallization during heating from room temperature or during cooling from the melt. This finding means that the polysaccharides can be utilized as low‐cost fillers for PLLA‐based materials, without disturbing the crystallization of the PLLA. The effect of additives in accelerating the overall PLLA crystallization during cooling from the melt, decreased in the following order: PDLA > talc > C₆₀ > montmorillonite > polysaccharides.

Polarization optical photomicrographs of pure PLLA, and the PLLA‐F film, with the fullerene additive, during cooling from the melt (Process IIB). Both of the photomicrographs were taken at 120 °C.     

Crystallization of poly(3‐hydroxybutyrate) with stereocomplexed polylactide as biodegradable nucleation agent

Crystallization kinetics behavior and morphology of poly(3‐hydroxybutyrate) (PHB) blended with of 2–10 wt% loadings of poly(L ‐ and D ‐lactic acid) (PLLA and PDLA) stereocomplex crystallites, as biodegradable nucleating agents, were studied using differential scanning calorimetry, polarizing‐light optical microscopy (POM), and wide‐angle X‐ray diffraction (WAXD). Blending PLLA with PDLA at 1:1 weight ratio led to formation of stereocomplexed PLA (sc‐PLA), which was incorporated as small crystalline nuclei into PHB for investigating melt‐crystallization kinetics. The Avrami equation was used to analyze the isothermal crystallization of PHB. The stereocomplexed crystallites acted as nucleation sites in blends and accelerated the crystallization rates of PHB by increasing the crystallization rate constant k and decreasing the half‐time (t_1/2). The PHB crystallization was nucleated most effectively with 10 wt% stereocomplexed crystallites, as evidenced byPOM results. The sc‐PLA complexes (nucleated PHB crystals) exhibit much small spherulite sizes but possess the same crystal cell morphology as that of neat PHB based on the WAXD result. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Highly Enhanced Accelerating Effect of Melt‐Recrystallized Stereocomplex Crystallites on Poly(L‐lactic acid) Crystallization, 2–Effects of Poly(D‐lactic acid) Concentration

The effects of varying concentrations of incorporated PDLA on the acceleration of PLLA homo‐crystallization due to stereocomplex (SC) crystallite formation are investigated in PLLA films doped with PDLA over the wide concentration range of 1–10 wt%. PLLA homo‐crystallization is accelerated for all the PDLA concentrations when the processing temperature T_p is just above the endset melting temperature of the SC crystallites (T_p = 226–238 °C), although the appropriate T_p range becomes narrow at low concentrations of PDLA. The accelerating effects of SC crystallites depend on the SC crystalline thickness and the interaction between the SC crystalline regions and PLLA amorphous regions for T_ps below and above the melting peak temperature of the SC crystallites, respectively.

     

Lamella reorientation in thin films of a symmetric poly(l-lactic acid)-block-polystyrene upon crystallization at different temperatures

The thin films of a symmetric crystalline-coil diblock copolymer of poly(l-lactic acid) and polystyrene (PLLA-b-PS) formed lamellae parallel to the substrate surface in melt. When annealed at temperatures well above the glass transition temperature of PLLA block (T_g^PLLA), the PLLA chains started to crystallize, leading to reorientation of lamellae. Such reorientation behavior exhibited dependence on the correlation between the crystallization temperature (T_c), the glass transition temperature of PS (T_g^PS), the peak melting point of PLLA crystals (T_m^PLLA), and the end melting point of PLLA crystals (T_m,end^PLLA). When annealed at (T_c=) 80 °C (T_c < T_g^PS < T_ODT, order-disorder transition temperature), 123 °C (T_g^PS < T_c < T_m^PLLA < T_ODT), 165 °C (T_g^PS < T_m^PLLA < T_c < T_m,end^PLLA < T_ODT), the parallel lamellae became perpendicular to the substrate surface, exclusively starting at the edge of surface relief patterns. Meanwhile, the corresponding lamellar spacing was significantly enhanced. The PLLA crystallization between PS layers was hypothesized to account for the lamella reorientation during annealing. The crystallization, chain conformation, and possible chain folding mechanisms were discussed, based on detailed analysis of the lamellar structure before and after crystallization.     

Highly enhanced accelerating effect of melt‐recrystallized stereocomplex crystallites on poly(L‐lactic acid) crystallization: effects of molecular weight of poly(D‐lactic acid)

