Synthesis of reactive and stimuli-responsive polymer thin films by initiated chemical vapor deposition and their sensor applications

Abstract

Stimuli-responsive polymer thin films provide the ability to control the interaction of a surface with its environment. Synthetic techniques with fine compositional control are required to engineer specific responses to stimuli. Initiated chemical vapor deposition (iCVD) is a novel vacuum deposition technique in which polymer films are synthesized bottom-up as monomer and initiator vapors are introduced into a vacuum coating chamber. iCVD can conformally coat nanoscale, three-dimensional geometries with a high level of compositional control. Such coating capabilities are difficult by solution-based film synthesis techniques, and compositional control is difficult by other CVD techniques. In this thesis work, the synthesis and application of stimuli-responsive and reactive polymer thin films by iCVD have been studied. First, functionally versatile alternating copolymers of poly(styrene-alt-maleic anhydride) were synthesized. This was the first demonstration of alternating copolymer synthesis by CVD. Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, and 13C NMR characterization verified that well-defined alternating structure was present, as predicted from standard solution phase polymerizations. Next, polymer crosslinking was tuned in the synthesis of pH-responsive hydrogel thin films composed of poly(maleic anhydride-co-dimethylacrylamide-co-di(ethylene glycol) divinyl ether). These films had swelling ratios, defined as the thickness in the water-swollen state over the thickness in the dry state, in excess of 11. These films were employed as ultrathin permeable, size-selective skin layers in composite membranes.

(cont.) The final two projects involved the synthesis of polymer thin films as chemically selective layers in microscale chemical sensors. In a sensing system based on the detection of amine compounds through their reaction with reactive polymer coatings on microcantilevers, it was shown that increased crosslinking of the polymer layer leads to greater cantilever deflection. This increased deflection enabled the design of simple, resistance-based signal readout schemes with low power requirements. New polymer compositions and sensing strategies were also developed for the detection of nitroaromatic explosives vapors. Poly(4-vinylpyridine) was shown to swell when exposed to nitroaromatics, while showing little response to common interferents. In contrast to conventional nitroaromatic-selective layers, it was shown that poly(4-vinylpyridine) does not interact with nitroaromatics through hydrogen bonding, which is important for multicomponent sensor arrays. A new microscale sensing concept utilizing this swelling was designed, fabricated, and characterized.