Influence of alkali treatment on flax fiber for use as reinforcements in polylactide stereocomplex composites

The flax and equivalent proportion of poly(l ‐lactic acid)/poly(d ‐lactic acid) (PLLA/PDLA) were melt compounded and injection molded to prepare flax‐reinforced polylactide stereocomplex (sc‐PLA) bio‐composite, and the effect of alkali treatment on the structure and properties of flax as well as the flax/sc‐PLA composite was investigated. SEM and FTIR results showed hemicellulose in flax was almost completely removed after alkali treatment and the treated flax (ALK‐flax) bundles were more separated with a cleaner surface than untreated flax (UN‐flax). DSC results showed homo‐crystallites (hc, T_m = 160–170°C) and stereocomplex crystallites (sc, T_m ～210°C) coexisted in sc‐PLA and flax/sc‐PLA composites. Compared with sc‐PLA, the total crystallinity and sc‐crystallinity of flax/sc‐PLA composite increased regardless of whether the flax were treated with alkali, whereas ALK‐flax/sc‐PLA composite showed a little higher crystallinity than UN‐flax/sc‐PLA composite. TGA results confirmed ALK‐flax/sc‐PLA composite had a higher thermal degradation temperature than UN‐flax/sc‐PLA composite. The mechanical tests indicated although the mechanical properties of sc‐PLA increased significantly by reinforcing with flax, the ALK‐flax/sc‐PLA composite showed little lower mechanical properties than UN‐flax/sc‐PLA composite. The alkali treatment of flax had no obvious influence on the Vicat softening temperature (VST) of flax/sc‐PLA composites, a higher heat resistance with VST at ～155°C could be obtained for flax/sc‐PLA composite. POLYM. ENG. SCI., 55:2553–2558, 2015. © 2015 Society of Plastics Engineers     

Supercritical fluid technology parameters affecting size and behavior of stereocomplex polylactide particles and their composites

Polylactide (PLA) is a eco‐friendly and biodegradable material that can be synthesized from renewable resources. PLA features poly(d ‐lactic acid) (PDLA) and poly(l ‐lactic acid) (PLLA) enantiomers. Supercritical fluid (SCF) technology is a very promising method for the stereocomplexation between PDLA and PLLA enantiomers. This study acquires stereocomplex (sc‐)PLA particles with diverse sizes and behaviors by controlling the experimental conditions. Various parameters including polymer concentration, reaction temperature, stirring speed, pressure reducing speed, and final temperature were controlled to adjust size and behavior of sc‐PLA particles. Additionally, we analyzed the effect of subsequent processing following SCF (such as homogenization, mechanical stirring, and sonication) on the size and morphological behavior of sc‐PLA particles. Finally, the mechanical strengths of different PLA composites featuring different sc‐PLA filler sizes were determined. The mechanical strength of PLA composites was significantly improved when using smaller filler sizes. POLYM. ENG. SCI., 58:1193–1200, 2018. © 2017 Society of Plastics Engineers     

Properties of a poly(L‐lactic acid)/poly(D‐lactic acid) stereocomplex and the stereocomplex crosslinked with triallyl isocyanurate by irradiation

A poly(L ‐lactic acid) (PLLA)/poly(D ‐lactic acid) (PDLA) stereocomplex was prepared from an equimolar mixture of commercial‐grade PLLA and PDLA by melt processing for the first time. Crosslinked samples were obtained by the radiation‐induced crosslinking of the poly(lactic acid) (PLA) stereocomplex mixed with triallyl isocyanurate (TAIC). The PLA stereocomplex and its crosslinked samples were characterized by their gel behavior, thermal and mechanical measurements, and enzymatic degradation. The crosslinking density of the crosslinked stereocomplex was described as the gel fraction, which increased with the TAIC content and radiation dose. The maximum crosslinking density was obtained in crosslinked samples of PLA/3% TAIC and PLA/5% TAIC irradiated at doses higher than 30 kGy. The stable crosslinking networks that formed in the irradiated PLA/TAIC substantially suppressed the segmental mobility for the crystallization of single crystals as well as stereocomplex crystals. The crosslinking network also significantly improved the mechanical properties and inhibited the enzymatic degradation of crosslinked PLA/3% TAIC. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci 2008     

