Ultrasonic study of anharmonicity in amorphous polymers: temperature, pressure and molecular weight effects

Abstract We measure the specific volume, V , and complex ultrasonic modulus, M * , of amorphous polystyrenes with molecular weights, M n , from 0.6 to 1,500 kg/mol. Experiments, at temperatures T , 200⩽ T ⩽600 K, and pressures up to P =100 MPa, probe the transformation from the glass to the melt state, through the glass transition region, near the glass transition temperature, T g . We compute the thermal expansion, θ , which defines T g . We evaluate the thermoelastic anharmonicity parameter, Γ=− 1 2 ( ∂ ln M/ ∂ ln V), that describes coupling between thermal vibrations, i.e. motional pressure, and volume forces, i.e. internal pressure. In the glass, Γ is temperature independent, illustrating equilibration of internal and motional pressure. Near T g , however, Γ increases indicating that the system evolves toward instability. In the melt, Γ becomes typical of equilibrium van der Waals interactions. We discuss implications for the pressure and molecular weight dependence of T g .