Deducting the temperature dependence of the structural relaxation time in equilibrium far below the nominal T-g by aging the decoupled conductivity relaxation to equilibrium

作者：Wojnarowska Z^*; Ngai K L; Paluch M

来源：Journal of Chemical Physics, 2014, 140(17): 174502.

DOI：10.1063/1.4872260

Using broadband dielectric spectroscopy we investigate the changes in the conductivity relaxation times tau(sigma) observed during the physical aging of the protic ionic conductor carvedilol dihydrogen phosphate (CP). Due to the large decoupling of ion diffusion from host molecule reorientation, the ion conductivity relaxation time tau(sigma)(T-age, t(age)) can be directly measured at temperatures T-age below T-g for exceedingly long aging times t(age) till tau(sigma)(T-age, t(age)) has reached the equilibrium value tau(eq)(sigma)(T-age). The dependence of tau(sigma)(T-age, t(age)) on t(age) is well described by the stretched exponential function, tau(sigma)(T-age, t(age)) = Aexp[-(t(age)/tau(age)(T-age))(beta)] + tau(eq)(sigma)(T-age), where beta is a constant and tau(age)(T-age) can be taken as the structural a-relaxation time of the equilibrium liquid at T = T-age. The value of tau(eq)(sigma)(T-age) obtained after 63 days long annealing of CP, deviates from the Vogel-Fulcher-Tammann-Hesse (VFTH sigma) dependence of tau(sigma)(T) determined from data taken above T-g and extrapolated down to T-age. Concurrently, tau(age)(T-age) also deviates from the Vogel-Fulcher-Tammann-Hesse (VFTH alpha) dependence. The results help to answer the longstanding question of whether the VFTH dependence of tau(sigma)(T) as well as the structural a-relaxation time tau(alpha)(T) holds or not in the equilibrium liquid state far below T-g.

• 出版日期2014-5-7