Adiabatic nucleation and glasses

Abstract

A recently developed Adiabatic Nucleation Theory (ANT), which has been shown to be in good agreement with the largest experimental supercoolings of pure liquid elements (mostly metals), is applied to organic, inorganic and metallic glasses. It is demonstrated that, in order to obtain a glass by cooling from the melt, the (normal, slow cooling) glass transition temperature ( T g) has to be systematically: (i) higher or (ii) only slightly lower than the maximum supercooling temperature ( T −), predicted by ANT. In the former case, rapid quenching is sometimes necessary in order to avoid (or reduce) crystallization initiated by heterogeneous nucleation and in the second case, rapid quenching is always necessary in order to also avoid homogeneous nucleation. The latter case can be explained by the fact that T g can be “raised” above T − by fast quenching. Only polymers containing co-polymers may show T g's which are considerably lower than T −, because these co-polymers may stabilize (eventually small) amorphous domains of the principal polymer below T −. Other materials with low T g's ( T g ⪡ T −) just do not form glasses by quenching from the liquid, but can eventually be obtained in the amorphous, glassy form by condensation from the vapour phase on a cold substrate. In this latter case, the glass is really “composed” well below T − and T g. In conclusion it can be stated that the glass forming ability is well characterized by T g/ T −.