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 (T-cp) increases linearly with increasing halide ion concentration. The T-cp also increases with increasing molecular weights of the PIL. The phase diagram of aqueous PIL solution shows the highest T-cp at 6 wt%. Interestingly, the T-cp 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 nano-tubes 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.