Orientation and structural development of semicrystalline poly(lactic acid) under uniaxial drawing assessed by infrared spectroscopy and X-ray diffraction

The effects of drawing temperature (T_d) and draw strain on the orientation and structure of semicrystalline poly(lactic acid) (PLA) films were investigated by wide angle X-ray diffraction and polarized Fourier transform infrared spectroscopy. Semicrystalline PLA samples with two initial levels of crystallinity, X_c = 1% and 11%, were prepared by cold crystallization at 80 ^°C. Whatever X_c and T_d, the total amount of the ordered phases (i.e. crystalline + mesophase) increased with draw strain, which could be ascribed to the formation of strain-induced mesophase at T_d = 60 or 70 ^°C but crystalline at 80 ^°C. Also, the molecular orientation of both the amorphous and ordered phases increased with draw strain. Whatever X_c, the orientation of the ordered phases was insensitive to T_d, whereas higher orientation in the amorphous phase was achieved at lower T_d, and the trend was more significant for X_c = 1% compared with 11%.     

Chemical modification of poly(lactic acid) and its use as matrix in poly(lactic acid) poly(butylene adipate-co-terephthalate) blends

Poly(lactic acid), PLA, was chemically modified with maleic anhydride (MA) by reactive extrusion. The effect of this modification on molar mass (MM) and acidity was assessed by means of size-exclusion chromatography (SEC) and titration, respectively. PLA MM decreased in the presence of MA solely and of MA and peroxide. Reduction in MM was monitored by the increase in acidity. PLA blends containing poly(butylene adipate-co-terephthalate) (PBAT) were prepared through different mixing protocols, PLA/PBAT, PLA-g-MA/PBAT and PLA/PBAT/MA/peroxide (PLA/PBAT in situ). SEC results and rheological properties revealed reduction in MM and viscosity of the modified blends. PLA/PBAT presented increase in MM and bimodal MM distribution. The calculated interfacial tension was significantly lower for the modified blends, despite the lower average particle area of PLA/PBAT. Surprisingly, the modified blends presented higher yield strength than that predicted by the rule of mixtures, which might indicate interfacial reactions.     

降解可控乳酸类聚合物

聚乳酸作为生物医用材料，其降解性能最为人们关注。根据降解性能的不同，其应用也有不同的用途。本文从共聚和共混两方面综述了聚乳酸在降解可控性方面研究的最新进展，对未来的工作进行了展望。     

Biodegradation of poly(lactic acid) and its nanocomposites

PLA nanocomposites based on organically modified montmorillonites at 5% w/w loading were prepared by melt blending using an internal mixer and then degraded in a commercial compost. The addition of nanoclays was found to increase the PLA degradation rate, especially for the highest dispersed clay in the polymer matrix. Biodegradation by microorganisms isolated from the compost showed the bacterium Bacillus licheniformis as one of the responsible for PLA biodegradation in compost. It was also found that clays can influence the polymer bacterial degradation depending on their chemical structure and affinity of the bacterium towards the clay.     

Compatibility and phase structure of binary blends of poly(lactic acid) and glycidyl methacrylate grafted poly(ethylene octane)

This work study is the compatibility, phase structure, and component interaction of poly(lactic acid) (PLA) and glycidyl methacrylate grafted poly(ethylene octane) (GMA-g-POE denoted as mPOE) blend by Fourier transform infrared (FTIR) spectra, dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), and wide-angle X-ray diffraction (WAXD), respectively. All the binary blend compositions exhibit two distinct glass transition temperatures corresponding to the mPOE-rich and PLA-rich phases, respectively. Moreover, these two peaks approach each other with increasing mPOE content, indicating partial compatibility between the PLA and mPOE. Chemical reactions between the end carboxyl groups of the PLA and epoxy groups of the mPOE are considered as the driving force of the enhanced compatibility. They lead to an increase in viscosity of the blends and a decrease in the structural symmetry of PLA. This result brings about a decrease in the spherulite growth rate and the degree of crystallinity. Glass transition temperature (T_g) depression of mPOE is attributed to the negative pressure imposed on the dispersed rubber phase, resulting from differential contraction due to the thermal shrinkage mismatch upon cooling from the melt state. The negative pressure in the dispersed particles, in turn, would cause a dilational effect for the matrix ligament between the particles, and therefore increases the ductility and toughness of PLA.     

