Structure of Polymeric Solids

Abstract Properties of solid polymers are determined by their structure—crystalline or amorphous—and their molecular conformations and entanglements. Molecules with regularity pack together to form crystals, but crystallinity is always less than 100%. The crystals are usually lamellae, arranged radially within spherulites. Melting temperatures and crystallinity are usually measured from the peak in heat flow on melting, caused by the latent heat of melting. Crystallization theory predicts the observed peak in crystallization rate on cooling. Crosslinks prevent flow, so thermosets at high temperatures are soft solids. These and polymers with molecular irregularity fail to crystallize on cooling, and instead become glassy materials at the glass transition temperature. They show physical ageing near this temperature. Such amorphous polymers have a wide range of molecular conformations, that are especially important in understanding elasticity of elastomers. They can be represented as freely jointed chains, and modelled mathematically in terms of the Gaussian chain model.