Tanaka Fumihiko, Koga Tsuyoshi, Winnik Françoise M
Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan.
Phys Rev Lett. 2008 Jul 11;101(2):028302. doi: 10.1103/PhysRevLett.101.028302.
If two good solvents become poor for a polymer when mixed, the solvent pair is called a cononsolvent pair. The sharp reentrant coil-to-globule-to-coil transition of a poly(N-isopropylacrylamide) chain observed in the mixed solvent of water and methanol is shown to be caused by the competitive hydrogen bonding by water and methanol molecules onto the polymer chain. On the basis of a new statistical-mechanical model for competitive hydrogen bonds, the mean square end-to-end distance is theoretically calculated and compared with experiment. The chain sharply collapses at the molar fraction xm approximately 0.2 of methanol, stays collapsed up to xm approximately 0.4, and finally recovers the swollen state at xm approximately 0.6. Such a reentrant coil-globule transition takes place because the total number of hydrogen bonds along the chain exhibits a similar square-well-type depression as a result of the competition.
如果两种良溶剂混合后对聚合物而言变成了不良溶剂，那么这对溶剂就被称为共溶剂对。在水和甲醇的混合溶剂中观察到的聚（N-异丙基丙烯酰胺）链从舒展线团到紧密小球再到舒展线团的急剧折返转变，被证明是由水分子和甲醇分子在聚合物链上的竞争性氢键作用导致的。基于一种新的竞争性氢键统计力学模型，从理论上计算了均方末端距并与实验结果进行了比较。在甲醇的摩尔分数xm约为0.2时，链急剧塌陷，在xm约为0.4之前一直保持塌陷状态，最终在xm约为0.6时恢复到舒展状态。这种折返的线团-小球转变之所以发生，是因为由于竞争作用，沿链的氢键总数呈现出类似方阱型的凹陷。
