Polyelectrolyte brushes have received extensive attention due to their swelling behavior in aqueous solutions, which is a result of their ionic nature and nonelectrostatic (polymer–polymer and polymer–solvent) interactions. While the nonelectrostatic contributions are typically assumed to be negligible compared to the ion osmotic pressure, we present herein a systematic investigation of how the nonelectrostatic interactions dramatically affect the swelling behavior of polyelectrolyte brushes. Using a modular synthesis procedure, polyelectrolyte brushes with similar chain lengths, grafting densities, and charge densities but with various side chains were synthesized. The swelling behaviors of the brushes were then thoroughly investigated as a function of grafting density and ionic strength using ellipsometry. A theoretical mean-field approach was also developed to quantify the contributions of the ion osmotic and nonelectrostatic effects. Accordingly, it was both experimentally and theoretically demonstrated how fundamentally different swelling behaviors can exist depending on the relative contributions of ion osmotic and nonelectrostatic effects. This new insight reveals new opportunities for applications of polyelectrolyte brushes based on their tunable hydrophobicity and provides new ideas for fundamental investigations of these systems.
A weak polyelectrolyte brush is composed of a layer of polyacids or polybases grafted by one end of their chains to a substrate surface. For such brush layers immersed in an aqueous solution, the dissociation behavior of the acidic or basic groups and the structural and physical properties of the brush layer will thus be strongly dependent on the environmental conditions. For a polyacid brush layer consisting of, e.g., poly(acrylic acid), this means that the chains in the brush layer will be charged at high pH and uncharged at low pH. However, theoretical scaling laws not only foresee the structural changes occurring in response to the pH-induced dissociation behavior but also how the dissociation behavior of the brush layer depends on the ionic strength of the aqueous solution and the density of acidic groups within the brush layer. We have herein employed spectroscopic ellipsometry and a quartz crystal microbalance with dissipation monitoring (QCM-D) to experimentally evaluate the theoretically predicted dissociation and structural behavior of PAA brushes. Spectroscopic ellipsometry allows us to study the brush thickness as a function of pH and ionic strength, while QCM-D gives us an opportunity to investigate the swelling behavior of PAA brushes at various penetration depths of propagating acoustic waves. Our studies show that the dissociation degree of the carboxylic acid groups in a PAA brush increases with increasing distance from the substrate. Moreover, the ionic strength enhances carboxylic acid dissociation, such that a higher ionic strength leads to a narrower distribution and higher average dissociation degree. In conclusion, our results provide an experimental verification of the theoretically predicted gradient in the degree of dissociation of the acid groups in weak polyacid brush layers and shows that at a pH value equal to approximately the average pK a value of the brush, the state of the acid groups varies from being almost uncharged to almost fully dissociated depending on the ionic strength and vertical position in the brush.
It is well known that specific types of counterions affect the hydration of polyelectrolytes both in the bulk and at interfaces, but the mechanisms of this effect have not yet been fully understood. In this work, we have designed a model system, consisting of imidazolium-based cationic polyelectrolyte brushes with controlled grafting densities, to systematically investigate how specific counterion properties affect well-established swelling mechanisms in brushes. With this approach, we show that two swelling mechanisms, namely, counterion influence on the ion osmotic pressure and counterion influence on brush−solvent nonelectrostatic interactions, are simultaneously at play. Here, we demonstrate that the former effect can be related to the polarizability of the counterions, while the latter effect can be correlated to the hydration enthalpy of the counterions. We further demonstrate that the interplay of these two mechanisms depends on the brush grafting density and ionic strength of the medium such that under certain conditions, one effect can dominate over the other. Specifically, at low ionic strength and low grafting density, swelling of the brush is significantly influenced by the polarizability of counterions, while at high grafting density and high ionic strength, the hydration enthalpy of ions is the dominating factor. Moreover, by employing a theoretical model, we rationalize the experimental findings and further quantify the contribution of specific counterion effects as a function of grafting density and ionic strength. We believe such an approach improves the general understanding of the influence of ions on the polyelectrolyte brush swelling and even beyond.
In this study, we systematically investigate the interactions between mobile ions generated from added salts and immobile charges within a sulfobetaine-based polyzwitterionic film in the presence of five salts (KCl, KBr, KSCN, LiCl, and CsCl). The sulfobetaine groups contain quaternary alkylammonium and sulfonate groups, giving the positive and negative charges. The swelling of the zwitterionic film in the presence of different salts is compared with the swelling behavior of a polycationic or polyanionic film containing the same charged groups. For such a comparative study, we design cross-linked terpolymer films with similar thicknesses, cross-link densities, and charge fractions, but with varying charged moieties. While the addition of salt in general leads to a collapse of both cationic and anionic films, the presence of specific types of mobile anions (Cl–, Br–, and SCN–) considerably influences the swelling behavior of polycationic films. We attribute this observation to a different degree of ion-pair formations between the different types of anionic counterions and the immobile cationic quaternary alkylammonium groups in the films where highly polarizable counterions such as SCN– lead to a high degree of ion pairing and less polarizable counterions, such as Cl–, cause a low degree of ion pairing. Conversely, we do not observe any substantial effect of varying the type of cationic counterions (K+, Li+, and Cs+), which we assign to the lack of ion pairing between the weakly polarizable cations and the immobile anionic sulfonate groups in the films. In addition, we observe that the zwitterionic films swell with increasing ionic strength and the degree of swelling is anion dependent, which is in agreement with previous reports on the “antipolyelectrolyte effect”. Herein, we explain this ion-specific swelling behavior with the different cation and anion abilities to form ion pairs with quaternary alkylammonium and sulfonate in the sulfobetaine groups.
Poly(dimethylsiloxane) (PDMS) is an attractive, versatile, and convenient material for use in biomedical devices that are in direct contact with the user. A crucial component in such a device is its surface in terms of antimicrobial properties preventing infection. Moreover, due to its inherent hydrophobicity, PDMS is rather prone to microbial colonization. Thus, developing an antimicrobial PDMS surface in a simple, large-scale, and applicable manner is an essential step in fully exploiting PDMS in the biomedical device industry. Current chemical modification methods for PDMS surfaces are limited; therefore, we present herein a new method for introducing an atom transfer radical polymerization (ATRP) initiator onto the PDMS surface via the base-catalyzed grafting of [(chloromethyl)phenylethyl]trimethoxysilane to the PDMS. The initiator surface was grafted with poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) brushes via a surface-initiated supplemental activator and reducing agent ATRP (SI-SARA-ATRP). The use of sodium sulfite as a novel reducing agent in SI-SARA-ATRP allowed for polymerization during complete exposure to air. Moreover, a fast and linear growth was observed for the polymer over time, leading to a 400 nm thick polymer layer in a 120 min reaction time. Furthermore, the grafted PDMAEMA was quaternized, using various alkylhalides, in order to study the effect on surface antimicrobial properties. It was shown that antimicrobial activity not only depended highly on the charge density but also on the amphiphilicity of the surface. The fast reaction rate, high oxygen tolerance, increased antimicrobial activity, and the overall robustness and simplicity of the presented method collectively move PDMS closer to its full-scale exploitation in biomedical devices.
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