Cold crystallization and postmelting crystallization of PLA plasticized by compressed carbon dioxide

The cold crystallization at temperature T_cc (melting > T_cc > glass transition) and the postmelting crystallization of polylactic acid plasticized by compressed carbon dioxide (CO₂) were studied using a high-pressure differential scanning calorimeter. The kinetics of the two kinds of crystallization were evaluated by the Avrami equation as a function of pressure at certain temperatures. The effects of using talc as a nucleation agent on the two types of crystallization under pressure were also investigated. The results show that compressed CO₂ increased the mobility of the polymer chains in solid state, resulting in an increased rate of cold crystallization. The decreased rate of postmelting crystallization was mainly in the nucleation-controlled region, which indicates that the number of nuclei was decreased by the compressed CO₂. The growth rate of the two crystallization types followed the Avrami equation, but the kinetics of each depended upon temperature and pressure. The inclusion of talc accelerated postmelting crystallization but had little effect on cold crystallization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2630–2636, 2008     

The effects of dioctyl phthalate plasticization on the morphology and thermal,mechanical, and rheological properties of chemical crosslinked polylactide

The tensile strength and thermal stability of polylactide (PLA) were significantly improved through chemical crosslinking. However, it became much more rigid and brittle. To obtain a material with good thermal stability and enhanced ability to plastic deformation, chemical crosslinked PLA with 0.5 wt % triallyl isocyanurate and 0.5 wt % dicumyl peroxide was blended with different contents of dioctyl phthalate (DOP). The advantage of using DOP is that it does not crystallize, has low glass transition temperature, and is miscible with PLA. The morphology and the thermal and mechanical properties of the crosslinked PLA and the blends of crosslinked PLA with various contents of DOP were investigated by means of scanning electron microscope, differential scanning calorimetry, tensile test, and dynamic mechanical analysis. The rheological properties of samples were also explored by using a capillary rheometer. The results showed that the DOP was an effective plasticizer for the chemical crosslinked PLA, resulting in a significantly decreased T_g, lower yield stress, and improved elongation at break. The plasticization effect was enhanced by adding higher DOP content. In addition, the DOP enhanced the crystallinity of crosslinked PLA, and all the crosslinked samples showed better heat stability than neat PLA. The apparent viscosity of the blends decreased with the increase of DOP content and a phase separation occurred when the content of DOP exceeded 12.5 wt %. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1136–1145, 2009     

Preparation of biodegradable magnetic microspheres with poly(lactic acid)-coated magnetite

Poly(lactic acid) (PLA)-coated magnetic nanoparticles were made using uncapped PLA with free carboxylate groups. The physical properties of these particles were compared to those of oleate-coated or oleate/sulphonate bilayer (W40) coated magnetic particles. Magnetic microspheres (MMS) with the matrix material poly(lactide-co-glycolide) (PLGA) or PLA were then formed by the emulsion solvent extraction method with encapsulation efficiencies of 40%, 83% and 96% for oleate, PLA and oleate/sulfonate-coated magnetic particles, respectively. MMS made from PLA-coated magnetite were hemocompatible and produced no hemolysis, whereas the other MMS were hemolytic above 0.3 mg/mL of blood.     

Difference in Glass Transition Behavior Between Semi Crystalline and Amorphous poly(lactic acid) Thin Films

Summary: Semi crystalline and amorphous poly(lactic acid) (PLA) thin films exhibit different glass transition temperature and behaviour, as revealed by ellipsometry. For semi-crystalline poly(L-lactic acid) (PLLA) thin film (with crystalline content between 40 and 60%), the glass transition temperature (T_g) is found to decrease below a film thickness of 50 nm. This depression was interpreted in term of disentenglement effect which is likely to occur upon confining the amorphous PLA phase near a non interacting surface. New results performed on non completed films, i.e. isolated objects, also reveal the lower transition temperature, thus underlying the importance of the entanglement state of the polymer chains on their mobility. For amorphous PLA thin film, obtained from the L and D copolymer, two distinct glass transitions were observed, with the highest Tg attributed to the presence of some nano-phase domains, formed by a possible cooperation of the D and L blocks to form stereocomplexes sequences, within the film. Furthermore, if these T_g remained constant as film thicknesses decrease down to 50 nm, they were also found to slightly decrease for isolated objects, thus supporting the importance of the entanglement hypothesis on the glass transition.     

Effect of wood flour content on surface properties of 3D printed materials produced from wood flour/PLA filament

Amount of wood flour was gradually increased from 0 to 50?wt% in the production of polylactic acid (PLA) based filaments with 1.75?mm diameter using twin screw extruder. Surface roughness and wettability were measured of the specimens. Surface roughness of the specimens significantly increased with incorporation of wood flour into PLA filament. Wettability of the specimens significantly decreased with increasing wood flour content. This was mainly attributed to the higher surface free energy of PLA than the beech wood. Contact angle values of the 3D printed wood/PLA specimens having wood flour content up to 30?wt% were less than 90°.     

