Crystal Forms and Microphase Structures of Poly(vinylidene fluoride-co-hexafluoropropylene) Physically and Chemically Incorporated with Ionic Liquids

The effects of incorporating two imidazolium-type ionic liquids (ILs), 1-vinyl-3-butylimidazolium bis-(trifluoromethylsulfonyl)imide (IL-TFSI) and 1-vinyl-3-butylimidazolium chloride (IL-Cl), on the crystal forms and phase microstructures of poly(vinylidene fluoride-co-hexafluoropropylene), P(VDF-co-HFP), were investigated. No change in the crystal form was observed with the incorporation of IL-TFSI because of the selective interaction between the IL and HFP moieties of the P(VDF-co-HFP) molecular chains. In contrast, the incorporation of IL-Cl induces an alpha-to-beta crystal form transition because of the interactions between IL-Cl and the VDF moieties. Furthermore, the incorporation of IL-TFSI led to distinct phase separation of P(VDF-co-HFP), whereas a homogeneous phase structure was maintained with the incorporation of IL-Cl. Element mapping images reveal that the specific interactions of IL-TFSI with the HFP moieties induce enrichment of the HFP moieties, which results in well-defined HFP/IL-TFSI nanodomains in the P(VDF-co-HFP) matrix. In contrast, a homogeneous morphology was observed in the P(VDF-co-HFP)/IL-Cl- blends. The smaller anion of IL-Cl can interact with VDF moieties and form a thermodynamically miscible state with the P(VDF-co-HFP) matrix. In addition, ILs chemically grafted on P(VDF-co-HFP) were in situ synthesized using electron-beam irradiation of the blends of P(VDF-co-HFP) with ILs at room temperature because both ILs contain double bonds. The phase structures of the IL-grafted copolymers were further studied. After chemical grafting and melting, the original microphase-separated structure of the P(VDF-co-HFP)/IL-TFSI blends was maintained, whereas discrete nanodomains with diameters of 10-20 nm were formed for the original homogeneous P(VDF-co-HFP)/IL-Cl blends. The interesting crystal forms and phase structures of P(VDF-co-HFP) physically or chemically incorporated with different ILs demonstrate the effectiveness of this strategy for the design of polymer microstructures using simple blending and radiation-induced grafting processes.