Birefringence of Stretched Cellulose Films

Abstract

Birefringence in three orthogonal directions was determined at normal incidence by the compensator method on thin sections of cellophane sheets, before and after subjecting them to swelling and stretching treatments. Considerable increase in crys tallinity and longitudinal orientation of molecular chain axes was achieved by stretch ing. From the course of variation in the three birefringence values with increasing stretch up to 42.5% imparted during treatment, it was possible to ascertain the biaxial orientation of the molecular chains in the crystalline and amorphous celluloses. If the crystallites are oriented with the (101) plane parallel to the film surface, the glucose rings in the amorphous regions align themselves parallel to the film surface. The stacking tendency of crystallites leading to the planar orientation of the (101) planes parallel to the film surface is also evident from crystallite dimension data.