Super-Oriented Crystallization of Crosslinked Polyethylene under Constant Stress

Abstract

The super-oriented crstallization, induced by cooling a melt under a constant stress, was investigated with regard to crosslinked polyethylene to obtain tenacious material in which chains are sharply oriented to the direction of stress. The elongation induced in terms of oriented crystallization was measured by cooling a high density polyethylene, crosslinked by means of r-ray irradiation of 10-30 Mrad, from 160 to 110° C under the stress of 2 x 10⁶∼I4 X 10⁶ dyne/cm².
The draw ratio after oriented crystallization increases with increasing stress and reaches a constant maximum value at a higher stress. The maximum draw ratio decreases with increasing irradiation dose, and coincides with that calculated from the average molecular weight of crosslinked chains. Therefore, the crosslinked chains are fully extended and crystallized upon applying a sufficient stress. The calculation of rubber elasticity reveals that crosslinked polyethylene containes many dangling chains, and that the contents of the chains decrease from 75 to 51% with increasing irradiation dose from 10 to 30 Mrad. The tensile strength of fully extended polyethylene increases from 27 to 48 kg/mm² with increasing irradiation dose from 10 to 30 Mrad, because the number of dangling chain which does not support the tensile force Oecreases with increasing irradiation.
It was concluded from these results that the extended chain crystals, composed of crosslinked chains, are produced at an initial stage of cooling owing to the elevation of crystallization temperature due to the stress applied to the crosslinked chains; then the folded chain crystals, composed of dangling chains, are produced on the extended chain crystals in terms of epitaxial growth at the following stage.