Investigating the crystalline structure of poly(vinylidene fluoride) (PVDF) in PVDF/silica binary and PVDF/poly(methyl methacrylate)/silica ternary hybrid composites using FTIR and solid-state 19F MAS NMR spectroscopy

We investigated the crystalline structures of poly(vinylidene fluoride) (PVDF) in PVDF/silica (SiO2) binary and PVDF/poly(methyl methacrylate) (PMMA)/SiO2 ternary hybrid composite films using infrared (IR) analysis and solid-state, high-speed magic-angle-spinning (MAS) F-19 NMR spectroscopy. These hybrid films were prepared by sol-gel processes. We used three different blending sequences to prepare the ternary hybrid composite films: (1) The Type 1 hybrid was prepared from a one-pot mixture of PVDF, PMMA, and tetraethoxysilane (TEOS), the silica precursor, in dimethylacetamide (DMAc). (2) The Type 2 hybrid composite was prepared by first forming a PVDF/silica hybrid and then mixing it with PMMA. (3) The Type 3 hybrid composite was prepared by mixing a preformed PMMA/silica hybrid with PVDF. The crystallinity of the PVDF/SiO2 hybrid composite films decreased upon increasing the silica content. By using the different methods to prepare the PVDF/PMMA/SiO2 ternary hybrid composite films, we were able to assign the gamma-phase of PVDF crystals in the solid-state F-19 MAS NMR spectrum, which shows seven resonances that are attributable to an amorphous domain (-88 ppm), crystalline domains (-101.3, -93.7, -84.2, and -79.6 ppm), and regioirregular structures (-112.4 and -110.4 ppm). F-19 MAS NMR spectra of the Type 2 and Type 3 PVDF/PMMA/SiO2 hybrid composite films show two new resonances, assignable to the gamma-phase at lower (-101.3 ppm) and higher (-84.2 ppm) frequencies, which show long F-19 spin-lattice relaxation times in the rotating frame (T-1rho(F)) and chemical shifts that are significantly different from those of alpha- and P-phases of the PVDF crystals. The disappearance of the alpha-phase of PVDF crystals in the Type 2 and Type 3 hybrid composite films can be explained by the large silica particles in the PVDF/PMMA matrix hindering crystallization of PVDF. Solid-state F-19 spin-lock NMR spectroscopy experiments also indicate significant differences exist between the values of T-1rho(F) of the crystalline and amorphous domains depending on the method of hybrid preparation.