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

Abstract

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 (Δχ) can be divided into three terms: Δχ1 due to the change in monomer identity after quaternization, ΔχBorn from the Born energy of counterions and χΓ arising from the local charge cohesion. For PEO-b-QP4VP-X BCPs, both Δχ1 and ΔχBorn are negative, but χΓ 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 Δχ1 plays an important role at high level of charge density. The rationality of the aforementioned thermodynamic mechanism of Δχ was also verified by the phase behavior of salt-doped PEO-b-QP4VP-X.