Chunling Xin

Dual electrode mode electrospinning of biodegradable polymers

A high speed dual electrode mode electrospinning was developed in this study. With coexistence of both positively and negatively charged nozzles, the electrospun fibers interact and bridge grafting between nozzles, interconnect with each other, and are stretched on a movable collecting roll. The results demonstrate that a highly intertwined, three-dimensional isotropic network structure can be obtained by using the dual electrode mode. Moreover, the productivity was increased more than 170 times compared to that using the conventional single electrode mode process, which is essential for commercialization of the electrospinning process.

Effect of processing history on the rheological properties, crystallization and foamability of branched polypropylene

The effects of processing history and post-processing annealing procedure on the rheological properties of long-chain branched polypropylene (LCB-PP) have been studied intensively. It is found that melt flow index, swelling ratio, storage modulus and complex viscosity at low frequency, as measures of melt viscoelasticity for LCB-PP, were greatly influenced even by the short-time processing in an internal mixer. Further, the relationship between the magnitude of shear modification and total shear strain was linear on a logarithmic scale. Moreover, the rheological properties of processed LCB-PP were recovered by vacuum-annealing to some extent. It also demonstrates that more intense processing history resulted in longer time required for the recovery of rheological properties. As far as the recovery process was concerned, high-pressure CO2 saturation annealing was much more effective than vacuum-annealing and more sufficient recovery was brought by higher saturation pressure. This superiority of high-pressure CO2 saturation annealing was attributed to the larger free volume for PP chain movement, which shorten the annealing time required for long branches to recover to their initial states. Besides, the peak temperature during the crystallization of LCB-PP was substantially improved after processing. Meanwhile, the foamability of LCB-PP was deteriorated after shearing process, which manifested as larger cell size and lower cell density.

Improved viscoelastic, thermal, and mechanical properties of in situ microfibrillar polypropylene/polyamide 6,6 composites via direct extrusion using a triple-screw extruder

In this work, a triangle arrayed triple-screw extruder (TTSE) was used to prepare in situ polypropylene/polyamide 6,6 (PP/PA66) microfibrillar composites (MFCs) by direct extrusion at a processing temperature between the melting points of the two phases. The dispersed phase, PA66 particles, deformed, merged and was stretched into fibrils under the alternating shearing and extensional flow field in the TTSE. The fibrillar morphology was controlled through adjusting the PA66 content and processing parameters (temperature and screw speed). Dynamic oscillatory shear rheological properties and extensional viscosity of PP/PA66 microfibrils with different aspect ratio were studied. The obtained results showed that the storage modulus and complex viscosity of PP/PA66 MFCs were improved with increasing fibrillar aspect ratio. Meanwhile, the loss tangent tan δ value decreased and the radius of the Cole–Cole plot circle increased with an increase in fibrillar aspect ratio. The PP/PA66 MFCs exhibited a pseudo-solid or gel-like behavior. The gel point concentration, which was determined by the Winter–Chambon criterion, decreased with the increasing fibrillar aspect ratio due to the increased interfacial area and enhanced entangled network structure. Additionally, high-aspect-ratio PA66 fibrils have dramatically improved PPs melting temperature, crystallization kinetics, and mechanical properties.

Effects of Organomontmorillonite Content on Morphology and Mechanical and Thermal Properties of Poly(2,6-dimethyl-1,4-phenylene oxide)/Polyamide-66 Nanocomposites

The nanocomposites consisting of polymer matrix and nanofiller have attracted great attention because of the improved physical properties. In this paper, organomontmorillonite (OMMT) was introduced into poly(2,6-dimethyl-1,4-phenylene oxide) grafted maleic anhydride (PPO-g-MA) compatibilized poly(2,6-dimethyl-1,4-phenylene oxide)/polyamide-66 (PPO/PA66) blends by melt extrusion. The morphology of PPO/PA66 nanocomposites with different amounts of OMMT was investigated using transmission electron microscopy (TEM), wide-angle X-ray diffraction (WAXD), and scanning electron microscopy (SEM). The OMMT platelets exhibited an exfoliated structure in the PA66 matrix and an intercalated structure on the surface of PPO domains at low OMMT loading (2 phr). However, the exfoliated platelets in matrix were found to transform into intercalated stacks by adding 6 phr of OMMT. The mechanical properties and thermal stability were significantly improved with the coexistence of exfoliated and intercalated OMMT at low OMMT loading (2–4 phr). The exfoliated OMMT platelets imposed a confinement effect on the macromolecular chains and thereby increased the storage modulus and complex viscosity of nanocomposites.