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 effective activation volume ΔV=20 cm3 mol-1 (33 Å3) at 398 K. The Vogel–Fulcher–Tammann equation is used to evaluate the pressure derivative of the glass transition temperature, dTg/dP=4.3×10-8 K Pa-1 (0.0044 K atm-1). It is further concluded that about one quarter of the increase in the mobility of PDMS chains with increasing temperature at constant pressure can be attributed to volume expansion; the remainder is due to an effect of temperature itself. Results from viscoelastic and neutron scattering measurements are also compared.