Low-Temperature Heat Capacity, Glass-Transition Cooperaivity, and Glass-Structure Vault Breakdown in a Series of Poly(n-alkyl methacrylate)s

摘要

Heat capacity in the 1 K range and close to T_g was studied in a series of poly(n-alkyl methacrylate)s, PnAMA, including some random copolymers. We report on two findings: (i) The glass transition temperature T_g and the cooperativity of he #alpha# process that freezes-in at T_g, N_#alpha#(T_g), decrease smoothly between methyl (PMMA) and hexyl member (PnHMA); the cooperativity from N_#alpha#(T_g) = 35 to N_#alpha#(T_g) (approx =) 1. (ii) The tunnel density measured in the 1 K range as c_1 constant of the heat capacity function increase between PMMA and the ethyl member (PEMA), shows, after a maximum near PEMA, a sharp drop (factor of 6) to the butyl member (PnBMA), and remains constant at a low level up to the octyl member (PnOMA). The cooperativity of the #alpha# process near T_g and the tunnel density will be correlated assuming that freezing-in (vitrification) fixes the dynamic heterogeneity in the self-organized equilibrium liquid near T_g. We try to explain how freezing of cynamic heterogeneities in the PnAMA series is responsible for the trend in the tunnel density including the sharp drop: Freezing-in of cooperativity shells around Glarum defects forms some vaults that enlarge the free volume near these defects in comparison to a situation without vaults. The maximum in c_1 is promoted by this additional free volume; the drop in c_1 is caused by a vault breakdown effect if the cooperativity shell is too small for vault fomation, N_#alpha#(T_g) < 15.