Reports: DNI552352-DNI5: Green Solventless Fabrication of Ionic Liquid/Polymer Composites Using Vapor Phase Polymerization

Malancha Gupta, PhD, University of Southern California

The goal of our proposal is to understand the mechanism and kinetics that govern the formation of robust polymer/ionic liquid (IL) composites for alternative energy applications using the initiated chemical vapor deposition (iCVD) process. In the iCVD process, monomer and initiator vapors are flown into a vacuum chamber. A heated filament array breaks the initiator molecules into free radicals to start the polymerization process. The iCVD process is typically used to coat solid substrates, but we recently introduced ionic liquids as substrates due to their extremely low vapor pressures. In order to determine the conditions that allow for the formation of polymer/IL composites, we first studied the deposition of soluble and insoluble monomers onto IL substrates.  We used 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) as a model IL, 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) as a model insoluble monomer, and ethylene glycol diacrylate (EGDA) as a model soluble monomer. The structure of the deposited films was examined using X-ray photoelectron spectroscopy (XPS). Our analysis showed that there was no [emim][BF4] incorporation into the PPFDA film since the insolubility of the PFDA monomer limits polymerization to only the IL surface. In contrast, the PEGDA film had [emim][BF4] incorporation since polymerization occurs both at the IL surface and within the bulk IL since the monomer and initiator can absorb into the IL and react within the IL. The PEGDA polymer that forms within the bulk becomes integrated into the PEGDA film that forms at the IL surface. The IL incorporation into the film is caused by the entrapment of IL between the polymer chains that form in the bulk as they become integrated into the film.

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.