Tuning of Thermo-Triggered Gel-to-Sol Transition of Aqueous Solution of Multi-Responsive Diblock Copolymer Poly(methoxytri(ethylene glycol) acrylate-co-acrylic acid)-b-poly(ethoxydi(ethylene glycol) acrylate)

Abstract

This article reports on the synthesis of a hydrophilic diblock copolymer composed of two distinct thermosensitive polymers with one block containing a small amount of carboxylic acid groups, poly(methoxytri(ethylene glycol) acrylate-co-acrylic acid)-b-poly(ethoxydi(ethylene glycol) acrylate) (P(TEGMA-co-AA)-b-PDEGEA), and the study of thermo-induced sol−gel−sol transitions of its moderately concentrated aqueous solutions at various pH values. The diblock copolymer was obtained by the removal of tert-butyl groups of P(TEGMA-co-tert-butyl acrylate)-b-PDEGEA, which was synthesized by reversible addition−fragmentation chain transfer polymerization. PTEGMA and PDEGEA are thermosensitive polymers with lower critical solution temperatures (LCSTs) of 58 and 9 °C, respectively, in water. The incorporation of a small amount of carboxylic acid groups into PTEGMA allowed the LCST of the P(TEGMA-co-AA) block to be tuned by changing the solution pH. We found that a 20 wt % aqueous solution of P(TEGMA-co-AA)-b-PDEGEA with pH of 3.11 (measured at 0 °C) underwent multiple phase transitions upon heating, from a clear, free-flowing liquid (<19 °C) to a clear, free-standing gel (19 to 39 °C), to a clear, free-flowing hot liquid (40 to 55 °C), and to a cloudy mixture (≥56 °C). With the increase of pH, the gel-to-sol transition (Tgel−sol) and the clouding temperature (Tclouding) of the sample shifted to higher values, while the sol-to-gel transition temperature (Tsol−gel) remained the same. These transitions and the tunability of Tgel−sol stemmed from the thermosensitive properties of the two blocks of the diblock copolymer and the pH dependence of the LCST of P(TEGMA-co-AA), which was confirmed by differential scanning calorimetry and dynamic light scattering studies. Using the vial inversion test method, we further mapped out the C-shaped sol−gel phase diagrams of (PTEGMA-co-AA)-b-PDEGEA in water in the moderate concentration range at three different pH values (3.11, 4.49, and 5.25, all measured at 0 °C). While the lower temperature boundaries overlapped, the upper temperature boundary shifted upward and the critical gelation concentration decreased with the increase of pH. In contrast, the sol−gel phase diagram of PTEGMA-b-PDEGEA, which contained no pH-responsive groups, showed no changes in Tsol−gel, Tgel−sol, and Tclouding with pH.