Phase Transition and Preferential Alcohol Adsorption of Poly(N,N-diethylacrylamide) Gel in Water/Alcohol Mixtures

Abstract

The phase separation of poly(N,N-diethylacrylamide) (PDEA) gel network in water/alcohol (methanol, ethanol, isopropanol, and n-propanol) mixtures has been systematically investigated by variable-temperature high-resolution 1H MAS NMR. The PDEA gel exhibits the cononsolvency effect and reentrant phase transition in water/isopropanol and water/n-propanol mixtures with larger alcohol hydrophobic groups but not in the water/methanol and water/ethanol bearing smaller alcohol hydrophobic groups, in contrast to the case of poly(N-isopropylacrylamide) where the cononsolvency effect was observed in all water/alcohol solutions. In the present PDEA gel, the phase separation is characterized by dehydration of the hydrophobic alkyl groups of PDEA, resulting in two distinct types of water/alcohol solutions above the lower critical solution temperature: confined binary solvents (3–5 solvent molecules per PDEA repeating unit) inside the gel network and free binary solvents outside the gel. The alcohol concentration in confined mixture solvents is markedly higher than that in free mixture solvents and increases with increasing hydrophobicity of the alcohol moiety. Interestingly, the alcohol enrichment inside the shrunk network does not cause additional dehydration of the PDEA network. Besides, the side chain mobility of shrunk PDEA gel network is enhanced by the appearance of alcohol and increases with the hydrophobicity of alcohol moiety. Our results reveal that there exists a strong interaction between alcohol and gel network.