Department of Polymer Chemistry , Kyoto University , Katsura , Kyoto 615-8510 , Japan.
Langmuir. 2018 Mar 6;34(9):3003-3009. doi: 10.1021/acs.langmuir.7b03815. Epub 2018 Feb 20.
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.
在水-甲醇混合溶剂中，聚 N-异丙基丙烯酰胺的浊点，在高达 0.25 重分数的聚合物浓度范围内进行了实验测量。结果表明，在 5 至 30°C 的温度范围内，浊点在三元相图上形成闭合环，因此完全共非溶剂化。随着冷却，混合溶剂的混合性环收缩，或者等效地，它们表现出较低的临界溶液温度行为。对于固定的聚合物浓度，在混合溶剂的组成中，浊点温度取最低值。实验结果表明，这种最低浊点温度组成的混合溶剂，至少在低于 0.25 重分数的测量稀溶液区域内，几乎与聚合物浓度无关。基于相分离与溶剂通过氢键的优先吸附密切相关的假设，我们采用 Flory-Huggins 型的模型溶液，辅以直接和协同的聚合物-溶剂氢键，构建了三元相图。对旋节线进行了理论计算，并将结果与实验得到的浊点数据进行了比较。还在相同的理论框架内研究了共溶剂分子体积的影响。对于具有较大分子体积的共溶剂，可能会出现上临界溶液温度共非溶剂化的可能性。
