Shifting Sol–Gel\nPhase Diagram of a Doubly Thermosensitive Hydrophilic Diblock Copolymer\nPoly(methoxytri(ethylene glycol) acrylate-<i>co</i>-acrylic\nacid)-<i>b</i>-poly(ethoxydi(ethylene glycol) acrylate-<i>co</i>-acrylic acid) in Aqueous Solution

Abstract

This article reports that the C-shaped sol–gel\nphase diagram of a doubly thermosensitive hydrophilic diblock copolymer\nwith each block containing a small amount of weak acid groups in aqueous\nsolution in the moderate concentration range can be readily and reversibly\nshifted by changing the solution pH. The diblock copolymer, poly­(methoxytri­(ethylene\nglycol) acrylate-<i>co</i>-acrylic acid)-<i>b</i>-poly­(ethoxydi­(ethylene glycol) acrylate-<i>co</i>-acrylic\nacid) (P­(TEGMA-<i>co</i>-AA)-<i>b</i>-P­(DEGEA-<i>co</i>-AA)), was synthesized by reversible addition–fragmentation\nchain transfer polymerization and postpolymerization modification.\nA 20 wt % aqueous solution of P­(TEGMA-<i>co</i>-AA)-<i>b</i>-P­(DEGEA-<i>co</i>-AA) with pH of 3.29 underwent\nsol-to-gel, gel-to-sol, and clear sol-to-cloudy sol transitions at\n17 °C (<i>T</i>_sol–gel), 38 °C\n(<i>T</i>_gel–sol), and 55 °C (<i>T</i>_clouding), respectively, upon heating. With\nthe increase of pH, all transition temperatures shifted to high values;\nfor instance, the <i>T</i>_sol–gel, <i>T</i>_gel–sol, and <i>T</i>_clouding were 20, 45, and 63 °C, respectively, at pH = 5.10, and 28,\n52, and 77 °C, respectively, at pH = 5.79. Using vial inversion\ntests, we mapped out sol–gel phase diagrams of the diblock\ncopolymer in aqueous solutions at three pH values (3.29, 5.10, and\n5.79, measured at ∼0 °C); the whole sol–gel phase\ndiagram shifted upward with the increase of pH. When the pH was lowered\nfrom 5.79 to 5.10, the diagram shifted back, though there was a 1\n°C difference at each selected concentration, compared with the\noriginal curve of pH = 5.10. The tunability of sol–gel–sol–clouding\ntransitions stemmed from the pH dependences of thermosensitive properties\nof two blocks, which were confirmed by a dynamic light scattering\nstudy. The results from small-angle X-ray scattering experiments indicated\nthat spherical micelles of P­(TEGMA-<i>co</i>-AA)-<i>b</i>-P­(DEGEA-<i>co</i>-AA) in 20 wt % aqueous solutions\nat selected pH and temperatures were packed into crystalline structures,\neither body-centered cubic or face-centered cubic, in the gel states.