Toward environment‐friendly composites of poly(ε‐caprolactone) reinforced with stereocomplex‐type poly(l‐lactide)/poly(d‐lactide)

In this work, stereocomplex‐poly(l ‐ and d ‐lactide) (sc‐PLA) was incorporated into poly(ε‐caprolactone) (PCL) to fabricate a novel biodegradable polymer composite. PCL/sc‐PLA composites were prepared by solution casting at sc‐PLA loadings of 5–30 wt %. Differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD) demonstrated the formation of the stereocomplex in the blends. DSC and WAXD curves also indicated that the addition of sc‐PLA did not alter the crystal structure of PCL. Rheology and mechanical properties of neat PCL and the PCL/sc‐PLA composites were investigated in detail. Rheological measurements indicated that the composites exhibited evident solid‐like response in the low frequency region as the sc‐PLA loadings reached up to 20 wt %. Moreover, the long‐range motion of PCL chains was highly restrained. Dynamic mechanical analysis showed that the storage modulus (E′) of PCL in the composites was improved and the glass transition temperature values were hardly changed after the addition of sc‐PLA. Tensile tests showed that the Young's modulus, and yield strength of the composites were enhanced by the addition of sc‐PLA while the tensile strength and elongation at break were reduced. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40208.     

Stereocomplexed polylactides (Neo‐PLA) as high‐performance bio‐based polymers: their formation,properties, and application

Polymeric materials prepared from renewable natural resources are now being accepted as “bio‐based polymers”, because they are superior to the conventional petroleum‐based polymers in reducing the emission of carbon dioxide. Among them, poly(L ‐lactide) (PLLA) prepared by fermentation and polymerization is paid an immediate attention. Although PLLA exhibits a broad range of physico‐chemical properties, its thermal and mechanical properties are somewhat poorer for use as ordinary structural materials. For improving these inferior properties, a stereocomplex form consisting of PLLA and its enantiomer poly(D ‐lactide) (PDLA) has high potential because of showing high melting nature (230 °C). It can be formed by simple polymer blend of PLLA and PDLA or more easily with stereoblock polylactides (sb‐PLA) which are PLLA/PDLA block copolymers. These novel PLA polymers, named “Neo‐PLA”, can provide a wide range of properties that have never be attained with single PLLA. Neo‐PLA retains sustainability or bio‐based nature, because both monomers L ‐ and D ‐lactic acids are manufactured from starch by fermentation. Copyright © 2006 Society of Chemical Industry     

New insight into the mechanism of enhanced crystallization of PLA in PLLA/PDLA mixture

Poly(lactic acid) (PLA) is a bio‐based and compostable polymer that has quickly developed into a competitive material, but the control of crystallinity is a bottleneck in extended utilization. The crystallization of PLA has been a rich topic because of the existence of two enantiomeric forms of poly(L‐lactic acid) (PLLA) and poly(d ‐lactic acid) (PDLA) can form stereocomplex (SC) crystal with high melting point that can be used to control the crystallization behaviors. The SC crystal was regarded as an effective nucleating agent for promoting the crystallization rate and crystallinity of PLA. We investigated cold crystallization of PLLA/PDLA (1:1) mixture with in situ WAXS measurements and found that the homo‐crystal of PLA formed earlier than the SC‐crystal in the mixture within the measured temperature range, which is different from the melting crystallization. The final crystalline structures are in correspondence with the melting and cold crystallization temperature, and the transition of homo‐PLA (δ to α) is not altered by the crystallization procedure. The SC‐crystal can be detected in both cold and melting crystallization of the mixture at the temperatures lower than 150 °C, which is conflict with the reported results. A new crystallization mechanism of the mixture was proposed to understand the crystallization behaviors in PLLA/PDLA mixtures. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45663.     

