Stereocomplex formation between enantiomeric poly(lactic acid). VIII. Complex fibers spun from mixed solution of poly(D-lactic acid) and poly(L-lactic acid)

To obtain poly(lactic acid) (PLA) complex fibers, spinning was performed by wet and dry methods from 5–10 g/dL chloroform solutions of poly(D-lactic acid) (PDLA) and poly(L-lactic), both with a viscosity-average molecular weight of 3 × 10⁵. The dope was extruded from a monohole nozzle into coagulation baths from ethanol and chloroform for wet spinning and into a drying column kept at 60°C for dry spinning. Scanning electron microscopic observation of the as-spun fibers showed that the surface of the wet-spun fiber had large basins with diameters of 50–100 μm and many pores with diameters from sub μm to 10 μm, whereas the surface of dry-spun fiber had a microporous structure with the pore diameter of 1–3 μm. The tensile strength of the wet-spun complex fiber was very low and could not be drawn at high temperatures, in contrast to the dry-spun fiber. The tensile strength of dry-spun complex fiber increased upon hot drawing and showed the tensile strength of 94 kg/mm² by drawing at 160°C to the draw ratio of 13. Differential scanning calorimetry revealed that the complex fibers contained both the stereocomplex crystallites (racemic crystallites) and the crystallites of the single polymers, PDLA and PLLA, regardless of the spinning methods. The ratio of the racemic crystallites to the single-polymer crystallites increased with the draw ratio of the complex fiber. © 1994 John Wiley & Sons, Inc.     

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

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

Synchronous and separate homo-crystallization of enantiomeric poly(l-lactic acid)/poly(d-lactic acid) blends

High-molecular-weight poly(l-lactic acid) (PLLA) and poly(d-lactic acid) (PDLA) are blended at different ratios and their crystallization behavior was investigated. Solely homo-crystallites mixtures of PLLA and PDLA were synchronously and separately formed during isothermal crystallization in the temperature (T_c) range of 90–130 °C, irrespective of blending ratio, whereas in addition to homo-crystallites, stereocomplex crystallites were formed in the equimolar blends at T_c above 150 and 160 °C. Interestingly, in isothermal crystallization at T_c = 130 °C, the spherulite morphology of blends became disordered, the periodical extinction (periodical twisting of lamellae) in spherulites disappeared, and the radial growth rate of spherulite (G) of the blends was reduced by the synchronous and separate crystallization of PLLA and PDLA and the coexistence of PLLA and PDLA homo-crystallites. However, the interplane distance (d), the crystallinity (X_c), the transition crystallization temperature (T_c) from α′-form to α-form, the alternately stacked structure of the crystalline and amorphous layers, and the nucleation mechanism were not altered by the synchronous and separate crystallization of PLLA and PDLA and the coexistence of PLLA and PDLA homo-crystallites. The unchanged d, X_c, transition T_c, long period of stacked lamellae, and nucleation mechanism strongly suggest that the chiral selection of PLLA or PDLA segments on the growth sites of PLLA or PDLA homo-crystallites to some extent was performed during solvent evaporation and this effect remained even after melting.     

Biodegradable polymer blends of poly(lactic acid) and poly(ethylene glycol)

Poly(lactic acid) (PLA) and poly(ethylene glycol) (PEG) were melt-blended and extruded into films in the PLA/PEG ratios of 100/0, 90/10, 70/30, 50/50, and 30/70. It was concluded from the differential scanning calorimetry and dynamic mechanical analysis results that PLA/PEG blends range from miscible to partially miscible, depending on the concentration. Below 50% PEG content the PEG plasticized the PLA, yielding higher elongations and lower modulus values. Above 50% PEG content the blend morphology was driven by the increasing crystallinity of PEG, resulting in an increase in modulus and a corresponding decrease in elongation at break. The tensile strength was found to decrease in a linear fashion with increasing PEG content. Results obtained from enzymatic degradation show that the weight loss for all of the blends was significantly greater than that for the pure PLA. When the PEG content was 30% or lower, weight loss was found to be primarily due to enzymatic degradation of the PLA. Above 30% PEG content, the weight loss was found to be mainly due to the dissolution of PEG. During hydrolytic degradation, for PLA/PEG blends up to 30% PEG, weight loss occurs as a combination of degradation of PLA and dissolution of PEG. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1495–1505, 1997     

