||Correlation between fragility and cooperativity in segmental dynamics of glass-forming para-substituted polystyrenes
Sasaki, Takashi ,
Ichimura, MizukiIrie, Satoshi
694 , 2015-10 , Nature Publishing Group
To understand the mechanism of the viscous slowdown in supercooled liquids that isresponsible for glass transition, we investigate the interrelation between glass transitiontemperature Tg, fragility, and cooperativity in segmental dynamics. Polymericglass-formers having a similar chemical structure are expected to exhibit clearcorrelations between the above parameters. In this paper, polystyrene (PS) derivativespossessing various para-substituents are studied using calorimetry in regard to thefragility parameter m, dynamic length scale ξ, and the number of cooperativelyrearranging 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 thebulkiness of the substituent increases. Wide-angle X-ray scattering measurementsrevealed that the structural correlation between backbone chains is reduced as thebulkiness increases. This may be responsible for the reduced cooperativity. In contrast,for poly(methacrylic acid ester)s, the relations between the above parameters appearedto be less clear. The clear trend observed for the PS system may be due to the rigidity ofthe phenylene unit, through which the substituent directly affects the backbonedynamics. The estimated activation energy per segment increased consistently with anincrease in the para-substituent's bulkiness.