Thermal and phase-separation behavior of injection-molded poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid)/poly(<Emphasis Type="SmallCaps">d</Emphasis>-lactic acid) blends with moderate optical purity

l-Lactide-rich poly(l-lactide) (LR-PLLA)/d-lactide-rich poly(d-lactide) (DR-PDLA) blends with moderate optical purity were prepared by conventional extrusion and followed by injection-molding process in this study. Thermal properties, crystalline structure, spherulite morphology, melt degradability, and thermal mechanical property were investigated by means of DSC, WAXD, POM, TG, and DMA. In comparison with LR-PLLA/DR-PDLA blends with higher optical purity, stereocomplex with less perfect structure was partially formed from the LR-PLLA/DR-PDLA blends with various compositions and showed lower melting temperature. Surprisingly, double melting peaks have appeared in blends with 40 or 50 wt% DR-PDLA. Annealing at higher temperature for blends with 50 wt% DR-PDLA resulted in three melting peaks. It is assumed that the optical purity would play a critical role, thus, producing limited amount of stereocomplex with less imperfect structure. Annealing would also induce the micro-phase separation behavior in LR-PLLA/DR-PDLA blends and significantly influence the thermal and degradable properties of blends.     

Synthesis of multi-arm poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactide) and its modification on linear polylactide

Multi-arm poly(l-lactide)s (PLLAs) were prepared by using alcohols with different numbers of hydroxyl groups as initiator and Sn(Oct)₂ as catalyst. The structure and composition was confirmed by NMR and GPC analysis. A series of blends of PLLA/multi-arm PLLA were prepared by melt-mixing. Several techniques were applied to investigate the effects of multi-arm PLLA on the melt rheology, crystallization, thermal stability, morphology and mechanical properties of the linear PLLA. Multi-arm PLLA enhanced crystallization rate of PLLA as a nucleating agent. Rheological analysis showed that the viscosity of PLLA at low frequencies increased after addition of multi-arm PLLA. In addition, the tensile strength was increased with the increase of multi-arm PLLA and can achieve a maximum. FE-SEM measurements revealed that the surface of blends was homogenous, indicating good compatibility between PLLA and multi-arm PLLA. The results had an important guiding role in PLA modification, which provided an opportunity for generating new PLA blending systems with enhanced properties.     

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.     

Synthesis of poly(ethylene adipate-<Emphasis Type="Italic">co</Emphasis>-<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid) copolymers via ring opening polymerization

Poly(ethylene adipate-co-l-lactic acid) (PLEA) copolymers were prepared via ring opening polymerization from l-lactide and hydroxyl terminated poly(ethylene adipate) prepolymer as starting materials. The composition and microstructure of the PLEA copolymers were characterized by nuclear magnetic resonance (¹H NMR) spectra. Results confirmed the incorporation of lactic acid segments into the chain of PLEA copolymers as well as the existence of ester exchange reaction. The thermal behaviors and thermal stability of the resultant PLEA copolymers were evaluated by differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA), respectively; and the crystal structure was confirmed by using wide-angle X-ray diffraction (WAXD). Results showed that those properties of the PLEA copolymers showed high dependence on the composition of the copolymers.     

Synthesis,characterization of star-shaped copolymers of <Emphasis Type="SmallCaps">l</Emphasis>-lactide and epoxidized soybean oil

Biodegradable star-shaped PLLA–ESO copolymers were synthesized by the bulk copolymerization of l-lactide (l-LA) and epoxidized soybean oil (ESO) with stannous octanoate as the catalyst. Effects of molar ratios of monomer to catalyst, and various amounts of ESO on copolymerization were studied. The resulting copolymers were characterized by FTIR, ¹H NMR, GPC, etc., which confirmed the successful synthesis of star-shaped copolymers of l-LA and ESO. The thermal and mechanical properties of samples were also investigated by means of DSC, TGA and tensile testing. The results showed that the PLLA–ESO copolymers possed lower glass transition temperature, melting point, crystallinity, and maximum decomposition temperature than those of neat polylactide. Tensile testing demonstrated that PLLA–ESO copolymer had better ductility than linear PLLA. It was also found that the amount of catalyst almost had no influence on the weight average molecular weight of PLLA–ESO copolymers, but which could be controlled by variation of molar ratios of l-LA to ESO.     

