Avai - RS Publication

International Journal of Advanced Scientific and Technical Research Available online on http://www.rspublication.com/ijst/index.html

(Special issue- Issue 5 volume 5) ISSN 2249-9954

Influence of ZnO nanoparticles on thermal behavior of Poly Vinyl Alcohol films K.S. Hemalatha1, Narsimha Parvatikar2, K. Rukmani3 1

Department of Physics, Bangalore University, Bangalore 560056, India, +91-80-26549800, [email protected] 2 Department of Physics, APS College of Engineering, Somanahalli, Bangalore 560082, India, +91-9448576930, [email protected] 3 Department of Physics, Bangalore University, Bangalore 560056, India, +91-80-22961482, [email protected]

ABSTRACT Thermal behavior and thermal stability of PVA and PVA-ZnO(5mol% - 20mol%) nanocomposite films prepared by solution casting method were studied by various techniques such as Differential Scanning Calorimetry (DSC), Thermogravimetry (TG) and Differential thermal analysis(DTA). Nano ZnO used as filler was prepared by low temperature solution combustion synthesis. Dehydration temperature (TD), glass transition temperature (Tg), and melting temperature(Tm) were estimated using DSC. Heat of enthalpy and specific heat capacity (Cp) of the samples are also estimated from these studies. Thermogravimetric analysis showed significant decrease in weight loss in nanocomposite films compared to pure polyvinyl alcohol indicating increase in thermal stability of the host matrix due to the incorporation of nano ZnO. The composite of 15mol% ZnO is found to exhibit the highest thermal stability. First order derivative of weight loss supports the results obtained from the DSC studies and shows extension of final degradation temperature of composite films in comparison with pure PVA. Differential thermal analysis reveals the existence of three endothermic peaks and an exothermic one corresponding to various physical and chemical changes taking place in pure polymer and composite films subjected to heating. Key words: Differential scanning calorimetry, TG/DTA, Thermal properties, Thermal degradation, specific heat capacity.

Corresponding Author: K. Rukmani INTRODUCTION Poly Vinyl alcohol (PVA; -[CH2-CHOH-]n-) a synthetic polar polymer is a good host material due to its good thermal-stability, chemical resistance, film forming ability, non-toxic, biocompatibility, bio-degradability and availability in different molecular weights. PVA is an important material in view of its large scale applications and addition of a small amount of dopant into PVA matrix can tune its physical, chemical, electrical and thermal properties to a great extent [1-8]. Incorporation of transition metal salts in PVA matrix has shown increase in CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS S J C INSTITUTE OF TECHNOLOGY”

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the thermal stability as revealed by Differential Scanning Calorimetry (DSC) and thermogravimetric analysis (TGA) studies [9]. Nanomaterials are attractive dopants because of their high surface reactivity, which is attributed to high surface to volume ratio. The DSC and TGA study on CdO-PVA nanocomposites showed that the combined nature of surface catalytic effect and barrier effect of nano CdO on PVA matrix increased the thermal resistance of PVA [10]. The addition of iron oxide nanoparticles into PVA matrix has favoured the cross-linkage of the bulk PVA matrices, resulting in a higher melting temperature, and an enhanced thermal stability of the polymer matrix [11]. High thermal stability is reported in NiO oxide and SiO2 doped PVA composites compared to host PVA matrix [12-13]. Various studies on the effect of addition of nano ZnO into PVA matrix have been reported [14-17]. M.I Abd-Elrhman reports an increase in the thermal stability of ZnO doped PVA [low molecular weight of 14,000] composites, Prepared by ball milling method and characterized using thermogravimetry and differential analysis of different heating rates[17] This paper studies the thermal behavior of PVA-ZnO nanocomposite films as a function of dopant concentration to understand the influence of nano ZnO on PVA matrix. PVA of higher molecular weight has been used and the nanoparticles of approximately 30nm have been prepared by solution combustion method while the samples are thick films of the nanocomposite. MATERIAL AND METHODS Poly Vinyl Alcohol (PVA) with an average molecular weight of 125,000 and 98% hydrolysis was obtained from Aldrich. Zinc nitrate [Zn (NO3)2, 6H2O] was purchased from CDH Ltd., India and Oxalyl di-hydrazide (ODH) [C2H6N4O2] was prepared in the lab according to the instruction given in the book [18]. ZnO nanopowder was prepared by low temperature self combustion method using ODH as a unique fuel and Zinc Nitrate as a precursor. Preparation of pure PVA films and ZnO doped PVA composite films was by solution casting technique reported in detail elsewhere [14]. DSC studies were carried out using Mettler Toledo DSC 822e in the temperature range-50C to 250C at a heating rate of 10C/minute. Thermogravimetric (TG) and DTA measurements were carried out using a Perkin Elmer, Diamond TG/DTA analysis apparatus in the temperature range 50C to 750C in the presence of nitrogen atmosphere with a flow rate of 100ml per minute and a heating rate of 10C minute. RESULTS AND DISCUSSION Differential scanning calorimetry (DSC) DSC thermograms of PVA and PVA/ZnO nanocomposite films are shown in Figure 1. Pure PVA shows a small dip at 30C, a broad peak at 114C (midpoint of the step at 82C) and a sharp melting endothermic peak at 222C corresponding to dehydration temperature, glass transition temperature and melting temperature respectively. These values are in good agreement with literature [19-21]. The composite films also show similar features in their thermograms. The melting temperature is seen to be nearly the same in PVA and the PVA-ZnO nanocomposite films indicating that the influence of ZnO is negligible. The peak in the glass transition temperature varies from100C to 120C (midpoint of step - 71C to 84C) for various concentrations of ZnO. The glass transition temperature increases for 5mol% of ZnO but decreases as the concentration is increased to 10mol% of ZnO and higher. This is attributed to easier motion of polymer chains. The shift in Tg values of polymer nanocomposite films CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS S J C INSTITUTE OF TECHNOLOGY”

