ELASTIC AND VISCOELASTIC PROPERTIES OF ADVANCED THERMOPLASTICS AND THEIR SHORT FIBER COMPOSITES (POLYETHERETHERKETONE, CRYSTALLINITY, MICROSTRUCTURE, CARBON)

The influence of microstructural features on the elastic and viscoelastic properties of Polyetheretherketone (PEEK) and short carbon fiber reinforced PEEK has been investigated. A "composites" approach has been employed to first model the properties of the semicrystalline polymer as a composite material and later to model the properties of the short fiber composite.;Short carbon fiber reinforced PEEK has been characterized for the elastic and viscoelastic response. The problems encountered in processing have been handled by suitable modifications in the molding cycle. Microstructural characterizations of fiber orientation and aspect ratio distributions have been used in conjunction with elastic and viscoelastic properties of neat PEEK to predict the effective composite properties. The elastic property predictions compare well with the corresponding experimental values. The predicted viscoelastic response shows deviation from the experimental response. The deviation has been attributed to differences in the properties of the polymer in the bulk and in the composite form.;The Aggregate Model has been used in conjunction with experimental moduli for the amorphous and semicrystalline polymer to extract an estimate for the crystalline modulus. The crystalline modulus was subsequently used to predict the creep response at intermediate levels of crystallinity. Creep and dynamic mechanical responses at elevated temperatures have been interrelated by the use of temperature-time-frequency superposition principle to obtain the long-term prediction for the creep response. The viscoelastic response was found to be affected by the physical aging phenomenon. The effect of aging on elastic and viscoelastic properties has been observed at all attainable levels of crystallinity. Calorimetric studies have been conducted to assess the influence of aging on the enthalpy relaxation of amorphous and semicrystalline PEEK.