Effect of the molecular chains grafted on graphene nanosheets on the properties of poly(l‐lactic acid) nanocomposites

We prepared thermally reduced graphene oxide (TRG) grafted with polymethyl methacrylate (PMMA) and polyvinyl acetate (PVAc) (TRG‐g‐PMMA and TRG‐g‐PVAc) by γ‐ray irradiation‐induced graft polymerization and studied their effects on poly(l ‐lactic acid) (PLLA) nanocomposites. PMMA and PVAc chains were proved to be grafted on the TRG surface successfully. TRG‐g‐PMMA and TRG‐g‐PVAc was found to restrict the crystallization behavior of PLLA compared with TRG. Moreover, tensile‐test results showed that TRG‐g‐PMMA and TRG‐g‐PVAc could enhance the elongation at break of PLLA nanocomposites without reducing the tensile strength and modulus compared with TRG, which indicated that the grafting of PMMA and PVAc chains on TRG could improve the toughness of PLLA nanocomposites. POLYM. COMPOS., 38:5–12, 2017. © 2015 Society of Plastics Engineers     

Fully biobased biodegradable poly(l‐lactide)‐b‐poly(ethylene brassylate)‐b‐poly(l‐lactide) triblock copolymers: synthesis and investigation of relationship between crystallization morphology and thermal properties

Well‐defined poly(l ‐lactide‐b‐ethylene brassylate‐b‐l ‐lactide) (PLLA‐b‐PEB‐b‐PLLA) triblock copolymer was synthesized by using double hydroxyl‐terminated PEBs with different molecular weights. Gel permeation chromatography and NMR characterization were employed to confirm the structure and composition of the triblock copolymers. DSC, wide‐angle X‐ray diffraction, TGA and polarized optical microscopy were also employed to demonstrate the relationship between the composition and properties. According to the DSC curves, the cold crystallization peak vanished gradually with decrease of the PLLA block, illustrating that the relatively smaller content of PLLA may lead to the formation of a deficient PLLA type crystal, leading to a decrease of melting enthalpy and melting temperature. Multi‐step thermal decompositions were determined by TGA, and the PEB unit exhibited much better thermal stability than the PLLA unit. Polarized optical microscopy images of all the triblock samples showed that spherulites which develop radially and with an extinction pattern in the form of a Maltese cross exhibit no ring bond. The growth rate of the spherulites of all triblock samples was investigated. The crystallization capacity of PLLA improved with incorporation of PLLA, which accords with the DSC and wide‐angle X‐ray diffraction results. © 2019 Society of Chemical Industry     

Poly(L‐lactide)/Poly(D‐lactide)/clay nanocomposites: Enhanced dispersion,crystallization, mechanical properties,and hydrolytic degradation

Poly(L ‐lactide) (PLLA)/poly(D ‐lactide) (PDLA)/clay nanocomposites are prepared via simple melt blending method at PDLA loadings from 5 to 20 wt%. Formation of the stereocomplex crystals in the nanocomposites is confirmed by differential scanning calorimetry and wide‐angle X‐ray diffraction (WAXD). The internal structure of the nanocomposites has been established by using WAXD and transmission electron microscope analyses. The dispersion of clay in the PLLA/PDLA/clay nanocomposites can be improved as a result of increased intensity of shear during melt blending. The overall crystallization rates are faster in the PLLA/PDLA/clay nanocomposites than in PLLA/clay nanocomposite and increase with an increase in the PDLA loading up to 10 wt%; however, the crystallization mechanism and crystal structure of these nanocomposites remain unchanged despite the presence of PDLA. The storage modulus has been apparently improved in the PLLA/PDLA/clay nanocomposites with respect to PLLA/clay nanocomposite. Moreover, it is found that the hydrolytic degradation rates have been enhanced obviously in the PLLA/PDLA/clay nanocomposites than in PLLA/clay nanocomposite. POLYM. ENG. SCI., 54:914–924, 2014. © 2013 Society of Plastics Engineers     

