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 >CO 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 α 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 Tg 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 α 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.