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indicates the interaction between the polymer and the dopant ZnO. The weak endothermic peak at 30C in pure PVA and 28C to 34C in composite films is attributed to removal of physically adsorbed water. The DSC results are tabulated in Table 1.

Fig 1: DSC thermogram of PVA and composite films Table1.Dehydration temperature (TD), glass transition temperature(Tg), melting temperature(Tm), enthalpy of glass transition(Hg) and enthalpy of melting(Hm) obtained from DSC. Sample

Dehydration temp.TD(C)

PVA

30.30

Glass transition temp. Tg(C) Peak temp. Midpoint of step 114.25 82.00

Melting temperature Tm(C)

Enthalpy (Hm) –Jg-1

Enthalpy (Hg) - Jg-1

222.13

62.85

131.78

PVA-5mol% 28.53 120.84 84.00 222.50 63.67 122.57 ZnO PVA-10mol% 34.28 108.56 75.95 223.09 57.32 126.02 ZnO PVA-15mol% 33.78 108.58 74.28 221.80 53.64 114.32 ZnO PVA-20mol% 34.78 100.08 71.27 222.98 53.80 114.33 ZnO The specific heat capacity of the samples can be estimated from the DSC data by comparing the heat flow rate of a blank (without the sample in a sample holder) and a sample in a particular temperature range. The heat flow rate into the sample is given by [22] CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS S J C INSTITUTE OF TECHNOLOGY”

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International Journal of Advanced Scientific and Technical Research Available online on http://www.rspublication.com/ijst/index.html 𝑑𝐻


𝑑𝑇

= 𝑚𝐶𝑝 𝑑𝑡 -------------------------- (1) Where dH/dt is the heat flow rate, m is the sample mass, Cp is the specific heat and dT/dt is the rate of heating. dH/dt between any two temperatures is calculated by finding out the area under dH/dt curve at that instant in the DSC thermogram as shown in Figure 2. The specific heat capacity is found to be initially increased for 5mol% (0.79 calories gram -1C-1) and decreases (0.66 to 0.57 calories gram-1C-1 ) for other concentration (10mol% -20mol%) of fillers and it is found to be highest for 5mol% ZnO concentration. The specific heat capacity of the composite films is found to be increased compared to pure PVA, indicating the more heat holding capacity of the composite films indicating higher thermal stability of the composite films. 𝑑𝑡

Fig 2: Variation of heat flow versus time of PVA and composite films with reference to blank Thermogravimetry analysis (TGA) Figure 3 shows the experimental results of TGA in PVA and PVA/ZnO nanocomposite films.

