Solid + liquid equilibria, phase transitions metastable equilibrium

The crystallization process of flexible long-chain molecules is rarely if ever complete. The transition from the entangled liquid-like state where individual chains adopt the random coil conformation, to the crystalline or ordered state, is mainly driven by kinetic rather than thermodynamic factors. During the course of this transition the molecules are unable to fully disentangle, and in the final state liquid-like regions coexist with well-ordered crystalline ones. The fact that solid- (crystalline) and liquid-like (amorphous) regions coexist at temperatures below equilibrium is a violation of Gibb s phase rule. Consequently, a metastable polycrystalline, partially ordered system is the one that actually develops. Semicrystalline polymers are crystalline systems well removed from equilibrium. 

It would certainly be desirable to see some definitive work establishing the validity of Equation 25 in at least a few systems. The above two examples suggest, furthermore, that it may be entirely possible to have metastable films at one or the other solid interface-for example, if the liquid-film transition involves a structural phase change [11], so that nucleation may be required to produce an equilibrium film. 

The phase diagram of stable equilibrium states (solid lines) of sulfur is superimposed by a phase diagram of metastable states (dashed lines) in which direct transition from rhombic to liquid sulfur can be formed by rapidly increasing the temperature from point A to C. This transition takes place at point D situated on the fusion pressure curve of the metastable rhombic sulfur, the corresponding metastable triple point being Tm (Fig. 5). 

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