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     

Effect of nucleating agents on the crystallization behavior and heat resistance of poly(l‐lactide)

Poly (l ‐lactide) (PLLA) blends with various nucleators were prepared by melt processing. The effect of different nucleators on the crystallization behavior and heat resistance as well as thermomechanical properties of PLLA was studied systematically by differential scanning calorimetry, X‐ray diffraction, heat deflection temperature tester, and dynamic mechanical analysis. It was found that poly(d ‐lactide), talcum powder (Talc), a multiamide compound (TMC‐328, abbreviated as TMC) can significantly improve the crystallization rate and crystallinity of PLLA, thus improving thermal–resistant property. The heat deflection temperature of nucleated PLLA can be as high as 150°C. The storage modulus of nucleated PLLA is higher than that of PLLA at the temperature above T_g of PLLA. Compared with other nucleating agents, TMC was much more efficient at enhancing the crystallization of PLLA and the PLLA containing TMC showed the best heat resistance. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42999.     

Thermal,crystalline, and mechanical properties of octa(3‐chloropropylsilsesquioxane)/ poly(L‐lactic acid) hybrid films

A series of organic‐inorganic hybrid films were prepared based on octa(3‐chloropropylsilsesquioxane) (OCPS) and poly(L‐lactic acid) (PLLA) via simply solution blending method. The thermal, crystalline and mechanical properties of OCPS/PLLA hybrid films were characterized by Fourier transform infrared, scanning electron microscopy, energy dispersive spectrometer, differential scanning calorimetry (DSC), X‐ray diffraction, polarized optical microscopy, thermogravimetric analysis (TGA), and tensile tests. The results showed that OCPS could be dispersed well at molecular level when its content was less than 3 wt % and began to crystallize in PLLA matrix when the content increased to 5 wt %. DSC study revealed that OCPS acted as a plasticizer to decrease both T_g and T_m of the PLLA matrix at various heating rates. The addition of OCPS did not change the crystal form of PLLA, while had an great influence on the cold crystallization and melting behaviors of PLLA in the second heating cycles. Moreover, the initial crystallinity of OCPS/PLLA was higher than that of pure PLLA. The results suggested that OCPS could be an effective heterogeneous nucleating reagent to promote the crystallization of PLLA. TGA showed that the PLLA thermal degradation mechanism remained unchanged, whereas the weight loss temperatures and residual weights were improved. Tensile tests indicated that the incorporation of OCPS into PLLA matrix changed the tensile behavior of the hybrid films from brittle to ductile, and the strain at break was improved remarkably as a result of the plasticizer effect of OCPS. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Surface modification of halloysite nanotubes with l‐lactic acid: An effective route to high‐performance poly(l‐lactide) composites

To improve the interfacial bonding between halloysite nanotubes (HNTs) and poly(l ‐lactide) (PLLA), a simple surface modification of HNTs with l ‐lactic acid via direct condensation polymerization has been developed. Two modified HNTs were obtained: HNTs grafting with l ‐lactic acid (l‐HNTs) and HNTs grafting with poly(l ‐lactide) (p‐HNTs). The structures and properties of l‐HNTs and p‐HNTs were investigated. Then, a series of HNTs/PLLA, l‐HNTs/PLLA and p‐HNTs/PLLA composites were prepared using a solution casting method and were characterized by polarized optical microscopy (POM), field scanning electron microscopy, and tensile testing. Results showed that l ‐lactic acid and PLLA could be easily grafted onto the surface of HNTs by forming an Al carboxylate bond and following with condensation polymerization, and the amounts of the l ‐lactic acid and PLLA grafted on the surface of the HNTs were 5.08 and 14.47%, respectively. The surface‐grafted l ‐lactic acid and PLLA played the important role in improving the interfacial bonding between the nanotubes and matrix. The l‐HNTs and p‐HNTs can disperse more uniformly in and show better compatibility with the PLLA matrix than untreated HNTs. As a result, the l‐HNTs/PLLA and p‐HNTs/PLLA composites had better tensile properties than that of the HNTs/PLLA composites. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41451.     