Methanolysis of poly(lactic acid) (PLA) catalyzed by ionic liquids

Poly(lactic acid) (PLA) was depolymerized by methanol in the presence of a novel catalyst: ionic liquids. It was found that the purification method of the main products in the methanolysis catalyzed by ionic liquids was simpler than that of traditional compounds, such as sulfuric acid. Qualitative analysis indicated that the main product in the methanolysis process was methyl lactate. The influences of experimental parameters, such as the amount of ionic liquids, methanolysis time, reaction temperature, and dosages of methanol on the conversion of PLA, yield of methyl lactate were investigated. Under the optimum conditions, using ionic liquid 1-butyl-3-methylimidazolium acetate ([Bmim][Ac]) as catalyst, results showed that the ionic liquid could be reused up to 6 times without apparent decrease in the conversion of PLA and yield of methyl lactate. The kinetics of the reaction was also investigated. The results indicated that the methanolysis of PLA was a first-order kinetic reaction with activation energy of 38.29 kJ/mol. In addition, a possible catalysis mechanism of the methanolysis of PLA was proposed.     

The effect of free radical reactions on structure and properties of poly(lactic acid) (PLA) based blends

The blending of PLA with poly(butylene-adipate-co-terephthalate) (PBAT) is a promising strategy to achieve a toughened multiphase material. The blends ductility could be further improved through reactive compatibilization, i.e. inducing the formation of comb PLA-PBAT copolymers during the melt blending. In the present work a non-selective strategy was adopted which consisted in the use of a peroxide, 2,5-Dimethyl-2,5-di(tert-butylperoxy)hexane. The phase morphology development and the final properties (torque, fluidity in the melt, tensile behaviour, thermal and dynamical-mechanical features) of the blends were studied as a function of the peroxide concentration. The elongation at break was improved up to a maximum value thanks to this approach and a corresponding minimum was observed in the value of the dispersed phase diameter. A structural characterization of the macromolecules formed during the reactive process was attempted by using size exclusion chromatography of the blends and comparison with the pure polymers obtained by processing in the presence of the peroxide.     

Morphological behaviour of poly(lactic acid) during hydrolytic degradation

The hydrolytic degradation and the morphological behaviour of a packaging grade of poly(lactic acid) (PLA) were characterized by a series of techniques. During the initial degradation process (stage 1) at a temperature near the glass transition temperature (T_g), the molecular weight of PLA decreased as degradation time increased following a bulk erosion mechanism while the crystallinity increased simultaneously, but no observable weight loss occurred at stage 1. Mainly α-form PLA crystal structure was formed for the crystalline PLA with a low content of d stereo-isomers, but the material displayed a lower regularity, smaller domain size, lower melting temperatures T_m and different motional dynamics as compared to the original PLA with a similar level of crystallinity achieved by annealing. The amorphous PLA with a higher amount of d stereo-isomers also yielded the α crystalline phase as well as stereo-complex crystals at stage 1. When the molecular weight and the crystallinity reached a stable level, PLA started erosion into the degrading aqueous medium. During this stage of degradation (stage 2), the crystalline structure in PLA residues was further modified and both pH and temperature influenced the modification. The degradation at stage 2 was likely to follow a surface erosion mechanism with lactic acid as the major product of the weight loss. Besides the crystallinity effect on the degradation, temperature also played a key role in determining the rate of PLA degradation in both stages. The process was very slow at temperatures below the T_g of PLA but the rate was greatly enhanced at temperatures above the T_g.     

