(551a) The Development of Polymer Microstructure: Where Thermodynamics and Kinetics Meet

The production methods for many polymer materials do not lead to equilibrium structures, but rather give materials with microstructures and properties that depend on process history. While there are sophisticated theories and simulation methods for describing polymer thermodynamics that span molecular to continuum length scales, approaches for systems with kinetic effects are considerably less well developed. One approach is to use coarse-grained molecular simulations, such as Brownian dynamics, to simulate the full thermodynamics and kinetics of the system. Such approaches are extremely valuable, but are typically too fine-grained to reach the length and time scales that characterize microstructural evolution.

In an ongoing collaboration with the Fredrickson group at UCSB, we have been developing field-based approaches that can model the kinetics of polymer materials with complex thermodynamic processes such as microphase and macrophase separation. Building on lessons learned with dynamic density functional theories and two-fluid models, we have constructed a series of phase field models that are capable of using field-theory based density-explicit free energy functionals that couple to continuum transport equations. In addition, we have built efficient, highly-parallelized simulation software for these models that is versatile enough to capture diffusive and convective transport, chemical kinetics, and multiple boundary conditions coupled to thermodynamic descriptions of a variety of polymeric solutions and melts.

We will discuss some of our preliminary work using this approach to study the process of nonsolvent induced phase separation (NIPS), widely used for the production of polymer membranes and porous micro/nanoparticles. In NIPS, a polymer solution consisting of a homopolymer and a good solvent is brought into contact with a solvent-miscible nonsolvent. The subsequent interchange of solvent and nonsolvent leads to the precipitation of a polymer-rich phase, which then solidifies and arrests the phase separation, fixing the microstructure of the material. Our model is able to capture the coupled kinetics of mass transfer and phase separation, unique Marangoni flows generated in these systems, and the effects of finite droplet size and shape on the subsequent microstructure. In addition, we will discuss some of our ongoing work incorporating block polymers and materials containing nanoparticles.

We acknowledge funding from the Board of Trustees at Brigham Young University as well as computational resources from the Brigham Young University Office of Research Computing.