The amorphous state equivalent of crystallization: new glass types by first order transition from liquids, crystals, and biopolymers

We review the normal state of glasses and explain some exceptional cases by referring to a mode of glass formation, which is distinct from the normal and involves a first order transition route. Important materials like amorphous water and silicon belong to the distinct class, which we expect will prove to have many members, and which we expect will occupy a position part way between quasi-crystals and ordinary glasses. There may also be many mesoscopic examples of this class of material, because the low energy tertiary structures obtained by the (first order) folding of specialized heteropolypeptides (proteins) satisfy many of the criteria that we utilize in defining the class. The mesoscopic examples have the advantage of undergoing the transition to the low energy state under conditions of relatively long-lived metastability so that the phenomenon can be studied at leisure. There is no obvious reason why the phenomenon should be confined to biomolecules. We discuss the relation of the new glass types to ordinary glasses, plastic crystals, folding proteins and quasi-crystals, within the energy landscape paradigm. The first order transition occurs in the lower levels of the landscape in all cases, implying that 'funnels' are the general rule. (C) 2000 Editions scientifiques et medicales Elsevier SAS. All rights reserved.