SMALL-ANGLE X-RAY-SCATTERING ON 2-PHASE SYSTEMS UNDER PRESSURE - DETERMINATION OF THERMODYNAMIC PARAMETERS

A small-angle X-ray scattering study on two different polyethylene sam ples under pressure up to about 750 MPa reveals a decrease of the Pore d invariant Q(p) with increasing pressure at constant temperatures. Th e volume-pressure relationship as well as the isothermal compressibili ty and the Gruneisen parameter for a large pressure range and the over all density range are calculated from Q(p) with the help of the Tait e quation; From the Porod invariant, the parameter B(t) of the Tait equa tion for the sample as well as of the purely amorphous and purely crys talline polyethylene are obtained. A general relationship between B(t) and the sample density has been found.