POLYMORPHISM OF A GLASS-FORMING PLASTIC CRYSTAL - A KINETIC INVESTIGATION

The glassy crystalline state designates the frozen state of the rotato r phase of some molecular crystals. These systems are very suitable fo r investigating the vitrification and the crystallization processes as well as the interrelations between these two processes. This paper sh eds light on this problem through a kinetic investigation of the glass forming plastic crystal (cyanoadamantane)(1-x) (chloroadamantane)(x) for x = 0.25. A careful study of both the equilibrium phase diagram an d the mode of transformation upon deep quenching conditions has been p erformed by time resolved x-ray diffraction and differential scanning calorimetry in a variety of thermal treatments. The results reveal a c omplex kinetic behavior corresponding to the imbrication of the kineti cs toward two low temperature phases: (IV) and (III). Phase (IV) is fo und to be metastable with respect to phase (III) and only appears upon specific thermal treatments which are clearly established. The condit ions in which the monotropic transition between the transient metastab le phase (IV) acid the undercooled rotator phase (I) can be seen are d escribed. (C) 1996 American Institute of Physics.