Stress Birefringence in Polyethylene

The birefringence and strain produced in polyethylene by different stresses have been measured at temperatures between 20° and 90° C It has been found that the relation between strain and birefringence is linear up to strains of 0.25 and for greater strains there is still no hysteresis This indicates that no true flow occurs in this temperature range The stress-birefringence relation, on the other hand, rapidly departs from linearity and shows large hysteresis, so that the nature of the birefringence effects is in direct contrast with that observed in rubber An expression has been derived for the birefringence in terms of the orientation of rod-like crystallites embedded in an elastic matrix and this was found to be in qualitative agreement with the experimental results.