The effects of the molecular weight of poly(D ‐lactic acid) (PDLA), which forms stereocomplex (SC) crystallites with poly(L ‐lactic acid) (PLLA), and those of processing temperature T_p on the acceleration (or nucleation) of PLLA homocrystallization were investigated using PLLA films containing 10 wt% PDLA with number‐average molecular weight (M_n) values of 5.47 × 10⁵, 9.67 × 10⁴ and 3.67 × 10⁴ g mol^–1 (PDLA‐H, PDLA‐M and PDLA‐L, respectively). For the PLLA/PDLA‐H and PLLA/PDLA‐M films, the SC crystallites that were ‘non’‐melted and those that were ‘completely’ melted at T_p values just above their endset melting temperature and recrystallized during cooling were found to act as effective accelerating (or nucleation) agents for PLLA homocrystallization. In contrast, SC crystallites formed from PDLA‐L, having the lowest M_n, were effective accelerating agents without any restrictions on T_p. In this case, the accelerating effects can be attributed to the plasticizer effect of PDLA‐L with the lowest M_n. The accelerating effects of SC crystallites in the PLLA/PDLA‐H and PLLA/PDLA‐M films was dependent on crystalline thickness for T_p values below the melting peak temperature of SC crystallites, whereas for T_p values above the melting peak temperature the accelerating effects are suggested to be affected by the interaction between the SC crystalline regions and PLLA amorphous regions.     

Crystallization, spherulite growth, and structure of blends of crystalline and amorphous poly(lactide)s

The effects of incorporated amorphous poly(dl-lactide) (PDLLA) on the isothermal crystallization and spherulite growth of crystalline poly(l-lactide) (PLLA) and the structure of the PLLA/PDLLA blends were investigated in the crystallization temperature (T_c) range of 90-150 °C. The differential scanning calorimetry results indicated that PLLA and PDLLA were phase-separated during crystallization. The small-angle X-ray scattering results revealed that for T_c of 130 °C, the long period associated with the lamellae stacks and the mean lamellar thickness values of pure PLLA and PLLA/PDLLA blend films did not depend on the PDLLA content. This finding is indicative of the fact that the coexisting PDLLA should have been excluded from the PLLA lamellae and inter-lamella regions during crystallization. The decrease in the spherulite growth rate and the increase in the disorder of spherulite morphology with an increase in PDLLA content strongly suggest that the presence of a very small amount of PDLLA chains in PLLA-rich phase disturbed the diffusion of PLLA chains to the growth sites of crystallites and the lamella orientation. However, the wide-angle X-ray scattering analysis indicated that the crystalline form of PLLA remained unvaried in the presence of PDLLA.     

Synthesis of branched poly(lactide) using polyglycidol and thermal, mechanical properties of its solution-cast film

Branched poly(lactide)(PLA)s with various lengths of graft chain were synthesized by ring-opening polymerization of l- or d-lactide (l- or d-LA) in bulk using polyglycidol as a macroinitiator. The properties of polymer films of branched PLLA or PDLA obtained and their stereocomplex were investigated through thermal analysis and tensile testing. The branched PLLA or PDLA film exhibited a lower glass transition temperature (T_g), melting temperature (T_m), crystallinity, Young's modulus and a higher strain at break than the corresponding linear PLLA or PDLA film. The branched PLLA/branched PDLA stereocomplex film showed a high maximum stress and a high Young's modulus keeping its high strain at break. Moreover, the usefulness of branched PLLA or PDLA as a plasticizer of linear PLLA was investigated with 1:9 blend or stereocomplex film prepared from the branched PLLA or branched PDLA and linear PLLA. The blend or linear PLLA/branched PDLA stereocomplex film showed a higher strain at break compared with linear PLLA film. The mechanical properties of the blend or linear PLLA/branched PDLA stereocomplex film could easily be controlled by changing the molecular weight of branched PLA.     

The electrical properties and crystallization of stereocomplex poly(lactic acid) filled with carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) filled poly(l-lactic acid) (PLLA) and PLLA/poly(d-lactic acid) (PDLA) composites were prepared through a directly melt mixing process. A special crystalline structure of stereocomplex was formed by PLLA and PDLA, which was easily found when mixing two polymers with identical chemical composition but different steric structures. The electrical conductivities were greatly improved by the formation of stereocomplex compared to that of PLLA/MWCNT composites at same MWCNT content. The percolation threshold of the PLLA/PDLA/MWCNT composite at a PLLA/PDLA weight ratio of 50/50 was 0.35 wt%, while being 1.43 wt% of PLLA/MWCNT composites. The X-ray diffraction, non-isothermal and isothermal crystallization results showed that the formation of stereocomplex greatly increased the crystallinity of the composites, meanwhile MWCNTs acted as heterogeneous nucleating agent, which significantly accelerated the nucleation and spherulite growth. Therefore, the PLLA/PDLA/MWCNT composites have a very low percolation threshold due to the volume exclusion effect.     

Melting behavior of poly(l-lactic acid): X-ray and DSC analyses of the melting process

To clarify the melting behavior of poly(l-lactic acid) (PLLA), the wide-angle X-ray diffraction patterns of the isothermally crystallized PLLA samples (ICSs) were successively obtained during heating. We have already suggested the discrete change in the crystallization behavior of PLLA at a crystallization temperature (T_c) of 113 °C (= T_b) and formation of two crystal modifications for the ICSs obtained in the temperature range T_c ≤ T_b and T_c ≥ T_b. It was elucidated from the change in the X-ray diffraction pattern that the phase transition from the low-temperature crystal modification (α′-form) to the high-temperature one (α-form) occurred in a range 155-165 °C for the ICSs(T_c ≤ T_b), and that the crystal structure for the ICSs(T_c ≥ T_b) did not change. Recrystallization during heating, which is the origin of the multiple melting behavior, was proved by the increase in the diffraction intensity before steep decrease due to the final melting. A temperature derivative curve of the X-ray diffraction intensity almost coincided with the DSC melting curve.     

