Quasielastic neutron scattering study of poly(dimethyl siloxane) at high pressure

The chain dynamics of poly(dimethyl siloxane) (PDMS) in the melt is studied in the pressure range 0.1 to 350 MPa using quasielastic neutron scattering . The pressure dependence of the average relaxation frequency gives an effe ctive activation volume Delta V-# = 20 cm(3) mol(-1) (33 Angstrom(3)) at 39 8 K. The Vogel-Fulcher-Tammann equation is used to evaluate the pressure de rivative of the glass transition temperature, partial derivative T-g/partia l derivative P = 4.3 x 10(-8) K Pa-1 (0.0044 K atm(-1)). It is further conc luded that about one quarter of the increase in the mobility of PDMS chains with increasing temperature at constant pressure can be attributed to volu me expansion; the remainder is due to an effect of temperature itself. Resu lts from viscoelastic and neutron scattering measurements are also compared .