Doping effect on optical properties of pure and malachite green doped polyvinylidene fluoride films

The films of pure polyvinylidene fluoride (PVDF) and various concentration of malachite green (MG) doped have been prepared by solution cast method. The absorption spectra recorded by UV-VIS spectrophotometer 1700 Shimadzu. Absorbance versus wavelength spectra of pure and various concentration of MG doped PVDF films were observed in wavelength range 200-800 nm. With increasing malachite green in PVDF, intensity of absorbance edge and refractive index increases.


Introduction
Polymeric material serve insulation in transportation, air craft and automobiles, communication and medical fields [1].A polymer is made up of many small molecules which have combined to form a single long or large molecule.The single small molecules (single unit) from which polymer is form known as monomers and the process by which the monomer molecules are link to form a big polymer is called polymerization [2].For the study of charge storage and transportation the insulating polymers are most attractive.Such polymers also yield the best electret for practical application because of their insulating properties and high concentration of deep trapping sites [3].In recent years, due to application in optical device, studies on the electrical and optical properties of polymers have attracted much attention.The optical properties of polymers can be suitably modified by attention of dopants / another polymer with the host matrix [4].Polyvinylidene fluoride (PVDF) is semicrystalline, thermoplastics polymer belonging to the family of fluoropolymer due to its excellent piezoelectric properties, thermal stability and mechanical strength [5].It has a good processability and it also possess chemical resistance property to different materials such as different acids, bases, organic solvents, oil and fat [3].PVDF has well physical and electrical properties of a kind of many applications [6].Its electrical properties have led to many applications as an electrical insulator, greatly as a wire coating [3].Since its commercialization in the mid-1960, PVDF 1291 (2023) 012024 IOP Publishing doi:10.1088/1757-899X/1291/1/012024 2 has been used as a verity of chemical process industries sectors due to its versatility and broad attributes [7].Glass transition temperature (Tg) of PVDF about is -38 0 C and melting temperature is 171 0 C. PVDF has low density 1.74 g/ml at 25 0 C. Structure formula of PVDF is (CH2-CF2) n.Malachite green (MG) is an organic compound of low molecular weight that is used as a dyestuff.MG is an Azo-dye which are the largest class of synthetic dyes.

Materials and film preparation
Polyvinylidene fluoride (PVDF) in powder form (molecular weight 534,000 MW) has been used as a host polymer in the present Thin films of PVDF and malachite doped PVDF film has been prepared by using common solvent Dimethyl formamide (DMF).Solution cast method has been used for preparation of pure and malachite green doped PVDF films.The solution was kept on magnetic stirrer at fixed temperature to get homogeneous and transparent solution.Homogeneous solution then poured onto an optically plane glass plate floating on mercury.The solvent was allowed to evaporate in an oven at desired temperature for 6 hrs to yield the desired film.This was followed by room temperature out gassing at 10 -5 torr for further period of 12 hours remove any residual solvent.The plate was then slowly drawn out of the solution leaving a uniform film on the plate.Thus, film obtained transparent, uniformly smooth and can be easily peeled from the glass surface.The doped sample were prepared by malachite green of 0.067%, 0.117 % and 0.167 % mixing respectively in 06 gm of PVDF dissolved in 50 ml of dimethyl formamide (DMF).The doped films were green in colour, uniformly smooth and can be easily peeled from the glass surface.

Results and Discussion
The absorbance spectra of pure PVDF and malachite green doped PVDF films recorded in wavelength range 200-800 nm by UV-VIS spectrophotometer 1700 Make Shimadzu.The absorption spectra for pure as well malachite green doped films of PVDF in different configuration is shown in figure 3.For pure PVDF maximum absorption edge occur at 230 nm wavelength [8].With increasing wavelength absorbance decreases continuously.However, addition of malachite green in PVDF gives rise to four absorption bands at wavelength around 320, 410, 530 and 620 nm for all doped samples.First absorption band observed in ultra violet region at 320 nm for 0.067 %, 0.117 % and 0.167 % doped films.
Remaining three bands occurs in visible region (410-620 nm).Intensity of absorption band increases with addition of malachite green [9].Maximum intensity of absorption band occurred for fourth band at 620 nm wavelength for all doped films.Malachite green is a cationic triphenyl methane dye [10].As the dye is cationic, the coloured ion is charged positively [9].The cationic nature of dye suggest that it is electron donor dye.Polymeric materials are likely to accept electrons, hence malachite green may form donor-acceptor complexes with PVDF when mixed under suitable condition [11].Also, malachite green has a one amino group containing unshared pair of electrons which may be responsible for charge transfer complex (CTC) formation [12].In this present study maximum absorption observed peak observed at wavelength 620 nm for 0.167 % concentration of malachite green.Variation in refractive index for pure PVDF with different concentration of MG doped samples have been detected.Spectra of refractive index versus wavelength for pure and MG doped PVDF shown in figure 4.

Conclusion
With increasing concentration of malachite green, the intensity of absorption bands and refractive index increases.Region for change in intensity of absorption band with increasing concentration of MG in PVDF may be due to formation of charge transfer complexes.

Figure 3 .
Figure 3. Absorbance versus wavelength spectra of pure and malachite green doped PVDF film.

Figure 4 .
Figure 4. Spectra for refractive index versus wavelength of pure and MG doped PVDF samples.

Table 1 .
For pure PVDF sample maximum refractive index occurs at 230 nm wavelength.When increasing concentration of malachite green in PVDF refractive index increases.Maximum refractive index occurs in visible range at 620 nm for all doped samples.Refractive index for pure PVDF and all MG doped samples presented in table 2. Absorbance versus wavelength UV spectra of pure and malachite green doped samples

Table 2 .
Refractive index for pure and malachite green doped PVDF samples