Some composting and biodegradation effects of physically or chemically crosslinked poly(lactic acid)

Polylactide (PLA) crosslinked by using both triallyl isocyanurate (TAIC) and electron radiation or using dicumyl peroxide (DCP) was studied with the aim of examining the behaviour of the modified polymer under various environmental conditions. Thus, the polymer samples were subjected to composting in an industrial pile, exposed to proteinase K, or incubated in sea water. The number-average molecular weight (M_n), melt flow index (MFI), crystallinity (χ), tensile strength (σ_M) and mass loss (in the case of samples treated with proteinase K) were determined. It was found that neat PLA irradiated with high-energy electrons underwent degradation that increased during composting. As a result, the value of M_n of this polymer dramatically decreased. It appeared that PLA crosslinked with TAIC and electron radiation contained, in addition to the crosslinked phase, a phase strongly degraded by this radiation, which facilitated hydrolytic degradation during composting. The σ_M value of PLA crosslinked with TAIC and electron radiation rapidly decreased during composting, whereas that of PLA crosslinked chemically and composted for three weeks slightly increased. As the electron radiation dose increased, the mass loss of PLA containing TAIC and treated with proteinase K decreased, which indicated that the physical crosslinking of PLA hindered enzymatic degradation of this polymer. Important changes in both neat and physically crosslinked PLA incubated in sea water for nine weeks were not detected.     

聚乳酸共混体系的研究进展

聚乳酸是人工合成的可生物降解的热塑性脂肪族聚酯 ,是一种环境友好的材料 ,逐渐成为人们研究的热点。本文根据聚乳酸共混体系中另一组分的生物降解性 ,将聚乳酸共混体系分为完全可生物降解共混体系和部分可生物降解共混体系两类。针对体系的相容性和结晶结构以及在提高PLA的性能和降低成本等方面 .综述了聚乳酸共混体系的最新研究进展 ,并对其发展动态进行了简要介绍     

Photo-grafting Poly(acrylic acid) onto Poly(lactic acid) Chains in Solution

Poly(lactic acid)(PLA)is one of the most important bio-plastics,and chemical modification of the already-polymerized poly(lactic acid)chains may enable optimization of its material properties and expand its application areas.In this study,we demonstrated that poly(lactic acid)can be readily dissolved in acrylic acid at room temperature,and acrylic acid can be graft-polymerized onto poly(lactic acid)chains in solution with the help of photoinitiator benzophenone under 254 nm ultraviolet(UV)irradiation.Similar photo-grafting polymerization of acrylic acid(PAA)has only been studied before in the surface modification of polymer films.The graft ratio could be controlled by various reaction parameters,including irradiation time,benzophenone content,and monomer/polymer ratios.This photo-grafting reaction resulted in high graft ratio(graft ratio PAA/PLA up to 180%)without formation of homopolymers of acrylic acid.When the PAA/PLA graft ratio was higher than 100%,the resulting PLA-g-PAA polymer was found dispersible in water.The pros and cons of the photo-grafting reaction were also discussed.     

溶剂诱导的聚乳酸/聚乳酸衍生物共结晶行为

通过溶液浇铸法制备不同组分的左旋聚乳酸(PLLA)和聚(L-2-羟基-3-甲基丁酸)(PL-2H3MB)共混物.运用差示扫描量热仪(DSC)、偏光显微镜(POM)、广角X射线衍射(WAXD)和热重分析仪(TGA)分析共混物的结晶、熔融行为和热稳定性.通过观察到DSC加热曲线中新的熔融峰判断PLLA和PL-2H3MB共晶...     

Preparation and thermal characterisation of poly(lactic acid) nanocomposites prepared from organoclays based on an amphoteric surfactant

New polymer-clay nanocomposites composed of poly(lactic acid) and a novel organoclay based on cocamidopropylbetaine (CAB) and sodium montmorillonite (MMT) were prepared by solution casting and characterised by X-Ray Diffraction Analysis (XRDA), Transmission Electron Microscopy (TEM) and Thermogravimetric Analysis (TGA). A similar series of composites based on PLA and Cloisite 30B, a commercially available organoclay, were prepared for comparison. The thermal stability of the CAB-MMT organoclays decreased with increasing surfactant loading. Experimental organoclays with an organic content similar to that of the commercial organoclay were found to be of comparable thermal stability. XRDA analysis of the PLA-organoclay nanocomposites showed that PLA intercalated the gallery space of both types of organoclay to similar extents and the increased spacing was confirmed by TEM. The thermal stabilities of the PLA-organoclay composites based on CAB-MMT were higher than those based on the commercial organoclay.     

