Piezoelectric β Polymorph in Poly(vinylidene fluoride)-Functionalized Multiwalled Carbon\nNanotube Nanocomposite Films

Abstract

Poly(vinylidene fluoride) (PVF₂)−multiwalled carbon nanotube (MWNT) nanocomposites (PCNCs) are\nprepared using ester (−COOC₂H₅)-functionalized MWNT (F-MWNT). Due to the specific interaction of the\n>CO group in F-MWNT and the >CF₂ group of PVF₂, the dispersion of F-MWNT in PVF₂ matrix is\nuniform. Transmission electron microscopy study reveals that the F-MWNTs in the composite are fatter than\nthe pure F-MWNTs indicating that the PVF₂ chains are wrapped on the surface of MWNTs. In the\nnanocomposites, the spherulitic morphology of PVF₂ is lost and the fibrils are curled and smaller in length\nthan those of pure PVF₂ sample. Field emission scanning electron microscopy study indicates good dispersion\nof carbon nanotubes in the F-MWNT samples. The solvent-cast films have the β-polymorphic (piezoelectric)\nstructure for F-MWNT concentration ≥1% (w/w) and have a mixture of α and β polymorphs below that\nconcentration of F-MWNT. In the melt-cooled specimen, there occurs a mixture of α and β polymorphs and\nthe latter is totally absent in the corresponding unfunctionalized MWNT−PVF₂ composites (UCP). Possible\nexplanation of β polymorph formation in the PCNCs has been offered. The <i>T</i>_g of F-MWNT−PVF₂\nnanocomposite 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\nα phase, with increasing F-MWNT content and reaches a saturation value of ∼50% at 5% F-MWNT content\nin the PCNC. The storage modulus and loss modulus values increase with increase in F-MWNT concentration,\nand the percent increase in storage modulus is much greater than that of unfunctionalized MWNT−PVF₂\nnanocomposites, particularly for higher MWNT content (>0.1% (w/w)). The current (<i>I</i>)−voltage (<i>V</i>)\ncharacteristic curves are very interesting, and their nature depends on the amount of F-MWNT in the composite.\nThe 1% and 2% F-MWNT content samples have negative hysteresis in the <i>I</i>−<i>V</i> curves for the complete\ncycle of forward and reverse bias while the corresponding CP5 samples exhibit a memory effect in both the\nnegative and the positive bias, rendering the material to be useful in fabricating memory devices. The aged\nCP5 sample (∼3 months) requires a higher inflection voltage to show memory effect, while the UCP5 sample\ndoes not exhibit any memory effect because of longer conducting path and higher conductivity.