ABSTRACT
This chapter deals with transitions between the liquid and solid state in polymer systems. For most polymers crystallization is a relatively slow process and the growth of spherulites can be observed and their rate of growth often measured using an optical microscope. Crystallization depends upon concentration fluctuations within the melt such that at some critical temperature these result in the formation of what is called a primary nucleus that is stable and can then grow. For practical purposes the fact that the melting temperature of a particular sample of a polymer varies with thermal history is usually not that critical. The melting temperature is inversely related to the entropy change on going from the crystalline state to the melt for a pure polymer. Upon heating a glassy solid, there is no such sharp transition in these properties. It just appears that the material “softens” in a certain temperature range until it eventually appears to be a liquid.
