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.     

Study on crystalline morphology of poly(l-lactide)-poly(ethylene glycol) diblock copolymer

Crystallization behavior, structural development and morphology evolution in a series of diblock copolymers of poly(l-lactide)-block-poly(ethylene glycol) (PLLA-b-PEG) were investigated via differential scanning calorimetry, wide-angle X-ray diffraction, polarized optical microscopy and atomic force microscopy. In these copolymers, both blocks are crystallizable and biocompatible. It was interesting that these PLLA-b-PEG diblock copolymers could form spherulites with banded textures, which was undercooling dependent. Single crystals with an abundance of screw dislocations were also observed via AFM. Such results indicated that these ringed spherulites and single crystals were formed during the crystallization of the PLLA blocks.     

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.     

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.     

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.     

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.     

Unique crystallization behavior of poly(l-lactide)/poly(d-lactide) stereocomplex depending on initial melt states

A unique crystallization behavior of poly(l-lactide) (PLLA)/poly(d-lactide) (PDLA) stereocomplex was observed when a PLLA/PDLA blend (50/50) was subjected to specific melting conditions. PLLA and PDLA were synthesized by ring opening polymerization of l- or d-lactide using zinc lactate as catalyst. PLLA/PDLA blend was prepared through solution mixing followed by vacuum drying. The blend was melted under various melting conditions and subsequent crystallization behaviors were analyzed by using DSC, XRD, NMR and ESEM. Stereocomplex was exclusively formed from the 50/50 blend of PLLA and PDLA with relatively low molecular weights. Surprisingly, stereocomplex crystallization was distinctly depressed when higher melting temperature and longer melting period were applied, in contrast to homopolymer crystallization. Considering predominant interactions between PLLA and PDLA chains, a novel model of melting process is proposed to illustrate this behavior. It is assumed that PLLA and PDLA chain couples would preserve their interactions (melt memory) when the stereocomplex crystal melts smoothly, thus resulting in a heterogeneous melt which can easily crystallize. The melt could gradually become homogeneous at higher temperature or longer melting time. The strong interactions between PLLA and PDLA chain segments are randomly distributed in a homogeneous melt, thus preventing subsequent stereocomplex crystallization. However, the homogeneous melt can recover its ability to crystallize via dissolution in a solvent.     

Single crystals morphology of biodegradable double crystalline PLLA-b-PCL diblock copolymers

The morphology of solution grown single crystals of a series of double crystalline diblock copolymers derived from l-lactide and ?-caprolactone has been investigated by means of transmission electron microscopy. The copolymers had a variable composition with a poly(l-lactide) weight percentage that ranged between 81 and 10%. All samples had a low polydispersity index (1.4-1.1) and a similar number average molecular weight (20,000-35,000 g/mol).Bulk crystallization and melting behaviour of diblock copolymers were evaluated by DSC and the results demonstrated the double crystalline nature of the samples. Fractionated crystallization clearly occurred in copolymers having an intermediate composition.Isothermal crystallizations were performed in dilute n-hexanol solutions at temperatures that ranged between 80 and 50 °C. Crystal morphologies were dependent on the crystallization temperature and even on the composition. Thus, the inability of poly(?-caprolactone) (PCL) blocks to crystallize between 80 and 70 °C rendered lozenge, truncated and spindle-shaped crystals associated to the poly(l-lactide) (PLLA) block. These usually had thicker edges due to PLLA overgrowths that mainly took place in their periphery. However, an overgrowth of irregular PCL crystals during subsequent cooling and crystallization at room temperature was also detected. Complex morphologies constituted by lamellar crystals of both PCL and PLLA blocks were developed at intermediate temperatures (70-65 °C), whereas elongated hexagonal morphologies mainly associated to the PCL block were detected at the lowest crystallization temperature. In general, electron diffraction patterns showed for all samples’ reflections associated to both poly(?-caprolactone) and poly(l-lactide) (α-form) crystals. The relative intensity between the two types of reflections varied according to the copolymer composition.     