Enhanced toughness and strength of poly (d‐lactide) by stereocomplexation with 5‐arm poly (l‐lactide)

The enhancement of mechanical properties were achieved by solution blending of poly(d ‐lactide) (PDLA) and 5‐arm poly(l ‐lactide) (5‐arm PLLA). Differential scanning calorimetry (DSC) and wide‐angle X‐ray diffraction (WAXD) results indicated almost complete stereocomplex could be obtained when 5‐arm PLLA exceeded 30wt %. Tensile test results showed that the addition of 5‐arm PLLA in linear PDLA gave dramatically improvement both on tensile strength and elongation at break, which generally could not be increased simultaneously. Furthermore, this work transformed PDLA from brittle polymer into tough and flexible materials. The mechanism was proposed based on the TEM results: the stereocomplex crystallites formed during solvent evaporation on the blends were small enough (100–200 nm), which played the role of physical crosslinking points and increased the interaction strength between PDLA and 5‐arm PLLA molecules, giving the blends high tensile strength and elongation at break. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 132, 42857.     

Largely improved crystallization behavior and thermal stability of poly(L‐lactide) via the synergistic effects of graphene oxide and carbon nanotubes

Graphene oxide (GO) and carbon nanotubes (CNTs) and their compound were introduced into semicrystalline poly(l ‐lactide) (PLLA) to prepare the corresponding binary and/or ternary nanocomposites, respectively. The dispersion states of nanofillers in different nanocomposites were investigated using UV‐Vis spectroscopy, scanning electron microscopy (SEM) and rheological measurement. The results showed that when GO and CNTs were simultaneously present in the PLLA matrix, good dispersion states of both GO and CNTs could be achieved and the ternary nanocomposites exhibited percolated network structure. The effects of different nanofillers on the crystallization behavior of PLLA matrix were comparatively investigated under the different crystallization conditions including melt crystallization process (nonisothermal and isothermal crystallization from the melt) and cold crystallization (crystallization occurring from an amorphous state during the annealing process). The results showed that GO and CNTs exhibited apparent synergistic effects in improving crystallization ability and enhancing crystallinity of PLLA matrix. Study on the thermal stability of nanocomposites showed that the presence of nanofillers greatly improved the thermal stability of PLLA matrix. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40143.     

Morphology,rheological, crystallization behavior,and mechanical properties of poly(L‐lactide)/ethylene‐co‐vinyl acetate blends with different VA contents

Poly(L‐lactide)/ethylene‐co‐vinyl acetate (PLLA/EVA) blends with different contents of Vinyl Acetate (VA) in EVA phase were prepared through melt blending process. Although the composition of the blends was invariant (70/30), different phase morphologies were observed, namely, sea‐island morphologies for the blends with VA contents of 7.5, 18, and 28 wt %, whereas approximate co‐continuous morphology for the blend with VA content of 40 wt % was observed. The interfacial interaction between PLLA and EVA was visualized by Fourier transform infrared and rheological measurements. The nonisothermal and isothermal crystallization behaviors of the blends were investigated by wide angle X‐ray diffraction, Differential scanning calorimetry, and polarization optical microscope. Post‐thermal treatment was applied to improve the crystalline structure of PLLA. The results show that all the samples are mainly in amorphous state during the injection molding process. However, annealing promotes the second crystallization of PLLA matrix, leading to the improvement of the crystalline structure. Especially, the effect of annealing on crystalline structure of PLLA matrix is greatly dependent on the VA content of EVA. As expected, addition of EVA results in the improvement of the ductility and fracture toughness of the blends. The decreased tensile modulus and tensile strength can be enhanced through annealing process. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Isothermal crystallization behavior of poly(L‐lactic acid)/organo‐montmorillonite nanocomposites

