The FESEM micrographs of the pure PVDF [Fig.1 (a)] and PVDF/PEG PB with varying weight fractions of 0.2 and 0.4 are shown in Figure 1 (b-d). The large created unfilled like structures and higher heterogeneity and immiscibility is clearly observable from the micrographs for the sample 0.2 [Fig.1 (b)], while for PB with 0.4 of PEG becomes more compatible and homogenous [Fig.1 (c) & (d)], suggesting the homogenousness of PB at higher loadings of PEG. The diameter of the created structures in the PB, are of the order of ~0.1 μm. From these micrographs of PVDF/PEG PB, it is clear that various types of interfaces have appeared in the blends, which can result in an increase of the absorbance. The FESEM images of PVDF/GO composites with different weight fractions 0.02, 0.05, 0.10, 0.20 are shown in the Figure 1 (e-h). All these micrographs of PVDF/GO nanocomposites show excellent homogenous microstructures of uniformly dispersed GO of the order of 1 μm range. With the increase of loading of GO, the homogeneity increases from 0.0 to 0.05[Fig.1 (e-f)] and then the homogeneity decreases from 0.05 to 0.2, resulting in the agglorameration of GO [Fig.1 (g-h)]. This also clearly indicates that on addition of GO, that may result to increase in absorbance of these PNC, due to higher conductivity of GO.
3.2 UV-Visible Spectroscopy
The UV–visible spectroscopy is one of the methods of finding the optical activity/response of the samples by collecting the information of absorbance/transmittance/reflectance of the samples in the ultraviolet-visible spectral region. Generally, the light in the visible and adjacent [near-UV and near-infrared] ranges of wavelengths are passed through the samples. The light interacts with the samples [some possible interactions are molecules containing π-electrons or non-bonding electrons can absorb the energy in the form of ultraviolet or visible light to excite these electrons to higher anti-bonding molecular orbitals]. The result of light/matter interaction are represented in terms of various optical parameters, such as; absorbance/transmittance/reflectance as a function of wavelength in the ultraviolet-visible spectral region of interest. The more easily the electrons are excited, the longer is the wavelength of light that it can absorb. When light passes through or is reflected from a sample, the amount of light absorbed is the difference between the incident radiation (I0) and the transmitted radiation (I). The amount of light absorbed is expressed as either transmittance or absorbance. Transmittance usually is given in terms of a fraction/as a percentage and is defined as . The absorbance is related to the transmittance and is defined as , which is given in the Figure 2. Figure 2 shows the UV-Visible absorbance spectra of both the series of samples in the visible region of wavelength range of 400 nm to 700 nm of light. There is an increase of absorbance linearly with increase of PEG into the PB, and the absorbance reaches maximum as 0.25 for 0.4 [Fig. 2 (a)]. However, for all the PB, the absorbance is highly dispersive in nature, suggesting as well attributed to the structure of PB to be highly heterogeneous and incompatible, confirmed from the Fig. 1 (a-d). A continuous decrease of absorbance with increasing wavelength up to 550 nm is also clearly visible. In a comparison to the PNC, the increase of absorbance also occurs linearly with increase of GO into the PNC, and the absorbance remains constant and nondispersive over whole visible region of wavelength range of 400 nm to 700 nm of light. From graph [Fig. 2 (b)], it is observed that the absorbance of pure PVDF solution (black line) decreases with wavelength from approximately 0.2 at 400 nm to 0.04 at 700nm. But with the increase of wt% of GO in the PNC, the absorbance approaches as high as 3.5 at 400 nm to 3.0 value at 700 nm for the PNC with 10%GO. It shows the distinct curves for different loadings of fillers of PB/ PNC and the absorbance decreases with wavelength in a systematic manner from higher to lower value with decreasing wt% of the fillers. The absorbance curves for PB are stiffer as compared to the PNC, attributed to their heterogeneity, confirmed from the microstructures [Fig. 1].
The increase in absorbance in both the cases, with increase of filler amount can be attributed to the electrical conductivity of the filler. Also, the higher conductivity of GO as compared to the PEG, is responsible for giving more than 10 times higher absorbance in case of PNC as compared to PB. The exclusive micro structure of both the series of samples confirms these interesting consequences also [Fig. 1].
