THE ROLE OF MOLECULAR SHAPE IN THE GLASS-TRANSITION PROCESS

The role played by the molecular shape in the evolution of the secondary (beta -) relaxation process has been examined by analysing the data corresponding to the binary glass forming systems. In these systems, it is observed that the activation energy of the solute molecule saturates at a viscosity (eta-s) with an increase in the solvent viscosity. A comparison of the binary-single component-systems and H-bonded systems has been made and it is concluded that the beta -relaxation returns to a perfect arrhenius behaviour in a region where the viscosity of the supercooled liquid reaches a values eta-s and also it is observed that the increase in the activation energy E-beta depends on the shape of the molecule. It is suggested that the eta-s corresponds to the temperature T11, where a change in the behaviour of the system has previously been observed (Murthy, 1988,89). It is also suggested that these results are consistent with the amorphous cluster model suggested previously. In the case of non-spherical, non-H-bonded systems, the temperature T11 approximately corresponds to the fusion temperature of the beta -process with that of the alpha -process.