Tuning of Thermally Induced Sol-to-Gel Transitions of Moderately Concentrated Aqueous Solutions of Doubly Thermosensitive Hydrophilic 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 transitions of moderately concentrated aqueous solutions of doubly thermosensitive hydrophilic diblock copolymers that consist of two blocks exhibiting distinct lower critical solution temperatures (LCSTs) in water. A small amount of weak acid groups is statistically incorporated into the lower LCST block so that its LCST can be tuned by varying solution pH. Well-defined diblock copolymers, poly­(methoxytri­(ethylene glycol) acrylate)-<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 chain transfer polymerization and postpolymerization modification. PTEGMA and PDEGEA are thermosensitive water-soluble polymers with LCSTs of 58 and 9 °C, respectively, in water. A 25 wt % aqueous solution of PTEGMA-<i>b</i>-P­(DEGEA-<i>co</i>-AA) with a molar ratio of DEGEA to AA units of 100:5.2 at pH = 3.24 underwent multiple phase transitions upon heating, from a clear, free-flowing liquid (<15 °C) to a clear, free-standing gel (15–46 °C) to a clear, free-flowing hot liquid (47–56 °C), and a cloudy mixture (≥57 °C). With the increase of pH, the 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 (<i>T</i>_clouding) of the sample remained essentially the same. These transitions and the tunability of <i>T</i>_sol–gel originated from the thermosensitive properties of two blocks of the diblock copolymer and the pH dependence of the LCST of P­(DEGEA-<i>co</i>-AA), which were confirmed by dynamic light scattering and differential scanning calorimetry studies. Using the vial inversion test method, we mapped out the C-shaped sol–gel phase diagrams of the diblock copolymer in aqueous buffers in the moderate concentration range at three different pH values (3.24, 5.58, and 5.82, all measured at ∼0 °C). While the upper temperature boundaries overlapped, the lower temperature boundary shifted upward and the critical gelation concentration increased with the increase of 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 dependence of <i>T</i>_sol–gel. For PTEGMA-<i>b</i>-P­(DEGEA-<i>co</i>-AA) with a molar ratio of DEGEA to AA units of 100:10, the <i>T</i>_sol–gel of its 25 wt % aqueous solution increased faster with the increase of 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