Preferential Adsorption and Co-nonsolvency of Thermoresponsive Polymers in Mixed Solvents of Water/Methanol

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If two good solvents become poor for a polymer when mixed, the solvent pair is called a co-nonsolvent pair for the polymer. The sharp depression of the LCST by the co-nonsolvency in solutions of poly( N -isopropylacrylamide) in the mixed solvent of water and methanol is shown to be caused by the competitive hydrogen bonding of water and methanol molecules onto the polymer chains. On the basis of a new statistical-mechanical model for competitive hydrogen bonds, the degree of hydration θ (w) and of methanol binding θ (m), excess degree Δθ E of solvent binding, preferential adsorption coefficients Γ, LCST spinodal lines, and cloud-point depression Δ T cl are theoretically calculated and compared with the experimental results. The optimal composition x m ( M ) of methanol at which LCST takes the minimum value is studied as a function of the polymer molecular weight M. In the high molecular weight limit, it takes x m ≃ 0.35. The solution recovers a uniform state in the region of higher methanol composition. Such a peculiar phase separation is caused by the dehydration of the polymer chains by the mixed methanol molecules in a cooperative way.