Malancha Gupta, PhD, University of Southern California
Our analysis of the PPFDA and PEGDA depositions above showed that polymer/IL composites could only be formed if the monomer is soluble in the IL. We used this knowledge to make polymer/ionic composites via deposition of 2-hydroxyethyl methacrylate (HEMA) onto thin layers of [emim][BF4]. The solubility of HEMA in the IL enabled polymerization at both the IL-vapor interface and within the IL layer, leading to composite formation. We first spin coated thin layers of IL onto silicon wafers and then systematically studied the effect of varying the polymer concentration by increasing the deposition time. We observed a transition from a viscous liquid to a flexible solid-like gel with increasing polymer concentration and these gels were robust enough to be handled with tweezers. We studied the molecular weight of the polymer chains using gel permeation chromatography (GPC) and dynamic light scattering (DLS). The polymer chains within the IL were orders of magnitude larger than the chains at the IL-vapor interface likely due to increased propagation rates and decreased termination rates within the bulk IL. We found that at short deposition times, there were two distinct molecular weights reflecting polymerization at the IL-vapor interface and within the IL layer, while at longer deposition times the molecular weight distribution within the IL layer broadened likely due to changes in viscosity of the IL during deposition.
Our results so far have shown that we can tune the molecular weight and polymer concentration of our composites by varying process parameters such as deposition time and pressure. For example, it is important to increase the ratio of the IL to polymer in order to increase the conductivity of the composites. The ratio of IL can be increased by increasing the molecular weight of the polymer which decreases the polymer concentration required to form a solid-like gel. For example, we found that increasing the reactor pressure increased the molecular weight of the polymer, leading to a lower amount of polymer needed to form a solid-like gel and therefore a more conductive composite. Our future work will focus on determining how the substrate temperature affects the concentration and molecular weight of the polymer. We expect that the temperature will affect the adsorption/absorption of the monomer, the viscosity of the IL, and the polymerization kinetics of the system.