Comment on 'temperature divergence of the dynamics of a poly(vinyl acetate) glass: Dielectric vs. mechanical behaviors' [J. Chem. Phys. 136, 154901 (2012)]

摘要

The temperature dependence of polymer dynamics below the standard glass transition temperature T_g is of considerable interest.~(1, 2) For a better understanding of physical aging and the glass transition phenomenology, it is important to assess the temperature at which the relaxation times will tend to diverge, in particular whether this will occur near T_g or not until T = 0.~3 An inherent problem in obtaining relaxation time data below the calorimetric glass transition is the prohibitively long time required to attain equilibrium and to measure such a correlation decay. In a recent paper, Zhao and McKenna have addressed this issue and measured the equilibrium dynamics of poly(vinyl acetate) (PVAc) in the range from Tg ? 15 K to T_g + 30 K,~4 using Struik’s protocol~5 to determine that physical aging had been completed prior to recording the dielectric retardation, ε(t), at the target temperature. The resulting shift factors, aT, display a transition to a lower apparent activation energy near T < T_g ? 10 K, where log10(aT) exceeds 2.5. Figure 9 of the paper by Zhao and McKenna~4 is declared as a comparison of their results with dielectric relaxation (M = 1/ε) data from time domain dielectric modulus experiments by Wagner and Richert,~6 but the dielectric retardation (ε) data set from that work is shown instead, which is limited to T > T_g. This erroneous comparison is reproduced in Fig. 1(a) and could suggest that previous dielectric experiments aimed at measuring very long quasi-equilibrium time constants have not been able to observe responses below T_g ? 10 K for PVAc and thus missed this transition to a more Arrhenius like behavior.