An AFM Study of Poly(L-lactic acid) and Poly(D-lactic acid) Macromolecules and Their Stereocomplexes at the Solid-Air Interface

The self assembly of PLA enantiomers have been studied at the nanometer scale using atomic force microscopy. At first, the conformation of D and L PLA macromolecules in dilute regime and the initial state of aggregation of both enantiomers were successively observed and compared with the aggregation of PLLA/PDLA blends. Our results points out differences in the aggregates structure between the homo-aggregates of each enantiomers and the 3 Dimensional stereocomplexes formed with racemic mixture of D and L entities. On the one hand, D or L PLA chains, which adopt a rigid conformation in dilute regime, form gradually aggregates that tend to grow from a nucleation center. On the other hand, stereocomplexes have a non-compact structure and are elongated with height variations within the fibrils that support the side by side aggregation of D and L helical structures to form thicker fibrils.     

Ablation with femtosecond pulses: The effect of temporal contrast

An inherent property of the ultrashort pulse lasers used presently for materials processing is that the main pulse is accompanied by an amplified spontaneous emission background. The temporal quality of the pulses is characterized by the temporal contrast. Due to the 5-6 orders of magnitude difference in the duration of the main pulse and the pedestal, high intensity contrast does not mean necessarily high enough difference in the respective energies. In materials processing applications consideration of the energy contrast is advisable. The importance of the contrast in determining the ablation process is exemplified by the dependence of the ablation rate of boron carbide on the cleanness of the pulses of a high brightness hybrid dye/excimer laser system. It is shown that serious consideration of the effects of the nanosecond background is mandatory when evaluating the results.     

Phospholipidomic identification of potential plasma biomarkers associated with type 2 diabetes mellitus and diabetic nephropathy

Type 2 diabetes mellitus (T2DM) and its attendant complications, such as diabetic nephropathy (DN), impose a significant societal and economic burden. The investigation of discovering potential biomarkers for T2DM and DN will facilitate the prediction and prevention of diabetes. Phospholipids (PLs) and their metabolisms are closely allied to nosogenesis and aggravation of T2DM and DN. The aim of this study is to characterize the human plasma phospholipids in T2DM and DN to identify potential biomarkers of T2DM and DN. Normal phase liquid chromatography coupled with time of flight mass spectrometry (NPLC-TOF/MS) was applied to the plasma phospholipids metabolic profiling of T2DM and DN. The plasma samples from control (n = 30), T2DM subjects (n = 30), and DN subjects (n = 52) were collected and analyzed. The significant difference in metabolic profiling was observed between healthy control group and DM group as well as between control group and DN group by the help of partial least squares discriminant analysis (PLS-DA). PLS-DA and one-way analysis of variance (ANOVA) were successfully used to screen out potential biomarkers from complex mass spectrometry data. The identification of molecular components of potential biomarkers was performed on Ion trap-MS/MS. An external standard method was applied to quantitative analysis of potential biomarkers. As a result, 18 compounds in 7 PL classes with significant regulation in patients compared with healthy controls were regarded as potential biomarkers for T2DM or DN. Among them, 3 DM-specific biomarkers, 8 DN-specific biomarkers and 7 common biomarkers to DM and DN were identified. Ultimately, 2 novel biomarkers, i.e., PI C18:0/22:6 and SM dC18:0/20:2, can be used to discriminate healthy individuals, T2DM cases and DN cases from each other group.     

直接法共聚改性聚乳酸研究新进展

聚乳酸(PLA)是一种重要的生物降解材料.广泛应用于生物医学、纤维、塑料等领域.为了改善PLA的性能,人们进行了大量的共聚改性研究.其中,直接从乳酸出发不经丙交酯中间体路线的共聚改性方法,因其简单易行、经济实用,越来越引起科学家的重视.针对直接法共聚改性聚乳酸物中物质种类的不同,结合本课题组的一些乳酸直接熔融共聚研究工...     

Effect of Organoclays on Silicate Layer Structures and Thermal Stabilities of in-situ Polymerized Poly(D-lactide)/Clay Nanocomposites

Six kinds of organoclays were prepared through three kinds of polyols (PTMG, PEA and PCL) to investigate the effects of molecular weight and the chemical structure of organifiers. PTMG based organoclays showed higher ion-exchanged fraction than other organoclays and long chain organifier showed better efficiency in ion-exchanged fraction in the case of PTMG based organifiers. From WAXD and TEM analysis, it was confirmed that PTMG based organoclays formed partially exfoliated or fully exfoliated silicate layer structures. PDLA/clay nanocomposites were prepared by in-situ ring-opening polymerization of D-lactide with PTMG based organoclays as macro-initiators in the presence of equimolar Sn(Oct)₂/PPh₃ complex catalysts. The molecular weight of PDLA/clay nanocomposite decreased as increasing the feeding amount of organoclay because organoclay had hydroxyl terminal groups which can initiate the ring-opening polymerization of D-lactide. From TGA analysis, thermal stabilities of PDLA/clay nanocomposites improved with increasing organoclay content. From WAXD and TEM analysis, organoclay which was prepared by high molecular weight of PTMG based organifier was effective on the exfoliation of silicate layers in the in-situ polymerized PDLA/clay nanocomposite.