Chemical composition characterization of poly(vinylidene fluoride-chlorotrifluoroethylene)-based copolymers with F–H decoupled 1H NMR

Abstract

Poly(vinylidene fluoride-chlorotrifluoroethylene) (P(VDF–CTFE)) has been widely utilized to synthesize functionalized fluoropolymers, including trifluoroethylene (TrFE) containing ferroelectric polymers and other vinyl monomer grafted copolymers, exploiting C–Cl as active sites. The chemical composition of the functionalized copolymers has been found to exhibit a remarkable influence on their properties. With the absence of protons on the CTFE units and the complicated linking sequence of multiple monomers, the precise composition, especially the molar content of CTFE in the resultant copolymers, has to be determined indirectly by measuring the TrFE molar content converted from the remaining CTFE through an extra hydrogenation reaction. In this study, F–H decoupled ¹H NMR spectra are obtained to directly detect the molar content of CTFE units in the resultant copolymers. After decoupling from F atoms, the overlapped proton peaks on VDF connected CTFE and VDF due to the strong coupling of H atoms between the finely separated F atoms, which allowed their integrals to be obtained accurately. The composition determined with the present method is found to be consistent with the results obtained from ¹⁹F NMR and the indirect CTFE hydrogenation strategy. The CTFE content was underestimated by about 10%, due to CTFE being connected with VDF in the –CH₂CF₂–CFClCF₂– sequence. For the possible connection of CTFE with another CTFE at a high loading molar ratio of the CTFE in P(VDF–CTFE), the method could be utilized for determining the CTFE content below 20 mol% with promising accuracy (the relative systematic error being within 10%). The successful application of this method in calculating the CTFE content in not only P(VDF–CTFE) copolymers and P(VDF–TrFE–CTFE) terpolymers but also their grafted copolymers strongly suggests that the method might provide a robust and facile tool for quickly determining the composition of P(VDF–CTFE) based copolymers.