Crystallization of Ca(PO 3 ) 2 Glass Below the Glass Transition Temperature

Block specimens of Ca(PO 3 ) 2 glass crystallize easily at temperatures considerably lower than the glass transition temperature, T g ; thin blown films of the glass do not exhibit this behavior. The crystals, which are β‐Ca(PO 3 ) 2 with a trace of the γ form, are identical to those formed above T g ; they exhibit “hemispherulitic” growth from the glass surface. The activation energy for crystal growth below T g is 30 kcal/mol, which indicates that the growth mechanism differs from that in blown film samples above T g (activation energy = 110 kcal/mol). A model for crystallization below T g is proposed; the phenomenon is explained in terms of induced tension in the molecular chains at the glass‐ crystal interface.