The isothermal crystallization behavior of poly(L ‐lactic acid)/organo‐montmorillonite nanocomposites (PLLA/OMMT) with different content of OMMT, using a kind of twice‐functionalized organoclay (TFC), prepared by melt intercalation process has been investigated by optical depolarizer. In isothermal crystallization from melt, the induction periods (t_i) and half times for overall PLLA crystallization (100°C ≤ T_c ≤ 120°C) were affected by the temperature and the content of TFC in nanocomposites. The kinetic of isothermal crystallization of PLLA/TFC nanocomposites was studied by Avrami theory. Also, polarized optical photomicrographs supplied a direct way to know the role of TFC in PLLA isothermal crystallization process. Wide angle X‐ray diffraction (WAXD) patterns showed the nanostructure of PLLA/TFC material, and the PLLA crystalline integrality was changed as the presence of TFC. Adding TFC led to the decrease of equilibrium melting point of nanocomposites, indicating that the layered structure of clay restricted the full formation of crystalline structure of polymer. The specific interaction between PLLA and TFC was characterized by the Flory‐Huggins interaction parameter (B), which was determined by the equilibrium melting point depression of nanocomposites. The final values of B showed that PLLA was more compatible with TFC than normal OMMT. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers     

Effect of zinc phenylphosphonate on the crystallization behavior of poly(l‐lactide)

The effect of zinc phenylphosphonate (PPZn) on the crystallization behavior of poly(l ‐lactide) (PLLA) was investigated using differential scanning calorimetry (DSC) and Polarized Optical Microscopy (POM) measurements. The non‐isothermal cold crystallization results showed that the addition of PPZn obviously decreased the cold crystallization temperature of PLLA and increased the degree of crystallinity of PLLA. The isothermal crystallization kinetics results showed that the crystallization rate of PLLA with small amount of PPZn was much higher than that of neat PLLA, and the half‐time (t_1/2) of PLLA/PPZn sample is far less than that of neat PLLA. As an effective nucleating agent, PPZn particles had also some influence on nucleation mechanism and crystal growth of PLLA. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2744–2752, 2013     

Enhanced nonisothermal and isothermal cold crystallization kinetics of biodegradable poly(l‐lactide) by trisilanolisobutyl‐polyhedral oligomeric silsesquioxanes in their nanocomposites

In this work, the nonisothermal and isothermal cold crystallization behaviors of poly(l ‐lactide) (PLLA)/trisilanolisobutyl‐polyhedral oligomeric silsesquioxanes (tsib‐POSS) nanocomposites with low tsib‐POSS contents were fully investigated. For all the samples, the variations of heating rate and the tsib‐POSS loading may influence the nonisothermal cold crystallization of PLLA. During the nonisothermal crystallization kinetics study, the Ozawa equation failed to fit the nonisothermal crystallization process of PLLA, while the Tobin equation could fit it well. For the isothermal crystallization kinetics study, the crystallization rates of all the samples increased with increasing crystallization temperature. The cold crystallization activation energy of PLLA was increased with 1 wt % tsib‐POSS. Moreover, the addition of tsib‐POSS and the increment of tsib‐POSS loading could increase the crystallization rate of PLLA, indicating the nucleating agent effect of tsib‐POSS. However, the crystallization mechanism and crystal structure of PLLA remained unchanged in the nanocomposites. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43896.     

