pubs.acs.org/Langmuir 1 H MAS NMR Studies of the Phase Separation of Poly (N-isopropylacrylamide) Gel in Binary Solvents Nian Wang, †,‡ Geying Ru, †,‡ Liying Wang, † and Jiwen Feng* ,† † State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Science, Wuhan 430071, PR China, and ‡ Graduate School, Chinese Academy of Science, Beijing 100029, PR China Received November 19, 2008. Revised Manuscript Received February 27, 2009 Preferential interactions of solvents with poly(N-isopropylacrylamide) (PNIPAM) gel networks in binary water/ alcohol (water/methanol and water/ethanol) mixtures have been investigated using variable-temperature high-resolu- tion 1 H MAS NMR. NMR results for PNIPAM gel in the binary solvents reveal the existence of two distinct types of water/alcohol mixtures above the LCST: confined binary solvents bound inside the gel, and free binary solvents expelled from the gel. It is interesting to find that the alcohol concentration in confined solution is significantly higher than that in free solution. Moreover, of the two alcohols, ethanol is more significantly concentrated in the confined solution. These results demonstrate that the polymer preferentially interacts with alcohol molecules over water and that the alcohol with higher hydrophobicity exhibits higher preferential absorption on PNIPAM. Our results also show that 1 H NMR measurements made on two distinct types of solution provide a convenient, direct means of characterizing the preferential adsorption of solvent on polymer. Introduction Poly(N-isopropylacrylamide) (PNIPAM) gel exhibits a very large volume change in pure water at a lower critical solution temperature (LCST) of about 33 °C. 1 The PNIPAM gel network shrinks from a hydrophilic coiled state to a hydrophobic globule state as the temperature is raised above the LCST. When the temperature falls to below the LCST, the gel can return to the swollen state. Thus, the PNIPAM gel is also called a thermo- reversible gel. 2,3 Materials of this kind could find a wide variety of applications in intelligent microfluidic switching, controlled drug delivery, bioseparations, and biomedical fields. 4-8 The PNIPAM gel also undergoes novel volume-phase transi- tions in water/alcohol mixtures in response to changes in tem- perature or solvent composition. 9 The PNIPAM gel is extremely soluble in pure water and pure alcohol but insoluble in certain mixtures of them (known as co-nonsolvency). 10 As a result, in PNIPAM a reentrant swelling-shrinking-swelling transition takes place as the alcohol content increases at the proper temperature. Utilizing this special property, the PNIPAM gel could be purified by a mixture of two good solvents. In fact, alcohol/water mixtures themselves are of great interest because they show various uncommon thermodynamic phenomena and have many important applications in industry and biochemistry. To understand the LCST phenomenon, properties of both PNIPAM chains and the PNIPAM gel in water have been widely studied by various techniques such as rheology, 11 differ- ential scanning calorimetry (DSC), 12 dynamic light scattering (DLS), 13,14 small-angle neutron scattering (SANS), 15 and NMR. 16 The effects of temperature, pH value, ionic strength, molecular weight, surfactant additives, and cross-link density have also been investigated extensively, 17-23 but the exact origin of the LCST of PNIPAM is not yet clear. 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