Thermal treatment, melting polymorph

Typical methods for generation of polymorphs include sublimation, crystallization from different solvents, vapor diffusion, thermal treatment, melt crystallization, and rapid precipitation. High-throughput screening methods have been reported for polymorph screening.50... 

In order to learn more about the am polymorph we crystallized PDES isothermally from the melt. A somewhat different phase diagram results then. By quenching the material from the melt to the isothermal crystallization temperature, bypassing thus the mesomorphic state, the crystallinity of the sample diminishes and the peaks move to lower temperature (266 K for the high temperature to mesomorphic phase transition and 290 K for the low to high temperature transition). This could be due to smaller crystals. The position of the peaks however, is not affected by the crystallization conditions and the subsequent thermal treatment. The ratio of the heats of fusion of the two peaks has also changed. These are indications that the phase... 

The degree of crystallinity and the order in existing polymer crystallites can be increased by a thermal treatment of the solid polymer at a fixed or changed temperature (annealing) without complete melting or dissolution. Crystalline polymers can change to more stable crystal polymorphs. 

Some surfactants show crystal polymorphism. Transitions between different crystalline structures are observed as sharp endothermic peaks on DSC thermograms. Frequently, crystal-crystal transitions are not reproducible because impurities or thermal treatment can promote the formation of metastable states. Visual observations are recommended to distinguish crystal-crystal transitions from melting. 

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