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     

Crystallization of polylactide films: An atomic force microscopy study of the effects of temperature and blending

Surface crystallinity on films of poly(l-lactide), poly(l/d-lactide) and their blends with poly(d-lactide) was studied. The isothermal spherulitic growth rate and its dependence on temperature were studied using tapping mode atomic force microscopy and ex situ isothermal crystallization. Using this technique, it is possible to extend spherulitic growth rate measurements to the region of significantly higher supercooling where nucleation concentration makes the use of in situ hot stage optical microscopy impossible. It was confirmed that while a poly(l/d-lactide) copolymer exhibits the typical “bell” shaped crystallization rate–temperature dependence, poly(l-lactide) exhibits a nonsymmetrical behavior having two crystallization rate maxima at 105 °C and 130 °C. As expected, the spherulitic growth rate of poly(l-lactide) was significantly higher than that of poly(l/d-lactide). The different types of crystalline formations exhibited at the surface of polylactide films are shown and discussed. The crystalline long spacing of poly(l-lactide) was also measured directly using tapping mode AFM and was found to be 19 nm at 165–170 °C. At low supercooling, several different scenarios of individual crystal formation were observed: purely flat-on stacks, purely edge-on stacks and scenarios where edge-on crystals flip to flat-on crystals and vice versa, where flat-on crystals yield edge-on sprouts. The preferred direction of growth of lamellae of both poly(l-lactide) and poly(d-lactide) was found to be counter-clockwise relative to the free surface.Finally, the crystallization kinetics of blends of poly(l-lactide) and poly(l/d-lactide) with poly(d-lactide) were studied. In such blends a triclinic stereocomplex crystalline structure forms between chains of opposite chirality and a pseudo-orthorhombic α-crystal structure forms between chains of like chirality. The presence of the stereocomplex crystals affects both the nucleation and the growth of the α-crystals. In fact depending on the stereocomplex content and the crystallization temperature the α-crystallization can either be enhanced or be inhibited. Interestingly it was found that the presence of the stereocomplex had a much stronger effect on the α-crystallization of poly(l/d-lactide) than on the α-crystallization of poly(l-lactide).     

Crystallisation and spherulite morphology of polylactide stereocomplex

In this study we investigated the crystallisation behaviours of stereocomplex crystals in poly(l ‐lactic acid)/poly(d ‐lactic acid) ( PLLA/PDLA) blends (LD blends) of various weight ratios. The crystallisation and melting behaviours were studied using DSC, and the crystal structure was analysed through wide‐angle X‐ray diffraction. The morphology of homocrystals and stereocomplex crystals in the blends was examined using a hot‐stage polarising microscope and a scanning electron microscope. The DSC results showed that homocrystals and stereocomplex crystals were present in all LD blends except that with 50 wt% PLLA/50 wt% PDLA; in this blend, only stereocomplex crystals were present. The regime II → III transition temperature of stereocomplex crystals in a Lauritzen–Hoffman plot of the LD blends was determined to be 165 °C. A concentric spherulite consisting of stereocomplex crystals and homocrystals formed under two‐step isothermal crystallisation conditions with three growth stages was observed. The confined spherulitic growth rate in the concentric spherulite and the unrestricted spherulitic growth rate in individual spherulites were also analysed. © 2018 Society of Chemical Industry     

Crystallization of poly(l-lactide-dimethyl siloxane-l-lactide) triblock copolymers and its effect on morphology of microphase separation

This investigation characterizes the molten morphologies following isothermal crystallization of poly(l-lactide-block-dimethyl siloxane-block-l-lactide) triblock copolymers, which were synthesized by ring-opening polymerization of l-lactide using hydroxyl-telechelic PDMS as macroinitiators, via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The break-out and preservation of the nanostructure of the triblock copolymer depended on the segregation strength, which was manipulated by varying the degree of polymerization. The crystallization kinetics of these semicrystalline copolymers and the effect of isothermal crystallization on their melting behaviors were also studied using DSC, FT-IR and WAXS. The exclusive presence of α-phase PLLA crystallite was verified by identifying the absence of the WAXS diffraction signal at 2θ = 24.5° and the presence of IR absorption at 1749 cm⁻¹ when the PLLA segment of the block copolymers was present as a minor component. The dependence of the crystallization rate (Rc) on the chemical composition of the triblock copolymers reveals that the Rc of the triblock copolymers was lower than that of PLLA homopolymer and the Rc were substantially reduced when the minor component of the crystallizable PLLA domains was dispersed in the PDMS matrix.     

