Shifting Sol–Gel Phase Diagram of a Doubly Thermosensitive Hydrophilic Diblock Copolymer Poly(methoxytri(ethylene glycol) acrylate-co-acrylic acid)-b-poly(ethoxydi(ethylene glycol) acrylate-co-acrylic acid) in Aqueous Solution

Abstract

This article reports that the C-shaped sol–gel phase diagram of a doubly thermosensitive hydrophilic diblock copolymer with each block containing a small amount of weak acid groups in aqueous solution in the moderate concentration range can be readily and reversibly shifted by changing the solution pH. The diblock copolymer, poly(methoxytri(ethylene glycol) acrylate-co-acrylic acid)-b-poly(ethoxydi(ethylene glycol) acrylate-co-acrylic acid) (P(TEGMA-co-AA)-b-P(DEGEA-co-AA)), was synthesized by reversible addition–fragmentation chain transfer polymerization and postpolymerization modification. A 20 wt % aqueous solution of P(TEGMA-co-AA)-b-P(DEGEA-co-AA) with pH of 3.29 underwent sol-to-gel, gel-to-sol, and clear sol-to-cloudy sol transitions at 17 °C (Tsol–gel), 38 °C (Tgel–sol), and 55 °C (Tclouding), respectively, upon heating. With the increase of pH, all transition temperatures shifted to high values; for instance, the Tsol–gel, Tgel–sol, and Tclouding were 20, 45, and 63 °C, respectively, at pH = 5.10, and 28, 52, and 77 °C, respectively, at pH = 5.79. Using vial inversion tests, we mapped out sol–gel phase diagrams of the diblock copolymer in aqueous solutions at three pH values (3.29, 5.10, and 5.79, measured at ∼0 °C); the whole sol–gel phase diagram shifted upward with the increase of pH. When the pH was lowered from 5.79 to 5.10, the diagram shifted back, though there was a 1 °C difference at each selected concentration, compared with the original curve of pH = 5.10. The tunability of sol–gel–sol–clouding transitions stemmed from the pH dependences of thermosensitive properties of two blocks, which were confirmed by a dynamic light scattering study. The results from small-angle X-ray scattering experiments indicated that spherical micelles of P(TEGMA-co-AA)-b-P(DEGEA-co-AA) in 20 wt % aqueous solutions at selected pH and temperatures were packed into crystalline structures, either body-centered cubic or face-centered cubic, in the gel states.