聚乳酸立构复合物的合成与表征

采用直接熔融缩聚法制备不同分子量特征的聚乳酸预聚物,将分子量相近的PLLA和PDLA预聚物等量混合,经过熔融共混后再进行固相聚合,制备得到聚乳酸立构复合物.结果表明:sc-PLA与两种PLA预聚物相比,熔点提高50℃左右,通过固相聚合PLA的分子量也得到显著提高.该方法工艺简单,产物纯净,是改善聚乳酸耐热性的一种有效途径.     

Preparation and characterization of poly(ethylene carbonate)/poly(lactic acid) blends

Poly(ethylene carbonate)/poly(lactic acid) blends were successfully prepared by means of a solution film-casting method, and their physicochemical properties were investigated. PEC/PLA blends exhibit partial miscibility and are characterized by the interaction of the ester and carbonic ester groups. One such interaction is between partial charges in –C–O– in –O–C=O of PLA and the carbonyl –C=O of PEC. Another is between –C–O– in –O–C=O of PLA and –C–O– in –CH₂–O– of PEC. The value of T_g varies by more than 10 °C across the blends. PEC does not significantly influence the melting temperature of neat PLA, but non-spherical spherulites are formed in PEC-rich blends, whereas the spherulites are spherical with an average size of 30 μm in PLA-rich blends. Crystallization of PLA is influenced by the addition of flexible PEC and by the proportion of PLA in the blends. Interestingly, addition of at least 10 wt% PLA increased T_g, with a crystallinity, X_c of 47% and better thermal degradation properties, with the temperature at 5 wt% weight loss (T_d5) more than 30 °C higher than for neat PEC.     

Enhanced mechanical properties and degradability of poly(butylene succinate) and poly(lactic acid) blends

To improve the tensile properties and degradability of poly(butylene succinate) (PBS) for biomedical usage, biodegradable polymer blends have been developed. A series of PBS and poly(lactic acid) (PLA) blends were prepared, and their degradation behaviors in simulated body fluid for 16 months were investigated based on morphology, tensile test, weight analysis, and molecular weight. The results showed that the incorporation of PLA into PBS increased the initial tensile strength to some extent, and the blends lost their tensile properties earlier than their parent polymers with the proceeding of hydrolysis. Both blends and parent polymers went through a plateau and subsequent rise stage in mass loss and water absorption, but the blends hydrolyzed faster than the parent polymers. The molecular weight variations also demonstrated faster hydrolysis of the blends. Moreover, both blends and their parent polymers underwent a slow-to-fast transition in their hydrolysis rates. When the M _n of PBS and PLA reached 4.0 × 10⁴ and 9.0 × 10⁴, the hydrolysis of parent polymers and blends began to accelerate, which is the start of auto-acceleration. The blends hydrolyzed faster in both stages. The interface between the components initiated accelerating hydrolysis in the first stage, and the reciprocal auto-acceleration effect resulted in faster hydrolysis of the blends in the second stage.     

Hydrolysis of lactic acid based poly(ester-urethane)s

The hydrolysis behaviour of lactic acid based poly(ester-urethane)s has been studied in a buffer solution of pH 7·00 at 37 and 55°C. Samples were prepared using a straight two step lactic acid polymerization process. The lactic acid was first polymerized by condensation with a low molecular weight by hydroxyl terminated telechelic prepolymer and the molecular weight then was increased with a chain extender such as a diisocyanate. In the hydrolysis study, the effect on the hydrolysis rate of different stereostructures (different amount of D -units in the polymer chain) and the length of the ester units were studied. The rate of hydrolysis was examined by various techniques including weighing (water absorption and weight loss), GPC (molecular weight and polydispersity), and DSC (thermal properties). GPC measurements showed that at 37°C the weight average molecular weight of the poly(ester-urethane)s started to decrease slowly during the first week of hydrolysis, but that at 55°C the weight average molecular weight decreased dramatically during the first week of hydrolysis. Significant mass loss occurred later at both temperatures. © 1998 Society of Chemical Industry     

