The effect of hydrostatic pressure on the visco-elastic properties of polymers

Abstract

This paper describes an experimental study of the effect of hydrostatic pressure on the visco-elastic properties of some common thermoplastic polymers, and natural rubber. A torsion pendulum was constructed to operate inside a thick walled cylindrical pressure vessel. The pressure could be varied from atmospheric to 1500 atmospheres and the temperature from −20°C to 120°C. The pressure medium was nitrogen gas but in some cases the polymer specimen was surrounded with a hydrocarbon oil to prevent the nitrogen from coming into contact with the polymer. The results obtained show that the glass transition temperatures of rigid polymers and rubber, are raised with pressure by amounts which vary between 15 and 30°C per thousand atmospheres depending upon the material. The values of this shift, for individual polymers, are in agreement with quasi-theoretical predictions based on the ‘Free-Volume’ theory of the glass transition temperature; they also agree with experimental studies of a related nature, by other workers. The experiments have also revealed a new phenomenon with pife: this polymer is rapidly plasticized by nitrogen at pressures of a few hundred atmospheres.