A polymer's dielectric normal modes depend on its film thickness when confined between nonwetting surfaces

Abstract

The dielectric loss peaks of both normal-mode relaxation (fluctuations of the end-to-end dipole vector perpendicular to the confining surfaces) and segmental motion (fluctuations perpendicular to the chain backbone) of cis-polyisoprene were measured with special attention paid to contrast between responses of the bulk samples and films approximate to 100 nm thick. The polymers, narrow-distribution samples with number-average molecular weight M-n = 2600, 6000, and 10 000 g mol(-1), were spin-cast onto atomically smooth mica, coated with a second mica sheet, and quenched to temperatures at which the resulting sandwich geometry was kinetically stable although the polymer films dewet these surfaces at equilibrium. The segmental relaxation process was the same for bulk and thin films, but the normal mode (the end-to-end dipole vector relaxation) slowed down, more so as temperature decreased. This loss mode in the capacitance, C " (f), did not for thin films display the expected terminal tail observed in the bulk samples (C " proportional to f(m) with m < 1 at low frequency f). The power m decreased from 0.9 to 0.5 as temperature was lowered from 260 to 235 K. The inability to quantitatively define the average frequency of this apparently inhomogeneous process led us to analyze the temperature dependence of the frequency at peak of the normal mode. In studies of its temperature dependence, the activation energy of the thin films was found to exceed by 10-20% that for bulk samples and, unlike the bulk state for samples in this range of relatively low molecular weight, to be independent of molecular weight. We interpret these results to indicate that the normal mode not only slowed down but also became more inhomogeneous in this temperature range of 100-30 K above the bulk glass transition temperature, Tg. The contrasting thickness and temperature dependence of the normal-mode and segmental relaxation modes indicates strong breakdown of time-temperature superposition.