Graphene nanoplatelets dispersion in poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid): preparation method and its influence on electrical,crystallinity and thermomechanical properties

Poly(l-lactic acid) (PLLA)/graphene nanoplatelets (GnP) nanocomposites were prepared through solvent casting and coagulation methods. The better dispersion of graphene was achieved by ultrasounds and its effect on crystallinity, thermomechanical and electrical properties of PLLA were studied and compared in both methods. Differential scanning calorimetry (DSC) was used to investigate the crystallinity of PLLA and its composites. Field emission gun scanning electron microscope (FEG-SEM) and wide-angle X-ray scattering (WAXS) were employed to characterize the microstructure of PLLA crystallites. Dynamic mechanical thermal analysis (DMTA) was performed to study the thermomechanical properties of the nanocomposites. FEG-SEM images illustrated finer dispersion of GnP in samples obtained by coagulation method with respect to solvent casting method. Graphene imparted higher electrical conductivity to nanocomposites obtained by solvent casting under ultrasound due to better formation of graphene network. DSC thermograms and their resulting data showed positive effects of GnP on crystallization kinetics of PLLA in both methods enhanced by the nucleating effect of graphene particles. Meanwhile, the effect of GnP, as nucleating agent, was more prominent in samples produced by coagulation method without utilization of ultrasounds. WAXS patterns represented the same characteristic peaks of PLLA in nanocomposite specimens suggesting similar crystalline structure of PLLA in presence of graphene, and the intensified peaks of nanocomposites compared to neat PLLA confirmed the DSC results regarding its improved crystallinity. Graphene increased storage modulus in rubbery region and glass transition temperature of nanocomposites in the coagulation method due to restricted mobility of PLLA chains.     

Preparation and thermo-oxidative degradation of poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid)/poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid)-grafted SiO<Subscript>2</Subscript> nanocomposites

Poly(l-lactic acid)/poly(l-lactic acid)-grafted SiO₂ nanocomposites were prepared by in situ melt polycondensation, in which “free” poly(l-lactic acid) and poly(l-lactic acid)-grafted SiO₂ nanoparticles were formed simultaneously. The maximum values of grafting ratio and grafting efficiency of poly(l-lactic acid) were up to 37.67% and 26.60%, respectively. In the polycondensation system, SiO₂ content was a critical parameter of getting nanocomposites with uniformly dispersed SiO₂ nanoparticles. At lower SiO₂ content, Mn of grafted poly(l-lactic acid) was close to that of “free” poly(l-lactic acid), and poly(l-lactic acid)-grafted SiO₂ nanoparticles could be well dispersed in poly(l-lactic acid) matrix. While at higher SiO₂ content, Mn of “free” poly(l-lactic acid) and grafted poly(l-lactic acid) decreased seriously, especially GPC curves of “free” poly(l-lactic acid) exhibited two peaks due to the aggregation of SiO₂ nanoparticles during the polycondensation process. The grafting ratio and SiO₂ content exhibited a clear effect on the thermo-oxidative degradation of nanocomposites. The existence of poly(l-lactic acid)-grafted SiO₂ nanoparticles dramatically improved the thermo-oxidative stability of poly(l-lactic acid). Compared with that of pure poly(l-lactic acid), T _g, T _c, and T _m of nanocomposites varied slightly.     

