ABSTRACT
Glass transition – the transition of a substance from the liquid state into the solid (but not crystalline) state – is characteristic of both polymers and many low-molecular-mass substances. A substance proves glassy when the energy of molecular motion, decreasing in cooling, approaches the energy of molecular interaction in the order of magnitude. Glass transition occurs when the cooling rate of a substance exceeds that of crystallization, i.e., it can be observed for substances with a low crystallization rate or those that do not crystallize at all. The higher the viscosity of a liquid, the more probable it is that it will pass into the glassy state when cooled. Due to the high viscosity of the melt, glass transition is realized, e.g., for low-molecular-mass silicate glasses. Vice versa, rapidly crystallizable substances are very difficult – and sometimes impossible – to obtain in the glassy state. This explains the fact that crystals are much more frequent than glasses among low-molecular-mass substances in the solid state.
