Ming Wang

Dynamic vulcanization of castor oil in a polylactide matrix for toughening

Dynamic vulcanization of biomass-derived castor oil in the presence of 4,4′-diphenylmethane diisocyanate (MDI) in a polylactide (PLA) matrix was performed with the aim of toughening PLA sustainably. Crosslinking of castor oil with MDI took place rapidly under dynamic vulcanization conditions, leading to the formation of a tough blend, which showed a phase-separated morphology with castor oil derived polyurethane (COP) as a dispersed phase and PLA as a continuous phase. The content of COP in the blends played an important role not only in the size of dispersed COP particles but also in the mechanical properties, rheological and crystallization behaviors of the formed PLA/COP blends. The size of dispersed COP, melt viscosity, and storage modulus of the blends increased significantly with COP content. The crystallization rate of PLA was enhanced by incorporation of COP due to the increased nucleation effect arising from interfacial nucleation of the phase-separated blends. The thermal stability of the blends was slightly reduced compared to neat PLA. The elongation at break of PLA was considerably increased by 45 times to 338%, compared to 7.5% of neat PLA, with the addition of only 5 wt% COP; meanwhile, the mechanical strength and modulus were largely retained. Cavitation, arising from dispersed phase debonding from the matrix, induced matrix plastic deformation was the toughening mechanism for the present PLA/COP blends.

Poly(sodium 4-styrenesulfonate) modified graphene for reinforced biodegradable poly(ε-caprolactone) nanocomposites

A homogeneous and stable water dispersion of graphene nanosheets (GNS) was prepared through a non-covalent functionalization technique by ultrasonic processing of GNS in the presence of poly(sodium 4-styrenesulfonate) (PSS) as the modifier. The dispersion was then used to compound with poly(e-caprolactone) (PCL) through solution coagulation to fabricate PCL/GNS nanocomposites. Scanning and transmission electron microscope observations indicated that PSS modified GNS dispersed uniformly in and showed strong interfacial adhesion with the PCL matrix when the loading of GNS was not more than 0.5 wt%. While when the loading of GNS increased to 1.0 wt%, aggregation morphology of the GNS in the PCL matrix was detected. The crystallization temperature of PCL, as investigated using a differential scanning calorimeter, increased apparently by incorporation of PSS modified GNS. Investigation on isothermal crystallization kinetics of neat PCL and its composites indicated that the crystallization of PCL was accelerated considerably. Addition of only 0.05 wt% GNS caused a 5.8 times improvement in crystallization rate compared to neat PCL. The crystallization mechanism almost remained unchanged. The improvement in crystallization rate was ascribed to the enhanced nucleation density by incorporation of PSS modified GNS, as evidenced using a polarizing optical microscope (POM). Tensile tests showed that both the tensile strength and the Youngs modulus of PCL were reinforced and increased gradually with increasing GNS loading within 0.5 wt%, meanwhile the elongation at break did not reduce but increased slightly. While when the loading of GNS increased to 1.0 wt%, the tensile strength and elongation at break reduced considerably due to the aggregation of GNS with increasing loading. Dynamic mechanical analysis indicated that the storage modulus of PCL was reinforced in the full temperature range by incorporation of PSS modified GNS.

Formation of thermally conductive networks in isotactic polypropylene/hexagonal boron nitride composites via “Bridge Effect” of multi-wall carbon nanotubes and graphene nanoplatelets

Fabrication of thermally conductive networks in polymer matrices is thought to be an efficient way to improve the thermal conductivity of polymer composites. Here we show a new approach to form thermally conductive networks in isotactic polypropylene (iPP)/hexagonal boron nitride (h-BN) composites via “bridge effect” of multi-wall carbon nanotubes (MWCNTs) or graphene nanoplatelets (GNPs). The isolated h-BN particles can be connected by MWCNTs or GNPs to form three-dimensional thermally conductive networks. It is found that the thermal conductivity of the iPP/h-BN composites is obviously enhanced but maintaining the electrical insulation by adding small amount of MWCNTs or GNPs. Because of the large content area of GNPs, the “bridge effect” of GNPs is more obvious than that of MWCNTs. The thermal conductivity of the iPP/h-BN composites with 10 wt% and 30 wt% h-BN particles show 14% and 23% enhancement by incorporation of 5.0 phr MWCNTs, respectively. Meanwhile, the thermal conductivity of the iPP/h-BN composites with 10 wt% and 30 wt% h-BN particles are enhanced by 59% and 70% when adding 5.0 phr GNPs, respectively. The electrical conductivities of the iPP/h-BN composites with MWCNTs and GNPs were maintained below 2.5 × 10−13 and 2.6 × 10−15 S cm−1, respectively.