Kinetic Control of Crystalline Order in Olefin-Based Polymers

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TECHNICAL SUMMARYResearch is proposed to advance knowledge of the interplay between chain microstructure, phase structure, crystallization kinetics and morphology of non-oriented bulk crystallized polyolefins. The aim is to understand this interplay using well-characterized systems that serve as models to predict the behavior of more complex commercial systems. Molecules could then be tailored with unique crystalline morphologies and properties. We seek to understand from first principles the role of crystallization kinetics on the development of the ultimate crystalline state. Of interest is the effect of undercooling on nucleus size, nature of folding when enabled, and impact in packing of sequence-specific macromolecules. Using polyethylene-based molecules with exquisite control in halogen substitution as model systems to predict kinetically controlled polymorphic transitions, further insights into primary chain deposition and subsequent arrangements are sought as they relate to classical and novel views of the path to polymer crystallization. In blends of metallocene-made poly(propylene) and higher alpha-olefin random copolymers, the role of sequence length distribution in polymorphic behavior and the crystalline phase structure that evolves will be addressed for a wide range of compositions.NON- TECHNICAL SUMMARYThe main proposed work involves the two leading commercial polyolefins, polyethylenes and polypropylenes. As they comprise over half of the annual production of all synthetic polymers, any improvement in the product, either by branching architecture, rate of processing, or judicious component blending will lead to a significant impact in the US and world economy. Most properties of these polyolefins are directed by the fraction and type of crystalline order that they assemble, which is a function of branching content and distribution. Commercial polyolefins are too complex in chain length and branching distributions to generate fundamental relations between chain-structure and properties. In the work proposed, polyethylenes and polypropylenes synthesized with exquisite control of branching distribution, either at a precise regular spacing or randomly placed will be studied as models to establish fundamental behaviors. Special emphasis is given to the role of crystallization kinetics, as it relates to processing rate, in the type of crystals assembled. Crystals with different degrees of symmetry are predicted with the model systems, thus bringing the opportunity to develop new polyolefins in which polymer primary structure can be manipulated to control physical properties. This fundamental research is also aimed to provide learning opportunities for both graduate and undergraduate students, many of them minority.