MOLECULAR MODELING OF POLYMER CRYSTALS AND CRYSTALLIZATION

Polymer materials are receiving increased attention in the field of materials science, both in academia and industry, with its widespread application from commercial plastics to advanced biomaterials. These include composites in airplanes and automobiles, functional films on monitors in mobile phones and computers, as well as adhesive and coating materials in civil engineering. Despite significate efforts, the major questions and challenges in understanding key properties of polymer materials are still not solved. Such lack of understanding hinders advances in delicate design and controlling of polymers for advanced functional applications. The development of polymer science began with the pioneering work made by Flory and his coworkers at 1950s as commercial synthetic polymer industry started to develop and grow. During the following decades, experimental work guided by theoretical predictions had been the major contribution of our further understanding while the great challenges in experimental techniques at molecular level always blurred critical information in polymer materials. With enhanced ability in computational science, simulation starts to become an essential investigation method to provide thermodynamic insights at this molecular level. Along with great progress in properties prediction with improved accuracy, great challenges still exist in modeling processing of polymer systems, especially in accurate description of dynamic evolution incorporated with various processing conditions resulting macroscopic structural changes like carbon fiber processing from polyacrylonitrile (PAN) precursor in which crystalline regions represent more than 55% of the material by volume. In terms of crystallinity in polymers, with the heated debates over classical crystal-growth models, major questions and challenges are still not solved including the control and determination of molecular conformations and crystal structures as well as mesoscale morphologies, detailed understanding of melting and crystallization. It is clear that molecular scale investigations on crystal structure and crystallization mechanisms as well as predictive simulations of that will be a huge demand in the near future to explore mechanical, optical, and other physical properties in polymeric materials.

The purpose of my dissertation is to summarize my major research contributions to our current understanding of crystalline polymers in the aspects of crystal structure determination and crystallization processes at molecular level, and to introduce our effort on simulation software development and indicate possible future directions in the field of molecular modeling of polymer crystals. Three major research topics will be included as the following

1. Crystalline and pseudo-crystalline phases of polyacrylonitrile from molecular dynamics; 2. Novel mode of non-crystallographic branching in the initial stages of polymer fibril

growth;
3. Polymer crystal structure generator and analysis software (PolymerXtal).