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

Abstract

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 À11 to 21 8C. 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 8C. The average relaxation times calculated from a Williams-Watts fit follow a VFT temperature dependence, with the stretching parameter decreasing with decreasing temperature. The distribution of relaxation times reveals a merging of the and -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 -relaxation present in higher order methacrylate polymers. The effect of the -relaxation is discussed in terms of the dynamic behavior over 12 decades in time. V V C 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1504-1519, 2005

Application of fractional calculus to the modeling of the complex rheological behavior of polymers: From the glass transition to flow behavior. I. The theoretical model

“…Polymer processing requires a complete understanding of the complex rheological behavior of polymers in a temperature range from subvitreous temperature to the terminal flow zone. However, because of the long‐chain character of polymer molecules, polymers are viscoelastic materials with rheological behaviors characterized by nonexponential relaxation processes exhibited in numerous experiments, such as dynamic mechanical analysis (DMA),1–4 dielectric spectroscopy,1, 5–7 quasielastic light scattering,8, 9 and specific heat measurements 1, 3, 7, 10. Relaxation processes are associated with molecular motions that lead to a new structural equilibrium with a low energy content.…”

Dynamics of ultra-thin polystyrene with and without a (artificial) dead layer studied by resonance enhanced dynamic light scattering

“…Although enormous theoretical and experimental efforts have been made to reach a concise view of the dynamic behavior of supported and free-standing ultrathin polymeric films, controversial results have been reported. [8][9][10][11][12][13][14][15][16][17][18][19] In particular the so-called α-relaxation, related to the glass transition which in polymers is attributed to the reorientational motion of chain segments and therefore to the main chain backbone 20 as well as mechanical properties, is in the focus of interest. There is an ongoing discussion whether and to which degree T g of ultrathin films is influenced by the confinement imposed by the nanoscopic dimensions of the films, the presence of a solid support, or the free-surface.…”