Polymer Brushes and Gels in Competing Solvents: The Role of Different Interactions and Quantitative Predictions for Poly(N-isopropylacrylamide) in Alcohol–Water Mixtures

Abstract

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.

Pressure Dependence of Water Dynamics in Concentrated Aqueous Poly(N-isopropylacrylamide) Solutions with a Methanol Cosolvent

“…Thus, near T cp , the bulk solvent phase is enriched with methanol, which, in turn, means that water is preferentially adsorbed on the polymer chains. This supports models attributing the reversal of the co-non-solvency effect at high pressures to enhanced polymer–water interactions. ,, Over a temperature interval of 20 °C above T cp , τ d gradually approaches the same value as in the neat 80:20 v/v H 2 O/CD 3 OD mixture, i.e., the original solvent partitioning is regained.…”

“…This supports models attributing the reversal of the conon-solvency effect at high pressures to enhanced polymer− water interactions. 57,58,60 Over a temperature interval of 20 °C above T cp , τ d gradually approaches the same value as in the neat 80:20 v/v H 2 O/CD 3 OD mixture, i.e., the original solvent partitioning is regained.…”

“…Partial replacement of cosolvent molecules decorating the PNIPAM chain by water molecules, as pressure is increased, was predicted to reduce bridge formation by cosolvent molecules between distant segments in the PNIPAM chain . In an extension of the adsorption–attraction model, it was suggested that, at high pressures, the interaction strengths between water and the polymer segments as well as those between water and the adsorbed cosolvent are increased, whereas the preferential adsorption of methanol to the polymer and thus the co-non-solvency effect are reduced …”

“…57 In an extension of the adsorption−attraction model, it was suggested that, at high pressures, the interaction strengths between water and the polymer segments as well as those between water and the adsorbed cosolvent are increased, whereas the preferential adsorption of methanol to the polymer and thus the co-nonsolvency effect are reduced. 60 Thus, the interactions of the two solvents with the polymer and with each other, both on the chain and in the bulk solvent mixture, play a role. In our recent SANS experiments on a PNIPAM solution in a water−methanol mixture (80:20 v/v), we characterized the temperature dependence of the concentration fluctuations below the cloud point at various pressures up to 200 MPa.…”

Regulating the Translocation of DNA through Poly(N-isopropylacrylamide)-Decorated Switchable Nanopores by Cononsolvency Effect

“…It is observed that in Figure 9a, an increase of grafting density of nanopore brushes weakens the collapse transition of the brush layer in ethanol/tris buffer mixtures, this follows the analytic prediction of a mean-field model for cononsolvency transition by our previous studies. 7,43 Also, as shown in Figures 5b and 9b, a PNiPAAm polymer layer with very low grafting density below the brush state still displays reentrance behavior; however, the corresponding phasetransition behaviors are not pronounced. It becomes clear that a pronounced switching effect can only be realized in a window of moderate grafting densities.…”