Hydrophobically modified copolymers provide a versatile platform of hydrogel materials for diverse applications, but the influence of salts on the swelling and material properties of this class of hydrogels has not been extensively studied. Here, we investigate model hydrogels with three different sodium salts with anions chosen from the classic Hofmeister series to determine how these counterions influence the swelling and mechanical properties of neutral hydrogels. The gel chosen was based on a statistical copolymer of dimethylacrylamide and 2-(N-ethylperfluorooctane sulfonamido) ethyl acrylate (FOSA). Our measurements utilize a quartz crystal microbalance with dissipation (QCM-D) to quantify both swelling and rheological properties of these gels. We find that a 1 mol/L solution of Na2SO4, corresponding to a kosmotropic anion, leads to nearly a 2.6-fold gel deswelling and correspondingly, the complex modulus increases by an order of magnitude under these solution conditions. In contrast, an initial increase in swelling and then a swelling maximum is observed for a 0.02 mol/L concentration in the case of a chaotropic anion, NaClO4, but the changes in the degree of gel swelling in this system are not directly correlated with changes in the gel shear modulus. The addition of NaBr, an anion salt closer to the middle of the chaotropic to kosmotropic range, leads to hydrogel deswelling where the degree of deswelling and the shear modulus are both nearly independent of salt concentration. Overall, the observed trends are broadly consistent with more kosmotropic ions causing diminished solubility (“salting out”) and strongly chaotropic ions causing improved solubility (“salting in”), a trend characteristic of the Hoffmeister series governing the solubility of many proteins and synthetic water-soluble polymers, but trends in the shear stiffness with gel swelling are clearly different from those normally observed in chemically cross-linked gels and are correspondingly difficult to interpret. The salt specificity of swelling and mechanical properties of nonionic hydrogels is important for any potential application in which a wide range of salt concentrations and types are encountered.

Non-covalently crosslinked hydrogels can exhibit toughness and mechanical adaptability typically associated with biological tissues, which make them promising for a variety of applications. However, molecules in the environment can interact to significantly alter the properties of these hydrogels, which could adversely impact their performance. Here, we illustrate how two common ionic surfactants, sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), influence the swelling and rheological properties of hydrogel films crosslinked by hydrophobic associations based on a random copolymer of poly(N,N-dimethylacryalmide-co-2-(N-ethylperfluorooctanesulfonamido)ethyl acrylate (DMA-FOSA) using quartz crystal 2 microbalance with dissipation (QCM-D). The effect of the surfactants on the swelling and stability of the hydrogels is contrasted with aqueous 2-propanol (IPA), which can dissolve the copolymer readily. The addition of IPA, SDS and CTAB at low concentrations increases the swelling of the hydrogel film, decreases the elastic modulus and increases the rheological phase angle (more fluidlike). A transition from swelling to (partial) dissolution occurs at higher concentrations, even for originally <200 nm thick copolymer films, with a threshold of approximately 7 wt% IPA, 0.1´ CMC for SDS, and 0.5´ CMC for CTAB to promote partial dissolution. With IPA, initial swelling is always observed after the solvent is added, whereas the initial swelling can be significantly delayed with long incubation times to dissolution, on the order of hours, at low concentrations of surfactant. These results illustrate that simple ionic surfactants can dissolve these hydrophobically crosslinked hydrogels, but identification of the maximum concentration of surfactant that will not dissolve the hydrogels may be challenged by the long incubation times that increase as the concentration decreases. The long times to dissolve these thin films suggest that stability of bulk physically crosslinked hydrogels in complex aqueous environments may be challenging to accurately assess if surfactant diffusion is limiting factor.

Polymer membranes represent a low cost, low energy solution to separating biologically derived chemicals from fermentation. However, the performance of these membranes is commonly limited by fouling caused by biological components, but additives are also included in typical fermentation broths that can interact with the polymer membrane. Here, we examine how a common polyol antifoam agent (Antifoam 204) dramatically swells and plasticizes a high performing biobutanol membrane copolymer of hydroxyhexafluoroisopropyl and n-butyl substituted norbornene. When equilibrated against 1 wt % (aq) butanol, the solvent in the copolymer increases from <10 vol % without the antifoam to >40 vol % at 1 ppm of antifoam and >80 vol % at 100 ppm of antifoam. The effect of the antifoam on the properties of the copolymer as determined by QCM-D is much more significant than that of the butanol concentration. Even with 4 wt % butanol (greater than typically viable for biobutanol), the copolymer swells <25 vol % without the antifoam. The rheological properties of the copolymer are also influenced with a phase angle of <15° (viscoelastic solid) with a nearly GPa modulus when swollen with aqueous butanol solution, while the phase angle increases to >50° (viscoelastic liquid) with an MPa modulus with the addition of ppm of antifoam. These results demonstrate the striking effect of solution additives on the properties of advanced polymers for membranes and illustrate the importance of considering the potential interactions of all components with a membrane beyond those of interest for the separation.

For nanoparticle (NP) polymer composites, the state of dispersion vs aggregation significantly affects optical, electronic, thermal, and mechanical properties. The switching of NP distribution states thus far was limited to polymer solutions or bulky polymer-grafted NPs. Herein, for the first time, NP distribution states within polymer films are switched by adjusting fluorophobic interactions and the enthalpy of mixing with immersion annealing. The fluorophobic effect is the tendency of fluorinated molecules to strongly phase-separate from non/less fluorinated molecules. A highly fluorophobic homopolymer, poly­(perfluorooctyl acrylate) (PFOA), was combined with gold NPs of variable fluorophobic character, prepared using mixtures of small-molecule ligands (xF-NP, where x is the mol % fluorinated ligands). Low-to-moderately fluorophobic F-NPs with PFOA were aggregated after spin coating where film swelling via immersion annealing with moderately fluorophobic trifluoro toluene (TFT) generally led to a dispersed state. In contrast, the highly fluorophobic 100F-NPs were dispersed regardless of immersion annealing. This behavior was attributed to the PFOA acting like a surfactant to enable dispersion of highly fluorophobic NPs in TFT. Since these two distinct behaviors favor nonoverlapping ranges of xF-NP compositions, the NPs with intermediate compositions exhibited limited dispersibility. This fluorophobic switchability could enable time- and chemical-selective sensing of fluorinated compounds in the future.