Morphologies and structures in poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactide-<Emphasis Type="Italic">b</Emphasis>-ethylene oxide) copolymers determined by crystallization,microphase separation,and vitrification

Morphologies and structures determined by crystallization of the blocks, microphase separation of the copolymers, and vitrification of PLLA block in poly(l-lactide-b-ethylene oxide) (PLLA-b-PEO) copolymers were investigated using microscopic techniques and synchrotron small angle X-ray scattering. The PLLA-b-PEO copolymer films were crystallized from two different annealing processes: melt crystallization (process A) or crystallized from glass state of PLLA block after quenching from melt state (process B). The relationship between the crystalline morphology and microstructure of the copolymers were explored using SAXS. The morphology and phase structure are predominated by crystallization of PLLA block, and greatly influenced by microphase separation of the copolymers. In process B, lozenge-shape and truncated lozenge-shaped PLLA crystals of nanometer scale can be observed. The crystalline morphology is markedly affected by the microstructure formed during the annealing process. Star-shaped morphologies stacked with PLLA single crystals were observed.     

Effect of poly(l‐lactide)/poly(d‐lactide) block length ratio on crystallization behavior of star‐shaped asymmetric poly(l(d)‐lactide) stereoblock copolymers

Effect of Poly(l ‐lactide)/Poly(d ‐lactide) (PLLA/PDLA) block length ratio on the crystallization behavior of star‐shaped poly(propylene oxide) block poly(d ‐lactide) block poly (l ‐lactide) (PPO–PDLA–PLLA) stereoblock copolymers with molecular weights (M_n) ranging from 6.2 × 10⁴ to 1.4 × 10⁵ g mol^?1 was investigated. Crystallization behaviors were studied utilizing differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide‐angle X‐ray diffraction (WAXD). Only stereocomplex crystallites formed in isothermal crystallization at 140 to 156°C for all samples. On one hand, the overall crystallization rate decreased as PLLA/PDLA block length ratio increased. As PLLA/PDLA block length ratio increased from 7:7 to 28:7, the value of half time of crystallization (t_1/2) delayed form 2.85 to 5.31 min at 140°C. On the other hand, according to the Lauritzen–Hoffman theory, the fold‐surface energy (σ_e) was calculated. σ_e decreased from 77.7 to 73.3 erg/cm² with an increase in PLLA/PDLA block length ratio. Correspondingly increase in nucleation density was observed by the polarized optical microscope. Results indicated that the PLLA/PDLA block length ratio had a significant impact on the crystallization behavior of PPO–PDLA–PLLA copolymers. POLYM. ENG. SCI., 55:2534–2541, 2015. © 2015 Society of Plastics Engineers     

Influence of Temperature and Time on Isothermal Crystallization of Poly-L-lactide

The thermal behavior of poly-L-lactide (PLLA) isothermal crystallization upon cooling from the melt was investigated using differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD) and polarizing microscope (POM) by changing the crystallization temperature and time. It was indicated that 110°C should be a critical temperature for PLLA melting crystallization. The melting point of crystallized PLLA discontinuously changed with crystallization temperature, increased with temperature, but decreased at about 110°C, and thereafter again increased with higher crystallization temperatures. At 110°C a multiple endothermic peak was observed. PLLA crystals of higher perfection form when crystallized under higher temperature, which reflects the effects of high chain mobility in higher temperatures. During isothermal crystallization, PLLA crystallites become increasingly perfect, and thicken with prolonged time, leading to an increasing melting point.     

Crystallization and structure formation of poly(l-lactide-co-meso-lactide) random copolymers: a time-resolved wide- and small-angle X-ray scattering study

Time-resolved synchrotron simultaneous wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) experiments were used to investigate the crystallization behavior and microstructure development of poly(l-lactide), PLLA, and two random copolymers containing l-lactide (predominately) and randomly placed R stereochemical defects. The general features of crystallization of PLLA and the copolymers are similar except that the copolymers crystallize much more slowly and to a lesser extent than PLLA, as expected. Lamellar thicknesses derived from SAXS experiments are in very good agreement with mean thicknesses determined in a tapping mode AFM study of the same materials. The reduction in lamellar thickness and crystallinity with increasing meso-lactide content supports significant exclusion of the R stereoisomer from crystalline lamellae. In a separate series of time-resolved WAXD/SAXS experiments, each (co)polymer was crystallized for a fixed time, then heated to above its melting point. The observed behavior suggests a model for crystallization of the copolymers in which thinner lamellae form between ‘primary’ lamellar stacks during crystallization, with an average lamellar thickness that decreases with increasing R stereoisomer content.     

