The effects of incorporated poly(d-lactic acid) (PDLA) as poly(lactic acid) (PLA) stereocomplex crystallites on the isothermal and non-isothermal crystallization behavior of poly(l-lactic acid) (PLLA) from the melt were investigated for a wide PDLA contents from 0.1 to 10 wt%. In isothermal crystallization from the melt, the radius growth rate of PLLA spherulites (crystallization temperature (Tc)≥125 °C), the induction period for PLLA spherulite formation (ti) (Tc≥125 °C), the growth mechanism of PLLA crystallites (90 °C≤Tc≤150 °C), and the mechanical properties of the PLLA films were not affected by the incorporation of PDLA or the presence of stereocomplex crystallites as a nucleating agent. In contrast, the presence of stereocomplex crystallites significantly increased the number of PLLA spherulites per unit area or volume. In isothermal crystallization from the melt, at PDLA content of 10 wt%, the starting, half, and ending times for overall PLLA crystallization (tc(S), tc(1/2), and tc(E), respectively) were much shorter than those at PDLA content of 0 wt%, due to the increased number of PLLA spherulites. Reversely, at PDLA content of 0.1 wt%, the tc(S), tc(1/2), and tc(E) were longer than or similar to those at PDLA content of 0 wt%, probably due to the long ti and the decreased number of spherulites. This seems to have been caused by free PDLA chains, which did not form stereocomplex crystallites. On the other hand, at PDLA contents of 0.3-3 wt%, the tc(S), tc(1/2), and tc(E) were shorter than or similar to those at PDLA content of 0 wt% for the Tc range below 95 °C and above 125 °C, whereas this inclination was reversed for the Tc range of 100-120 °C. In the non-isothermal crystallization of as-cast or amorphous-made PLLA films during cooling from the melt, the addition of PDLA above 1 wt% was effective to accelerate overall PLLA crystallization. The X-ray diffractometry could trace the formation of stereocomplex crystallites in the melt-quenched PLLA films at PDLA contents above 1 wt%. This study revealed that the addition of small amounts of PDLA is effective to accelerate overall PLLA crystallization when the PDLA content and crystallization conditions are scrupulously selected. 相似文献
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 Tp on the acceleration (or nucleation) of PLLA homocrystallization were investigated using PLLA films containing 10 wt% PDLA with number‐average molecular weight (Mn) 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 Tp 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 Mn, were effective accelerating agents without any restrictions on Tp. In this case, the accelerating effects can be attributed to the plasticizer effect of PDLA‐L with the lowest Mn. The accelerating effects of SC crystallites in the PLLA/PDLA‐H and PLLA/PDLA‐M films was dependent on crystalline thickness for Tp values below the melting peak temperature of SC crystallites, whereas for Tp 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. 相似文献
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. 相似文献
Crystallization of nonequimolar compositions of poly(d-lactic acid) with low-molecular-weight poly(l-lactic acid) (PDLA/LMw-PLLA) blends leads to formation of various fractions of stereocomplexed PLA (sc-crystallites) and homocrystallites (PDLA or PLLA). For the PDLA/LMw-PLLA blends within the composition window of LMw-PLLA content between 30 and 50 wt%, only sc-crystal exists and no homocrystal is present. On the other hand, for PDLA/LMw-PLLA blends with excess PDLA, e.g. PDLA/LMw-PLLA = 90/10, atomic-force microscopy (AFM) characterization on various stages of crystallization of sc-PLA crystal with PDLA homocrystal shows a repetitive stacking of excess PDLA on pre-formed sc-PLA crystal serving as crystallizing templates. The crystallization initially begins with string-like (fibril-like) PDLA lamellae, followed with PDLA aggregating on sc-PLA crystal into a bead-on-string crystal, then growing to thicker irregularly-shaped dough-like lamellae. Repetitive growth cycle from strings to bead-on-string lamellae continues on top of the dough-like lamellae as new substrates, until ending impingement of the PDLA spherulites. 相似文献
PLLA and stereocomplexed polylactide (sc‐PLA) nanofibers were formed by electrospinning solutions of the polymers in HFIP. A highly semi‐crystalline sc‐PLA nanofiber having only sc crystallites was confirmed by WAXD analysis. The diameters of the nanofibers of both polymers decreased slightly when they were annealed at 60 °C, which was near Tg. Enzyme degradation of both as‐spun PLLA and sc‐PLA nanofibers by proteinase K from Tritirachium album was carried out. The rate of degradation of the nanofibers can be controlled by varying annealing conditions, hence the extent of crystallinity.
Effects of poly(3-hydroxybutyrate) (PHB) on crystalline morphology of stereocomplexing capacity of poly(L- and D-lactic acid) (PLLA and PDLA) were studied by differential scanning calorimetry (DSC), polarizing-light optical microscopy (POM), atomic-force microscopy (AFM) and wide-angle X-ray diffraction (WAXD). When crystallized at high Tc (130 °C or above), morphology transition of stereocomplexed PLA (sc-PLA) occurs from original well-rounded Maltese-cross spherulites to dendritic form in blends of high PHB contents (50 wt.% or higher), where PHB acts as an amorphous species. Microscopy characterizations show that morphology of sc-PLA in PHB/sc-PLA blends crystallized at Tc = 170 °C no longer retain original complexed Maltese-cross well-rounded spherulites; instead, the spherulites are disintegrated and restructured into two types of dendrites: (1) edge-on feather-like dendrites (early growth) and (2) flat-on wedge-like crystal plates (later growth) by growing along different directions and exhibiting different optical brightness. The concentration and/or distribution of amorphous PHB at the crystal growth front, corresponding to variation of the slopes of spherulitic growth rates, is a factor resulting in alteration and restructuring of the sc-PLA spherulites in the blends. Despite of spherulite disintegration, WAXD result shows that these two PHB-induced dendrites still retain the original unit cells of complexes, and thus these two new dendrites are sc-PLA. 相似文献
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 Tm 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. 相似文献
Effect of the addition of poly(D-lactic acid) (PDLA) as stereocomplex (SC) on crystallization behavior of poly(L-lactic acid) (PLLA) had been systemically investigated. The result indicated that the inclusion of PDLA with higher MW into PLLA exhibited lower t1/2 and showed accelerated crystallization behavior. Meanwhile, SC formed in blends with higher MW of PDLA exhibited enhanced nucleation activity. In combination with both DSC and WAXD analysis, it was believed that nucleation process was more related to the crystalline size of SC. The result in this study would provide guidance for the application of SC as nucleating agent for the PLA-based products. 相似文献
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 (Mn) 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 (Mn = 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 Mn of PDLA segments as low as 2.0 × 103 and 3.5 × 103 g mol?1, respectively. With decreasing Mn 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 (ΔHm) of SC crystallites during heating and the crystallinity (Xc) of SC crystallites after slow cooling increased, whereas ΔHm of PLLA homo‐crystallites during heating and Xc of PLLA homo‐crystallites after slow cooling decreased. The total ΔHm of SC crystallites and PLLA homo‐crystallites during heating and the total Xc 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 ΔHm and Xc at d ‐lactyl unit content of 10–15% are attributable to the formation of two crystalline species of SC crystallites and PLLA homo‐crystallites. 相似文献
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. 相似文献