The Influence of Reinforcing Fibers on the Morphology and Crystallization of Thermoplastic Polymer Composites
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THE INFLUENCE OF REINFORCING FIBERS ON THE MORPHOLOGY AND CRYSTALLIZATION OF THERMOPLASTIC POLYMER COMPOSITES LUDWIG REBENFELD, GLENN P. DESIO, and VERONIKA E. REINSCH TRI/Princeton, and Department of Chemical Engineering, Princeton University, P.O. Box 625, Princeton, New Jersey 08542 USA Abstract Semi-crystalline thermoplastic polymers are being used increasingly as matrices in high performance fiber reinforced composites. The crystallization kinetics and morphology of these polymers have been studied extensively, but relatively little attention has been given to the effects of the reinforcing fibers on the crystallization process. We have studied the effects of glass, carbon and aramid fibers on the rates of crystallization, the degree of crystallinity, and the glass transition temperature of such typical thermoplastics as poly(phenylene sulfide) and poly(ethylene terephthalate). Based on the isothermal crystallization studies using DSC, we find that, in general, reinforcing fibers increase the rates of crystallization and decrease the degree of crystallinity, the extent of these effects being dependent on the weight fraction of fiber in the composite, the specific type of fiber, and the nature of surface finishes (sizes) that may have been applied. The spherulitic morphology that develops in these polymers during the crystallization process, as characterized by polarized light microscopy, is also affected by the reinforcing fibers. In many cases, transcrystalline regions develop near the fiber surface due to nucleation effects. Introduction Fiber reinforced composites are a rapidly growing class of engineering materials that are finding widespread applications in building construction, automobiles, aircraft, boats, recreational equipment, and a nearly endless list of specialty products. Engineering plastic composites invariably become the materials of choice when high strength and low weight are required, and when relative ease of fabrication into complex geometric configurations is necessary. The performance characteristics of fiber composites depend on many factors, including the physical properties of the fibers, the geometric arrangement of the fibers in the reinforcing network, the chemical, thermal, and physical properties of the plastic matrix, and the interaction between the two major components of a composite - the fibers and the matrix. Indeed, it is probably correct to say that interfacial effects are critically important, and that the quality of the interface between fibers and matrix controls ultimate performance and durability of the composite. A wide range of thermosetting and thermoplastic resins have found extensive use in composite materials. Epoxies and polyimides are among the most important thermosets, while polycarbonates, polyetheretherketones, aromatic and liquid crystalline polyesters, polyphenylene sulfides, and certain polyimides are among the important thermoplastics. Many different types of fibers have found application as reinforcing elements in composites. In addition to the more
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