Evidence for the γ-relaxation in the light scattering spectra of poly (n-hexyl methacrylate) near the glass transition

The full range of relaxation processes present in optically pure poly(n-hexyl methacrylate) (PHMA) was studied using Rayleigh-Brillouin and photon correlation spectroscopy (PCS). Brillouin shifts, linewidths, and Landau-Placzek ratios (LPR) were measured over the temperature range from -11 to 21 degrees C. The Brillouin splitting and linewidth were consistent with previous studies of PHMA, but the LPR was much lower, indicating that the scattered light primarily comes from intrinsic density fluctuations. Relaxation functions of the same PHMA sample were measured using PCS over the temperature range 0.5-52.5 degrees C. The average relaxation times calculated from a Williams-Watts fit follow a VFT temperature dependence, with the stretching parameter beta decreasing with decreasing temperature. The distribution of relaxation times reveals a merging of the alpha and beta-relaxations over this temperature range, and the temperature dependent width confirms that there are at least two processes with separate temperature dependences. Furthermore, there appears a process at short times in the correlation function window at low temperatures. This upturn at the fastest relaxation times is attributed to the gamma-relaxation present in higher order methacrylate polymers. The effect of the gamma-relaxation is discussed in terms of the dynamic behavior over 12 decades in time. (c) 2005 Wiley Periodicals, Inc.