Spherulitic crystallisation of polymers from the melt

The overall rate of crystallisation of a super-cooled liquid is determined by the two factors mentioned the rate of formation of nuclei (above the critical size) and the rate of growth of such nuclei to the final crystalline aggregates. 

When a polymer sample cools down from the molten state to the temperature of measurement, crystallisation is very slow at first. After an induction period the process speeds up to a maximum rate and slows down again as it approaches the final equilibrium state. 

Most polymers crystallise at measurable rates over a range of temperatures that is characteristic of each polymer. It may extend from about 30 °C above the glass temperature (Tg) to about 10 °C below the melting point (Tm). 

The rate of crystallisation increases as the temperature decreases below Tm, reaching a maximum at T, and decreases again when the temperature is lowered still further. 

There is hardly a class of materials in which bulk properties are as kinetically determined as in that of the macromolecules. The consequences of the nature of nucleation and that of growth are so persistent that virtually no amount of subsequent annealing can eliminate their effects. 

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