METHYL-GROUP DYNAMICS IN GLASSY POLYISOPRENE - A NEUTRON BACKSCATTERING INVESTIGATION

Energy-resolved, elastic neutron backscattering was employed to investigate the methyl group dynamics in polyisoprene between T = 2 K and room temperature. The use of partially deuterated samples (PI-d5, PI-d3, and PI-d8) and of a fully protonated sample (PI-h8) allowed the separation of the dynamics arising from the methyl group and from the backbone. A two-step relaxation is observed and attributed to the methyl group rotation at low temperatures and to the main-chain relaxation close to the glass transition. An Arrhenius-like increase of the methyl group rotational correlation time tau = tau0 exp(E(act)/kT), with E(act)/k = 1550 K is similar to 12 kJ/mol and GAMMA0 is similar to 1/tau0 = 23.5 meV (tau0 is similar to 1.76 X 10(-13) s) describes well the midposition of the first elastic intensity decrease but not its breadth. A 3-fold jump model with a broad Gaussian distribution of activation energies (dE/E is similar to 25%) around 1500 K can account for the observed temperature decrease. Inconstancies in the Q-dependence might be due to disorder effects. The torsional mode of the methyl group rotation is directly observed at GAMMA0 = 23.5 meV by time-of-flight. Near the glass transition temperature a further decrease of the elastic scattering is observed due to the onset of a fast dynamics of the backbone in the picosecond range.