Tuning of Thermally Induced\nSol-to-Gel Transitions\nof Moderately Concentrated Aqueous Solutions of Doubly Thermosensitive\nHydrophilic Diblock Copolymers Poly(methoxytri(ethylene glycol) acrylate)-<i>b</i>-poly(ethoxydi(ethylene glycol) acrylate-<i>co</i>-acrylic acid)

Abstract

We report in this article a method to tune the sol-to-gel\ntransitions\nof moderately concentrated aqueous solutions of doubly thermosensitive\nhydrophilic diblock copolymers that consist of two blocks exhibiting\ndistinct lower critical solution temperatures (LCSTs) in water. A\nsmall amount of weak acid groups is statistically incorporated into\nthe lower LCST block so that its LCST can be tuned by varying solution\npH. Well-defined diblock copolymers, poly­(methoxytri­(ethylene glycol)\nacrylate)-<i>b</i>-poly­(ethoxydi­(ethylene glycol) acrylate-<i>co</i>-acrylic acid) (PTEGMA-<i>b</i>-P­(DEGEA-<i>co</i>-AA)), were prepared by reversible addition–fragmentation\nchain transfer polymerization and postpolymerization modification.\nPTEGMA and PDEGEA are thermosensitive water-soluble polymers with\nLCSTs of 58 and 9 °C, respectively, in water. A 25 wt % aqueous\nsolution of PTEGMA-<i>b</i>-P­(DEGEA-<i>co</i>-AA)\nwith a molar ratio of DEGEA to AA units of 100:5.2 at pH = 3.24 underwent\nmultiple phase transitions upon heating, from a clear, free-flowing\nliquid (<15 °C) to a clear, free-standing gel (15–46\n°C) to a clear, free-flowing hot liquid (47–56 °C),\nand a cloudy mixture (≥57 °C). With the increase of pH,\nthe sol-to-gel transition temperature (<i>T</i>_sol–gel) shifted to higher values, while the gel-to-sol transition (<i>T</i>_gel–sol) and the clouding temperature\n(<i>T</i>_clouding) of the sample remained essentially\nthe same. These transitions and the tunability of <i>T</i>_sol–gel originated from the thermosensitive properties\nof two blocks of the diblock copolymer and the pH dependence of the\nLCST of P­(DEGEA-<i>co</i>-AA), which were confirmed by dynamic\nlight scattering and differential scanning calorimetry studies. Using\nthe vial inversion test method, we mapped out the C-shaped sol–gel\nphase diagrams of the diblock copolymer in aqueous buffers in the\nmoderate concentration range at three different pH values (3.24, 5.58,\nand 5.82, all measured at ∼0 °C). While the upper temperature\nboundaries overlapped, the lower temperature boundary shifted upward\nand the critical gelation concentration increased with the increase\nof pH. The AA content in PTEGMA-<i>b</i>-P­(DEGEA-<i>co</i>-AA) was found to have a significant effect on the pH\ndependence of <i>T</i>_sol–gel. For PTEGMA-<i>b</i>-P­(DEGEA-<i>co</i>-AA) with a molar ratio of\nDEGEA to AA units of 100:10, the <i>T</i>_sol–gel of its 25 wt % aqueous solution increased faster with the increase\nof pH than that of PTEGMA-<i>b</i>-P­(DEGEA-<i>co</i>-AA) with a DEGEA-to-AA molar ratio of 100:5.2.