Piezoelectric β polymorph in poly(vinylidene fluoride)-functionalized multiwalled carbon nanotube nanocomposite films

Abstract

Poly(vinylidene fluoride) (PVF₂)-multiwalled carbon nanotube (MWNT) nanocomposites (PCNCs) are prepared using ester (−COOC₂H₅)-functionalized MWNT (F-MWNT). Due to the specific interaction of the >C=O group in F-MWNT and the >CF₂ group of PVF₂, the dispersion of F-MWNT in PVF₂ matrix is uniform. Transmission electron microscopy study reveals that the F-MWNTs in the composite are fatter than the pure F-MWNTs indicating that the PVF₂ chains are wrapped on the surface of MWNTs. In the nanocomposites, the spherulitic morphology of PVF₂ is lost and the fibrils are curled and smaller in length than those of pure PVF₂ sample. Field emission scanning electron microscopy study indicates good dispersion of carbon nanotubes in the F-MWNT samples. The solvent-cast films have the β-polymorphic (piezoelectric) structure for F-MWNT concentration ≥1% (w/w) and have a mixture of α and β polymorphs below that concentration of F-MWNT. In the melt-cooled specimen, there occurs a mixture of a and β polymorphs and the latter is totally absent in the corresponding unfunctionalized MWNT-PVF₂ composites (UCP). Possible explanation of β polymorph formation in the PCNCs has been offered. The T_g of F-MWNT-PVF₂ nanocomposite was higher than that of pure PVF₂, and it is also greater than those of unfunctionalized MWNT-PVF₂ nanocomposites. In the melt-cooled samples, the β phase increases, with a concomitant decrease of the a phase, with increasing F-MWNT content and reaches a saturation value of 50% at 5% F-MWNT content in the PCNC. The storage modulus and loss modulus values increase with increase in F-MWNT concentration, and the percent increase in storage modulus is much greater than that of unfunctionalized MWNT-PVF₂ nanocomposites, particularly for higher MWNT content (>0.1% (w/w)). The current (I)-voltage (V) characteristic curves are very interesting, and their nature depends on the amount of F-MWNT in the composite. The 1% and 2% F-MWNT content samples have negative hysteresis in the I-V curves for the complete cycle of forward and reverse bias while the corresponding CP5 samples exhibit a memory effect in both the negative and the positive bias, rendering the material to be useful in fabricating memory devices. The aged CP5 sample (3 months) requires a higher inflection voltage to show memory effect, while the UCP5 sample does not exhibit any memory effect because of longer conducting path and higher conductivity.