The Critical Role of Glass Transition Phenomena on the Dynamics of Miscible Polymer Blends

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9977928KumarIntermolecular friction in glass-forming liquids is poorly understood - a fact that is strongly demonstrated in miscible mixtures of two polymers. The effective friction coefficient between two monomers can vary over several orders of magnitude in a nonmonotonic fashion as blend composition is changed at constant T - Tg. A concentration fluctuation model appears to offer a promising framework for understanding such phenomena, and has been tested with experiments. A comprehensive study of two model polyolefin blend systems, tested with experiments. A comprehensive study of two model polyolefin blend systems, using a suite of experimental probes, is proposed here. This will delineate the dynamics at all relevant length scales, from the segmental motion of monomers to the motion of entire chains. These blends are chosen to vary concentration fluctuation in a systematic manner, and will serve as a greatly needed database for blend dynamics. Information on segmental dynamics will be used to test and improve the current concentration fluctuation model for local dynamics. Chain dynamics data will allow testing of proposed extensions of this model to the chain level, enabling prediction of intermolecular friction, and thus the viscosity and diffusion coefficients of miscible blends. The least-understood part of the current model is the notion of a cooperatively rearranging region, which refers to a region in space where all monomers have to move cooperatively to relax stress. Thus, molecular simulations will be utilized to understand cooperative motion, crucial for understanding dynamics in polymer blends, and also in all glass-forming liquids. The goal of this proposal is to construct a predictive model for chain dynamics, which will make the connection between concentration fluctuations and effective intermolecular friction. This will not only result in a predictive model for the viscosity of miscible polymer blends, needed in the polymers industry, but also have far-reaching consequences for understanding the dynamics of all glass-forming liquids.Polymer blends are used in an increasing number of consumer products, including metals replacement in the transportation industry that enables weight to be reduced without sacrificing strength, thereby using less fuel in transportation. Polymer blends are also vital for plastics recycling. The proposed research will greatly expand our understanding of these complicated materials.