Phase Behavior of Co-Nonsolvent Systems: Poly(N-isopropylacrylamide) in Mixed Solvents of Water and Methanol

Abstract

Cloud points of poly( N-isopropylacrylamide) in aqueous mixed solvents, with methanol as the cosolvent, are experimentally measured for polymer concentrations varied up to as high as the weight fraction 0.25. They are shown to form closed loops on the ternary phase plane in the temperature region between 5 and 30 °C, and hence co-nonsolvency is complete. Miscibility loops shrink by cooling, or equivalently, they exhibit lower critical solution temperature behavior. For a fixed polymer concentration, there is a composition of the mixed solvent at which the cloud-point temperature takes the lowest value. This minimum cloud-point temperature composition of the mixed solvent turned out to be almost independent of the polymer concentration, at least within the measured dilute region below the weight fraction 0.25. On the basis of the assumption that the phase separation is closely related to the preferential adsorption of the solvents by hydrogen bonding, we employ a model solution of Flory-Huggins type, augmented with direct and cooperative polymer-solvent hydrogen bonds, to construct the ternary phase diagrams. Theoretical calculation of the spinodal curves is performed, and the results are compared with the obtained experimental cloud-point data. The effect of molecular volume of the cosolvent is also studied within the same theoretical framework. Possibility for a upper critical solution temperature co-nonsolvency to appear for cosolvents with larger molecular volume is discussed.

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“…The phenomenon of cononsolvency has been previously reported in thermal-responsive homopolymer PNiPAM, ,, other polymers, , and organic small molecules . Many theoretical and experimental studies explained that cononsolvency was induced by strong alcohol–water interaction, where alcohol increases the hydrogen-bond network among the water molecules, thereby reducing the interaction of water and solute and thus resulting the reduction of hydrophobic hydration. ,, …”

Microneedle‐Array Patch Fabricated with Enzyme‐Free Polymeric Components Capable of On‐Demand Insulin Delivery

“…The resultant hydrogels yielded microporous and interconnected structures, as shown in the cross‐sectional scanning electron microscope (SEM) images ( Figure A), with the structures being more prominent in the presence of SF . This characteristic porous morphology probably stems from a microscopic phase separation taking place during polymerization, which is commonly observed for poly(acrylamide) derivatives in methanol (MeOH)‐water mixed solvent systems . During polymerization, SF chains are also expected to interdiffuse into the polymer network, leading to increased wall thickness that may contribute to the improved mechanical strength.…”

On Going to a New Era of Microgel Exhibiting Volume Phase Transition

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