Zhong-Ming Li

Poly(ethylene terephthalate)/polyethylene composite based on in-situ microfiber formation

Poly(ethylene terephthalate) (PET) microfiber was in-situ formed by compounding PET with polyethylene (PE) through a single screw extruder of a Haake rheometer system, where a rod die with comparatively smaller diameter (2.1 mm) was used, and the extrudate was drawn in a certain drawing ratio (3.1:1) and quickly cooled in cold water. Subsequently, the in-situ PET/PE composite was injection molded into specimens at temperatures that were lower than the melting temperature of the PET to keep the original shape of the PET fibers. The result from morphology observations of the composite showed that when the die diameter of the extruder is 2.1 mm and the drawing ratio of the extrudate is 3.1:1, PET was more or less changed into microfibers. The PET almost changed into fibers when the concentration was 15 wt%; concentrations below and above which decreased the fiber content. For the PET/PE blend prepared by conventional mixing technology, the dispersed PET formed spheres and no microfibrilar structure were found. The reinforcing effect of the PET fibers on the corresponding composite was significant at 15 wt% PET concentration observed from the correlation between the PET content and the tensile properties of the PET/PE in-situ composite. Besides, in general, the tensile strength and modulus of the composite increased with the PET concentration, and was higher than the conventional PET/PE blend without microfibers.

Temperature-Resistivity Behaviour of CNTs/UHMWPE Composites with a Two-Dimensional Conductive Network

Temperature-resistivity behaviour of carbon nanotubes (CNTs)/ultrahigh molecular weight polyethylene (UHMWPE) conductive composite with a 2-dimensional conductive network is investigated. The composite, in which CNTs were only dispersed in the interface of matrix particles, experiences a positive temperature coefficient of resistivity (PTCR) around the melting point followed by a negative temperature coefficient of resistivity (NTCR), and the resistivity begins to rise abruptly at the maximum crystallization temperature during cooling. The composite has a low PTC and NTC effec, which is determined by its 2-dimensional conductive network and the properties of the matrix.

Structural Hierarchy and Polymorphic Transformation in Shear-Induced Shish-Kebab of Stereocomplex Poly(Lactic Acid).

The realization of hierarchical shish-kebab structures for stereocomplex poly(lactic acid) (PLA) is achieved by the application of a shear flow (100 s(-1) for 1 s) mimicking what can be expected during polymer processing. Compared to the normal shearing scenarios, this transient and strong shear flow enables the creation of dense shish precursors in time- and energy-saving manner. The distribution of crystal form associated with the hierarchical structure is revealed by 2D Fourier transform infrared spectroscopy imaging, creating a unique visualization for both spatial resolution and polymorphism identification. Interestingly, in the shear stereocomplex chains are preferentially extended and crystallized as stable central cores with weak temperature dependence, whereas the development of lateral kebabs is defined by the distinct relation to the crystallization temperature. Below the melting point of homocrystals, both homo and stereocomplex crystallization are engaged in lamellar packing. Above that, exclusive stereocomplex crystals are organized into ordered lamellae. Combining the direct observations at multiscale, the ordered alignment of stereocomplex chains is recognized as the molecular origin of fibrillar extended chain bundles that constitute the central row-nuclei. The proposed hypothesis affords elucidation of shish-kebab formation and unique polymorphism in sheared stereocomplex PLA, which generates opportunities for engendering hierarchically structured PLA with improved performance.

Electrically Conductive Carbon Black/Poly(Ethylene Terephthalate)/Polyethylene Microfibrillar Composite: The influence of CaCO3 Nanoparticles

The influence of a complex filler system on the electrical properties of a microfibrillar conductive polymer composite (MCPC) is discussed. By adding insulating filler, nano-CaCO3, to carbon black (CB)-filled MCPC, the morphology of the poly(ethylene terephthalate) (PET) microfibrillar phase was tailored according to the ratio of CB/nano-CaCO3, and so were the electrical properties of MCPC. It was found that nano-CaCO3 did not influence electrical properties in a monotone way. With an increase in nano-CaCO3 content, on one hand, the surface of the microfibrils became smoother, which jeopardized the conductivity of the MCPC. At the same time, the nano-CaCO3 particles substituted for the CB particles on the surface of the microfibrils and further decreased conductivity. On the other hand, longer and better-defined microfibrils could form, which enhanced the conductive network and increased the conductivity of the MCPC. As a result, the percolation threshold changed little compared to the common CB-filled MCPC.

Role of microstructure of electrically microfibrillar conductive polymer composite in its conductivity

Abstract How to improve the electrical properties of conductive polymer composite (CPC) such as lowering the percolation threshold and endowing the composite with unique properties is a most important research area in developing this kind of material. Various methods have been employed, among which changing the processing procedure of the material is the most simple. The present paper describes how the authors, by eliminating the mixing procedure before compression moulding, managed to fabricate a material with different percolation thresholds and much more stable volume resistivity temperature behaviours compared to that utilising the mixing procedure. Microstructures of these two materials were investigated. The authors found that the composite produced using the mixing procedure had much shorter conductive fibrils, while that produced without mixing had a hierarchical structure, in which long and well defined conductive fibrils composing the conductive sheet structure first and conductive sheet overlapped together to form the conductive network throughout the composite.