Coalescence-induced coalescence in polymeric membrane formation

The morphological development of isotactic polypropylene (iPP) membranes form via liquid-liquid thermally induced phase separation (TIPS) was studied in an effort to control the structures and transport properties of these membranes. In TIPS, a homogenous polymer solution is quenched to induce phase separation and subsequent growth of the dispersed phase. Upon cooling to the point of polymer solidification, the droplets are extracted and the extractant evaporated, leaving behind the cells of the membrane. The size and size distribution of these droplets ultimately determine the membrane morphology and performance. It is believed that in some polymer solutions, the droplet coarsening that follows phase separation and precedes polymer solidification is driven by a mechanism known as coalescence-induced coalescence (CIC). The theory behind CIC is as follows: when two droplets approach within a critical distance (defined as h_cr) of each other, attractive forces between them become strong enough to initiate coalescence. As the droplets begin to coalesce, they form a composite droplet with an energetically unfavorable shape. Curvature gradients cause the composite droplet to relax to a sphere, inducing flow of the surrounding matrix fluid. This fluid displaces nearby droplets, causing more coalescence events to occur. A cascade of coalescence events results, increasing the average droplet size, and in some cases broadening the droplet size distribution.; A molecular dynamics-like simulation based on CIC was constructed in an effort to understand the factors that govern the TIPS process. The simulation begins with a random dispersion of equally sized droplets to simulate the condition of the system immediately following the temperature quench. Some of these droplets coalesce and create velocity fields due to the relaxation of the composite droplets, causing more coalescence events to occur and propagating the cascade process. The simulation reports both the coarsening rate and the evolution of the droplet size distribution. This simulation was run for several quench conditions and the results compared favorably with those obtained from coarsening experiments performed with iPP-DPE at identical conditions. In order to compare the dimensional experimental data with the dimensionless simulation results, the complex viscosities of phase-separated iPP-DPE solutions were measured with a dynamic rheological technique and the interfacial tensions of these solutions were estimated based on relaxation times of coalescing droplets. It was found that the rheological studies provided additional information about the phase transition temperatures of a polymer-diluent system.; Additionally, in order to predict the structure and performance of coarsened membranes formed via TIPS, three more simulations were developed. Porous media were modeled using the droplet positions as given by the CIC simulation, and their structural and transport characteristics were estimated. Overall, this work tested hypotheses about the relationships between coarsening, polymer solution properties, and membrane microstructure and performance. It was concluded that CIC likely is at work in the iPP-DPE system under certain quench conditions and that further investigation into the effects of CIC on membrane performance is warranted.