Introduction

In this chapter we shall be concerned with the basic principles that govern the crystallization behavior of flexible long-chain molecules. The more-rigid type of polymers will be discussed in Chapter 5. The subject matter divides itself naturally into several interrelated subdivisions. These include thermodynamics of crystallization, kinetics and mechanisms of crystallization, structure and morphology, and microscopic and macroscopic properties. We shall discuss each of these topics in terms of fundamental physical and chemical concepts. There is an interrelation among the various aspects of polymer crystallization as is indicated by the chart given in Fig. 4.1.

Essentially all properties are controlled by the molecular morphology. In turn the molecular morphology is determined by the crystallization mechanisms. Such mechanisms are deduced from detailed studies of crystallization kinetics. Knowledge of the equilibrium requirements, or the thermodynamics of crystallization, is necessary in order to analyze the kinetics properly. Although this information is important and can be obtained theoretically, the actual equilibrium state of a crystalline polymer is rarely, if ever, achieved. This well-known situation is due to kinetic factors involved in crystallization from the melt that lead to the formation of relatively small crystallites with accompanying high interfacial free energies. Crystallites involving extended chains are difficult to achieve. The difficulties in establishing the equilibrium state led early on to the recognition that one is dealing with a metastable state [1]. The various aspects of the problems are clearly interdependent upon one another. This interdependence will become apparent as we examine experimental results.