STUDIES OF A.C. CONDUCTIVITY OF POLY(VINYL BORATE) AND ITS CALCIUM DERIVATIVE IN SOLID STATE

Chinese Journal of Polymer Science Vol. 26, No. 4, (2008), 501 506 Chinese Journal of Polymer Science 2008 World Scientific Note STUDIES OF A.C. CONDUCTIVITY OF POLY(VINYL BORATE) AND ITS CALCIUM DERIVATIVE IN SOLID STATE Prafulla Chetri, Neelotpal Sen Sarma and Narendra Nath Dass * Material Science Division, Polymer Unit, IASST, Paschim Boragaon, Guwahati-781 035, Assam, India Abstract An attempt has been made in the present work to prepare poly(vinyl borate), PVBO and its calcium derivative by homogeneous esterification of PVA with boric acid in non-aqueous medium in the presence of a catalyst ethyl nitrate dimethyl sulfoxide. The compounds were characterized by IR and 1 H-NMR spectra. Conductivities were determined from 30 C to 90 C in solid state within a frequency range of 42 Hz to 100 khz. The compounds so formed showed ionic conductivity and their conductivities were dependent on frequencies used. It is found that the addition of Ca 2+ ion increases the ionic conductivity of PVBO appreciably. The conductivity of PVBO-Ca increases rapidly after 50 C. The total ionic transport number and activation energy of the copolymers were also determined. Keywords: Ethyl nitrate dimethyl sulfoxide; Poly(vinyl alcohol); Poly(vinyl borate); Calcium derivative of poly(vinyl borate); Conductivity. INTRODUCTION Poly(vinyl alcohol) (PVA), unlike many polymers, is soluble in water [1]. Therefore, derivatives of PVA are generally prepared in aqueous medium [2]. Industrially, the important and higher poly(vinyl esters) are prepared either by the polymerization of monomers of vinyl ester or by the transvinylation method [3, 4]. A survey of literature indicates that poly(vinyl borate) (PVBO) can be prepared by the interaction between PVA and boric acid in aqueous medium [5 7]. Experimentally it was found that PVA can be dissolved in an organic solvent in the presence of catalytic concentration of Ethyl nitrate dimethyl sulfoxide, C 2 H 5 ONO 2 DMSO (EN DMSO) [8]. Alternatively synthesis of a good solid polyelectrolyte is very important due to its wide use in various applications like solid state batteries [9] etc. Normally polyelectrolytes are prepared by addition of salts, acids and alkalies. The ionic polymers are generally mixtures of covalent polymers associated with its ionic salts. A very common example of this type is polyethelene oxide (PEO) based polyelectrolytes where PEO is mixed with divalent cationic (Pb 2+, Mg 2+, Zn 2+ ) salts [10 12]. In polymer-solvent-salt matrix system, the role of polymer is secondary in so far as conducting property is concerned. It acts as a stiffener for solvents of high dielectric constants. But in case of PVBO system no additional salt is added, and instead the virgin polymer acts as a polyelectrolyte. EXPERIMENTAL Materials and Methods Poly(vinyl alcohol), (PVA) used, is in white crystalline form (CDH reagent grade with viscosity average molecular weight of 14000 containing 1% of residual vinyl acetate). It was used without purification. Boric acid (BDH reagent grade) was recrystallized from benzene. Dimethyl formamide, DMF (BDH reagent grade), * Corresponding author: Narendra Nath Dass, E-mail: narendas@sify.com Received October 7, 2007; Revised December 13, 2007; Accepted December 17, 2007

