Control of Structural Morphology in Shear-Induced Crystallization of Polymers

Critical examination finds that the longest chains play a catalytic role in the formation of shish kebabs recruiting other chains into the formation of this morphology. The longest chains in an ensemble are stretched by shear flow to form the “shish” upon which the bulk of the material crystallizes as “kebabs”. A universal parameter for the formation of shish kebab structures, the specific mechanical work, and a method by which it may be measured for any given ensemble of polymers is provided. In rotating parallel-plate flow a clear boundary is observed between oriented and unoriented material which is dependent on both the shear rate and the total strain. It has been found that the necessary conditions for the formation of oriented nuclei is that the shear rate should be larger than the inverse Rouse time of the longest chain in the ensemble and that mechanical work above a critical threshold is required. The experimental procedure required to make such measurements, and the precautions necessary to avoid artifacts such as elongated spherulites, elastic instabilities and photoelasticity, are examined in detail. The concept of the critical work being a control parameter has been previously demonstrated using model linear−linear hydrogenated polybutadiene blends and this concept is extended to industrial polymers, low-density polyethylene and polypropylene, using both small-angle X-ray scattering and polarized light imaging to measure orientation. The approach proposed is simple, is elegant, and can be easily implemented in the laboratory to study the fundamental processes of flow-induced crystallization and to test commercial materials before processing in real applications.