Glass transition and fragility in the simple molecular glassformer CS2 from CS2–S2Cl2 solution studies

摘要

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–S2Cl2 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 Tg obtained from both thermal studies and from dielectric relaxation using Tg,diel=T=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 Tg 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 Cp vib, excess=dSvib, 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.