Effect of multi‐branched PDLA additives on the mechanical and thermomechanical properties of blends with PLLA

Stereocomplex formation between poly(l ‐lactic acid) (PLLA) and poly(d ‐lactic acid) (PDLA) in the melt state was investigated and altered via the addition of multi‐branched poly(d ‐lactide) (PDLA) additives. Two different multi‐branched PDLA additives, a 3‐arm and 4‐arm star‐shaped polymeric structure, were synthesized as potential heat resistance modifiers and incorporated into PLLA at 5, 10, and 20 (w/w) through melt blending. Mechanical and thermomechanical properties of these blends were compared with linear poly(l ‐lactide) (PLLA) as well as with blends formed by the addition of two linear PDLA analogs that had similar molecular weights to their branched counterparts. Blends with linear PDLA additives exhibited two distinct melting peaks at 170–180°C and 200–250°C which implied that two distinct crystalline domains were present, that of the homopolymer and that of the stereocomplex, the more stable crystalline structure formed by the co‐crystallization of both d ‐ and l ‐lactide enantiomers. In contrast, blends of PLLA with multi‐branched PDLA formed a single broad melting peak indicative of mainly formation of the stereocomplex, behavior which was confirmed by X‐ray diffraction (XRD) analysis. The heat deflection temperature determined by thermal mechanical analysis was improved for all blends compared to neat PLLA, with increases of up to180°C for 20% addition of the 3‐arm PLLA additive. Rheological properties of the blends, as characterized by complex viscosity (η*), remained stable over a wide temperature range. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 132, 42858.     

The effects on mechanical properties and crystallization of poly (l‐lactic acid) reinforced by cellulosic fibers with different scales

Bacterial cellulose (BC), microcrystalline cellulose (MCC), and bamboo cellulosic fibers (BCFs) were used to reinforce poly(l ‐lactic acid) (PLLA) based bio‐composites. The mechanical properties and crystallization of the composites were studied through mechanical testing, differential scanning calorimetry, X‐ray diffraction, scanning electron microscopy, and polarizing microscope. The incorporation of all three kinds of cellulose increased the stiffness of the composites compared to pure PLLA. The reinforcing effect of the MCC in the composites is most significant. The Young's modulus and impact toughness of the MCC/PLLA composites were increased by 44.4% and 58.8%, respectively. The tensile strength of the MCC/PLLA composites was increased to 71 MPa from 61 MPa of PLLA. However, the tensile strength of the composites reinforced with BCF or BC was lower than PLLA. The three kinds of cellulosic fibers improved the crystallization of PLLA. The BC with smallest size provided the composites with smallest grain and highest crystallinity. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 41077.     

Crystallization behavior of poly(L‐lactic acid)‐based ecocomposites prepared with kenaf fiber and rice straw

Several composites of poly(L ‐lactic acid) (PLLA) with natural fibers (kenaf and rice straw) and pigments have been prepared and analyzed. The study of the thermal behavior has shown a rather important nucleation ability of these fillers for the crystallization of the PLLA component in the composites. Thus, the cooling from the melt of pure PLLA at 10°C/min leads to an almost completely amorphous sample, while a high crystallinity (around 60%) is exhibited by the sample PLLA and rice straw (PLLA‐RS)‐yellow under those conditions. The analysis of the isothermal crystallization from the melt indicates that a maximum rate of crystallization is obtained for all the samples at around 105°C, although the rate is three times faster for samples PLLA and kenaf fiber (PLLA‐KF), PLLA‐KF‐red, and PLLA‐RS, in comparison with pure PLLA. The rate is increased by another factor of three for sample PLLA‐RS‐yellow. The analysis of the melting temperatures and crystallinities as a function of the crystallization temperature shows that there is a break at around 115°C, which seems to be related to the formation of ordered crystals at higher temperatures and disordered ones at lower temperatures. Besides, the natural fibers are environmentally friendly and nonexpensive materials, and the higher crystallization rates of the composites will result in shorter production cycles of end‐use articles. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