502 P. Chetri et al. benzene (BDH reagent grade), dimethyl sulfoxide, DMSO (BDH reagent grade) and methanol (BDH reagent grade) were purified by distillation under vacuum. Acrylic acid (BDH reagent grade) was purified according to the procedure adopted by O'Neil [13]. Para toluene sulfonic acid, P-TSA (BDH reagent grade) was recrystallized from benzene. Nitric acid (BDH reagent grade), calcium hydroxide (BDH reagent grade) and hydrogen peroxide (E. Merck) were used without further purification. EN DMSO was prepared by interaction of acrylic acid with conc. HNO 3 in DMSO and subsequent decarboxylation with H 2 O 2 solution [14]. The product was a white crystalline solid. The IR spectra of PVBO and PVBO-Ca were recorded with a PERKIN ELMER 883 IR spectrophotometer. The 300 MHz proton NMR spectra were recorded with a BRUKER DPX-300 NMR spectrometer using DMSO (d) as the solvent. The bulk electrical conductivity of these compounds was evaluated from the complex impedanceadmittance plots recorded at different temperatures using a HIOKI 3520, frequency response analyzer. The plots were recorded in the frequency range from 42 Hz to 100 khz keeping the signal amplitude of 20 mv. The geometry of the cell for the measurement of conductivity was Pt polymer film Pt, where platinum plate was used as electrodes. The experiment was carried out under a relative humidity of 58% [15]. The total ionic transport number, t ion was evaluated by the standard Wagner polarization technique [16]. The cell SS PVBO SS and SS PVBO-Ca SS (SS stands for Stainless Steel) was polarized by a step potential (about 1.0 V) and the resulting potentiostatic current was monitored as a function of time. The stainless steel acts as blocking electrodes for the above cell. The t ion was evaluated using the formula: t ion = (i T i e )/i T where i T and i e are total and residual current respectively. Preparation of PVBO and PVBO-Ca PVA 4.4 g (0.10 mol, based on CH 2 CHOH as the repeat unit) was dissolved in 150 ml of a solvent mixture of DMF and benzene (4:1 V/V) in the presence of EN DMSO. The ratio of [PVA] to [EN DMSO] was maintained at 1:(1.4 10 3 ). Boric acid 6.8 g (0.11 mol) taken in 100 ml DMF, was then added slowly to the PVA solution. Homogeneous esterification was carried out by heating the reaction mixture for about 24 h at a temperature around 90 C. The water produced during reaction was removed from the reaction medium as it was formed using Dean and Stark principle [17]. After completion of the reaction, the solvent was removed by distilling under vacuum. The ester was precipitated by pouring into a mixed solvent of acetone and petroleum ether (1:2 V/V), with constant stirring. The ester was then dissolved in methanol and filtered to remove the unreacted PVA. Methanol was removed by rotary distillation apparatus. The insoluble white product was filtered off, dissolved in methanol and reprecipitated with benzene. This process was repeated several times to ensure the complete removal of PVA, boric acid, P-TSA and EN DMSO [18]. The ester was dried at 40 C and stored over anhydrous calcium chloride. The PVBO was dissolved in 100 ml of methanol and 20 ml of 0.01 mol/l Ca(OH) 2 solution was added drop wise to the solution with constant stirring. On cooling in the refrigerator, the solution set to a gel. The gel was washed twice with absolute alcohol, and twice with diethyl ether. It was then dried in a vacuum desiccator. RESULTS AND DISCUSSION The reactions for the production of PVBO and PVBO-Ca may be summed up as Scheme 1.

Studies of A.C. Conductivity of Poly(vinyl borate) and Its Calcium Derivative in Solid State 503 Scheme 1 Reactions showing the formation of PVBO and PVBO-Ca from PVA and boric acid Analysis of PVBO and PVBO-Ca The degree of esterification of PVBO was found to be 57 mol% by acetylation method [19]. Since not all OH groups were transferred to borate, the polymer PVBO may be termed as copolymer. The IR spectra for PVBO and PVBO-Ca are shown in Fig. 1. Both PVBO and PVBO-Ca molecules showed a broad band at 3380 cm 1 due to its unreacted OH groups. It also showed strong bands at 2960 2800 cm 1 due to C H stretching. The band at 1600 cm 1 is accounted for the cyclic ester group [20]. The bands at 1480 cm 1 are for OH and C H bending whereas at 1420 cm 1 is for CH 2 bending. The peaks at 1060 cm 1 and 880 cm 1 are due to the B O and B OH groups [21] respectively, and the appearance of the peaks confirmed the formation of the ester. Due Fig. 1 IR spectra of (a) PVBO and (b) PVBO-Ca

504 P. Chetri et al. to the formation of the gel, PVBO-Ca the peak at 880 cm 1 disappeared as all the B OH groups are converted to B O groups. Therefore, only the peak at 1060 cm 1 which is for B O group present in the IR spectra of the gel. Proton NMR spectra for PVBO and PVBO-Ca are shown in Fig. 2. The signals observed at δ = 1.3 1.9 and δ = 3.0 3.5 are due to the methylene ( CH 2 ) and methine ( CH ) protons respectively [22]. The signal at δ = 2.4 is due to the unreacted hydroxyl groups ( OH). On the other hand, estimated amount of boron present in PVBO and PVBO-Ca are 4.91% and 2.72% respectively. The estimated calcium in PVBO-Ca is 4.26% as against the expected value of 5.03% assuming 58% esterification. This indicates that there is some probability for the production of free ions [23] due to the fact that some of the calcium ions may be singly bonded for the non availability of adjacent BO 4 group. Fig. 2 1 H-NMR spectra of (a) PVBO and (b) PVBO-Ca The total ionic transport number, as evaluated by Wagner polarization technique was found to be 0.789 in case of PVBO and that of PVBO-Ca was 0.886. The experiments were repeated thrice and the plot of time versus current is shown in Fig. 3. The variation of electrical conductivity with time has been taken as a measure of ionic conduction. This graph indicates fast exponential type decrease in electrical conductivity initially, saturating later to almost constant values, which could be separated as electronic and ionic part by extrapolating

