Pengwu Xu

Bio-based poly(lactide)/ethylene-co-vinyl acetate thermoplastic vulcanizates by dynamic crosslinking: structure vs. property

Bio-based thermoplastic vulcanizates (TPV) from poly(lactide) (PLA) and ethylene-co-vinyl acetate rubber (EVA) were fabricated using dicumyl peroxide (DCP) as a curing agent; it is the first time that the application of PLA in elastic materials is demonstrated. A two-stage competing reaction mechanism as a function of peroxide content is revealed via gel analysis. The crosslinking of EVA is dominant at low DCP content (<1 wt%), which levels off when the DCP content exceeds 1 wt%. The gel fractions of the EVA and PLA phases can be tuned by the DCP content and fabrication technique. A desirable phase inversion of the PLA/EVA blends due to selective dynamic curing was monitored by both atomic force microscopy (AFM) and dynamic mechanical analysis (DMA). Consequently, PLA/EVA thermoplastic vulcanizates with high strength, high elongation at break, low tensile set and intermediate hardness were obtained. Moreover, their mechanical properties can be tuned by the DCP content and plasticization. The correlation between their structures and properties was investigated.

Crystallization modification of poly(lactide) by using nucleating agents and stereocomplexation

Abstract Poly(lactide), PLA, as one of the most promising biopolymers, has been receiving increasing attention in recent years because of its excellent performances in renewability, mechanical properties, biocompatibility and biodegradability. However, its application is limited by its brittleness and low heat distortion temperatures (HDT). The low HDT mainly results from a low crystallization rate and lack of crystallinity after fast processing, e.g. injection molding. Consequently, considerable attention was paid, in recent years, to achieve fast(er) crystallization of PLA. In here, we briefly review the research progress in the crystallization modification of PLA notably by means of adding nucleating agents and stereocomplexation.

Modification of graphene oxide by a facile coprecipitation method and click chemistry for use as a drug carrier

A ternary composite based on graphene oxide (GO), Fe3O4 nanoparticles and poly[2-(dimethylamino) ethyl methacrylate] (PDMAEMA) was synthesized via anchoring Fe3O4 nanoparticles onto a GO nanosheet by a facile coprecipitation method and subsequently grafting PDMAEMA to the surface by click chemistry. It was shown that the composite had a defined structure and morphology. Thermogravimetric analysis (TGA) results showed that the weight ratio of attached Fe3O4 nanoparticles was above 10.86 wt% and the surface grafting density reached above 2 chains per 10 000 carbon atoms of GO sheet. The composite exhibited good water dispersive stability depending on surface grafting density and grafted chain length. Loading and release behaviours of levofloxacin (LOFX) demonstrated that the composites had high loading capacity and pH triggered controlled release properties.

Interfacial modification on polyhydroxyalkanoates/starch blend by grafting in-situ

The interfacial adhesion between polyhydroxyalkanoates (PHAs) and native starch is poor. To improve the interfacial adhesion, PHAs were in-situ grafted onto starch using dicumyl peroxide (DCP) as a free radical initiator. The grafting reaction was carefully characterized and confirmed by gel analysis and Fourier transform infrared spectroscopy (FT-IR). The gel yield of the PHAs/starch/DCP blend increased with the DCP concentration up to 2wt%. Meanwhile, obvious plastic deformation (stretched fibrils) was observed at the interface in the PHAs/starch/DCP blend in comparison with complete interfacial debonding in the PHAs/starch physical blend. The improved interfacial adhesion after grafting was further confirmed by a reduction in adhesion factor (Af) obtained from dynamic mechanical analysis (DMA). The mechanical strength and the crystallization rate of the PHAs were deteriorated after incorporation of starch, and were backed up by the interfacial improvement. A linear relationship between the mechanical properties and the gel yield was discovered. In addition, the PHAs/starch/DCP blend exhibited higher decomposition active energy (Ea) and thus better thermal stability in comparison with the PHAs and the PHAs/starch physical blend. Therefore, this study provides a simple route to utilize low-cost starch as a component in biopolymer blend.

Crystallization behaviours of bacterially synthesized poly(hydroxyalkanoate)s in the presence of oxalamide compounds with different configurations

Bacterially synthesized poly(hydroxyalkanoate)s (PHAs) suffers from low crystallization rate which is enhanced by using tailor-made oxalamide compounds as nucleators. The influence of nucleator configurations on the crystallization behaviour of the PHAs was investigated using differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction (XRD). The oxalamide compounds with ringy terminal structures (cyclohexyl and phenyl), notably the phenyl group, show higher nucleation efficiency and a better compatibility in the PHAs matrix, while the linear terminal structure (n-hexane) has poor nucleation effect. The crystallization temperature (Tc) and the crystallinity (Xc) of the PHAs are increased from 58°C to 71°C and from 5% to 48%, respectively, after addition of 0.75wt% of the nucleator (phenyl group) upon cooling from the melt. Meanwhile, the half-life isothermal crystallization time (t0.5) of the PHAs at 110°C is decreased by 70%. The oxalamide compounds increases the nuclei density of the PHAs accompanied with a reduction in spherulitic size. In addition, the crystal form and crystallization mechanism of the PHAs are not altered obviously after addition of the nulceators as confirmed by the POM, XRD and Avrami analysis.

Effects of modified nanocrystalline cellulose on the hydrophilicity, crystallization and mechanical behaviors of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate)

Fine dispersion of nanocrystalline cellulose (NCC) in a bacterially synthesized poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) matrix is challenging due to their incompatibility. To improve their compatibility, NCC was first grafted with PHBH, marked as NCC-g-PHBH, using (3-aminopropyl)triethoxysilane (APES) as a coupling agent, leading to a grafting degree of ∼25 wt%. The grafting mechanism and the mechanical and crystallization behaviors of the PHBH/NCC-g-PHBH blends, prepared via solution casting, were systematically investigated. The NCC was uniformly dispersed in the PHBH matrix after grafting with PHBH. As a consequence, the mechanical and crystallization properties of the PHBH were greatly improved in the presence of the NCC-g-PHBH. With the addition of 1.0 wt% NCC-g-PHBH, the Youngs modulus of the PHBH/NCC-g-PHBH blend was increased by 15%, which is further evaluated by the Halpin–Tsai model, while the surface contact angles decreased by 30°. Furthermore, the presence of NCC-g-PHBH copolymers greatly increased the nuclei density and decreased the spherulite size of PHBH without changing the crystal structures. Therefore, this work provided a novel route to prepare fully biodegradable and bio-based PHBH nanocomposites, which may broaden the application range of PHBH.