Viscoelastic response of polyethylene to temperature and pressure
Specimens of high-density low-density polyethylene were uniaxially compressed at nominally constant strain-rates ranging from 0.000066 to 0.66 sec-1 at temperatures from 77° to 350°K and hydrostatic pressures from 0.001(latm) to 4 K bar.
It was shown that there were significant effects of temperature, pressure and strain rate on both polyethylenes.
At small strains (O.O1), the time-temperature and time-pressure superpositions were used successfully to construct the master curves of stress versus reduced strain-rate. These master curves, which were found to have been taken in the range of linear-viscoelastic behaviour of the polymers, were employed to derive the stress-relaxation moduli and the relaxation spectra.
The viscoelastic response to temperature and pressure of high-density polyethylene in the transition zone between the ? and ? relaxations, and that of low-density polyethylene in the transition zone between the ? and ? relaxations have been found to be controlled by the same molecular motion. This motion, an Arrhenius-type activated process, was identified as involving molecular segments of six to eight carbon atoms with an activation energy of 22 Kcal/mole and an activation volume of 110 cm3/mole.