Oriented crystallization and contractility

A characteristic property of amorphous polymers is the ability to sustain large strains. For cross-linked three-dimensional networks the strain is usually recoverable and the deformation process reversible. The tendency toward crystallization is greatly enhanced by deformation since chains between points of cross-linkages are distorted from their most probable conformations. A decrease in conformational entropy consequently ensues. Hence, if the deformation is maintained, less entropy is sacrificed in the transformation to the crystalline state. The decrease in the total entropy of fusion allows crystallization, and melting, to occur at a higher temperature than would normally be observed for the same polymer in the absence of any deformation. This enhanced tendency toward crystallization is exemplified by natural rubber and polyisobutylene. These two polymers crystallize very slowly in the absence of an external stress. However, they crystallize extremely rapidly upon stretching. 

A stress-strain isotherm for the uniaxial deformation of natural rubber, at ambient temperature, that was cross-linked in the liquid state is shown in Fig. 8.1.(5) Here f is the nominal stress defined as the tensile force,/, in the stretching direction divided by the initial cross-section, and a is the extension ratio. Using the most rudimentary form of molecular rubber elasticity theory f can be expressed as (6-9) 

For a further detailed discussion of rubber elasticity theory, see Refs. (6,7,8,9). 

In order to properly analyze the melting of an oriented system, it must be ascertained whether the process is reversible, i.e. whether oriented crystallites are formed on recrystallization. This concern exists since it is possible that the original oriented 

Although irreversible melting is commonly associated with oriented crystalline polymers, the possibility of conducting the transformation under reversible conditions that approach equilibrium cannot be disregarded. In fact, the treatment of this problem as one of phase equilibria lead to important relations between crystallization, deformation, and dimensional changes.(3,4) 

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