Production of optically pure poly(lactic acid) from lactic acid

Optically pure poly(lactic acid) (PLA) was obtained from lactic acid via purification of the corresponding lactide. The optical purity of PLA was determined using polarimetry and NMR. In the depolymerization process, the effect of the reaction conditions and catalysts on optical purity of the lactide was examined with temperature having a significant effect. In addition, the degree of racemization increased with increasing molecular weight of the oligomeric PLA. The effects of temperature, time, solvent, and stirring speed (RPM) on the lactide purification process were examined in order to improve optical purity. Optical purity was maximized when separation was carried out at 25 °C. The optical purity of PLA was significantly affected by that of lactide used.     

Crystallization behavior of poly(lactic acid)/elastomer blends

Differential Scanning Calorimetry (DSC) was used to evaluate the crystallization behavior of poly(lactic acid) and its blends with elastomer. It has been observed that the cold crystallization temperature of the blends decreased as the weight fraction of elastomer increased as well as the onset temperature of cold crystallization also shifted to lower temperature. In non-isothermal crystallization experiments, the crystallinity of poly(lactic acid) increased with a decrease in the heating and cooling rate. The melt crystallization of poly(lactic acid) appeared in the low cooling rate (1, 5 and 7.5 °C/min). The presence of low elastomer tends also to increase the crystallinity of poly (lactic acid). The DSC thermogram at ramp of 10 °C/min showed the maximum crystallinity of poly(lactic acid) is 36.95% with 20 wt% elastomer contents in blends. In isothermal crystallization, the cold crystallization rate increased with increasing crystallization temperature in the blends. The Avrami analysis showed that the cold crystallization was in two stages process and it was clearly seen at low temperature. The Avrami exponent (n) at first stage was varying from 1.59 to 2 which described a one-dimensional crystallization growth with homogeneous nucleation, whereas at second stage was varying from 2.09 to 2.71 which described the transitional mechanism to three dimensional crystallization growth with heterogeneous nucleation mechanism. The equilibrium melting point of poly(lactic acid) was also evaluated at 176 °C.     

High impact poly(lactic acid)/poly(ethylene octene) blends prepared by reactive blending

Poly(ethylene octene) grafted with glycidyl methacrylate (POE‐g‐GMA) was prepared and used to toughen poly (lactic acid) (PLA) via reactive blending. It was found that the notched Izod impact strength of PLA/POE‐g‐GMA blends improved dramatically when the content of elastomer was higher than 10 wt%. Reactive compatibilization between PLA and POE‐g‐GMA were studied by Fourier transform infrared spectroscopy (FTIR) and “Molau test,” the results showed the end carboxyl groups of PLA reacted with the epoxide groups of POE‐g‐GMA during blending. This considerably improved the compatibilization, leading to better wetting of the dispersed phase by the PLA matrix and finer dispersed POE‐g‐GMA particles with narrow distribution. Moreover, the critical interparticle distance (L_c) of the dispersed domains for PLA/POE‐g‐GMA blends system at room temperature was also identified. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Stereocomplex formation in atactic poly(methyl methacrylate)I. Effect of solvents

Summary The stereocomplexation in three kinds of atactic poly (methyl methacrylate) (a-PMMA) films with different tacticities isolated from acetone, benzene, and chloroform solution was studied by Fourier transformation infrared (FTIR) and differential calorimetry scanning (DSC) techniques. Based on the results of infrared spectra, it could be deduced that stereocomplexes were formed in the films cast from acetone and benzene solutions with the increase in the population of trans-trans (tt) conformation of the backbone of i- and s-segments. It was assumed that the stereocomplexes formed by the interactions between i- and s-segments, possibly including intramolecular complexation. The stereocomplexation was also confirmed by the evidence of a endotherm with melt temperature over the range 180°C to 200°C, which was corresponded to the melting stereocomplex aggregates formed in the solutions and stabilized by the solvent molecules. Received: 19 January 2000/Revised version: 22 April 2000/Accepted: 25 April 2000     