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     

Stereocomplexation of solvent‐cast poly(lactic acid) by addition of non‐solvents

Equimolar blends of poly(L ‐lactic acid) (PLLA) and poly(D ‐lactic acid) (PDLA) were obtained by solution casting from chloroform/methanol mixed solvents and analyzed using wide‐angle X‐ray diffraction (WAXD), differential scanning calorimetry (DSC) and polarizing optical microscopy. Chloroform and methanol are a solvent and non‐solvent, respectively, for poly(lactic acid). The WAXD and DSC results showed that stereocomplex crystallization between PLLA and PDLA occurred in addition to homo‐crystallization. On adding methanol to the casting solution, the stereocomplexation was gradually enhanced while the homo‐crystallization was suppressed. When a large amount of methanol was added, the homo‐crystallization was fully suppressed and the degree of stereocomplex crystallinity reached 90%. Similar results were obtained when another non‐solvent, hexane, was added to the casting solution in place of methanol. The effect of the addition of good and poor solvents such as tetrahydrofuran, ethanol, acetone and ethyl acetate was also studied. Copyright © 2011 Society of Chemical Industry     

Macromolecular design of specialty polylactides by means of controlled copolymerization and stereocomplexation

Among the biobased polymers developed, poly‐l ‐lactide (PLLA ) has been the most widely used in many fields because of its excellent cost ? property balance. However, the properties and functionalities of PLLA cannot easily be controlled like the conventional petroleum‐based polymers, retarding the progress of its manufacturing on a large scale. One approach to obtain high‐performance and specialty polylactides is to use stereocomplex‐type polylactides (sc‐PLA ) that can be obtained by mixing PLLA and its enantiomer poly‐d ‐lactide. The other approach is to use copolymers consisting of polylactides (PLA ) and other types of macromolecular chains. Here we demonstrate how such high‐performance and specialty PLA polymers can be designed and synthesized. These macromolecular designs and synthetic methodologies are highly effective for controlling the structure and properties of PLA polymers for use as biomedical and high‐end industrial materials. © 2016 Society of Chemical Industry     

Effect of stereoisomerism of poly(lactic acid) during neural guide conduit membrane synthesis

Poly(lactic acid) membranes are being developed as biomaterials for several purposes such as artificial implants for peripheral nerve injury, also known as neural guide conduits (NGC). These membranes need to meet standards of mechanical, degradability, and permeability properties, besides dimensional and structural requirements. Among the stereoisomers of polylactides, poly(l ‐lactic acid), and poly(d ,l ‐lactic acid) are the most used as biomaterials, having significant differences in solubility, crystallinity thermal, and mechanical properties. In this work, PLLA and PDLLA were compared for hollow fiber membrane synthesis by liquid induced phase separation. PLLA samples presented 18% of crystallinity, while PDLLA is amorphous. PDLLA and PLLA polymer solutions on N‐methyl‐pyrrolidone presented values of 3428 and 320.2 cP, respectively. In immersion of PLA‐NMP solutions in water, PLLA solution presented instantaneous demixing, while PDLLA showed a 28 s delay on precipitation. The PLLA–NMP–water has a larger demixing region compared to PDLLA–NMP–water system. Hollow fibers of both polymers presented closed external surface with finger‐like macropores morphology in their cross‐sections. PDLLA presented typical liquid–liquid demixing pores while PLLA presented spherulitical crystalline solid–liquid separation structures, which deeply compromised its mechanical properties. PDLLA presented as a good candidate for hollow fiber NGC material, as presented good mechanical resistance in tensile and suture simulating tests. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46190.     

Synergistic effects of N,N′‐bis (benzoyl) sebacic acid dihydrazide and talc on the physical and mechanical behaviors of poly(l‐latic acid)

The important practical problem of poor heat stability of poly(l ‐lactic acid) (PLLA) is addressed by the addition of N, N′‐bis (benzoyl) sebacic acid dihydrazide (BSAD) and talc as a nucleating agent system. The idea of incorporating talc into the PLLA/BSAD composites is that talc can provide supplementary nucleation effect with very small amount of BSAD (0.2 wt %) and therefore can improve the heat deflection resistance of PLLA materials. Effects of BSAD/talc on morphology, crystallization behavior, heat resistance, and mechanical properties of PLLA/BSAD/talc were investigated after annealing processes. The results indicated that the BSAD/talc system increased the crystallinity from 6.0% of pure PLLA to a maximum 42.9% by the synergistic effects of BSAD and talc increasing the growth of spherulites and nucleation density, respectively. After annealing at different temperatures, the heat deflection temperature (HDT) of PLLA was improved dramatically due to synergistic effects of BSAD/talc between restricted chain movement and acceleration of crystallization. At high temperature (above T_g), the thermo‐mechanical properties of PLLA is mainly determined by the crystallinity and the reinforcement effect of talc acted as a filler. Moreover, effects of BSAD/talc on mechanical properties were discussed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41454.     

