Temperature-induced phase-transitions of methoxyoligo(oxyethylene) styrene-based block copolymers in aqueous solution

Abstract

Novel well-defined diblock copolymers based upon styrene substituted with pendant methoxyoligo(oxyethylene) groups (4-CH₃O(EO)₄St, TrEGS) were prepared by sequential atom transfer radical polymerization. Polystyrene (PS) was synthesized as the first block in the copolymer using (1-bromoethyl)benzene as the initiator. This was followed by growth of a second block composed of PTrEGS. Each copolymer exhibits a lower critical solution temperature (LCST) in aqueous solution, and the LCST depends on the length of the PTrEGS block and the copolymer composition. Longer PTrEGS chains result in higher LCST values in aqueous solutions. The cloud points (T_cs) of PS-b-PTrEGS copolymers are lower than that of PTrEGS homopolymer as determined by UV-visible spectroscopy. Variable-temperature NMR spectra show that the intensity of proton signals from oxyethylene groups dramatically decreases at temperatures above the cloud point, indicating phase transition due to hydrogen bonding changes. Dynamic light scattering (DLS) measurements indicate that micelles are formed, where the PS backbone serves as the core at 25 °C. Each of these block copolymer solutions transformed from transparent bluish solutions to white gels above certain concentrations (≥10 wt%) and temperatures (lower critical gelation temperature, LCGT). The gelation process is thermally reversible, implying physical crosslinking through hydrophobic interactions.