Crystallization kinetics of poly(L‐lactide) in contact with pressurized CO2

The effect of CO₂ on the isothermal crystallization kinetics of poly(L‐lactide), PLLA, was investigated using a high‐pressure differential scanning calorimeter (DSC), which can perform calorimetric measurements while keeping the sample polymer in contact with pressurized CO₂. It was found that the crystallization rate followed the Avrami equation. However, the crystallization kinetic constant was changed depending upon the crystallization temperature and concentration of CO₂ dissolved in the PLLA. The crystallization rate was accelerated by CO₂ at the temperature in the crystal‐growth rate controlled region (self‐diffusion controlled region), and depressed in the nucleation‐controlled region. CO₂ has also decreased the glass transition temperature, T_g, and the melting temperature, T_m. As a result, the CO₂‐induced change in the crystallization rate can be predicted from the magnitudes of depression of both T_g and the equilibrium melting temperature. The crystalline structure and crystallinity of polymers crystallized in contact with pressurized CO₂ were also investigated using a wide angle X‐ray diffractometer (WAXD). The resulting crystallinity of the sample was increased with the pressure level of CO₂, although the presence of CO₂ did not change the crystalline structure.     

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.     

Degradation behaviors of linear low‐density polyethylene and poly(L‐lactic acid) blends

In this study, the degradability of linear low‐density polyethylene (LLDPE) and poly(L ‐lactic acid) (PLLA) blend films under controlled composting conditions was investigated according to modified ASTM D 5338 (2003). Differential scanning calorimetry, X‐ray diffraction, and Fourier transform infrared spectroscopy were used to determine the thermal and morphological properties of the plastic films. LLDPE 80 (80 wt % LLDPE and 20 wt % PLLA) degraded faster than grafted low‐density polyethylene–maleic anhydride (M‐g‐L) 80/4 (80 wt % LLDPE, 20 wt % PLLA, and 4 phr compatibilizer) and pure LLDPE (LLDPE 100). The mechanical properties and weight changes were determined after composting. The tensile strength of LLDPE 100, LLDPE 80, and M‐g‐L 80/4 decreased by 20, 54, and 35%, respectively. The films, as a result of degradation, exhibited a decrease in their mass. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Increasing the compatibility of poly(l‐lactide)/poly(para‐dioxanone) blends through the addition of poly(para‐dioxanone‐co‐l‐lactide)

To modify the mechanical properties of a poly(l ‐lactide) (PLLA)/poly(para‐dioxanone) (PPDO) 85/15 blend, poly(para‐dioxanone‐co‐l ‐lactide) (PDOLLA) was used as a compatibilizer. The 85/15 PLLA/PPDO blends containing 1–5 wt % of the random copolymer PDOLLA were prepared by solution coprecipitation. Then, the thermal, morphological, and mechanical properties of the blends with different contents of PDOLLA were studied via differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and tensile testing, respectively. The DSC result revealed that the addition of PDOLLA into the blends only slightly changed the thermal properties by inhibiting the crystallization degree of the poly(l ‐lactide) in the polymer blends. The SEM photos indicated that the addition of 3 wt % PDOLLA into the blend was ideal for making the interface between the PLLA and PPDO phases unclear. The tensile testing result demonstrated that the mechanical properties of the blends containing 3 wt % PDOLLA were much improved with a tensile strength of 48 MPa and a breaking elongation of 214%. Therefore, we concluded that the morphological and mechanical properties of the PLLA/PPDO 85/15 blends could be tailored by the addition of the PDOLLA as a compatibilizer and that the blend containing a proper content of PDOLLA had the potential to be used as a medical implant material. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41323.     

Microstructural characteristics and crystallization behaviors of poly(l‐lactide) scaffolds by thermally induced phase separation

