Interaction Forces on Polyampholytic Hydrogel Gradient Surfaces

Abstract

Rational design and informed development of nontoxic antifouling coatings requires a thorough understanding of the interactions between surfaces and fouling species. With more complex antifouling materials, such as composites or zwitterionic polymers, there follows also a need for better characterization of the materials as such. To further the understanding of the antifouling properties of charge-balanced polymers, we explore the properties of layered polyelectrolytes and their interactions with charged surfaces. These polymers were prepared via self-initiated photografting and photopolymerization (SIPGP); on top of a uniform bottom layer of anionic poly(methacrylic acid) (PMAA), a cationic poly(2-dimethylaminoethyl methacrylate) (PDMAEMA) thickness gradient was formed. Infrared microscopy and imaging spectroscopic ellipsometry were used to characterize chemical composition and swelling of the combined layer. Direct force measurements by colloidal probe atomic force microscopy were performed to investigate the forces between the polymer gradients and charged probes. The swelling of PMAA and PDMAEMA are very different, with steric and electrostatic forces varying in a nontrivial manner along the gradient. The gradients can be tuned to form a protein-resistant charge-neutral region, and we demonstrate that this region, where both electrostatic and steric forces are small, is highly compressed and the origin of the protein resistance of this region is most likely an effect of strong hydration of charged residues at the surface, rather than swelling or bulk hydration of the polymer. In the highly swollen regions far from charge-neutrality, steric forces dominate the interactions between the probe and the polymer. In these regions, the SIPGP polymer has qualitative similarities with brushes, but we were unable to quantitatively describe the polymer as a brush, supporting previous data suggesting that these polymers are cross-linked.

Structure and pH-Induced Swelling of Polymer Films Prepared from Sequentially Grafted Polyelectrolytes

“…This method could lead to better results in swelling experiments. 25,26 In the "U"-type swelling curves (Figures 6a,b and S8), the transitions at low pH are in most cases more diffuse than those at high pH, extending over a greater pH range, as illustrated in Figure 7. This is in agreement with previous studies, which demonstrate clear differences in the swelling between regions rich in cations and anions, respectively, with variations in pH.…”

“…In a recent publication, we investigated the contributions of steric and electrostatic forces to the interactions of a particle approaching such a gradient over both the swollen and the collapsed regions. 26 However, this does not permit an inference of the composition of the gradient at a certain position. To establish a correlation between the composition of the films, the pH-dependent swelling, and other properties, we investigate the swelling and the monomer distribution in a series of sequentially grafted polymer bilayers, where both layers are homogeneously polymerized over the surface.…”

Polyampholytic Poly(AEMA-co-SPMA) Thin Films and Their Potential for Antifouling Applications

“…The monomers are excited by UV irradiation to form radicals with enough energy to initiate free-radical polymerization in the solution, but which might also abstract hydrogen atoms from the substrate to form surface-bound radicals as starting points for grafting. This is an uncontrolled polymerization reaction where polymerization proceeds in the bulk and monomers, oligomers, or polymers are grafted, while possibly also cross-linking and branching of grafted chains occur. , In previous studies in our lab, this preparation method has been used to fabricate both excellent antifouling coatings , and charge-balanced materials. , Among the advantages of this method is that it is inexpensive and simple in setup and preparation, uses water as the solvent, and is compatible with a wide range of monomers, substrate surface chemistries (almost any organic material), and geometries . For use in sensitive environments and for biological assays, it also has a considerable advantage since it does not use initiators or catalysts, thus eliminating the risk of toxicity from residual chemicals, for example, Cu ions, and only the potential toxicity of remaining free monomers needs to be considered.…”

“…26,27 In previous studies in our lab, this preparation method has been used to fabricate both excellent antifouling coatings 28,29 and charge-balanced materials. 30,31 Among the advantages of this method is that it is inexpensive and simple in setup and preparation, uses water as the solvent, and is compatible with a wide range of monomers, substrate surface chemistries (almost any organic material), 32 and geometries. 33 For use in sensitive environments and for biological assays, it also has a considerable advantage since it does not use initiators or catalysts, thus eliminating the risk of toxicity from residual chemicals, for example, Cu ions, and only the potential toxicity of remaining free monomers needs to be considered.…”

Superlubricity of pH-responsive hydrogels in extreme environments

“…When the countersurface and hydrogel have the same charge, friction decreases with increasing surface charge density due to greater electrostatic repulsion between the two sliding interfaces and the formation of a solvent layer ( Yoo et al, 1997 ; Gong et al, 1999a ; Oogaki et al, 2009 ; Ahmed et al, 2014 ; Osaheni et al, 2020 ; Wang et al, 2020 ). Models have been developed to predict the thickness of this fluid layer, which depend on variables such as contact pressures and swelling ratios ( Gong et al, 1999a ; Sokoloff, 2010 ; Sokoloff, 2012 ; Sokoloff, 2013 ; Erbaş and Olvera De La Cruz, 2016 ; Tai et al, 2019 ).…”