Synthesis and characterization of amphiphilic PLA-(PαN3CL-g-PBA) copolymers by ring-opening polymerization and click reaction

The present study prepared novel amphiphilic block-graft PDLLA-b-(PαN₃CL-g-PBA) and PLLA-b-(PαN₃CL-g-PBA) functional polyesters, containing a hydrophilic poly(α-azido-ε-caprolactone-graft-alkyne) (PαN₃CL-g-alkyne) segment and a hydrophobic poly(dl-lactide) (PDLLA) or poly(l-lactide) (PLLA) segment, using ring-opening polymerization of α-chloro-ε-caprolactone (αClCL) with a hydroxyl-terminated macroinitiator of PDLLA or PLLA, substituting pendent chloride with sodium azide. The copolymers were subsequently used for grafting of 2-propynyl-terminal benzoate moieties by way of Cu(I)-catalyzed Huisgen's 1,3-dipolar cycloaddition, thus producing a “click” reaction. Differential scanning calorimetry (DSC) and ¹H NMR, FT-IR, and GPC examined the characteristics of the copolymers. The critical micelle concentration (CMC) ranged from 2.7 mg L^?1 to 24.6 mg L^?1 at 25 °C and the average micelle size ranged from 106 nm to 297 nm. The length of the hydrophilic segment and the configuration of the lactide both influenced micelle stability. The micelle of PLLA-b-(PαN₃CL-g-PBA) provided high drug entrapment efficiency and loading content. The results from in vitro cell viability assays indicated that PLA-b-(PαN₃CL-g-PBA) shows low cytotoxicity.     

Highly accelerated stereocomplex crystallization by blending star-shaped 4-armed stereo diblock poly(lactide)s with poly(d-lactide) and poly(l-lactide) cores

Star-shaped 4-armed stereo diblock poly(lactide)s with the core/shell types of poly(d-lactide) (PDLA)/poly(l-lactide) (PLLA) and PLLA/PDLA (abbreviated as 4-DL and 4-LD, respectively) and the number-average molecular weights of about 1 × 10⁴ g mol⁻¹ were synthesized and the crystallization behavior of neat 4-DL, 4-LD, and their 50/50 blend (abbreviated as 4-DL/4-LD blend) was investigated. Solely stereocomplex (SC) crystallites as crystalline species were formed in the neat 4-DL, 4-LD, and 4-DL/4-LD blend, irrespective of crystallization temperature (100–160 °C). The overall SC crystallization of 4-DL/4-LD blend was highly accelerated compared with that of neat 4-DL and 4-LD, due to the largely elevated spherulite nuclei number per unit mass in the blend. Such high density of nuclei formation in 4-DL/4-LD blend is attributable to the facile intermolecular interaction and subsequent SC nucleation between the PLLA shell of 4-DL and the PDLA shell of 4-LD. The blending method reported in the present study is applicable for various core/shell types of star-shaped stereo diblock stereocomplexationable polymers to accelerate overall SC crystallization and can counterbalance the lowered crystallization rate caused by the star-shaped architecture. Despite the highly accelerated overall SC crystallization of 4-DL/4-LD blend by blending 4-DL and 4-LD, the spherulite growth rate, induction period for spherulite growth, final crystallinity, crystalline species, growth morphology, and crystallization mechanism were not altered by blending 4-DL and 4-LD.     

Effect of nucleation and plasticization on the stereocomplex formation between enantiomeric poly(lactic acid)s

The effect of nucleation and plasticization on the stereocomplex formation between poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) was investigated in blends where PDLA is added as a minor phase in a major phase of PLLA. The use of small amounts of PDLA is aimed at creating a high melting point stereocomplex phase that in turn can serve as nucleating agent for the major phase of PLLA. Blends containing 5% PDLA with talc or organic phosphonate as nucleants and polyethylene glycol as plasticizer were prepared via melt-blending. Their crystallization behavior was investigated through Differential Scanning Calorimetry (DSC) using various thermal histories. Two peculiar stereocomplex melting endotherms were found. The peak temperature and enthalpy of these two endotherms were correlated to prior isothermal crystallization temperature. The different endotherms were also associated with two different crystalline morphologies observed by optical microscopy and referred to as Network and Spherulitic morphologies. The influence of plasticization and of heterogeneous nucleation on these morphologies was investigated through optical microscopy and calorimetric observations.