RELATION BETWEEN MOLECULAR ORIENTATION AND MECHANICAL PROPERTIES OF STRETCHED POLYVINYLCHLORIDE FILMS

Abstract

The relationship between the change in molecular orientation and the mechanical properties of polystyrene films prepared by various types of stretching was investigated. The specimens were prepared by stretching hot-pressed films to various ratios by different methods at 120°C and at a rate of 100%/min in a circulating hot air bath. The methods were uniaxial with the free and/or constant width, two-way successively biaxial and simultaneously biaxial stretching. For the estimation of the orientation the birefringences were measured by using a tilting technique and mechanical properties are discussed in terms of stress-strain curves.The following results were obtained:(1) The molecular orientation in the films uniaxially stretched with free width is uniaxial with cylindrical symmetry around the stretching axis, and the anisotropy of mechanical properties is greater than that of the others.(2) For the uniaxially stretched films with constant width, the orientation is roughly biaxial or uniaxial together with more or less planar orientation parallel to the film surface, Thus, the anisotropy of Young's modulus are nearly equal in both the machine and transverse directions.(3) For the two-way successive-biaxially stretched films, the “balance point of orientation” for the degree of chain orientation agree with the “balance point of mechanical properties” when the ratio of first and second stretchings are equal. Thus, the difference of mechanical property can be related to the difference of molecular orientation.(4) For the simultaneous-biaxially stretched films, the balanced mechanical properties, which are isotropic in the film plane, are independent of the stretch ratio. The molecular chain shows a random orientation within the film along with a planar orientation parallel to the film surface, as observed for other polymer films. The simultaneous-biaxial stretching is more excellent and preferable to successive-biaxial stretching for polystyrene film.