|Institution:||San Diego State University|
|Full text PDF:||http://hdl.handle.net/10211.3/145061|
Self-associating polymers have become a key component within modern material science. The ability of the molecule to add complex material properties has spawned interest in a multitude of applications, including a wide variety of consumer products and most recently within the medical field. Fluid-like viscosity to near solid-like elastic dynamics can be observed within polymeric materials. Relatively small changes in external parameters can result in large transitions in this behavior. The concentration of polymers within solution, the temperature of the material, and the lifetime between the attractive chemical groups are a few factors that contribute to their general properties. Despite being widely utilized, how these systems deform under external stresses and exhibit such a wide range of properties is still poorly understood. The following manuscript describes numerical studies where the response of associating polymers to shear deformation is explored. A computational model using a novel, hybrid molecular dynamics, Monte Carlo (MC) algorithm is employed. Polymer chains are modeled as course-grained, bead-spring systems. The attractive end groups form reversible, physical bonds according to MC rules. At high temperatures the system is shown to behave as a fluid. Decreasing the temperature results in self-assembly, forming a complex network structure that is transient. The nodes of this network consist of aggregates of end groups, while links between aggregates are formed by one or more bridging polymer chains. The aggregation dynamics of end groups vary with temperature and time. As a result, aggregate sizes change frequently throughout the simulation. Master equations are employed, defining changes in the number of aggregates of a certain size in terms of reaction rates. Variations in these rates are shown to give rise to the observed changes in the aggregate distribution that characterizes the transition between fluid to solid-like dynamics. Rheological experiments are then performed to obtain the system???s stress response to shear deformation. Studies provide insight on microstructural variations in the network topology that lead to observed phenomena within experimental systems. Data obtained in both the linear and nonlinear regimes are then correlated with observed changes in the material properties. Advisors/Committee Members: Baljon, Arlette, Cooksy, Andrew, Salamon, Peter, Percus, Allon, Landsberg, Adam.