Functional glycidyl triazolyl polymers exhibiting pH-controllable and time-dependent lower critical solution temperature

Abstract

In this study, poly(ethylene glycol)-based materials exhibiting pH-controllable and time-dependent lower critical solution temperature (LCST) were developed using glycidyl triazolyl polymers (GTPs) functionalized with tri(ethylene glycol) monomethyl ether (EG3) and carboxylic acid (COOH) side groups via a copper-free azide–alkyne cycloaddition reaction. The physical properties of the GTP-EG3 homopolymer and the GTP-EG3-co-COOH copolymer were characterized using NMR, IR, size exclusion chromatography, differential scanning calorimetry, and thermogravimetric analysis. The effects of copolymer composition, polymer concentration, ions in phosphate-buffered saline (PBS), and pH on LCST behavior were systematically investigated. PBS was found to promote aggregation of the GTP-EG3 homopolymer due to the salting-out effect described by the Hofmeister series. Since non-ionized COOH groups behave as hydrophobic moieties, the LCST decreased with increasing COOH content. Active control of LCST through pH variation was successfully demonstrated, revealing a linear relationship between LCST and the degree of COOH ionization. The equivalence point observed in the pH titration deviated from the theoretical value. Over time, this deviation gradually diminished. In connection with this phenomenon, a time-dependent change in LCST was also observed, even though no chemical reaction such as hydrolysis took place. The observed LCSTs ranged from 18 °C to 81 °C, encompassing the physiological temperature range.