Ionic liquid integrated multiwalled carbon nanotube in a poly(vinylidene fluoride) matrix: formation of a piezoelectric β-polymorph with significant reinforcement and conductivity improvement.

Multiwalled carbon nanotubes (MWNTs) are functionalized covalently with ionic liquid (IL, 3-aminoethyl imidazolium bromide) which helps good dispersion of IL-functionalized MWNTs (MWNT-IL) in the poly(vinylidene fluoride) (PVDF) matrix. Analysis of transmission electron microscopy (TEM) micrographs suggests ∼10 nm coating thickness of MWNTs by ILs, and the covalent linkage of ILs with MWNTs is confirmed from FT-IR and Raman spectra. PVDF nanocomposites with full β-polymorphic (piezoelectric) form are prepared using MWNT-IL by both the solvent cast and melt-blending methods. The FE-SEM and TEM micrographs indicate that IL-bound MWNTs are homogeneously dispersed within the PVDF matrix. Increasing MWNT-IL concentration in the composites results in increased β polymorph formation with a concomitant decrease of the α polymorph, and a 100% β polymorph formation occurs for 1 wt % MWNT-IL in both the fabrication conditions. A differential scanning calorimetry (DSC) study shows that the MWNT-ILs are an efficient nucleating agent for PVDF crystallization preferentially nucleating the β form due to its dipolar interactions with PVDF. The glass transition temperature (T(g)) gradually increases with an increase in MWNT-IL concentration, and the storage modulus (G') of the composites increases significantly, showing a maximum increase of 101.3% for 0.5 wt % MWNT-IL. The Young's modulus increases with MWNT-IL concentration, and analysis of the data using the Halpin-Tsai equation suggests that at low concentration they adopt an orientation parallel to the film surface; however, at higher MWNT-IL concentration it is randomly oriented. The tensile strength also increases with an increase in MWNT-IL concentration, and both the Young's modulus and the tensile strength of solvent cast films are lower than melt-blended samples. The elongation at break in the solvent cast samples shows a maximum, but in melt-blended samples it decreases continuously with increasing MWNT-IL concentration. The composites exhibit a very low conductivity percolation threshold at 0.05 wt %, and the three-dimensional conducting network is produced. Higher conductivity (∼1 S/cm for 1% MWNT-IL) than other MWNT/PVDF composites has been attributed to the anchored ionic liquid.