Flexible Nanogenerator and Nano-Pressure Sensor Based on Nanofiber Web of PVDF and its Copolymers - Soft Fibrillar Materials: Fabrication and Applications - page 234

carbon nanotube (MWCNT) as well as CaCl 2 + MWCNT content on the piezoelec-

tric characteristics of PVDF were also analyzed in this study. For faster evaporation
of the solvent during the electrospinning process, DMAc/acetone (6 : 4 w/w) solvent
mixture was used for preparing both PVDF-CaCl 2 and PVDF-MWCNT solutions.
FTIR-TS technique is effectively used to characterize the crystal structures of the
PVDF. Figure 8.6a shows the FTIR-TS absorption changes for PVDF-CaCl 2 electro-
spun fibers. Compared to the PVDF solution-cast film sample (Figure 8.6a(i)), the
β -phase peaks in electrospun cases (with lower CaCl 2 content than in solution-cast

PVDF film) were observed to be significantly stronger. Figure 8.6b,c shows the
FTIR-TS absorption changes for PVDF electrospun fibers without and with CaCl 2
(0.5 wt%) respectively as a function of varying MWCNT content. Qualitative anal-
ysis of their spectra clearly showed the absence of α -phase peaks and significantly

higher -phase peaks in all the electrospun cases.

The slower evaporation rate in PVDF-DMAc may have influenced the formation
of relaxed polymer chains in solution-cast samples resulting in comparatively less
orientation of the

β

-phase along the poling direction, which is their thermodynami-
cally stable state. In the case of using a mixed solvent system in the electrospinning
process (DMAc/acetone), the faster evaporation rate along with the simultaneous
stretching and poling effects may have caused much more favorable crystalline
orientation and increase in their

β

β -phase content compared to the film samples.

Quantitative estimation of the - (1279 cm −1 )

crystalline phase FTIR-TS absorbance changes relative to the internal thickness
band (1072 cm −1 ) is shown in Figure 8.6d-f for the respective spectra shown in
Figure 8.6a-c. As observed in Figure 8.6d, the effect of electrospinning-induced
β

α - (763 cm −1 ), γ - (1232 cm −1 ), and β

-phase formation in PVDF is clearly visible, but shows a reducing trend with the
initial addition of CaCl 2 (0.25 wt%). Interestingly, the further addition of CaCl 2 in
the electrospun PVDF showed an increasing -phase content, which may allow

us in our future work to analyze the changes in

β

-phase at higher CaCl 2 content
than that presently used in this study. Compared to MWCNTs (Figure 8.6e), the
samples with CaCl 2 (Figure 8.6d) showed relatively higher

β

β -phase content and

lower -crystalline

formation in PVDF. The quantitative analysis for the PVDF-CaCl 2 (0.5 wt%) with
varying MWCNT contents (Figure 8.6f) is rather inconclusive with the amount
of MWCNTs used and hence needs further studies with higher content of the
additives.
Figure 8.7 shows the SEM images of the electrospun PVDF nanofiber prepared
with varying CaCl 2 and MWCNT content. The average fiber diameter was found
to decrease gradually with increasing CaCl 2 content (Figure 8.7a-c), though
insignificant changes in the fiber diameter were observed in the case of MWCNT
addition (Figure 8.7d-f). It seems that the interaction between CaCl 2 and DMAc
has an influence on the surface morphology and fiber diameter of the electrospun
PVDF-CaCl 2 nanofiber mat. In an earlier study, irregularly oriented and non-
directional pores were observed in the electrospun PVDF membranes prepared
using acetone as a solvent [22].

α -and γ -phases in PVDF, which confirms the CaCl 2 -induced β