Fig 3: Variation of residual weight(%) versus temperature of PVA and composite films All the samples showed a four step weight loss as seen from Figure 3 and tabulated in Table 2. For pure PVA, initially, no weight loss was observed from 40C to 78C indicating the thermal stability of the sample. CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS S J C INSTITUTE OF TECHNOLOGY”

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Table2. Thermogravimetric(TG) data for pure PVA and PVA-ZnO composite films. Sample

Temperature range (C)

% Weight loss Partial

PVA

PVA-5mol%ZnO

PVA10mol%ZnO

PVA15mol%ZnO

PVA20mol%ZnO

I stage II stage III stage 1V stage I stage II stage III stage 1V stage I stage II stage III stage 1V stage I stage II stage III stage 1V stage I stage II stage III stage 1V stage

80-230 230-332 332-487 487-645 75-214 214-305 305-461 461-735 73-210 210-300 300-449 449-735 62-200 200-296 296-460 460-735 70-202 202-292 292-458 458-735

8.07 72.66 14.7 4.41 7.08 66.25 14.09 5.53 6.34 62.96 11.96 7.89 6.70 57.05 14.36 10.69 5.59 59.28 18.74 13.66

Total up to II stage 80.73

73.33

69.30

63.75

64.87

The first event was observed in 80C - 230C temperature range accompanied by 8.07% weight loss. This loss is due to the removal of the physically adsorbed water. The melting of PVA is around 200C to 230C and at higher temperatures the polymer undergoes a thermal decomposition. The thermal decomposition occurs at 3 different stages in pure PVA. Major weight loss was observed in the first decomposition stage in the range 230C to 332C with gradual weight loss of 72.66%. This was followed by a second weight loss of 14.7% up to 487C presumably via backbone fragmentation or chain scission reactions with hydrogen elimination and carbon or hydrogen yield [20, 23-24]. Complete degradation of the polymer occurs between 487C and 645C with 4.41% weight loss. Final residual weight is 0.15% indicating complete decrease in thermal stability of the polymer in this temperature range. In the case of PVA-ZnO composites, the first decomposition stage occurs in the temperature range 200C - 305C with weight losses of 57% to 66% for various composite films much less than the mass loss of pure PVA which is 72% under the same conditions. The 10 to 15% reduction in weight loss in composite films indicates increase in thermal stability during this stage. The next two decomposition stages also shows decrease in weight loss in the composite films compared to pure PVA. The final decomposition stage gets extended to 730C CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS S J C INSTITUTE OF TECHNOLOGY”

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in all the composite films. Larger residual mass was observed in all the composite films compared to pure PVA at 645C. The final residual product was found to be 5 to 15% in composite films compared to 0.15% of pure PVA at this temperature. Though the starting and ending temperatures of the different decomposition stages were lower, lesser weight loss at each stage and increase in product residue indicates more stability of the composite films.

Fig 4: Derivative weight thermogram of PVA and composite films The thermal stability of the composite films can also be analysed by studying peak temperature of derivative thermogravimetric (DTG) curves. The first-order derivative of TGA curves as shown in figure 4 reveal the temperature at which the maximum decrease of mass occurs. The DTG data clearly shows that the side chains of PVA decompose earlier than the main chains [25]. The maximum weight loss of the PVA-ZnO composites are shifted to a lower temperature compared to pure PVA and also decreases systematically with the addition of ZnO. For composite films also the main endothermic peak around 259C – 271C may be attributed to side chain scission reactions as in pure PVA. The decrease in the weight loss with the addition of ZnO may be due to the cross linking of ZnO and polyvinyl alcohol matrix leading to stronger intermolecular bonding [26]. Differential thermal analysis (DTA) Thermal stability of the PVA-ZnO nano composites and pure PVA can also be analyzed from DTA technique. The DTA thermograms are shown in figure 5. Three endothermic events and a major exothermic one were observed in DTA curve of each sample. In pure PVA, the endothermic peak at 118C is related to the removal of the physically adsorbed water and the thermal phase transition of polymer matrix from glassy to rubbery state. The second weak endothermic peak at 222C corresponds to melting of PVA. The decomposition process of PVA explored by TG analysis is also revealed by DTA curves. The peak at 281C is associated with decomposition of the side chain (scission of C-O) of PVA [27]. Finally the large exothermic peak at 367C indicates the beginning of degradation of the polymer. The phase transition and melting temperature obtained by DTA is in good agreement with the values recorded from DSC. CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS S J C INSTITUTE OF TECHNOLOGY”

