Glass transition and fragility in the simple molecular glassformer CS ₂ from CS₂-S₂Cl₂ solution studies

With an interest in finding the fragility for a simple, single component, molecular glassformer, we have determined the dielectric relaxation and glass transition behavior for a series of glasses in the CS₂-S ₂Cl₂ and CS₂ -toluene systems. Crystallization of CS₂ can be completely avoided down to the composition 20 mol% second component, and the fragility proves almost independent of CS₂ content in each system. Since the glass temperature T_g obtained from both thermal studies and from dielectric relaxation (using T_g,diel = T_{τ=100 s}) is quite linear over the whole composition range in each system, and since relaxation time data for pure CS₂ fall on the same master plot when scaled by the linearly extrapolated T_g value, we deduce that pure CS₂ has the same high fragility as the binary solutions. The value is m=86, as for ortho-terphenyl (OTP). Based on observations of independent studies for the vibrational density of states (VDoS) (of inherent structures for OTP and instantaneous, at-temperature structures for CS₂), we attribute the high fragility to an excess vibrational heat capacity (defined by C_p(vib, excess) = dS(vib, excess)/d ln T) originating in the behavior of the low frequency modes of the VDoS (the boson peak modes). Both low frequency DoS and anharmonicity increase with increasing temperature, augmenting the configurational entropy drive to the top of the system energy landscape. The surprising implication is that fragility is determined in the vibrational, not configurational, manifold of microstates.