Initiated Chemical Vapor Deposition of Linear and Cross-linked Poly(2-hydroxyethyl methacrylate) for Use as Thin-Film Hydrogels

Abstract

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.

Sequential Processing for Organic Photovoltaics: Design Rules for Morphology Control by Tailored Semi‐Orthogonal Solvent Blends

“…[ 74,[77][78][79] Figure 1 b shows a nearly linear relationship between the volume fraction of solvent within the fi lm, determined by the EMA, and the total fi lm dilation. This indicates that for P3HT, polymer crystallinity has little effect on the extent of fi lm dilation per unit volume of adsorbed solvent.…”

Thin Polymer Films with High Step Coverage in Microtrenches by Initiated CVD

“…[17] Decomposition of the initiator at an array of heated filaments produces radicals which initiate a free-radical polymerization of the monomer through its vinyl bonds. This method has been used to deposit a variety of polymers such as hydrogels, [18] alternating copolymers, [19] functionalizable materials, [20] antimicrobial films, [21] insulating organosilicones, [22] and sacrificial polymers for the production of air-gap structures. [23] In this work, we have deposited pCHMA using tertbutyl peroxide (TBPO) as the initiator.…”

Particle Surface Design using an All‐Dry Encapsulation Method

“…[21] Surprisingly, iCVD proves to be able to make a wide variety of polymers by simply using chemistries analogous to that of addition polymers from solution, making iCVD a versatile and powerful technique in the surface design of particles. Among the iCVD polymers reportedly obtained on flat substrates are poly(tetrafluoroethylene), [30,31] poly(organosilicones), [32] poly(methyl methacrylate), [33] poly(glycidyl methacrylate) (PGMA), [34] poly(2-hydroxyethyl methacrylate), [35] poly(perfluoroalkylethyl methacrylate), [36] and poly(n-alkyl acrylates). [37,38] Here, we demonstrate that microparticles and nanotubes well below 100 lm in size can be completely encapsulated with a clean polymer coating using iCVD without particle agglomeration.…”