Blends of Poly（lactic acid） with Thermoplastic Acetylated Starch

Blends of poly(lactic acid)(PLA) and thermoplastic acetylated starch(ATPS) were prepared by means of the melt mixing method. The results show that PLA and ATPS were partially miscible, which was confirmed with the measurement of Tg by dynamic mechanical analysis(DMA) and differrential scanning calorimetry(DSC). The mechanical and thermal properties of the blends were improved. With increasing the ATPS content, the elongation at break and impact strength were increased. The elongation at break increased from...     

Crystallization kinetics and morphology of biodegradable poly(lactic acid) with a hydrazide nucleating agent

The effect of tetramethylenedicarboxylic dibenzoylhydrazide (designated here as TMC) on the nonisothermal and isothermal crystallization behavior of PLA was investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM) and wide angle X-ray diffraction (WAXD). TMC shows excellent nucleating effect on PLA. With the addition of 0.05 wt% TMC, the crystallization half-time of PLA decreases from 26.06 to 6.13 min at 130 °C. The isothermal crystallization data were further analyzed by the Avrami model. The values of the Avrami exponent of the blends are comparable to that of neat PLA, indicating that the presence of TMC does not change the crystallization mechanism of the matrix. The observation from POM and WAXD measurements showed that the presence of TMC increases significantly the nuclei density of PLA but has no discernible effect on its crystalline structure.     

Kinetics of the thermal decomposition of processed poly(lactic acid)

The kinetics of the thermal decomposition of processed poly(lactic acid) has been studied and compared to that of raw material. Processing consisted of two different industrial processes: 1) Injection (with or without further annealing); 2) Extrusion followed by injection (with or without further annealing). For this study, an integral method (based on the general analytical solution), differential methods (based on the first conversion derivative and on the 2nd derivative) and special methods have been used. On the other hand, a method based on the maximum decomposition rate has been considered. By doing that, the kinetic parameters (reaction order, frequency factor and activation energy) have been determined. It has been demonstrated that there was only one first-order reaction for the entire conversion range. A new equation (based on the second conversion derivative plot as a function of temperature) was developed allowing the calculation of the reaction order. This method quantifies peak areas (and not peak heights, as reported by Kissinger). It is very useful because it considers both peak shape and width. Activation energy, as determined by using the general analytical solution, was 318 kJ/mol for unprocessed poly(lactic acid) whereas it was 280 ± 5 kJ/mol for processed materials. All the processed materials had approximately the same thermal stability (T₅ = 333.0-335.8 °C, at 95% confidence level), which was slightly lower than that of unprocessed materials (T₅ = 337.5 °C). PLA melting (during extrusion and injection) was responsible for depolymerization reactions (the small molecules formed during melting processes can volatilize readily).     

Gas permeation properties of poly(lactic acid)

The need for the development of polymeric materials based on renewable resources has led to the development of poly(lactic acid) (PLA) which is being produced from a feedstock of corn rather than petroleum. The present study examines the permeation of nitrogen, oxygen, carbon dioxide, and methane in amorphous films of PLA cast from solution. The properties of PLA are compared to other commodity plastics and it is shown that PLA permeation closely resembles that of polystyrene. At 30°C, N₂ permeation in PLA is 1.3 (10⁻¹⁰ cm³ (STP) cm/cm² s cmHg) and the activation energy is 11.2 kJ/mol. For oxygen the corresponding values are 3.3 (10⁻¹⁰ cm³(STP) cm/cm² s cm Hg) and 11.1 kJ/mol. The values for carbon dioxide permeation are 1.2 (10⁻¹⁰ cm³ (STP) cm/cm² s cmHg) and 6.1 kJ/mol. For methane values of 1.0 (10⁻¹⁰ cm³ (STP) cm/cm² s cmHg) and an activation energy of 13.0 kJ/mol are found. Studies with pure gases show that polymer chain branching and small changes in l:d stereochemical content have no effect on permeation properties. Crystallinity is found to dominate permeation properties in a biaxially oriented film. The separation factor for a CO₂/CH₄ mixed gas system is measured between 0 and 50°C and does not deviate significantly from the calculated ideal separation factor; at 0°C the separation factor is 16, a value that suggests continued studies of PLA as a separation medium are warranted.     

