The dynamic tensile deformation mechanism of spherulitic poly-alpha-olefins, high-density polyethylene, isotactic polypropylene, and isotactic polybutene-1, was investigated by an advanced dynamic X-ray diffraction technique at various temperatures and frequencies in order to assign the α and β mechanical dispersions of the materials, explicitly. The uniaxial orientation distribution function qj(ζj, 0) of the j-th crystal plane and its dynamic response Δqj′ (ζj, 0) in-phase with dynamic strain are observed for several crystal planes, and then the orientation distribution function ω(ζ,0,η) of crystal grains and its dynamic response Δω′(ζ,0,η), also in-phase with the dynamic strain, were determined by a well-known mathematical transformation procedure proposed by Roe and Krigbaum on the basis of the Legendre addition theorem. The temperature and frequency dependences of Δω'(ζ,0,η) are analyzed in terms of a spherulite deformation model combining affine orientation of crystal lamellae with several types of preferential reorientation of the crystal grains within the orienting lamellae. The following assignments are made: i) The α mechanical dispersion must be assigned to the dynamic orientation dispersions of the crystal grains within the lamellae, involving two types of preferential rotations of the grains to result in lamellar detwisting mostly in the equatorial zone of uniaxially deformed spherulites and lamellar tilting mostly in the polar zone of the spherulites. Both processes are 'intralamellar grain-boundary phenomena', and the former process of lamellar detwisting is hardly activated for polypropylene and polybutene-1 spherulites in contrast to polyethylene spherulites, ii) The β mechanical dispersion must be assigned to the dynamic orientation dispersion of the crystal lamellae behaving as rigid bodies, not accompanied by any of the reorientation mechanisms of the crystal grains within the orienting lamellae. This process is an 'interlamellar grain-boundary phenomenon' associated with orientational and/or distortional dispersions of noncrystalline materials between the lamellae. The more pronounced the lamellar orientation dispersion (β mechanical dispersion) and the less pronounced the reorientation dispersion of the crystal grains (α mechanical dispersion), as the specimen changes from polyethylene to polypropylene and to polybutene-1.
Polymer melts, combining high elasticity with high viscosity and a non-linear response to imposed stress, have provided rheology with two major stimulants during the last two decades: first was the question of how to characterize such materials; second was the requirement to express that characterization within the framework of continuum mechanics. These challenges have borne considerable fruit which the plastics industry has exploited by improving the processing characteristics of the resins and optimizing the processing technologies used to shape these resins. Those responses, which have largely effected the quantitative aspects of the plastics business in terms of increased output at higher efficiency, have themselves been checked by the combined brakes of recession and overcapacity. If the industry is to continue to advance we must change the emphases of our research from the quantitative aspects-how much-to the qualitative- how good. In this paper I shall consider three rheological challenges: the modification of existing commodity plastics to enhance the quality of the product as exemplified by acrylic processing aids in PVC; the exploitation and control of new molecular states as exemplified by liquid crystalline order in melts; and the extension of high performance resins into the field of composites as exemplified by the development of continuous fibre reinforced thermoplastics. The common thread through these diverse topics-and the thread which will be common to the new challenges of the next decade-is the close interaction between morphology and rheology: the microstructure of the material determines the flow processes and is itself determined by the flow history.