Mechanical and optical anisotropy in polypropylene fibres

Abstract

Synthetic textile fibres in the drawn or extended state show considerable optical and mechanical anisotropy. A series of polypropylene fibres was made by drawing an isotropic spun fibre over as wide a range of draw ratios as possible and the extensional modulus, torsional modulus and optical birefringence were measured at each draw ratio. An attempt to explain the development of both the mechanical and optical anisotropy on a theoretical basis, in which the polymer is considered as an aggregate of anisotropic units of structure, has met with limited success. The results suggest that in the early stages of drawing the development of anisotropy can be considered as simply related to the change of orientation of such units. At higher draw ratios, however, it appears that there are changes in morphology and molecular mobility taking place simultaneously which also affect the mechanical properties. A second series was produced by drawing fibres which were already anisotropic; their mechanical properties are compared with those of the first series and show significant differences. In particular there is evidence for the breakdown of the morphological structure even at the lowest draw ratios.