Abstract
We have prepared a series of ampholytic polymer films, using a self-initiated photografting and photopolymerization (SI-PGP) method to sequentially polymerize first anionic (deuterated methacrylic acid (dMAA)) and thereafter cationic (2-aminoethyl methacrylate (AEMA)) monomers to investigate the SI-PGP grafting process. Dry films were investigated by ellipsometry, X-ray, and neutron reflectometry, and their swelling was followed over a pH range from 4.5 to 10.5 with spectroscopic ellipsometry. The deuterated monomer allows us to separate the distributions of the two components by neutron reflectometry. Growth of both polymers proceeds via grafting of solution-polymerized fragments to the surface, and also the second layer is primarily grafted to the substrate and not as a continuation of the existing chains. The polymer films are stratified, with one layer of near 1:1 composition and the other layer enriched in one component and located either above or below the former layer. The ellipsometry results show swelling transitions at low and high pH but with no systematic variation in the pH values where these transitions occur. The results suggest that grafting density in SI-PGP-prepared homopolymers could be increased via repeated polymerization steps, but that this process does not necessarily increase the average chain length.
Highlights
The unwanted accumulation of biological material on surfaces is a concern in many fields of science and engineering, including marine environments,[1] medical applications,[2] and biosensing.[3]
For many of the samples, the refractive indices fall between reported values of 1.537 for pAEMA59 and 1.475 for pMAA,[60] but we note that samples S45, S54, and S55 have refractive indices lower than this range and much lower than the average value
Since the two-layer model generates overall better fits and reveals plausible internal structuring of the polymer layer, in that it accounts for the grafting depth of the poly(aminoethyl methacrylate) (pAEMA) layer and predicts the expected degradation of the pdMAA layer; we argue that it has superior explanatory power, and we consider that its use is justified
Summary
The unwanted accumulation of biological material on surfaces is a concern in many fields of science and engineering, including marine environments,[1] medical applications,[2] and biosensing.[3]. These prevent attachment by binding water molecules strongly to the surface, providing steric hindrance and increasing the enthalpic cost of attachment and in certain cases rely on entropic effects acting upon the expulsion of water from hydrated polymer films to hinder macromolecular adsorption This approach has a long history, with work on poly(2-hydroxyethyl methacrylate) (pHEMA) dating back over 60 years.[4] Later on, interest in the field has expanded to cover a wide range of polymers and hydrogel materials,[5−7] and considerable efforts have been spent in understanding and using poly(ethylene glycol) (PEG) for antifouling purposes due to its excellent fouling resistance.[8−10] For PEG, the efficacy of strong hydration as a major reason for its antifouling properties has been qualitatively and quantitatively demonstrated[11] and explained, both at the molecular level[12,13] and collectively for polymer brushes.[14] Growing concerns about immunogenicity[15] and stability[16] of PEG are gradually shifting interest toward other polymers. Most zwitterionic polymers are prepared from a very limited range of zwitterionic residues,[19] but small differences in the molecular structure of the polymers can significantly influence their properties;[20−22] efforts are made to explore and understand the behavior and properties of different zwitterionic monomer structure variants.[19,23]
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