The specification of orientation and its development in polymer processing

Abstract

AbstractA critical review of the specification of orientation and its development in polymer‐processing operations is presented. Orientation may in general be specified by orientation distribution functions, but is most conveniently expressed in terms of orientation factors which are second moments of the distribution. The Hermans orientation factor represents polymer‐chain orientation for systems with fiber symmetry (uniaxial orientation) and the Hermans‐Stein orientation factors express uniaxial orientation for each of the crystallographic axes of crystalline polymers. Biaxial orientation is, however, developed in tubular film extrusion, blowmolding and, indeed, all processing operations other than fiber formation. Orientation factors developed previously by the authors express biaxial orientation in terms of the angles between the machine and transverse directions and the polymer chain axis or crystallographic axes. In flowing polymer melts, the Rheo‐Optical Law, which relates birefringence and stress, represents a relationship between polymer‐chain orientation and stress. In vitrified polymeric glasses (e.g. polystyrene), the orientation factors are related linearly to the stress field at vitrification. This has been shown experimentally for melt spinning and tubular film extrusion. The results of studies of blowmolding and injection molding are consistent with this. The crystalline orientation factors have also been found to be determined by the stress field at solidification in melt spinning and tubular film extrusion.