Enhanced mechanical and photoluminescence effect of poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactide) reinforced with functionalized multiwalled carbon nanotubes

The poly(l-lactide) (PLLA) biocompatible and biodegradable polymer was reinforced with functionalized Multiwalled carbon nanotubes (MWCNTs) to overcome on insufficient mechanical properties of this polymer for high load bearing applications. To fully realize the potential of MWCNTs for this purpose, they have to be homogeneously dispersed in polymer matrix and have efficient load transfer across the MWCNTs/polymer interface. The pristine MWCNTs (pMWCNTs) were functionalized, at first, by Friedel–Crafts acylation, which introduced the aromatic amine groups on the sidewall of MWCNTs (MWCNT–NH₂) without shortening or cutting of pMWCNTs. And then, the PLLA chains covalently grafted from the sidewall of MWCNT–NH₂ by in situ ring-opening polymerization of l-lactide oligomers using stannous octanoate as the initiating system. The Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy spectra revealed that the PLLA chains grafted form the sidewall of MWCNTs strongly. The surface morphology of pristine and PLLA-grafted MWCNTs (MWCNT-g-PLLAs) was characterized by scanning electron microscopy and transmission electron microscopy. The tensile test of prepared composites of PLLA with various concentrations of MWCNT-g-PLLAs show a significant increment in tensile strength and elongation at failure of composites with increasing the concentration of MWCNT-g-PLLAs in composites. Also, it is found that the MWCNT-g-PLLAs increased the photoluminescence effect of PLLA and widened the luminescence region of PLLA.     

In vitro degradation behaviors of Poly-<Emphasis Type="SmallCaps">l</Emphasis>-lactide/bioactive glass composite materials in phosphate-buffered solution

In vitro degradation behaviors of composite materials composed of poly-l-lactide (PLLA) and bioactive glass (BG) were systematically investigated up to 20 weeks in phosphate-buffered solution (PBS) at 37 °C. The properties of PLLA/BG composites and PLLA materials, including weight loss, bending strength and modulus, shearing strength, polymer molecular weight and its distribution, and the morphologies, were investigated as a function of degradation time. The change of the pH value of the PBS media was also detected. The results showed that the presence of the bioactive glass modified the degradation of the matrix polymer. The degradation rate of the PLLA/BG composites was slower than the degradation rate of the sole PLLA materials.     

Mechanical properties, morphologies, and crystallization behavior of plasticized poly(l-lactide)/poly(butylene succinate-co-l-lactate) blends

The blends of poly(l-lactide) (PLLA) with poly(butylene succinate-co-l-lactate) (PBSL) containing the lactate unit of ca. 3 mol% and Rikemal PL710 (RKM) which is a plasticizer mainly composed of diglycerine tetraacetate were prepared by melt-mixing and subsequent injection molding. The studied RKM content of the PLLA/PBSL/RKM blends was 0-20 wt%, and the PLLA/PBSL weight ratio was 100/0 to 80/20. Although elongation at break in the tensile test did not increase by the addition of 10 wt% RKM to PLLA, the addition of a small amount of PBSL to the PLLA/RKM blend caused a considerable increase of the elongation. The SEM and DSC analyses revealed that all the PLLA/PBSL/RKM blends are immiscible blends where the PBSL particles are finely dispersed, and that there is some compatibility between PLLA-rich phase and PBSL-rich phase in the amorphous state when the RKM content is 20 wt%. As a result of investigation of the crystallization behavior by DSC and polarized optical microscopic measurements, it was revealed that the addition of RKM causes the acceleration of crystalline growth rate at a lower annealing temperature, and the addition of PBSL mainly enhances the formation of PLLA crystal nucleus.     

Characterization of the mechanical and thermal properties and morphological behavior of biodegradable poly(L‐lactide)/poly(ε‐caprolactone) and poly(L‐lactide)/poly(butylene succinate‐co‐L‐lactate) polymeric blends

Two series of biodegradable polymer blends were prepared from combinations of poly(L ‐lactide) (PLLA) with poly(?‐caprolactone) (PCL) and poly(butylene succinate‐co‐L ‐lactate) (PBSL) in proportions of 100/0, 90/10, 80/20, and 70/30 (based on the weight percentage). Their mechanical properties were investigated and related to their morphologies. The thermal properties, Fourier transform infrared spectroscopy, and melt flow index analysis of the binary blends and virgin polymers were then evaluated. The addition of PCL and PBSL to PLLA reduced the tensile strength and Young's modulus, whereas the elongation at break and melt flow index increased. The stress–strain curve showed that the blending of PLLA with ductile PCL and PBSL improved the toughness and increased the thermal stability of the blended polymers. A morphological analysis of the PLLA and the PLLA blends revealed that all the PLLA/PCL and PLLA/PBSL blends were immiscible with the PCL and PBSL phases finely dispersed in the PLLA‐rich phase. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effect of chain entanglement on the melt-crystallization behavior of poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactide) acid

