Fusion of homopolymers

The melting-crystallization process of a system of small molecules is formally described as a first-order phase transition. Appropriate laws then follow that can be applied to a variety of problems. For a one-component system at constant pressure, the transition temperature is independent of the relative abundance of either of the two phases that are maintained in equilibrium. Melting is very sharp. The characteristic temperature of equilibrium is defined as the melting temperature. For the above conditions to be experimentally satisfied an almost perfect internal 

The essence of the problem is whether the ordered regions in the crystalline polymers can be treated as a separate phase. The usual thermodynamic criteria will have to be satisfied. For a pure phase of one component the chemical potential must be uniform throughout the phase and only depend on the temperature and pressure. For a poorly developed crystalline system, whether it be polymer or low molecular weight species, this condition will obviously not be fulfilled. Under these circumstances the chemical potential will also depend on the degree of order and the crystallite size. The extent to which the idealized crystalline state can be approached must ultimately be judged by the sharpness of the fiasion process and the 

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Theories of Melting Points and Heats of Fusion of Homopolymers. 237... 

This portion of the chapter can be summarized by noting that there is a substantial body of evidence demonstrating that formal phase-equilibrium thermodynamics can be successfully applied to the fusion of homopolymers, copolymers, and polymer-diluent mixtures. This conclusion has many far-reaching consequences. It has also been found that the same principles of phase equilibrium can be applied to the analysis of the influence of hydrostratic pressure and various types of deformation on the process of fusion [11], However, equilibrium conditions are rarely obtained in crystalline polymer systems. Usually, one is dealing with a metastable state, in which the crystallization is not complete and the crystallite sizes are restricted. Consequently, the actual molecular stmcture and related morphology that is involved determines properties. Information that leads to an understanding of the structure in the crystalline state comes from studying the kinetics and mechanism of crystallization. This is the subject matter of the next section. 

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