The glassy state

The solid state: crystals and glasses

Organic matter often crystallizes only in part, or not at all. In particular, polymers often solidify in the form of a glass, but there are a number of low molecular weight organic materials that also form glasses, and there are inorganic glass formers as well; eg. Silicon oxide used for windows. A glass displays the mechanical properties of a solid – similar to crystalline solids, but the structure is disordered as in a liquid. In thermodynamic terms, the glassy state is a non equilibrium state. However, particularly for polymeric glasses, the relaxation time to reach equilibrium can diverge to infinity at the Vogel-Fulcher temperature, well above absolute zero; thus the non-equilibrium state is no longer transient. Thermodynamic theories have been tailored to accommodate the permanent non-equilibrium nature of the glassy state, and to describe the glass transition above which the material regains its fluidity and re-approaches thermodynamic equilibrium. These are powerful theories, but currently no comprehensive molecular theory of the glass transition based on first principles is available. ….the most important fact about the glassy staqte in thecontext of organic semiconductors is that he glass is structurally disordered like a liquid. Consequently, unlike crystalline materials there is no translation symmetry and Bloch’s theorem does not apply.

Kelsall, Nanoscale, Wiley