Fluorescence study on multiple phase behavior of dimethylacrylamide–methacrylic acid copolymer gel

Multiple phase behavior was found in copolymer gels consisting of dimethylacrylamide and methacrylic acid. They are characterized by distinct degrees of swelling; the gel can take one of a set of swelling values, but none of the intermediate values. Three different phases, denoted as phase051 (as-prepared), phase376 (swollen at high pH), phase440 (swollen after treatment by pH 12) were clearly resolved, where the three digits denote their linear swelling ratios in percentage with respect to the size at preparation. Each phase was stable and did not change its swelling ratio with pH or temperature as long as the values of pH or temperature were within limited ranges. Transitions among different phases were discrete with hysteresis loops. The microenvironment of these three phases was observed by steady-state and transient fluorescence spectroscopy, which indicated the multiple phase behavior appeared as the result of the coexistence of hydrophilic and less-hydrophilic (hydrophobic) domains in the gel and their fraction varied depending on pH.