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Long-chain molecules in polymers are responsible for flow-induced nucleation (FIN). The role of a short-chain matrix in FIN was investigated using bidisperse blends of polyethylenelike high molecular weight long chains in short-chain polymer matrices. It was found that the critical specific work, w(c), measured for the onset of FIN in the blends has a power law dependence on the molecular weight of the matrix polymer with w(c) proportional M(w)(2.59 ± 0.27) indicating that the matrix can affect the stretching of the long chains and limit their ability to act as nucleation sites for flow-induced crystallization.

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The effect of polydispersity on the formation of flow-induced, oriented morphology in polyolefins is investigated by polarized light imaging and small angle X-ray scattering. A torsional shear flow was applied at different temperatures to model polyethylene blends (bimodal and trimodal hydrogenated polybutadienes) comprising of two kinds of long chains with different molecular weight (1080 and 1770 kDa) in a matrix of short chains (18 KDa), and the results were compared to those of polydisperse materials. While a single boundary associated with the threshold flow conditions for the onset of oriented morphology is observed in the bimodal blends, two boundaries corresponding to the orientation of the longest chains (1770 kDa) and next longest chains (1080 kDa) are detected in the trimodal blends. The results obtained, herein, are extended by inference to polydisperse polymers. It is demonstrated that the shear rate dependence of the critical specific work parameter for the onset of oriented morphology in polydisperse polymers is dictated by the molecular weight distribution and that the longest chains mainly control the process with some contribution from shorter chains involved in the formation of flow-induced precursors at higher flow rates. KEYWORDS: crystallization; orientation; SAXS; viscoelastic properties INTRODUCTION Polyolefins are the most widely used polymer nowadays due to their excellent cost-benefit performance. The typical processing methods for semicrystalline polyolefins take a melt and shape it by means of either an extrusion or molding technique and the shape stabilization process is crystallization by cooling.1 The flow conditions in the extruder, and die or mold system have a profound effect on the morphology of the crystalline material and introduce different forms of crystals from isotropic spherulites to highly oriented ''shish-kebab structure'' in the polyolefin with the significant effect on materials through their mechanical, thermal, and optical properties.2 As processing conditions (temperature, pressure, flow rate, and duration of flow) can be adjusted and are relatively easy to control, the relationship between this group of parameters and characteristics of the polymer (chemical composition and molecular weight distribution) could significantly improve the control of structural morphologies and widen an understanding of flowinduced phenomena in polymeric materials.