Structural, morphological and mechanical characteristics of polyethylene, poly(lactic acid) and poly(ethylene-co-glycidyl methacrylate) blends

In this work, uncompatibilized and compatibilized blends of low density polyethylene (LDPE) and poly(lactic acid) (PLA) were subjected to several investigations: Fourier transform infrared (FTIR) spectroscopy, morphological analysis and mechanical testing (tensile, impact, microhardness). The copolymer (ethylene-co-glycidyl methacrylate) (EGMA) was used as compatibilizer. The percentages of PLA in LDPE/PLA samples ranged from 0 to 100 wt% while the EGMA was added to the blend 60/40 (LDPE/PLA) at concentrations of 2, 5, 7, 10, 15 and 20 parts per hundred (phr). FTIR analysis showed the absence of any interaction between LDPE and PLA, but after addition of compatibilizer, reactions between epoxy groups of EGMA and carboxylic or hydroxyl groups of PLA were confirmed. Tensile and impact tests revealed a loss of ductility of LDPE with the incorporation of PLA, except for the composition 80/20 (LDPE/PLA). However, the addition of 15 phr of EGMA led to the maximum increase in the elongation-at-break (about three times the value of uncompatibilized blend) and in the impact strength, but a marginal improvement was observed for tensile strength. SEM micrographs confirmed that the enhancement of mechanical properties is due to the improvement of the interfacial adhesion between different phases owing to the presence of EGMA. The microhardness values of the different blends (uncompatibilized or compatibilized) were in good agreement with the macroscopic mechanical properties (tensile and impact strengths).     

Effect of organoclay on non-linear rheological properties of poly(lactic acid)/poly(caprolactone) blends

The nonlinear viscoelastic properties of PLA/PCL blends with and without clay (montmorillonite, MMT) under large amplitude oscillatory shear (LAOS) flow were investigated. The G′ and G″ as a function of strain amplitude, Lissajous plots and FT-rheology methods were used to interpret nonlinear behavior of PLA/PCL blends with and without MMT. Additionally, scanning electron microscopy (SEM) images of PLA/PCL with MMT blends were taken to investigate the effects of clay on the internal structure of the PLA/PCL blends. A relationship between morphological changes and linear and nonlinear rheological properties was observed. SEM image analysis revealed that clay acted as a compatibilizer and then reduced the size of droplets in the PCL domain of the PLA matrix. As a result, nonlinear properties sensitively reflect morphological changes with increasing MMT amount. The nonlinear rheological properties of PLA/PCL/MMT/metallocene-LLDPE (mLLDPE) were also investigated when mLLDPE was used as an impact modifier to improve mechanical properties, and the nonlinear rheological properties of PLA/PCL/MMT and PLA/PCL/MMT/mLLDPE were also compared.     

Biodegradable polymer blends of poly(L‐lactic acid) and gelatinized starch

Gelatinized starches were prepared with various content of glycerol and were investigated in terms of the effect of the glycerol addition on characteristics of starch and its blends. Poly (L‐lactic acid) (PLA) with various ratios of linear/star shaped PLA and starch gelatinized with various ratios of water/glycerol were melt‐blended by using twin screw mixer. The blends were characterized by DSC thermal analysis, tensile test and morphological analysis. Gelatinization of starch was found to lead to destruction or diminution of hydrogen bonding in granules and a decrease of crystallinity of starch. DSC data showed that starch played a role as a nucleating agent and glycerol as plasticizer contributed to an improvement in crystallinity in PLA blends. When the content of starch increased, the size of spherulites in PLA blends was smaller and less regular. In the case of PLA/pure starch blends, the voids appeared, which were formed by the separation of starch particles from the matrix. But for PLA/gelainized starch blends, these voids were not observed. In the case of blends with linear PLA and starch gelatinized with water/glycerol ratio of 100/40, the greatest superiority of mechanical properties was shown and the toughness was improved compared with PLA/pure starch blends.     

