Comparison of the pressure and temperature dependence of the elastic incoherent scattering for the polymers polybutadiene and polyisobutylene

The pressure and temperature dependence of the elastic scattering of two amorphous polymers, polybutadiene (PB) and polyisobutylene (PIB), is analyzed on the neutron backscattering spectrometer IN 16 and with an energy resolution < HWHM = 0.5 mu eV in a Q-range from 0.2 to 1.9 Angstrom(-1), a temperature range from 2 to 400 K and for pressures between P-atm and P = 300 MPa. The Q-dependence of the scattering is analyzed in terms of the mean-squared displacement as a function of temperature and pressure in Gaussian approximation. We show that for polybutadiene the pressure dependence of the mean square displacement can be scaled to a mastercurve taking the thermodynamic glass transition temperature T-g as a scaling parameter. For polyisobutylene the application of pressure clearly separates an additional dynamic process from the fast and alpha-relaxation process. (C) 1999 Elsevier Science B.V. All rights reserved.