Co-nonsolvency occurs if a mixture of two good solvents causes the collapse or demixing of polymers into a polymer-rich phase in a certain range of compositions of these two solvents. In this work, we systematically study the co-nonsolvency behavior of poly­(N-isopropylacrylamide) brushes of different grafting densities in a series of alcohol–water binary mixtures with increasing hydrophobic parts ranging from methanol to 1-butanol by using ellipsometry. We report a strong collapse transition by increasing the alcohol concentration in the water-rich phase, which is enhanced for longer-chain alcohols. The analysis of the thermodynamic properties of the alcohol–water series displays that an increasing demixing tendency between alcohol and water is correlated with an enhancement of the collapse transition of the brush. The increase of grafting density weakens the transition behavior but does not shift the solvent composition point of maximum brush collapse, which is in agreement with the predictions of a recently proposed mean-field model based on the preferential adsorption concept. Among the fully miscible solvents, the most sensitive switching behavior of the brush is found for 1-propanol while 1-butanol already displays a miscibility gap at higher volume fractions.

Polymer brush surfaces that alter their physical properties in response to chemical stimuli have the capacity to be used as new surface-based sensing materials.F or such surfaces,d etecting the polymer conformation is key to their sensing capabilities.H erein, we report on FRET-integrated ultrathin (< 70 nm) polymer brush surfaces that exhibit stimuli-dependent FRET with changing brush conformation. Poly(N-isopropylacrylamide) polymers were chosen due their exceptional sensitivity to liquid mixture compositions and their ability to be assembled into well-defined polymer brushes.The brush transitions were used to optically sense changes in liquid mixture compositions with high spatial resolution (tens of micrometers), where the FRET coupling allowed for noninvasive observation of brush transitions around complex interfaces with real-time sensing of the liquid environment. Our methods have the potential to be leveraged towards greater surface-based sensing capabilities at intricate interfaces.

Cononsolvency occurs if a mixture of two good solvents causes the collapse or demixing of polymers into a polymer-rich phase in a certain range of compositions of these two solvents. The better solvent is usually called the cosolvent, and the other common solvent is called the solvent. An unsolved problem in the understanding of the cononsolvency transition of polymers is the role of various polymer–solvent and cosolvent–solvent interactions. In this work, using a mean-field model, we offer a comprehensive and quantitative theoretical study of the cononsolvency effect of neutral immobilized polymers, in particular, poly­(N-isopropylacrylamide) (PNiPAAm) brushes and macrogels. Our model quantitatively describes and predicts the phase-transition behaviors of PNiPAAm brushes and gels in various aqueous alcohol solutions. We demonstrated that in addition to the dominant polymer–cosolvent preferential adsorption and monomer–cosolvent–monomer triple contact (cosolvent-assisted temporary cross-linking effect), a nonideal mixing between the polymer and solvent shifts the collapse transition to the lower-concentration region of the cosolvent, while an increase of the demixing tendency between the cosolvent and solvent reduces the width of the cononsolvency transition. Moreover, weakening of the cononsolvency transition in cosolvent-poor aqueous solutions at high hydrostatic pressure can be explained by the suppression of demixing tendencies between the cosolvent and water, and between polymer and water in the case of PNiPAAm.

In this study, the cononsolvency transition of poly(N-isopropylacrylamide) (PNiPAAm) brushes in aqueous ethanol mixtures was studied by using Vis-spectroscopic ellipsometry (SE) discussed in conjunction with the adsorption-attraction model. We proved that the cononsolvency transition of PNiPAAm brushes showed features of a volume phase transition, such as a sharp collapse, reaching a maximum decrease in thickness for a very narrow ethanol volume composition range of 15% to 17%. These observations are in agreement with the recently published preferential adsorption model of the cononsolvency effect.