Crystallisation rate-temperature curve

Different from the dissolution of amorphous polymers is that of semi-crystalline ones. Dissolution of these polymers is much more difficult than that in the glassy state, as the enthalpy of melting has to be supplied by the solvent. Many solvents, which are able to dissolve tactic but glassy polymers, are unable to dissolve the same polymer in the crystalline state. Asmussen et al. (1965) have found that the velocity of dissolution of crystalline polymers as a function of temperature closely resembles the velocity of crystallisation versus temperature curves. Polymers formed at the highest rate of growth also dissolve at the highest rate. 

An analysis of crystallisation rates is conveniently performed in terms of the so-called time-temperature-transformation (TTT) curves, which relate the time taken to crystallise a given fraction of the undercooled liquid or the supersaturated solution to the temperature. Experimentally, the crystallisation rates are measured by quenching the liquid phase to some predetermined temperature T and measuring the time taken for the solid to crystallise at that temperature, either by monitoring the latent heat of crystallisation or by microscopic observation. The volume fraction 4>(T) that crystallises out in time 1 is given by one form of the Avrami equation ... 

Amorphous materials are metastable phases, since the crystalline state corresponds to the thermodynamic equilibrium state. To obtain an amorphous material, a drastic cooling from the liquid or the gas phase down to a temperature where diffusion is negligible has to be performed to avoid crystallisation. Fig. 6.17. Experimentally, a number of curves such as (a) and (b) are obtained as a function of the cooling rate, which suggests that different disordered atomic arrangements may exist. 

Figure 4.4. Change of onsets of melting and crystallisation as a function of the rate of temperature change. The data were obtained with a TA Instruments 2920 DSC in two different configurations, with air cooling (lower curve) and cooling with the liquid nitrogen accessory...

The first part of the curve is an induction period in which time is required for the formation of nuclei. The steepest part is related to the growth of the spheruhtes. The final part is when the spherulites begin to touch each other and the rate slows down. When the crystallisation temperature is increased, the shape... 

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