New Experimental Evidence for Thermodynamic Links to the Kinetic Fragility of Glass-Forming Polymers

Providing a thermodynamic basis to kinetic fragility has been pursued for decades. This objective is particularly challenging for glass-forming polymers because of the difficulty in gauging configurational excess entropy ΔSex. Herein, we report that enthalpy hysteresis ΔHR determined from a well-defined cooling and subsequent heating cycle of heat capacity curves bears a proportional relationship with α-related excess enthalpy at the glass transition temperature (Tg). Correlations of fragility (me) with ΔHR and heat capacity jump ΔCP at Tg are explored in a broad range of polymers, including monodispersed polystyrene with a molecular weight of 580–2 400 000, homopolymers with a fragility ranging from 50 to 200 and Tg ranging from 200 to >400 K, and copolymers and polymer/small-molecule mixtures with different compositions and varied intermolecular strengths. Surprisingly, all of the presented data follow me = 0.75ΔCP·Tg/ΔHR + 15, a formula that can be theoretically derived from the Adam–Gibbs equation if the prefactor τ0 is given at 10–13 s. These experimental results not only provide a possible routine for estimating the α-related excess entropy of noncrystallizable polymers but also validate the thermodynamic link to the kinetic fragility of glass-forming polymers.