Читать книгу Encyclopedia of Glass Science, Technology, History, and Culture - Группа авторов - Страница 30

This description of the glass transition applies to a variety of kinetically controlled processes in crystals. Plastic crystals, characterized by low entropy of fusion and an unusually high plasticity, are good examples of disordered systems with three‐dimensional long‐range order. When the high‐temperature form of cyclohexanol (ChI), for instance, crystallizes at 299 K, the C₆H₁₂O molecules order in a face‐centered cubic lattice but their regular shape allows them to maintain orientational mobility by rotations around the lattice points. It is through a transition to the low‐temperature polymorph (ChII), which is stable below 265 K, that this dynamics vanishes and the orientational disorder disappears [36]. With rapid cooling rates, the ChI form can be obtained metastably and kept for long periods of time below 180 K. On further cooling, a transition is eventually observed near 160 K (Figure 13). The orientational disorder of C₆H₁₂O molecules is then frozen in within the crystal. In contrast to the ChII form, whose entropy is zero at 0 K, ChI has a residual entropy of 4.7 J/mol K. The similarity with the glass transition phenomenology is such that the name of glassy crystals has been proposed for crystals where rotation of molecular groups is freed above a glass‐like transition temperature and gives rise to relaxation phenomena much more complex than summarized here (Chapter 8.6).

Figure 13 The calorimetric signature of orientational disorder in cyclohexanol plastic crystal. Measurements made upon heating with a gap from slightly above the glass‐transition temperature T_g and the melting temperature T_f because of rapid transformation into the stable, ordered polymorph.

Source: Data from [36].