Studies of A.C. Conductivity of Poly(vinyl borate) and Its Calcium Derivative in Solid State 505 the linear part to zero time for electronic and point wise subtraction for ionic conduction. It is evident that mobile ions are present in the material. Fig. 3 Time versus current plot of (a) PVBO and (b) PVBO-Ca The behaviors of electrical conductivities of these compounds with the rise of temperature are shown in Fig. 4. The curve b indicates that a decrease of conductivity with the rise of temperature at the initial stage, but it gradually tends to exhibit an increased conductivity at a later stage, and the phenomenon is just like the proton transfer mechanism in ammonium dihydrogen phosphate observed by other investigators [24]. This may be due to the loss of water molecule absorbed by the electrolytes. Fig. 4 lg(σt) versus 1/T plot for (a) PVBO and (b) PVBO-Ca Conductivity data for PVBO and PVBO-Ca with respect to temperature showed that the PVBO-Ca needs higher activation energy. The mechanism of conduction for the polymeric material is a mixture of ionic and electronic. The activation energy was computed from Arrhenius plot of lg(σt) versus 1/T in Fig. 4 of the compounds PVBO and PVBO-Ca using only the two points, one at the beginning and end of the curve [25] and the values are 0.0753 ev and 0.873 ev respectively. The negative activation energy of PVBO can be explained with the help of similar work done by Aoki et al. [26]. Apparent negative activation energy in PVBO indicates a conversion of conducting-to-insulating species with increase of temperature. To overcome this problem PVBO was converted to PVBO-Ca compound. The higher value of activation energy for PVBO-Ca indicates that the percentage of ionic conduction is predominant compared to PVBO. So, PVBO-Ca behaves as a moderately good polyelectrolyte. Conductivity study indicated that the mechanism for conduction is mainly ionic [27]. The electrode system used, consisted of two pieces of steel anvil. The conductivities increased with the rise of frequencies, which is a

506 P. Chetri et al. well-known behavior for all ionic materials [28]. As expected PVBO compound shows little ionic conductivity. But by the addition of Ca 2+ ions, it shows marked improvement for ionic conductivity. This is due to the production of ions surrounded in the gels. The addition of Ca(OH) 2 increases the cross linking producing polyelectrolyte with improved ionic conductivity. ACKNOWLEDGEMENT Authors are thankful to DAE, GoI for financial help. REFERENCES 1 Billmeyer, F.W. Jr., Text book of Polymer Science, Wiley Interscience, New York, 1984, p.393 2 Lindemann, M.K., Encycl. Polym. Sci. Technol., ed. by Bikales, N.M., John Wiley and sons, Inc., New York, 1971, Vol. 14, p.169 3 Chetri, P. and Dass, N.N., J. Appl. Polym. Sci., 2001, 81: 1182 4 Watanable, S. and Ichimura, K., J. Polym. Sci. Polym. Chem. Ed., 1982, 20: 3261 5 Marvel, C.S. and Denoon, Jr. C.E., J. Am. Chem. Soc., 1938, 60: 1048 6 Deuel, H. and Nenkom, H., Makromol. Chem., 1949, 3: 13 7 Thiele, H. and Lamp, H., Kolloid. Z., 1960, 173: 63 8 Chetri, P. and Dass, N.N., Polymer, 1997, 37: 5289 9 Suthanthiraraj, S.A., Trends in Solid State Ionics, Raj Publishing House, Chennai, 1997, p.108 10 Chandra, S. and Hashmi, S.A., J. Mater. Sci., 1990, 25: 2459 11 Huq, R. and Farrington, G.C., J. Electrochem. Soc., 1988, 135: 524 12 Huq, R. and Farrington, G.C., Solid State Ionics, 1988, 28: 990 13 O'Neil, T., J. Polym. Sci., A-l, 1972, 10: 569 14 Chetri, P., Islam, N. and Dass, N.N., J. Polym. Sci. A, 1996, 34: 1613 15 Foot, P., Ritchi, T. and Mohammad, F., Chem. Commun., 1988, 1536 16 Hashmi, S.A. and Chandra, S., Mater. Sci. Eng. B, 1995, 34: 18 17 Macmillan, N., An Advanced Organic Laboratory Course, Longmans, New York, 1972, p.10 18 Chetri, P. and Dass, N.N., J. Appl. Polym. Sci., 1996, 62: 2139 19 Mann, F.G. and Saunders, B.C., Practical Organic Chemistry, Longmans, London, 1974, p.450 20 Chetri, P., Sarma, N. and Dass, N.N., J. Polym. Mater., 1997, 14: 168 21 Ross, S.D., Inorganic Infrared Spectra, Interscience, London, 1972, p.262 22 Shiga, T., Fukumori, K., Hirose, Y., Okada, A. and Kuranchi, T., J. Polym. Sci. Polym. Phys. Ed., 1994, 32: 85 23 Sinton, S.W., Macromolecules, 1987, 20: 2430 24 Chandra, S. and Hashmi, S.A., J. Mater. Sci., 1990, 25: 2459 25 Chetri, P., Dass, N.N. and Sarma, N., Mater. Sci. & Eng. B, 2006, 128: 188 26 Aoki, K. and Li, J.Q., J. Electroanalytical Chem., 1998, 441: 161 27 Sarma, N., Chetri, P. and Dass, N.N., J. Polym. Mater., 1998, 15: 23 28 Ahsen, V., Ozdemir, M., Ziya Ozturk, Z., Gul, A. and Bekaroglu, O., J. Chem. Res. (S), 1995, 9: 348