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     

Spherulite growth of l-lactide copolymers: Effects of tacticity and comonomers

The spherulite growth behavior and mechanism of l-lactide copolymers, poly(l-lactide-co-d-lactide) [P(LLA-DLA)], poly(l-lactide-co-glycolide) [P(LLA-GA)], and poly(l-lactide-co-ε-caprolactone) [P(LLA-CL)] have been studied using polarization optical microscopy in comparison with poly(l-lactide) (PLLA) having different molecular weights to elucidate the effects of incorporated comonomer units. The incorporation of comonomer units reduced the radius growth rate of spherulites (G) and increased the induction period of spherulite formation (t_i), irrespective of the kind of comonomer unit. Such effects became remarkable with the content of comonomers. At a crystallization temperature (T_c) of 130 °C, the disturbance effects of comonomers on the spherulite growth decreased in the following order: d-lactide>glycolide>ε-caprolactone, when compared at the same comonomer unit or reciprocal of averaged l-lactyl unit sequence length (l_l). The t_i estimation indicated that the glycolide units have the lowest disturbance effects on the formation of spherulite (crystallite) nuclei. The PLLA having the number-average molecular weight (M_n) exceeding 3.1×10⁴ g mol⁻¹ showed the transition from regime II to regime III at T_c=120 °C, whereas PLLA with the lowest M_n of 9.2×10³ g mol⁻¹ crystallized solely in regime III kinetics and the copolymers excluding P(LLA-DLA) with 3% of d-lactide units crystallized solely according to regime II kinetics. The nucleation and front constant for regime II and III [K_g(II), K_g(III), G₀(II), and G₀(III), respectively] estimated with each (not with a fixed for high-molecular-weight PLLA) decreased with increasing the amount of defects per unit mass of the polymer for crystallization, i.e. with increasing the comonomer content and the density of terminal group through decreasing the molecular weight.     

Structure and properties of highly stereoregular isotactic poly(methyl methacrylate) and syndiotactic poly(methyl methacrylate) blends treated with supercritical CO2

The structure and properties of highly stereoregular isotactic poly(methyl methacrylate) (it-PMMA) and syndiotactic poly(methyl methacrylate) (st-PMMA) blends with crystalline stereocomplex formed by supercritical CO₂ treatment at temperatures ranging from 35 to 130 °C were investigated by means of differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and dynamic mechanical analysis (DMA) measurements. The melting temperature, T_m, and the heat of fusion, ΔH_m, had maximum values at about 200 °C and 25 J/g, respectively. The degree of crystallinity evaluated by WAXD ranged in value from 32 to 38%. The fringed-micellar stereocomplex crystallites were formed in case of treatment temperatures below 90 °C, and the orderliness perpendicular to the helix axis of the fringed-micellar crystallites was considered to be increased with increasing treatment temperature. In case of treatment temperature of 130 °C, the fringed-micellar crystallites and the lamellar crystallites with high orderliness parallel to the helix axis coupled with the perpendicular orderliness were formed, and the respective double endothermic peaks, T_m¹ and T_m³, were observed in DSC due to the melting of the two kinds of stereocomplex crystallites. The it-PMMA/st-PMMA blends containing the fringed-micellar crystallites maintained high values of storage modulus, E′, up to higher temperature compared with the amorphous blends. The E′ of the blend treated with CO₂ at 130 °C decreased twice at temperatures corresponding to T_m¹ and T_m³.     

Improvements of thermal property and crystallization behavior of PLLA based multiblock copolymer by forming stereocomplex with PDLA oligomer

Although poly(l-lactic acid) (PLLA) can be greatly toughened by copolymerization, its lower melting temperature and lower ability of crystallization limit its widespread application as commodity. In order to improve the melting point and ability of crystallization of PLLA based multiblock copolymers, Poly (d-lactic acid) (PDLA) oligomer was used to complex with PLLA-bisphenol A epoxy resin multiblock copolymer (PLLA-co-bis A) to form a stereocomplex. Differential scanning calorimeter (DSC), X-ray diffraction (XRD) and polarized optical microscopy (POM) were used to characterize the thermal properties and crystallization behavior of the stereocomplexes. The results indicated that the stereocomplex of PLLA-co-bis A and PDLA was formed. The formed stereocomplexes with good thermal properties (high T_m) and good crystallization properties (high crystallization rate and more stable crystals) are convinced to have high potential as high performance biodegradable polymers.