Thermosensitive Aqueous Gels with Tunable Sol Gel Transition Temperatures from Thermo- and pH-Responsive Hydrophilic ABA Triblock Copolymer

This article reports on the synthesis of a well-defined hydrophilic ABA triblock copolymer composed of a poly(ethylene oxide) (13E0) middle block and thermo- and pH-sensitive outer blocks and the study of sol get transitions of its aqueous solutions at various pH values. The doubly responsive linear triblock copolymer, poly(methoxydi(ethylene glycol) methacrylate-co-methacrylic acid)-h-PEO-h-poly(methoxydi(ethylene glycol) methacrylate-co-methacrylic acid) (P(DEGMMA-co-MAA)-b-PEO-h-P(DEGMMA-co-MAA)). was prepared by atom transfer radical polymerization of a mixture of DEGM MA and Jeri-butyl methacrylate with a molar ratio of 100:5 from a difunctional PEO macroinitiator and subsequent removal of tert-butyl groups using trifluoroacetic acid. Dynamic light scattering studies showed that the critical micellization temperature (CMT) of this ABA triblock copolymer in a 0.2 wt % aqueous solution was dependent on the solution pH and can be varied in a large temperature range ( > 20 degrees C). To study the sol gel transitions, a 12.0 wt % aqueous solution of the triblock copolymer with a pH of 4.89 was made: its pH value can be readily changed and well controlled by the injection of either a 1.0 M HCI or a 1.0 M KOH solution. From rheological measurements, the sol gel transition temperature (T,1_0) versus pH curve was found to closely trace the CMT versus pH curve, though there was a shift. By cycling the solution pH between 3.2 and 5.4, we showed that the Tsol-gel at a specific pH was reproducible. Moreover, multiple sol gel sol transitions were realized by judiciously controlling the temperature and pH simultaneously, demonstrating the possibility of achieving on-demand sot gel transitions by using two external stimuli. In addition, the effect of polymer concentration on Tsol-gel at pH = 4.0 was investigated. The sol gel transition temperature increased with the decrease of polymer concentration and the critical gelation concentration was found to be between 4 and 6 wt