Initiated chemical vapor deposition (iCVD) is able to synthesize linear and cross-linked poly(2-hydroxyethyl methacrylate) (PHEMA) thin films, in one step, from vapors of 2-hydroxyethyl methacrylate (HEMA), ethylene glycol diacrylate (EGDA), and tert-butyl peroxide (TBPO) without using any solvents. This all-dry technique also allows control of the cross-link density by adjusting the partial pressure of the cross-linking agent EGDA in the vapor phase. Films with specific cross-link densities and hence thermal, wetting, and swelling properties can be created in one single vacuum processing step. Through selective thermal decomposition of the initiator TBPO, films with well-defined chemical structures and full functionality retention can be deposited, which is evident in the Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses. These spectroscopic methods also facilitate determination of EGDA incorporation in the cross-linked films based on the fact that HEMA contains a hydroxyl group but EGDA does not. For the linear PHEMA depositions, the growth rate was found to be nonlinear in the partial pressure of HEMA, possibly due to nonlinear multilayer adsorption and/or primary termination. The EGDA/HEMA ratio in the films systematically increased from 0.00 to 0.46 as the EGDA partial pressure was raised. The onset temperatures of decomposition were between 270 and 302 degrees C for the linear and the most cross-linked films, respectively. Thermal annealing at approximately 430 degrees C resulted in minuscule amounts of residue for all films, linear or cross-linked. The most cross-linked film had approximately 99.50% thickness removed after annealing. The contact angle was found to increase with increasing cross-link density. Significant contact-angle hysteresis was observed, indicating surface reconfiguration, and the lowest receding angle was 17 degrees for the linear film. Swelling measurements using spectroscopic ellipsometry showed that the degree of swelling decreased with increasing EGDA incorporation. The water content decreased from 35% (v/v) for the linear film to below 10% (v/v) for the most cross-linked film. These results show that iCVD is able to produce PHEMA thin films that function as hydrogels when soaked in water. The spectroscopic results, the contact-angle results, and the swelling analysis altogether prove the retention of the hydrophilic pendant groups in the iCVD process.

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Initiated CVD (iCVD), a dry method, is able to produce poly(methyl methacrylate) (PMMA) thin films by utilizing a reactive gaseous mixture of the monomer methyl methacrylate and the initiator triethylamine. The deposition rate is twenty times faster with the use of the initiator. Fourier-transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) show high structural resemblance between iCVD PMMA and conventional PMMA, and the degree of functionality retention increases with decreasing residence time in the vacuum chamber. XPS detection of nitrogen incorporation is consistent with the incorporation of the initiator into the polymer chains. NMR spectroscopy on completely dissolved films shows that the tacticity of iCVD PMMA resembles that of conventional, radically polymerized PMMA. Altogether these observations support the hypothesis that, for iCVD PMMA, the polymerization is by a free-radical mechanism.

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This work studies the effects of changing equilibrium monomer surface concentration on the deposition rates and the number-average molecular weights (M n) of polymers deposited from glycidyl methacrylate and cyclohexyl methacrylate using initiated chemical vapor deposition (iCVD) with tert-butyl peroxide as the initiator. Both the surface temperature and the monomer partial pressure were varied to effect different equilibrium surface concentrations, measured using a quartz-crystal microbalance. In either case, the deposition rate and M n were found to be linear in equilibrium monomer surface concentration. This strong dependence concludes that chain propagation occurs predominantly on the surface and suggests that the surface concentration is at equilibrium during iCVD, which in turn infers that the adsorption of monomer is not the rate-limiting step in the polymerization process.