The β Form in PVDF Nanocomposites with Carbon Nanotubes: Structural Features and Properties

Abstract

Different amounts of carbon nanotubes (CNT) have been incorporated in materials based on poly(vinylidene fluoride) (PVDF) by solvent blending followed by their further precipitation. Final processing was performed by compression molding. The morphological aspects and crystalline characteristics have been examined, additionally exploring in these nanocomposites the common routes described in the pristine PVDF to induce the β polymorph. This polar β phase has been found to be promoted by the simple inclusion of CNT. Therefore, coexistence of the α and β lattices occurs for the analyzed materials. The real-time variable-temperature X-ray diffraction measurements with synchrotron radiation at a wide angle have undoubtedly allowed us to observe the presence of the two polymorphs and determine the melting temperature of both crystalline modifications. Furthermore, the CNT plays a nucleating role in the PVDF crystallization, and also acts as reinforcement, increasing the stiffness of the nanocomposites. Moreover, the mobility within the amorphous and crystalline PVDF regions is found to change with the CNT content. Finally, the presence of CNT leads to a very remarkable increase in the conductivity parameter, in such a way that the transition from insulator to electrical conductor is reached in these nanocomposites at a percolation threshold ranging from 1 to 2 wt.%, leading to the excellent value of conductivity of 0.05 S/cm in the material with the highest content in CNT (8 wt.%).

Keywords: CNT; PVDF; conductivity; nanocomposites; percolation; β and α polymorphs.

Figure 1

TEM micrographs of the (a) KCNT2 and (b) KCNT8 nanocomposites. The scale bar is referred to as 0.2 µm.

Figure 2

SEM micrographs of the KCNT2 nanocomposites at distinct augments: (a) scale bar of 2 µm; and, (b) scale bar of 200 nm.

Figure 3

(a) FTIR spectra of the neat PVDF and the different nanocomposites under study. From top to bottom: PVDF, KCNT05, KCNT1, KCNT2, KCNT4, and KCNT8. (b) Fraction of the β polymorh in the several materials under study.

Figure 4

Room-temperature X-ray diffractograms of the neat PVDF and the different KCNT nanocomposites. The characteristic reflections of each crystalline phase are indicated.

Figure 5

Real-time variable-temperature synchrotron profiles for the KCNT8^Qice specimen and their variation with temperature in the melting experiments at 20 °C/min. For clarity of the presentation, only one out of every three frames are plotted in the interval ranged from 20 to 160 °C, while all of the frames are represented from 160° to 180 °C.

Figure 6

Dependence on temperature of the intensity for the main diffraction of the α phase and that characteristic for the electroactive β lattice. The intersection of the slope for their decrease at the higher temperature interval is an accurate indication of the T_m of both phases.

Figure 7

Real-time variable-temperature synchrotron profiles for the KCNT05^stretched sample and their variation with temperature in the melting experiments at 20 °C/min. Only one out of every two frames are plotted for clarity of the presentation.

Figure 8

DSC curves obtained during the first heating run at 20 °C/min for the pristine PVDF and the different nanocomposites with CNT.

Figure 9

DSC curves obtained during the crystallization run at 20 °C/min for the pristine PVDF and the different nanocomposites with CNT.

Figure 10

Dependence on temperature of: (a) the storage modulus (real component of complex modulus); and (b) loss tangent curves at 3 Hz of the neat PVDF and its KCNT nanocomposites with CNT.

Figure 11

Electrical conductivity (σ′) measured at 25 °C as a function of frequency for the neat PVDF and the different KCNT nanocomposites with distinct contents in CNT.