Role of Chain Structure and Melt Topology in Polymer Crystallization

点击翻译按钮获取中文摘要

NON-TECHNICAL SUMMARY:Most commercial polymers are processed into useful materials by cooling them from a high temperature. During this process, a fraction of the long polymer molecules self-assemble in ordered crystalline regions connected by regions where the molecules remain disordered. The speed of the solidification process depends on the liquid phase structure of the melt and the entanglements (topology) that long polymer molecules adopt during heating. Molecules that retain memory of the ordered state in the melt act as seeds to speed up the crystallization. This project addresses how the seeded melt structure changes in polymers with short or long branches, or with polar or non-polar branches and their role on crystallization. Studies of model blends, and polyethylenes with pendant groups, either random or at a precise distance will enable building of fundamental relations between molecular structure and properties to predict the behavior of commercial complex polyethylene materials. These studies are significant also because the great expansion of shale gas extraction has placed the US as a very highly competitive provider of derivatives such as polyethylenes. Hence, advances in the understanding of the state of the melt and the crystallization of polyethylenes are of major relevance as they could lead to improved processing technologies and to reduced costs and energy utilization. The proposed research is also aimed at providing learning and training opportunities for graduate and undergraduate students, many of them underrepresented minorities.TECHNICAL SUMMARY:The proposed research program builds on recent developments on the strong melt memory of crystallization of random ethylene copolymers, and the unique layered crystallization of precision halogen substituted polyethylenes to further address the role of chain structure in the evolution and final crystal morphology of semicrystalline polymers with various types of short and long-chain branching. Models for random copolymers will be used to address: (a) the effect of branch length (ethyl to hexyl), chain architecture (stars, H-type, comb, pom-pom), and branch polarity (acetoxy) on melt diffusion, and self-nucleation; (b) the effect of comonomer content distribution on co-crystallization in model blends, and the interplay between sequence segregation and LLPS; (c) the generality of the role of sequence selection in crystalline melt memory. A second thrust aims to advance understanding of the evolution of the crystalline state of polyethylenes with pendant groups placed at the same equidistant length along the backbone. Taking advantage of model systems with pendant groups with controlled tacticity, the origin and drive for the formation of lamellar crystallites from isotactic as well as from atactic systems will be sought. Differences in sublamellar structure, and crystallization kinetics are of special interest as they relate to novel crystallites formed from ethylene-like copolymers with controlled tacticity that are not accessible in classical random ethylene copolymers. Molecular details at the crystal growth front will be extracted from the unique temperature gradient of growth rate kinetics.