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

Abstract

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) 19F 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 γ-phase of PVDF crystals in the solid-state 19F 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). 19F MAS NMR spectra of the Type 2 and Type 3 PVDF/PMMA/SiO2 hybrid composite films show two new resonances, assignable to the γ-phase at lower (−101.3 ppm) and higher (−84.2 ppm) frequencies, which show long 19F spin−lattice relaxation times in the rotating frame (T1ρF) and chemical shifts that are significantly different from those of α- and β-phases of the PVDF crystals. The disappearance of the α-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 19F spin-lock NMR spectroscopy experiments also indicate significant differences exist between the values of T1ρF of the crystalline and amorphous domains depending on the method of hybrid preparation.