Crystallization, hydrolytic degradation, and mechanical properties of poly (trimethylene terephthalate)/poly(lactic acid) blends

Crystallization, melting, hydrolytic degradation, and mechanical properties of poly(trimentylene terephthalate)/poly(lactic acid) (PTT/PLA) blends have been investigated. The blends show a single and composition-dependent glass-transition temperature (T _g) over the entire composition range, implying that these blends are fully miscible in the amorphous state. The observed T _g is found to increase with increasing PLA content and fitted well with the Gordon–Taylor equation, with the fitting parameter k being 0.91. The cold-crystallization peak temperature increases, while the melt-crystallization peak decreases with increasing the PLA content. Both the pure PTT and PTT/PLA blends cannot accomplish the crystallization during the cooling procedure and the recrystallization occurs again on the second heating. Therefore, on the thermogram recorded, there is exothermal peak followed by endothermal peak with a shoulder. However, to pure PLA, no crystallization takes place during cooling from the melt, therefore, no melting endothermic peak is found on the second heating curve. WAXD analysis indicates PLA and PTT components do not co-crystallize and the crystalline phase of the blends is that of their enriched pure component. With increasing PLA content, the hydrolytic degradation of the blend films increases, while both the tensile strength and the elongation at break of the blend films decrease. That is to say, the hydrolytic degradation of the PTT/PLA blends increases with the introduction of PLA at the cost of the decrease of the flexibility of PTT.     

Rheological and mechanical characterization of poly(lactic acid)/polypropylene polymer blends

Poly(lactic acid) (PLA) was melt blended with polypropylene (PP) with the aim of replacing commodity polymers in future applications. Since cost of PLA is quite high, it is not economically feasible to use it alone for day to day use as a packaging material without blending. This paper reports the preparation of poly(lactic acid)/polypropylene polymer blends (PLA/PP) using a laboratory scale single screw extruder. Rheological and mechanical properties of the prepared blends were determined. The rheological experiments were carried out on a capillary rheometer, the effect of shear rate, temperature and PLA content on the flow activation energy and true viscosity of the blends were described. Mechanical properties of the blends were investigated on dog bone-shaped samples obtained by injection molding; tensile tests were performed using Testometric M350-10KN. The effect of PLA content on Young’s modulus, strain at break and stress at break of the blends were described. The rheological results showed that the true viscosity of the blends is between that of the pure polymers, whereas the flow activation energy of the blends is less than that of the pure polymers. The mechanical results showed incompatibility between PLA and PP in the blend.     

Miscibility and toughness improvement of poly(lactic acid)/poly(3-Hydroxybutyrate) blends using a melt-induced degradation approach

Biodegradable polymer blends of high-molecular-weight poly(3-hydroxybutyrate) (PHB) and poly(lactic acid) (PLA) are not miscible in general. Yet, by decreasing the molecular weight of PHB, the low-molecular-weight PHB could have improved miscibility with the PLA. In this study, a melt-induced degradation process of PLA/PHB blends was therefore implemented, termed the in-situ self-compatibilization approach, to produce low-molecular-weight PHB during melt blending process. The solution blends of PLA and oligomer PHB (PLA/OPHB) were also prepared as a basis to understand the role of low-molecular-weight PHB to improve its miscibility with PLA in PLA/PHB blends. Only one single glass transition temperature (T_g) was found for the resulting PLA/PHB blends at compositions of 95/05 to 80/20, proving that the miscibility was greatly improved by decreasing molecular weight of PHB. Because the degraded PHB had a relatively lower T_g, it thus provided plasticization effect to the PLA and resulted in the decreased crystallization temperature. Moreover, with increasing PHB content to 20% in the blend, the elongation at break increased significantly from 7.2% to 227%, more than 30-fold. The extensive shear yielding and necking behavior were observed during tensile testing for the blend of 80/20. The localized plasticization within PLA/PHB matrix with the reduction of local yield stress and the well-dispersed PHB crystallites were the major contributing factors to trigger shear yielding phenomenon. Moreover, initial modulus decreased only 20%, from 1.68 to 1.35 GPa. A common problem of severely reduced stiffness from the added plasticizer encountered in the plasticized PLA blends was therefore not perceived here.