The mechanical behaviour of nano-particle modified thermoplastics

Abstract

Hybrids made up of polymers and nano-particles have received great interest both in academia and industry. The uniformly dispersed nano-sized particles create a large interfacial area per unit volume and could result in a remarkable improvement in thermal and mechanical properties when compared with virgin polymer or conventional micron-sized particle modified composites. This thesis presents and discusses the effects of adding rigid nano-sized ZnO particles into four thermoplastic systems at 23 °C and 80 °C. The matrix materials include two styrene acrylonitrile (SAN) copolymers, polymethyl methacrylate (PMMA) and polyamide 6 (PA 6). Three types of ZnO particles were used as fillers, denoted A, B and C. The ZnO-A and ZnO-B are cylindrical, while the ZnO-C particles are spherical. The microstructures and the thermal and fracture behaviour of the resulting systems under quasi-static and fatigue loadings were investigated using various techniques. The microstructure studies showed that ZnO nano-particles could be uniformly dispersed into the matrices up to about 0.30% by volume fraction, above which they tended to cluster into agglomerates. The addition of ZnO nano-particles had only a marginal effect on the glass transition temperature and yield stress of the modified thermoplastics. A notable increase in Young’s modulus was observed when the nano-particles were well bonded with the matrices. Remarkable improvements in toughness and fatigue threshold were also observed for some ZnO modified thermoplastic systems. However, the toughness decreased at relatively high concentrations of reinforcement. Microscopy studies showed that debonding of the nano-particles and subsequent plastic void growth in the matrix and multiple crazing initiated by the nano-particles were the main toughening mechanisms identified. At high volume fractions, the ZnO agglomerates acted as stress concentration sites, and the voids nucleated at lower stress levels, which led to a reduction in the toughness of the modified composites.