Tunable doubly responsive UCST-type phosphonium poly(ionic liquid): a thermosensitive dispersant for carbon nanotubes

A phosphonium poly(ionic liquid) (PIL), poly(triphenyl-4-vinylbenzylphosphonium chloride) (P[VBTP][Cl]), of varying and controllable molecular weights is synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization to afford a doubly responsive PIL that responds to both halide ions and temperature. The addition of halide ions transforms the transparent aqueous PIL solution into a turbid two-phase solution, forming insoluble microgel aggregates owing to the screening of the positively charged phosphonium groups of PILs and eventually forms intra- and/or inter-chain cross-linking among the PIL chains through halide ion bridges. Further, this turbid solution exhibits a distinct upper critical solution temperature (UCST)-type phase transition and transforms into a one-phase transparent solution due to the disruption of ion bridges upon heating. The rate of aggregation of P[VBTP][Cl] increases sharply with an increase of the size of the added halide ions in this order I− > Br− > Cl−. The cloud point temperature (Tcp) increases linearly with increasing halide ion concentration. The Tcp also increases with increasing molecular weights of the PIL. The phase diagram of aqueous PIL solution shows the highest Tcp at 6 wt%. Interestingly, the Tcp of the P[VBTP][Cl] in water decreases sharply with addition of small but increasing amounts of organic cosolvents. This PIL exhibits very good stabilizing ability for carbon nanotubes in water, whose dispersion state can be switched from dispersed to agglomerate and vice versa by adding halide ions and increasing the temperature respectively. The cross-linked hydrogel of P[VBTP][Cl] also shows dual responsiveness towards both halide ions and temperature.