Department of Chemistry, Texas A&M University, College Station, Texas 77842-3012, USA.
J Am Chem Soc. 2012 May 2;134(17):7378-83. doi: 10.1021/ja211315e. Epub 2012 Apr 23.
Pendant groups on polymers that have lower-critical solution temperature (LCST) properties experience a water-like environment below the LCST where the polymer is soluble but are less hydrated above the LCST when the polymer phase separates from solution. When these pendant groups are amphoteric groups like carboxylate salts or ammonium salts, the change in solvation that accompanies the polymer precipitation event significantly changes these groups' acidity or basicity. These changes in acidity or basicity can lead to carboxylate salts forming carboxylic acid groups by capturing protons from the bulk solvent or ammonium salts reverting to the neutral amine by release of protons to the bulk solvent, respectively. When polymers like poly(N-isopropylacrylamide) that contain a sufficient loading of such comonomers are dissolved in solutions whose pH is near the pK(a) of the pendant acid or basic group and undergo an LCST event, the LCST event can change the bulk solution pH. These changes are reversible. These effects were visually followed using common indicators with soluble polymers and or by monitoring solution pH as a function of temperature. LCST events triggered by the addition of a kosmotropic salt lead to similar reversible solution pH changes.
具有低临界溶液温度 (LCST) 特性的聚合物上的侧基基团在 LCST 以下的环境中经历类似于水的环境，此时聚合物是可溶的，但在 LCST 以上时，聚合物会从溶液中分离出来，此时聚合物相分离。当这些侧基基团是两性基团，如羧酸盐或铵盐时，伴随聚合物沉淀事件的溶剂化变化会显著改变这些基团的酸度或碱度。酸度或碱度的这些变化可以导致羧酸盐通过从主体溶剂中捕获质子形成羧酸基团，或者铵盐通过向主体溶剂释放质子而恢复为中性胺，分别。当含有足够此类共聚单体的聚合物（如聚（N-异丙基丙烯酰胺））溶解在 pH 值接近侧基酸或碱基团的 pK(a)的溶液中并经历 LCST 事件时，LCST 事件会改变主体溶液的 pH 值。这些变化是可逆的。使用可溶性聚合物中的常见指示剂或通过监测溶液 pH 值作为温度的函数，可以直观地跟踪这些效应。由添加亲溶盐引发的 LCST 事件会导致类似的可逆溶液 pH 变化。
