THE EFFECT OF MOLECULAR ORIENTATION ON THE MECHANICAL PROPERTIES OF

Abstract-—Oriented sheets of two commercial homopolystyrenes differing in molecular weight, rheological and glass transition temperature properties have been prepared by sheet extrusion and drawing. The properties of the resulting sheets have been studied at seven different laboratories in Europe. The degree of orientation, irrespective of how it is achieved, is defined in terms of the birefringence which is proportional to the maximum shrinkage stress through the stress optical coefficient, and to a sonic modulus function. The nominal draw ratio and the length reversion ratio are not unequivocal estimates of the degree of orientation. The orientation stress can be divided into two parts, the first is associated with Hookean elasticity and the other with rubber-like elasticity. It is the deformation due to this latter part that gives rise largely to the properties of the oriented solid polymers. From the value of the stress optical coefficient and the relation between maximum shrinkage stress and length reversion ratio estimates of the size of the random link and of the effect of temperature of orientation and strain rate on the degree of chain entanglement are obtained. Tensile properties including rupture stress, rupture strain and rupture energy have been determined over a wide range of strain rates for the various degrees of orientation. Increasing orientation generally improved rupture stress as also did strain rate but as orientation increased and rupture took place by yielding the rupture stress tended to fall