3.3 Spectroscopic Ellipsometric Studies
Spectroscopic ellipsometery was used to study the variation of the optical parameters of the thin films of PB and PNC [30-32]. The SE is a high precise and nondestructive powerful tool to extract the varius optical parameters of the films with accurately and reliably results. Ellipsometry (reflection ellipsometry) measures the changes in the state of polarization of light upon reflection from a surface. As a non-invasive and non-destructive tool, ellipsometry requires only a low-power light source and, consequently, it does not affect most processes, which renders ellipsometry a convenient tool for in situ studies. Figure 3 show the plot of ellipsometric parameters, psi (ψ) and delta (Δ) versus wavelength (λ) in the visible wavelength region from 500 nm to 800 nm for all the PB/PNC under investigation. The significant variation in the ellipsometric parameters Ψ and Δ for all the PB/PNC confirms the significant change in the amplitude as well as phase ratio of S and P polarized light. Further the extent of variation of Ψ and Δ for all the PB samples with 0.0, 0.10, 0.3 and 0.4 is highly varying, while for the PB with 0.2, the Ψ and Δ remains almost constant throughout the wavelengths [Fig. 3 (a-b)]. However the extent of variation of Ψ and Δ for all the PNC samples with 0.0,0.01,0.02,0.05 are remaining constant, while for the samples with higher loading of GO, i.e. for 0.1 and 0.2, the Ψ and Δ are highly varying throughout the wavelengths [Fig. 3 (c-d)]. The differences of these optical features are attributed to the homogenity of the samples and the differences of the electrical conductivity of their respective fillers. I found that the PB sample with 20 % of PEG and PNC sample with 5% GO, show same value of ellipsometric parameters (Ψ~30 and Δ~5) and these values also remains constant over the whole wavelength range. Thus these optical parameters also demand that the 20 % of PEG in the PB and 5% of GO in the PNC need to be highly homogenous, and these can be also well confirmed from the highly homogenous microstructures respectively [Fig. 1( b &f)]. Thus the extent of variation in parameters Ψ and Δ confirms the homogeneity and the morphological changes of the PB/PNC, and find their suitability for optical applications.
The variation of real and imaginary parts of complex refractive index (ň=n+ik) [Fig. 4] is shown for all the PB/PNC, where ‘n’ and ‘k’ represents the refractive index and the extinction coefficient of the PB/PNC films with wavelengths in the range 500 nm to 800 nm. It can be observed that there is a significant change in the refractive index and extinction coefficient for all PB/PNC samples with weight % of the filers in the PB/PNC over the wavelengths in the range of 500 nm to 800 nm. Similar type of observation as that of variation of Ψ and Δ was found for ‘n’ and ‘k’ for both PB/PNC samples. In the case of PB, 10%& 20 % of PEG in the PB sample, show higher value of refractive index [1.1-1.3] with low extinction coefficient [0.0-0.05] was observed and are found to be constant in the region of wavelength of 500 nm to 800 nm[Fig. 4(a-b)]. However for the PNC with wt% of GO as 0.02,0.03 and 0.05, higher value of refractive index [1.2-1.3] with low extinction coefficient [0.0-0.1] was observed and are found to be constant in the region of wavelength of 500 nm to 800 nm[Fig. 4(c-d)]. Hence these respective fluctuations and constancy of the optical parameters can be attributed to the extent of heterogeneity/homogeneity of the PB/PNC samples respectively [Fig. 1].
Similarly, the variation of real and imaginary parts of complex dielectric function (ε=ε'+iε'') [Fig. 5] are shown for all the PB/PNC, where ‘ε'’ and ‘ε''’ represents the real part and the imaginary parts of the complex dielectric function of the PB/PNC films with wavelengths in the ranges of 500 nm to 800 nm. From figure 5, the similar types of observations were recorded for both PB/PNC as that of Figure 3 and 4. The real and imaginary parts of complex dielectric function remains constant for the samples with wt% as 0.20, while large variations were obtained for all other PB. Similarly, the real and imaginary parts of complex dielectric function remains constant for the samples with wt% as 0.03 and 0.05, while large variations were obtained for all other PNC. From all the graphs [Fig. 3 to Fig. 5], it can be observed that all the optical parameters of the PB sample with 20wt% of PEG and for PNC sample with GO as 0.05, are highly stable over the complete wave length region and these achieved stabilized values are also attributed to the excellent homogeneity of the samples as prepared [Fig. 1 (b) and Fig. 1(f)].