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     

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     

Influence of heat treatment on the heat distortion temperature of poly(lactic acid)/bamboo fiber/talc hybrid biocomposites

Influence of heat treatment and fillers on the heat distortion temperature (HDT) of poly(lactic acid) hybrid biocomposites was intensively studied through HDT testing, polarizing microscope (POM), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). With loading 20 wt % BF or 20 wt % talc, improvement of HDT in PLA composite was about 10°C comparing with neat PLA after heat treatment. Moreover, there was a great improvement (above 45°C) of HDT in PLA composites with loading 20 wt % BF and 20 wt % talc simultaneously after heat treatment. Transcrystallization was observed during heat treatment and isothermal crystallization of PLA composites with loading BF and talc simultaneously. There was no similar phenomenon in other PLA composite with loading only one filler. The possible mechanism of forming transcrystallization was proposed. DSC and DMA were also used to clarify the variation in HDT before and after heat treatment, and the results suggest that the crystallinity, modulus and glass transition changed, especially formation of transcrystallization played a key role in improvement of HDT in PLA composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

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     

Characterization of poly(L‐lactic acid) fibers produced by melt spinning

Biodegradable poly(L ‐lactic acid) (PLLA) fibers were processed by a two‐step melt‐spinning method (melt extrusion and hot draw) from PLLA with three different viscosity‐average molecular weights (494,600, 304,700, and 262,800). Before spinning, the polymer flakes were first milled into powders and dried under vacuum. Viscosity‐average molecular weight of PLLA following the fabrication process was monitored. Tensile properties of as‐spun and hot‐drawn fibers were investigated. Morphology of the PLLA fibers was viewed under a scanning electron microscope. Crystallinity of these fibers was assessed by thermogram analysis of differential scanning calorimetry. Results showed that the extent of decrease in the viscosity‐average molecular weight of PLLA dropped sharply by 13.1–19.5% during pulverization and by 39.0–69.0% during melt‐extrusion. The hot‐draw process in this study had a little effect on the viscosity‐average molecular weight of PLLA. Smoother fibers could be obtained for the die temperature at least 230°C for raw materials with higher crystallinity (more than 75%) and at least 220°C for raw materials with lower crystallinity (about 60%). The as‐spun fibers showed crystallinity of 16.5–22.8% and the value increased to 50.3–63.7% after hot draw. Tensile moduli of the as‐spun fibers were in the range of 1.2–2.4 GPa, which were raised to 3.6–5.4 GPa after hot draw. The final PLLA fibers with 110–160 μm diameters showed tensile strengths of 300–600 MPa. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 251–260, 2001     

Stereocomplexation and morphology of enantiomeric poly(lactic acid)s with moderate‐molecular‐weight

The thermal behavior and spherulitic morphologies of poly(L ‐lactic acid) (PLLA)/poly(D ‐lactic acid) (PDLA) 1/1 blend with weight‐molecular‐weight of 10⁵ order, together with those of pure PLLA and PDLA, were investigated using differential scanning calorimetry and polarized optical microscopy. It was found that in the blend, stereocomplex crystallites could be formed exclusively or coexisted with homocrystallites depending on thermal history. Banded to nonbanded spherulitic morphological transition occurred for melt‐crystallized PLLA and PDLA, while the blend presented exclusively nonbanded spherulitic morphologies in the temperature range investigated. The spherulite growth of the blend occurred within a wider temperature range (≤180°C) compared with that of homopolymers (≤150°C), while the spherulite growth rates were comparable for both the blend and homopolymers. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008