Toward the two-step microdynamic phase transition mechanism of an oligo(ethylene glycol)methacrylate-based copolymer with a LCST-type poly(ionic liquid) block

Abstract

A new LCST-type thermoresponsive polyelectrolyte P[P_4,4,4,4][SS], poly(tetrabutyl phosphonium styrene sulfonate), was introduced to PMEO₂MA (poly(2-(2-methoxyethoxy)ethyl methacrylate)) via RAFT polymerization, in order to explore the transition behavior of the block copolymer PMEO₂MA-b-P[P_4,4,4,4][SS] with two distinct LCST-type segments. A relatively sharp LCST-type phase transition with only one transition point is observed in the turbidity curves, while the whole phase transition is completely different from the micro perspective. The phase transition temperature range is relatively broad, according to the unsynchronized changes of different protons of the two blocks in the temperature-variable ¹H NMR analysis. From PCMW analysis, it is found that there exists an obvious two-step phase transition behavior, especially in the region of the C–H groups. Accordingly, we divided the whole transition process into two subregions: 20–40 °C and 40–55 °C in 2Dcos analysis. At the first stage of 20–40 °C, the CH₃ groups mainly belonging to the backbones of PMEO₂MA blocks have the earliest response to the heating and drive the first step of the dehydration process of PMEO₂MA-b-P[P_4,4,4,4][SS], resulting in the formation of an intermediate micelle state composed of the collapsed PMEO₂MA core and hydrophilic P[P_4,4,4,4][SS] corona. In particular, the conformational changes and the more compact structures due to the interaction between the CO groups and P[P_4,4,4,4][SS] segments (ν(CO⋯D₂O-PILs)) were observed using IR analysis. With the continual increase of the temperature, when the second temperature range of 40–55 °C is reached, the P[P_4,4,4,4][SS] segments start to collapse and expel the water molecules, driven by the anions of the poly(ionic liquid)s, with the phosphonium cations being distributed over the relatively hydrophilic outside.