ADVANCE MATERIALS GROWTH & CHARACTERIZATION
OPTICAL AND STRUCTURAL CHARACTERIZATION OF KMnO4 DOPED PVA FILMS Blaise Lobo and M R Ranganath Department of Physics, Karnatak Science College, Dharwad 580001, Karnataka, India Email:
[email protected] Potassium permanganate (KMnO4) doped poly (vinyl alcohol) (PVA) films were prepared by solution casting method, in the doping range, from 0 wt% up to 17.6 wt%. The films were characterized by ATR-FTIR spectroscopy and UVVisible spectroscopy. The variation in the crystallinity of PVA with doping level (concentration of KMnO4) has been experimentally investigated using XRD. A survey of research literature reveals that, little work has been done on KMnO4 doped PVA. KMnO4 is a strong oxidizing agent. On doping with KMnO4, the fire resistance of PVA is known to increase1. Using a cone calorimeter, it was experimentally observed that there is a dramatic decrease of the heat release & significant increase in ignition time, for oxidized PVA in comparison to the original PVA. Hassan2 investigated the oxidation of PVA by KMnO4 in alkaline solution. The oxidation occurs through formation of two intermediate complexes, and finally leads to the formation of poly(vinyl ketone), which was isolated & identified by microanalysis and spectral data. Our earlier experimental investigation revealed that the optical bandgap of KMnO 4 doped PVA films is found to significantly decrease with doping level3. The FTIR & XRD spectra reveal significant changes in optical and microstructural properties of PVA, when doped with KMnO4. FTIR is used for identification of functional groups in the pure and doped samples. Figure 1 shows the FTIR scan of undoped PVA and that doped to maximum level (17.6 wt%), beyond which the material becomes brittle & does not form films. The undoped samples reveals absorption peaks at well-established frequencies, assigned to different functional groups like OH stretch (wide absorption band peaking at 3390 cm-1), C-H stretch between 3000 & 2850 cm-1, C-H bend between 1470-1450 cm-1 & C-H rock of the methyl group between 1370 & 1350 cm-1. In addition, absorption peaks at 1736 cm -1 and 1090 cm-1 are present, attributed to stretching vibrations of C=O group of ester and the O-C-O group of the remaining vinylacetate groups (unhydrolyzed portion), respectively. On doping with KMnO4, it is noticed that a sharp peak appears at 1612 cm -1, especially for doping levels beyond 5 wt%. Also the absorption peak at 1736 cm-1 is reduced on doping. These two changes become more pronounced with increase in doping level. For intermediate levels, FTIR spectra reveal increased absorption at 1650 cm-1, assigned to carbonyl groups, not associated with ester. These changes are used to propose mechanisms of chemical changes caused in PVA, due to oxidation by KMnO4. Difference spectroscopy, obtained by subtraction of the FTIR spectrum of doped films relative to that of undoped PVA has been utilized to highlight these changes. PVA granules and KMnO4 were obtained from Hi Media Laboratories Pvt. Ltd., Mumbai. Two grams of PVA granules were dissolved in 100 ml distilled water using a magnetic stirrer with hot stage, maintained at 70˚C. A standard solution of KMnO4 was prepared by dissolving 10 g of KMnO4 in 280 ml distilled water, and different volumes of the same was added, at room temperature, to the aqueous PVA solution prepared earlier. Thus different doping levels of KMnO4 in PVA could be achieved. Films were obtained by solution casting method, the drying taking place in an air cooled, temperature controlled oven. After drying, the films were peeled off from the glass substrate to give the KMnO 4 doped PVA films used in this study. FTIR spectra were recorded in the wavenumber range 400-4000 cm -1 using NicoletModel 5700 FTIR Spectrometer with an ATR accessory (Germanium crystal) available at the University Scientific Instruments Centre (USIC), Karnatak University Dharwad (KUD). Resolution of 0.5 cm-1 was used, and 64 scans were averaged to get good signal to noise ratio of the final spectrum. UV-Visible Spectra were recorded using an Hitachi U-3310 spectrometer available at USIC, KUD. UV-Visible spectra were recorded in the range 200-800 nm, with spectral bandpass of 5 nm at speed 120 nm/minute. The UV-Visible spectra were analysed to obtain optical bandgap information, and the same has been published elsewhere3. XRD spectra were recorded using the facility at Department of Physics, Alagappa University, Karaikudi. Copper Kalpha X-rays (1.54A) were used, generated at voltage 40kV & current 30 mA, with XPERT-PRO goniometer system (PW3050/60), with continuous scanning from (2 theta =)10 to 80 degree, at step size ( 2 theta=) 0.05 degree, and scan step time 10 seconds. The XRD spectra were analyzed to determine the crystallinity of the samples and d-spacing. The XRD spectra, for differently doped films are shown in figure 2. (KMnO4 doped PVA is denoted as KPVA in figure 2). For the first crystalline halo, 2θ = 19.685˚ and d spacing = 4.51A. Notice that the intensity of the crystalline halo decreases with increase in KMnO4 content, implying thereby that the degree of crystallinity of PVA decreases with increased KMnO4 concentration. 1 G E Zaikov & S M Lomakin 1997 Innovative type of low flammability varnish based on poly(vinyl alcohol) Polymer Degradation & Stability 57, 279. 2 R M Hassan, S A El-Gaiar, A M El-Summan 1993 New Co-ordination Polymers IV. Oxidation of PVA by permanganate ion in alkaline solutions. Kinetics & mechanism of decomposition of intermediate complex. Polymer International 32 (1), 39. 3 M R Ranganath & Blaise Lobo 2008 “Experimental Investigation of the optical band gap in potassium permanganate doped poly(vinyl alcohol) films” Proc. International Conference on Materials Science Research & Nanotechnology (ICMSRN-2008), 27-29 February 2008, Mother Teresa Women's University, Kodaikannal, TN, India, p 194-196 (ISBN 81-86520-14-7)
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Figure 1. ATR-FTIR scans of 0 wt% and 17.6 wt% KMnO4 doped PVA. Note appearance of absorption peak at 1612 cm-1 & decrease in 1736 cm-1 absorption peak intensity, on doping PVA with KMnO4.
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Figure 2. XRD scans of PVA doped to different levels, with KMnO4. Note that the degree of crystallinity decreases with increase in doping level. For the first crystalline halo, 2θ = 19.685˚ and d spacing = 4.51A. KMnO4 doped PVA is denoted as KPVA in this figure. Acknowledgements. Mr. Kadlimatti & Mrs. Bharathi of USIC, Karnatak University, Dharwad, for technical assistance enabling acquisition of ATR-FTIR & UV-Visible spectra respectively & Professor Dr. Sanjeeviraja, Department of Physics, Alagappa University, Karaikudi for arrangements to record the XRD spectra.