Influence of carbon black on the properties of plasticized poly(lactic acid) composites

Using acetyl tributyl citrate (ATBC) and poly(1,3-butylene adipate) (PBA) as the plasticizer of poly(lactic acid) (PLA) and carbon black (CB) as reinforced filler, high performance composites were prepared in melting blend. Fourier transform infrared spectroscopy revealed that the interaction existed between PLA and CB, and plasticizer could improve this interaction. The rheology showed that plasticizer could obviously improve the fluidity of the composites, but just the reverse for CB. Scanning electron microscopy revealed that the addition of plasticizer facilitated the dispersion of the CB in PLA. With the increasing of CB content, the enforcement effect, storage modulus and glass transition temperature increased. The elongation at break of PLA/PBA (30 wt%) could be above 600%, which was higher than the same weight ATBC plasticized PLA. Moreover, CB could restrain the thermally induced migration of plasticizer in plasticized PLA. Compared with ATBC, PBA was a thermal stable plasticizer for PLA.     

生物基超韧聚乳酸复合材料的制备与性能

采用来源于可再生资源的聚醚酰胺弹性体（PEBA）增韧聚乳酸（PLA）制备超韧聚乳酸（PLA/PEBA-GMA）复合材料.为了提高PEBA与PLA之间的相容性,选择极性单体甲基丙烯酸缩水甘油酯（GMA）、共接枝单体乙烯基吡咯烷酮（NVP）及引发剂过氧化二异丙苯（DCP）对PEBA进行接枝改性制备PEBA-GMA.研究了接枝单体组分的用量（m/g）对PLA/PEBA-GMA复合材料性能的影响.研究发现,随着接枝单体组分用量的提高,复合材料的缺口冲击强度逐渐增大,当接枝单体组分GMA,NVP和DCP的用量分别为2.5,2.5和0.25 g时,复合材料的冲击强度高达88.6 kJ/m²,断裂伸长率为164.1%.研究表明,在熔融共混过程中,聚乳酸的端基（—OH和—COOH）与PEBA-GMA上环氧基团发生反应,有效改善两相间的界面相容性,随着接枝单体组分比例的提高,分散相PEBA-GMA的粒子尺寸逐渐减小且分布均匀.     

Characterization of poly(lactic acid)s with reduced molecular weight fabricated through an autoclave process

Poly(lactic acids) (PLAs) with different molecular weights (MWs) were prepared by autoclaving a commercial PLA for different time periods. The harvested PLAs were characterized by different techniques. Gel permeation chromatography results showed that the MW of PLA decreased with increasing autoclaving time. The Mark–Houwink parameters for PLA in dichloromethane at 25 °C were determined for the first time. The melting endotherms, revealed by differential scanning calorimetry, were observed for the 60 and 120 min-autoclaved PLAs (cf. PLA60 and PLA120) but hardly detected for the original and 30 min-autoclaved PLA (PLA30). The PLA120 exhibited higher crystallinity than that of PLA60. Thermogravimetric analysis showed that the activation energy for thermal degradation decreased from 186 kJ mol⁻¹ to 140 kJ mol⁻¹ for original PLA and PLA120. The hydrophilicity of the PLA increased, indicating higher number of COOH groups, with increasing autoclaving time, as revealed by the contact angle measurement. Rheological data showed that the complex viscosity and storage modulus of PLA decreased with increasing autoclaving time due to the decreasing MW. Unlike original PLA and PLA30, PLA60 and PLA120 exhibited Newtonian fluid behavior at all test frequencies.