Chain entanglements and the entanglement degree determine many processes and behaviors of polymers. In this work, poly(l-lactide) acid (PLLA) samples with markedly decreased entanglements were obtained via a freeze extraction method and the kinetics of entanglement recovery process of freeze-extracted samples was monitored by dynamic rheology approach. The crystallization kinetics of freeze-extracted PLLA samples was further studied by polarized optical microscope, which revealed that the entanglement degree greatly influences the crystallization of PLLA and lower degree of entanglement or disentanglement was conducive to the melt-crystallization of PLLA. The spherulites grew faster in partially disentangled melt than in well entangled melt.     

<Emphasis Type="SmallCaps">l</Emphasis>-Aspartic acid incorporated optically active poly(amide-imide)s: synthesis and characterization

N-trimelliticimido-l-aspartic acid (1) was prepared from the reaction of trimellitic anhydride with l-aspartic acid in a mixture of glacial acetic acid and pyridine solution (3/2 ratio) under refluxing conditions. The solution polycondensation of the corresponding activated monomer with eight aromatic diamines were carried out in DMAc. The resulting poly(amide-imide)s were obtained in quantitative yields, showed admirable inherent viscosities (0.20–0.36 dl g⁻¹), good optical activity (+7.32o to +15.24o), and were readily soluble in polar aprotic solvents. They start to decompose (T _10%) above 170 °C and display glass-transition temperatures at 120–237 °C. All of the above polymers were fully characterized by UV, FT–IR, and ¹HNMR spectroscopy, elemental analysis, thermogravimetric analyses, DSC, inherent viscosity measurement, and specific rotation.     

Mechanical properties, morphology, and crystallization behavior of blends of poly(l-lactide) with poly(butylene succinate-co-l-lactate) and poly(butylene succinate)

The blends of poly(l-lactide) (PLLA) with poly(butylene succinate) (PBS) and poly(butylene succinate-co-l-lactate) (PBSL) containing the lactate unit of ca. 3 mol% were prepared by melt-mixing and subsequent injection molding, and their mechanical properties, morphology, and crystallization behavior have been compared. Dynamic viscoelasticity and SEM measurements of the blends revealed that the extent of the compatibility of PBSL and PBS with PLLA is almost the same, and that the PBSL and PBS components in the blends with a low content of PBSL or PBS (5-20 wt%) are homogenously dispersed as 0.1−0.4 μm particles. The tensile strength and modulus of the blends approximately followed the rule of mixtures over the whole composition range except that those of PLLA/PBS 99/1 blend were exceptionally higher than those of pure PLLA. All the blends showed considerably higher elongation at break than pure PLLA, PBSL, and PBS. Differential scanning calorimetric analysis of the blends revealed that the isothermal and non-isothermal crystallization of the PLLA component is promoted by the addition of a small amount of PBSL, while the addition of PBS was much less effective.     

Role of Catechol Structure in the Adsorption and Transformation Reactions of <Emphasis Type="SmallCaps">l</Emphasis>-D<Emphasis Type="SmallCaps">opa</Emphasis> in Soils