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Fig 5: DTA thermogram of (a) PVA and (b) 5mol% (C)10mol% (d)15mol% (e) 20mol% PVAZnO composite films The first endothermic peak observed in 100C-110C temperature range in case of nanocomposite films has been shifted to lower temperature compared to pure PVA. This indicates the removal of physically adsorbed water and thermal phase transition taking place at an early stage in nanocomposite films in comparison with PVA. The second endothermic peak corresponding to melting point is in the range 216C - 219C in nano composite films which is nearly same as pure PVA. Small variation in the melting temperature compared to DSC may be due to the processing condition or instrument employed. The third endothermic peak which appears at 281C in pure PVA has been shifted to 260C - 273C temperature range in composite films. This indicates early start of decomposition of composite films related to side chain scission reaction. Final exothermic peak was observed in the temperature range of 460C-496C in the composite films indicating increase in the beginning of complete degradation temperature compared to PVA. The results of DTA are shown in the Table 3. Table 3. Glass transition temperature (Tg), melting temperature(Tm), decomposition temperature (Td1) and complete degradation temperature (Td2)of pure PVA and PVA-ZnO nanocomposites obtained by differential thermal analysis. Sample

PVA PVA-5mol% ZnO PVA-10mol% ZnO PVA-15mol% ZnO PVA-20mol% ZnO

First endothermic peak (Tg-C) 118 108

Second endothermic peak (Tm-C) 223 218

Third endothermic peak(Td1-C) 281 273

Fourth exothermic peak (Td2-C) 367 496

109

216

269

475

107

216

259

465

100

219

260

460

CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS S J C INSTITUTE OF TECHNOLOGY”

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CONCLUSION PVA and PVA-ZnO nanocomposite films were synthesized by solution casting method and their thermal behavior was studied using various techniques such as DSC, TGA and DTA. The melting point of all the samples was nearly the same indicating negligible effect of ZnO on it. The glass transition temperature increases for 5mol% ZnO and then decreases on further addition of the filler. The specific heat capacity also shows a similar trend as a function of concentration of ZnO with all the composite films showing higher heat capacity than that of pure PVA. This indicates an increase in thermal stability of the composites as compared to PVA. The TGA study also confirms the increase in thermal stability of composite films due to low mass loss during different degradation stages. The glass transition temperatures and melting temperatures obtained from DSC and DTA are found to be approximately the same. The final degradation temperature of the composite was noticed to be increased and final residual mass was found to be more compared to pure PVA indicating the overall improvement in the thermal stability of composite films. The maximum thermal stability has been observed in the 15mol% ZnO composite films. ACKNOWLEDGEMENT Author K.S H would like to thank University Grants Commission (UGC), Government of India, for the award of UGC-FIP fellowship and STIC (Cochin) for providing experimental facility. REFERENCE [1] M. Abdelaziz, and E.M. Abdelrazek, Effect of dopant mixture on structural, optical and electron spin resonance properties of polyvinyl alcohol, Physica B., Vol.390(1-2), pp.19,2007. [2] M. Abdelaziz, and Magdy M. Ghannam, Influence of titanium chloride addition on the optical and dielectric properties of PVA films, Physica B, Vol.405(3), pp.958-964, 2010. [3] M. Abdelaziz, Cerium (III) doping effects on optical and thermal properties of PVA films, Physica B., Vol.406(6-7), pp.1300-1307, 2011. [4] F.H. Abd El-Kader, S.A. Gafer, A.F. Basha, S.J. Bannon, and M.A.F. Basha, Thermal and optical properties of gelatin/poly(vinyl alcohol) blends, J Appl Polym Sci., Vol.118(1), pp.413-420, 2010. [5] C. Uma Devi, A.K. Sharma, and V.V.R.N. Rao, Electrical and optical properties of pure and silver nitrate-doped polyvinyl alcohol films, Materials Letters., Vol.56(3), pp.167174, 2002. [6] M. Krumova, D. López, R. Benavente, C. Mijangos, and J.M. Pereña, Effect of crosslinking on the mechanical and thermal properties of poly(vinyl alcohol), Polymer., Vol.41(26), pp.9265–9272,2000. CONFERENCE PAPER PROCEEDINGS OF NATIONAL SEMINAR ON “RESEARCH ASPIRANTS OF NANO MATERIALS AND ITS APPLICATIONS" (NSRANA-2015) 21ST AND 22ND JULY 2015 Organized by “DEPARTMENT OF PHYSICS S J C INSTITUTE OF TECHNOLOGY”

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