Correlation between fragility and cooperativity in segmental dynamics of glass-forming para-substituted polystyrenes

Abstract

To understand the mechanism of the viscous slowdown in supercooled liquids that is responsible for glass transition, we investigate the interrelation between glass transition temperature Tg, fragility and cooperativity in segmental dynamics. Polymeric glass formers having a similar chemical structure are expected to exhibit clear correlations between the above parameters. In this paper, polystyrene (PS) derivatives possessing various para-substituents are studied using calorimetry with regard to the fragility parameter m, dynamic length scale ξ and the number of cooperatively rearranging segments NCRR at Tg. Positive correlations were revealed for both NCRR(Tg) vs m and ξ3(Tg) vs m. Both fragility and cooperativity were found to increase as the bulkiness of the substituent increases. Wide-angle X-ray scattering measurements revealed that the structural correlation between backbone chains is reduced as the bulkiness increases. This may be responsible for the reduced cooperativity. In contrast, for poly(methacrylic acid ester)s, the relations between the above parameters appeared to be less clear. The clear trend observed for the PS system may be due to the rigidity of the phenylene unit, through which the substituent directly affects the backbone dynamics. The estimated activation energy per segment increased consistently with an increase in the para-substituent’s bulkiness. The relations between glass transition temperature, fragility and cooperativity in segmental dynamics were investigated on polystyrene derivatives possessing various para-substituents using calorimetry. A positive correlation was observed between cooperativity and fragility. Both fragility and cooperativity were found to increase as the bulkiness of the substituent increases. Estimated activation energy per segment increased consistently with an increase in the substituent’s bulkiness.