3-(3′,4′-Dihydroxyphenyl)-l-alanine (l-DOPA), which is synthesized in velvet bean (Mucuna pruriens), inhibits plant growth. The concentration of l-DOPA in soil is reduced by adsorption and transformation reactions, which can result in the reduction of its plant-growth-inhibitory activity. To determine which part of the l-DOPA structure is involved in the adsorption and soil transformation reactions, we compared the kinetics of l-DOPA disappearance in a volcanic ash soil with that of l-phenylalanine (3-phenyl-l-alanine) and l-tyrosine (3-(4′-hydroxyphenyl)-l-alanine), compounds that are similar in structure to l-DOPA but do not have a catechol (o-dihydroxybenzene) moiety. l-Phenylalanine and l-tyrosine were not adsorbed and transformed in the soil at equilibrium pH values between 4 and 7. These results suggest that the adsorption and transformation reactions of l-DOPA in the soil involve the catechol moiety and not the amino and carboxylic acid groups, which are common to all three compounds. Like l-DOPA, (+)-catechin, another allelochemical that contains a catechol moiety, underwent adsorption and soil transformation reactions. Thus, we concluded that the concentrations of allelochemicals bearing a catechol moiety in soils will decrease rapidly owing to adsorption and transformation reactions, and this decrease will be faster in soils with a high pH value or high adsorption ability. Owing to this decrease in concentration, allelopathic phenomena may not occur.     

Miscibility, thermal characterization and crystallization of poly(l-lactide) and poly(tetramethylene adipate-co-terephthalate) blend membranes

Binary blend membranes of biodegradable poly(l-lactide) (PLLA) with poly(tetramethylene adipate-co-terephthalate) (PTAT) copolymer were prepared by solution casting via air evaporation. The miscibility of PLLA/PTAT blends was studied by dynamic mechanical analysis (DMA) and thermal mechanical analysis (TMA) in a tensile mode. Differential scanning calorimetry (DSC) measurement was carried out. The surface microstructure and tensile properties of the blend membranes were examined using atomic force microscopy (AFM) and tensile tester. It was concluded that PLLA/PTAT blends should be partially miscible for all ranges of compositions. Higher roughness and porosity were observed for the blend containing 50% PTAT, suggesting more phase separation occurred. The DSC analysis showed that the fusion enthalpy and crystallinity (X_c) of the PLLA-rich phase decreased with increasing PTAT content. Solidification process strongly suggested that the crystallization rate was accelerated by blending with 25% PTAT content, which served as the nucleation agent. Furthermore, the crystallization rate coefficient (CRC) depended on the blending miscibility and cooling rate in the non-isothermal crystallization process. Besides, PTAT addition could be proved to enhance the thermal stability and elongation of resulting blend membranes, even superior to those properties of poly(lactic acid-co-glycolic acid) (PLGA).     

<Emphasis Type="SmallCaps">l</Emphasis>-DOPA Increases Lignification Associated with <Emphasis Type="BoldItalic">Glycine max</Emphasis> Root Growth-Inhibition

l-3,4-dihydroxyphenylalanine (l-DOPA), an allelochemical exuded from the roots of velvet bean [Mucuna pruriens (L.) DC. var. utilis], presents a highly inhibitory action to plant growth. The effects of l-DOPA on phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) and peroxidase (POD, EC 1.11.1.7) activities, and phenolic compound and lignin content in soybean [Glycine max (L.) Merr.] roots were investigated to determine the possible phytotoxic mechanism. Three-day-old seedlings were cultivated in half-strength Hoagland nutrient solution (pH 6.0), without or with 0.1 to 1.0 mM l-DOPA in a growth chamber (25°C, 12-hr light to 12-hr darkness photoperiod, irradiance of 280 μmol m⁻² s⁻¹) for 24 hr. In general, the length, fresh weight, and dry weight of the roots decreased, whereas PAL and POD activities and phenolic compound and lignin content increased after l-DOPA treatments. Results showed the susceptibility of soybean to l-DOPA and reinforce the role of this nonprotein amino acid as a strong allelochemical. The present findings also suggest that l-DOPA-induced inhibition in soybean roots may be because of a cell wall stiffening process related to the formation of cross-linking between cell wall polymers linked to lignin production.     

