59th Annual Report on Research 2014

The goal of this project is to study the formation of ionic liquid/polymer composites using vapor phase polymerization. Our vapor phase polymerization technique is called initiated chemical vapor deposition (iCVD). In the iCVD process, monomer and initiator gases are introduced into a chamber and a hot filament array breaks the initiator into free radicals. Polymerization occurs on the surface of the substrate via a free radical polymerization mechanism.

In order to make the composites, we deposit polymers onto ionic liquid substrates. Ionic liquids can be used as substrates in our iCVD system because ILs have extremely low vapor pressures and are therefore stable under vacuum. We have shown that if the monomer is insoluble in the liquid, then polymerization only occurs on the surface of the liquid. However, if the monomer is soluble in the liquid, then polymerization occurs at both the surface of the liquid and within the liquid, which leads to the formation of IL/polymer gels.

We first demonstrated that we could polymerize 2-hydroxyethyl methacrylate (HEMA) within thin layers of 1-ethyl-3-methylimidazolium tetrafluoroborate [emim][BF₄] to form robust gels. We used dynamic mechanical analysis to study the transition from a viscous liquid to a gel as the polymer concentration was increased. We used gel permeation chromatography to study the molecular weights of the polymer chains. Our most interesting result was that there were two distinct molecular weights. The shorter chains were due to polymerization at the liquid surface whereas the longer chains were due to polymerization within the liquid. We also found that increasing our reactor pressure could increase the molecular weight of the chains.

We then found that we could controllably form heterogeneous gels by using several monomers and varying the order that these monomers are introduced into the reactor. Our study focused on two monomers: an insoluble monomer 1H,1H,2H,2H-perfluorodecyl acrylate (PFDA) that only polymerizes at the liquid surface and a soluble monomer ethylene glycol diacrylate (EGDA) that polymerizes at both the liquid surface and within the liquid. We varied the order that these monomers were introduced into the reactor. We used [emim][BF₄] as the IL substrate. When PFDA was introduced first, a film formed at the liquid surface and the EGDA monomer then diffused through this film and polymerized within the bulk, leading to a sandwich structure. When EGDA was introduced first and then PFDA, a layered film was formed whose structure followed the order in which the monomers were introduced. When the two monomers were introduced simultaneously, the EDGA polymerized in the bulk while a copolymer film formed at the surface of the liquid.

In the gels mentioned above, the [emim][BF₄] IL is incorporated into the gel but it is not covalently bonded to the polymer matrix. In order to make a material that has the IL and the polymer covalently bonded, we copolymerized an IL monomer 1-ethyl-3-vinylimidazolium bis(trifluoromethylsulfonyl)imide ([EVIm][TFSI]) with EGDA. We placed droplets of [EVIm][TFSI] in the reactor and introduced EGDA and tert-butyl peroxide initiator in the vapor phase. The heterogeneous films that formed had a homopolymer PEGDA top layer formed by the polymerization of adsorbed EGDA at the liquid surface and a poly([EVIm][TFSI]-co-EGDA) copolymer bottom layer that formed by the copolymerization of [EVIm][TFSI] with absorbed EGDA within the liquid. We also demonstrated that the composition can be further controlled by tuning the reaction time and pressure. Also, the gels can be formed on solid supports like wire meshes which is useful for practical applications.