Polymorphism of a glass forming plastic crystal: A kinetic investigation

The glassy crystalline state designates the frozen state of the rotator phase of some molecular crystals. These systems are very suitable for investigating the vitrification and the crystallization processes as well as the interrelations between these two processes. This paper sheds 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 and the mode of transformation upon deep quenching conditions has been performed by time resolved x-ray diffraction and differential scanning calorimetry in a variety of thermal treatments. The results reveal a complex kinetic behavior corresponding to the imbrication of the kinetics toward two low temperature phases: (IV) and (III). Phase (IV) is found to be metastable with respect to phase (III) and only appears upon specific thermal treatments which are clearly established. The conditions in which the monotropic transition between the transient metastable phase (IV) acid the undercooled rotator phase (I) can be seen are described. (C) 1996 American Institute of Physics.