Ring-opening polymerization of <Emphasis Type="SmallCaps">l</Emphasis>-lactide catalyzed by a novel molybdenum-based catalytic system

As a transparent material that can be completely biodegradable, poly(l-lactide) (PL-LA) has recently received considerable attention. In this study, it our first efforts to fabricate l-lactide (L-LA) by a novel molybdenum-based catalytic system consisting of molybdenum pentachloride (MoCl₅) as the main catalyst and m-cresol substituted alkyl aluminum Al(OPhCH₃)(i-Bu)₂ as the co-catalyst. The effects of different types of phosphorus ligands, Al:Mo molar ratios, catalyst contents,catalyst components (separate catalysis of m-cresol aluminum and cocatalysis of Al/Mo system) and polymerization temperature were investigated. The T_g and T_m of the resulting poly(l-lactide) (PL-LA) were characterized by differential scanning calorimetry (DSC), and the molecular weight and molecular weight distribution were determined by gel permeation chromatography (GPC). The GPC results showed that the molecular weight of the PL-LA was higher than that 10⁴ g/mol and the molecular weight distribution was narrow. The structures of PL-LA was detected by ¹H NMR spectroscopy (¹H NMR) and X-ray diffraction (XRD) validation, which demonstrated that a moalr ratio of Mo/Al/l-lactide?=?1:30:1000 showed the higher conversion rate and molecular weight. In comparison to the separate catalysis of m-cresol aluminum, the molecular weight of PL-LA prepared by the cocatalysis of Al/Mo system was slightly improved, and the molecular chains were relatively regular and the crystallinity was higher.     

Elevation of flow temperature of poly(<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid) gel by controlling its morphology and crystal form

Complex crystals (ε-crystals) of poly(l-lactic acid) (PLLA) with solvents such as N,N-dimethylformamide (DMF) induce gelation of PLLA by forming fibrous structure. Three procedures to elevate the flow temperature of the PLLA gel were described: formation of an additional network by immersion of a DMF solution of PLLA with lower molecular weight, changing the crystal form from ε-crystal to α-crystal by heating the gel at 46 °C, and increasing the concentration of the solution to form the gel followed by heating. Field emission electron microscopy and X-ray diffraction measurements were carried out to elucidate the structural change and driving force of the elevation of the flow temperature. Densification of the fibrous structure and change of the crystal form from ε-crystals to α-crystals with lower solubility at higher temperatures were observed in the gel after the formation of an additional network by immersion in a DMF solution of PLLA. The procedure to elevate the flow temperature, which has been developed in this study, can broaden the application of PLLA gels.     

Synthesis of poly(lactic acid) by heterogeneous acid catalysis from <Emphasis Type="SmallCaps">d</Emphasis>,<Emphasis Type="SmallCaps">l</Emphasis>-lactic acid

Poly(lactic acid) (PLA) is an important polymer because of its significant biocompatibility and biodegradability. Supported H₃PW₁₂O₄₀ (H₃PW) on activated carbon was utilized for the catalytic polymerization of D,L-lactic acid, resulting in blends of PLA. The stability of the polymer was monitored by thermogravimetry (TGA), and the decomposition temperature (T_d) was used to determine the optimal production conditions (i.e., temperature of 180 °C for 15 h; 0.1 wt. % catalyst; 20 wt. % H₃PW/carbon calcined at 400 °C). The best catalyst was reused three times with good activity and recovery (95 %) and was analyzed to confirm the consistency of its Keggin structure, dispersion, and acidity, which are important parameters that affect the catalyst’s activity. The obtained polymer was characterized by gel permeation chromatography (GPC), Fourier-transform infrared spectroscopy (FT-IR), ¹H/¹³C nuclear magnetic resonance (NMR) spectroscopy, specific optical rotation ([α]_D ²⁵), powder X-ray diffraction (XRD), and differential scanning calorimetry (DSC). The average molar mass of the polymer was 17,400 g mol^?1. Blends of poly(lactic acid) with 85 % poly(L-lactic acid) stereospecific isomer were obtained.

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Graphical Abstract Stereoselective synthesis of 85 % PLLA from polymerization of d,l-lactic acid using 12-tungstophosphoric acid supported on carbon as a catalyst