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 Delta S-ex. Herein, we report that enthalpy hysteresis Delta H-R determined from a well-defined cooling and subsequent heating cycle of heat capacity curves bears a proportional relationship with alpha-related excess enthalpy at the glass transition temperature (T-g). Correlations of fragility (m(e)) with Delta H-R and heat capacity jump ACp 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 SO 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) = 0.75 Delta Cp.T-g/Delta H-R + 15, a formula that can be theoretically derived Adam-Gibbs equation if the prefactor r o is given at 10(-13) s. These experimental results not only provide a possible routine for estimating the alpha-related excess entropy of noncrystallizable polymers but also validate the thermodynamic link to the kinetic fragility of glass-forming polymers.

Automated summary

This study explores the relationship between enthalpy hysteresis and excess enthalpy related to the glass transition temperature in glass-forming polymers. The experimental results provide a possible routine for estimating the alpha-related excess entropy of noncrystallizable polymers and validate the thermodynamic link to the kinetic fragility of glass-forming polymers.