In this study, poly(l ‐lactic acid) (PLLA) was prepared by four typical approach systems, namely, solid–liquid phase‐separation processes from PLLA–dioxane at ?80°C, PLLA–dioxane–water at ?80°C, PLLA–tetrahydrofuran (THF) at ?80°C, and PLLA–THF at 18°C. The microstructural characteristics and crystallization behaviors of PLLA were investigated by scanning electron microscopy, differential scanning calorimetry, X‐ray diffraction, and Fourier transform infrared spectroscopy. In the PLLA–dioxane binary system and PLLA–dioxane–water ternary system, the solvent froze immediately after quenching to a low temperature, and this restricted the PLLA chain arrangement. Thus, the PLLA amorphous phase dominated in the scaffolds, and solid‐walled structures were produced. THF was liquid throughout the entire process, which enabled free PLLA chain arrangement and further crystallization. Single crystals aggregated by crystal nucleation and growth at a critical temperature (T_c) of 18°C; this resulted in its most common and stable polymorph, the α form. However, α′‐form crystals, which were assumed to be limit‐disordered crystals of the α form, were produced at a low T_c (?80°C). Scaffolds with a plateletlike structure were produced at a T_c of 18°C, whereas a nanofibrous network was obtained at ?80°C. PLLA crystallization competed with phase separation; thus, the crystal structure and scaffold morphology depended on the codevelopment of these two processes. Finally, the effects of the scaffold morphologies on the cell behaviors were studied, and the nanofibrous scaffold was found to have better cell adhesion and viability than the other three scaffolds. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39436.     

Enhancing crystallization rate and melt strength of PLLA with four‐arm PLLA grafted silica: The effect of molecular weight of the grafting PLLA chains

To improve the crystallization rate and melt strength of polylactide (PLLA), nano‐size amino silica grafted by four‐arm PLLA (4A‐PLLA) with different molecular weight was synthesized. ¹H nuclear magnetic resonance proved that 4A‐PLLA had been grafted onto the surface of SiO₂ successfully, and the grafting ratios and the degradation behaviors of the grafted SiO₂ nanoparticles (g‐SiO₂) were studied. When the grafted silica was introduced into PLLA matrix, the crystallization rate and melt strength of composites were found to be improved and the length of grafted chain played an important role. The extension rheology indicated that long grafted 4A‐PLLA on the surface of SiO₂ was more efficient in enhancing the elongational viscosity of PLLA, owing to the stronger interactions between the grafted chains and the matrix. The crystallization behavior of ungrafted silica filled composite was similar to that of neat PLA, while g‐SiO₂ played a role of nucleating agent. The crystallinities and the crystallization rates of the composites depended on the content of g‐SiO₂ and the grafted chain length of 4A‐PLLA, especially the latter. Longer grafted chain acted as nucleation site in the matrix and significantly improved the crystallization behaviors of PLLA. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45675.     

Miscibility and phase structure of binary blends of poly(L‐lactide) and poly(vinyl alcohol)

Blend films of poly(L ‐lactide) (PLLA) and poly(vinyl alcohol) (PVA) were obtained by evaporation of hexafluoroisopropanol solutions of both components. The component interaction, crystallization behavior, and miscibility of these blends were studied by solid‐state NMR and other conventional methods, such as Fourier transform infrared (FTIR) spectra, differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WAXD). The existence of two series of isolated and constant glass‐transition temperatures (T_g's) independent of the blend composition indicates that PLLA and PVA are immiscible in the amorphous region. However, the DSC data still demonstrates that some degree of compatibility related to blend composition exists in both PLLA/atactic‐PVA (a‐PVA) and PLLA/syndiotactic‐PVA (s‐PVA) blend systems. Furthermore, the formation of interpolymer hydrogen bonding in the amorphous region, which is regarded as the driving force leading to some degree of component compatibility in these immiscible systems, is confirmed by FTIR and further analyzed by ¹³C solid‐state NMR analyses, especially for the blends with low PLLA contents. Although the crystallization kinetics of one component (especially PVA) were affected by another component, WAXD measurement shows that these blends still possess two isolated crystalline PLLA and PVA phases other than the so‐called cocrystalline phase. ¹³C solid‐state NMR analysis excludes the interpolymer hydrogen bonding in the crystalline region. The mechanical properties (tensile strength and elongation at break) of blend films are consistent with the immiscible but somewhat compatible nature of these blends. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 762–772, 2001