The term amorphous is used to describe materials that are completely non-crystalline, such as polymer glasses and rubbers. Perhaps the most widely-known amorphous glassy polymer is polystyrene which, although relatively brittle, is used widely because it can be produced relatively cheaply and has good processing properties. The use of poly(methyl methacrylate) (PMMA), with its excellent optical properties, has now been extended to general thermoforming and moulding applications. Polycarbonate also has been introduced, and its clarity and toughness have been exploited widely. Although we will see in subsequent chapters that the properties of these materials have been studied at length, it is difficult to characterize their detailed morphology in simple terms, as there is no welldefined molecular order in amorphous polymers.