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 ΔS ex. Herein, we report that enthalpy hysteresis ΔH R 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 (T g). Correlations of fragility (m e) with ΔH R and heat capacity jump ΔC P at T g 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 T g 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 m e = 0.75ΔC P·T g /ΔH R + 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.