Collaborative Research: Wetting Phenomena in Particle-Filled Polymers: Multifunctional Composites with Easy Processability

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Fibers are often mixed into plastics to make them stiffer, stronger, or better electrical conductors. Yet such fiber-containing plastic composites are difficult to manufacture by standard molding or other processing. Furthermore, fibers often suffer damage during manufacturing. This project is to create a bridging system that converts particle fillers into a network of reformable fibers i.e. fibers that are capable of reassembling after breaking. Composites with such network can retain the initial design properties (such as high strength and toughness or high conductivity) without suffering property loss during processing. The Principle Investigators will conduct scientific outreach at pre-college level, especially at the Pittsburgh SciTech High School and two K-8 schools with high minority enrollment, to encourage students to pursue education in STEM disciplines.The specific hypothesis of this research is that composites containing a fiber-like labile filler can be realized by dispersing solid particles into a polymer matrix, and bridging them together by menisci of a wetting fluid. Since the menisci continuously break and reform during flow, the composites will have excellent processability. The properties of the resulting polymer composites may be tailored by suitable choice of filler, and this project will test fillers that realize composites with high stiffness, or high toughness, or high electrical conductivity. The hypothesis will be tested experimentally using polystyrene as the matrix plastic, and silica or metal particles as the particulate filler. Various thermoplastic materials will be used as the wetting phase that forms menisci between particles. Polystyrene, particles, and the wetting phase will be blended by a melt extrusion process optimized to realize meniscus-bridging of particles. Characterization includes tensile and impact testing, electrical conductivity measurements, and morphological studies by scanning electron microscopy. The intellectual significance of this project is to demonstrate that meniscus-bridging can be exploited to achieve materials with the processability of ordinary particle filled polymers, with the mechanical properties of fiber reinforced composites, or the electrical conductivity approaching metals. Furthermore, this project will advance the fundamental knowledge of capillary interactions in filled polymers.