Thermodynamic understanding the phase behavior of fully quaternized poly(ethylene oxide)-b-poly(4-vinylpyridine) block copolymers

In this study, the poly(ionic liquid)s (PILs)-based block copolymers (BCPs) with a high charge density, poly (ethylene oxide)-b-quaternized poly(4-vinylpyridinium) with X- as counterion (PEO-b-QP4VP-X), were prepared by full quaternization of the poly(4-vinylpyridine) (P4VP) block in neutral PEO-b-P4VP BCP. The complete quaternization of P4VP block was confirmed by NMR and FT-IR. The phase separation behavior of PEO-b-QP4VP-X BCPs was examined, which was found to be strongly dependent on the counterion species. Specifically, as compared with the neutral PEO-b-P4VP, PEO-b-QP4VP-Br exhibits a stronger segregation strength, while the other three PEO-b-QP4VP-X (X = PF6, TFO and TFSI) BCPs show a weaker phase separation ability. The phase behavior of PEO-b-QP4VP-X was explained using the thermodynamic mechanism originally applied for salt doped BCPs of low charge density. The variation of the Flory-Huggins parameter (delta chi ) can be divided into three terms: delta(chi 1) due to the change in monomer identity after quaternization, delta chi Born from the Born energy of counterions and chi(gamma) arising from the local charge cohesion. For PEO-b-QP4VP-X BCPs, both delta(chi 1) and delta chi Born are negative, but chi(gamma) is positive, and they vary with counterion size in different ways. The competition of these three terms dominates the change tendency of phase behavior, among which & UDelta;chi 1 plays an important role at high level of charge density. The rationality of the aforementioned thermodynamic mechanism of delta chi was also verified by the phase behavior of salt-doped PEO-b-QP4VP-X.

Automated summary

In this study, high charge density PEO-b-QP4VP-X BCPs were prepared by full quaternization of the P4VP block in neutral PEO-b-P4VP BCP. The phase separation behavior of PEO-b-QP4VP-X BCPs was found to be strongly dependent on the counterion species. The phase behavior was explained using a thermodynamic mechanism, in which the competition of delta(chi 1), delta chi Born, and chi(gamma) dominates the change tendency of phase behavior.