Cr2O3 Nanocomposites-Positron Annihilation Spectroscopic Study

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ScienceDirect Materials Today: Proceedings 3 (2016) 3646–3651

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ICMRA 2016

Solution Combustion Synthesis of Cr2O3 Nanoparticles and Derived PVA/Cr2O3 Nanocomposites-Positron Annihilation Spectroscopic Study K.S Prashantha, S.S Maheshb* ,M.B Nanda Prakashc, L.M. Munirathnammac, S.Ningarajuc , H.B.Ravikumarc, R.S Somashekarc and B.M Nagabhushanad a

Department of Physics, New Horizon college of Engineering, Bangalore 560103, Karnataka, India b Department of Physics, Acharya Institute of Technology, Bangalore 560107 Karnataka, India c Department of Studies in Physics, University of Mysore, Manasagangotri, Mysore 570006, Karnataka,India d Department of Chemistry, M S Ramiah Institute of Technology, Bangalore 560064, Karnataka,India

Abstract Solution combustion based nanosize chromium oxide nanoparticles blended PVA/Cr2O3/ NaCl nanocomposite thin films of various weight percentages were synthesized using solvent cast method. The FTIR and SEM results are in confirmation with XRD results indicating the formation of nanocomposites. Positron Life time spectroscopy (PALS) are used for the microstructural characterization in the (PVA)/ Cr2O3 /NaCl polymer nanocomposites. The PALS results show that the free volume size and o-Ps lifetime (τ3) increases up to 4 wt% decreases after 6 wt% The decreased o-Ps lifetime (τ3) indicates the inhibition of o-Ps formation upon incorporation of Chromium Oxide nano particles into PVA matrix © 2016Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International conference on materials research and applications-2016. Keywords: Solution combustion method; Nanocomposites; PALS

1. Introduction Polymer nanocomposites recurrently show physical and chemical properties perilously distinct from conventional composites. Also polymer nanocomposites prepared by dispersion of metal oxide nanoparticles in * Corresponding author. Tel :+919886771045 ; fax: +918023700242. E-mail address: [email protected] 2214-7853 © 2016 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of International conference on materials research and applications-2016.

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pristine polymer matrix present remarkable properties, which notably depend on the size, morphology and distribution of the ensuing nanoparticles [1-5]. The incorporation of nano-fillers (of 0.1-200nm size) can increase the toughness, cross link, and dimensional stability of polymeric materials. Polymers filled with several inorganic compounds such as Zn, Al, Fe, SiO2 and Carbon black etc., have been extensively studied by the many researchers [6-8]. Solution combustion synthesis [9-12] is a versatile, simple and rapid process, which allows effective synthesis of a variety of nanosize materials. This process involves a self-sustained reaction in homogeneous solution of different oxidizers and fuel. Combustion synthesis provides molecular level of mixing and high degree of homogeneity, short reaction time that leads to reduction in crystallization temperature and prevents from segregation during heating; it is easy to control the stoichiometry and crystallite size [13]. Combustion method is a low temperature synthesis technique that offers a unique mechanism via a highly exothermic redox reaction to produce oxides. In our current study ecofriendly, water soluble polymer PVA was chosen as the host polymer and chromium oxide nanoparticles was dispersed in the polymer matrix as additives. The inorganic salt sodium chloride was used as spacer to prevent agglomeration of the nanoparticles and to facilitate uniform dispersion of nanoparticles in the prepared polymer nanocomposites. The interfacial interactions between the host polymer and nanomaterials is accountable for distinguished properties of the polymer nanocomposites through free volume size and distribution[14].Positron Absorption Life Time Spectroscopy furnish information about the free volume size, density and size distribution in the polymer nanocomposites. In the present study, we present the experimental investigations on Poly vinyl alcohol/Chromium oxide nanocomposites by making use of one of the well-established techniques viz., Positron Annihilation Lifetime Spectroscopy (PALS).

2. Experimental 2.1. Materials PVA of molecular weight (~ 60kDa) was obtained from Fischer Scientific India with degree of hydrolysis 85-89 cSt ;Viscosity 35-50 cSt. Chromium Nitrate from NICE chemicals, Kerala, India. Since reagents and chemicals were of analytical grade; they were used without purification. 2.2. Synthesis of Chromium Oxide nanoparticles The starting chemicals used were of analytical grade Chromium nitrate (Cr(NO3)2·6H2O) and oxalyl dihydrazide (C2H6N4O2; ODH). The ODH was used as a fuel, which was prepared in the laboratory. An aqueous solution containing stoichiometric amounts of Chromium nitrate Cr(NO3)2·6H2O and ODH in 2:1 ratio were taken in a Petri dish. Then the Petri dish is introduced into a pre heated muffle furnace maintained at 300 ± 100C. The reaction mixture undergoes thermal dehydration and ignites at one spot with liberation of gaseous products such as oxides of nitrogen and carbon. 2.3. Preparation of Nanocomposite films PVA/NaCl/Cr2O3 Nano composites were prepared by solution casting method. PVA was dissolved in distilled water by heating at 600 C for 16 h under stirring . This was used as stock solution 0 wt% .The blends with varying amounts viz., 2wt%,4wt% 6wt% and 8 wt% were prepared by stirring the composition of the blends for 30 min. The PVA/ Cr2O3/NaCl blend solution was casted on to a cleaned glass plate and dried at room temperature.

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2.4. XRD studies X-ray diffraction pattern were recorded on Rigaku Miniflex II Diffractometer with Ni filtered, CuKα radiation of wavelength 1.542 Å, and a graphite monochromator. The scattered beam from the sample was focused on to a detector. The specifications used for the recording were 30 kV and 15 mA. The sample was scanned in the 2θ range of 80 to 800 with a scanning step of 0.020. 2.5. FTIR Measurements The Fourier Transformation Infrared Spectroscopic measurements were performed on Perkin Elmer version 10.03.09 FTIR instrument with a resolution of 4 cm-1 over the spectral range of 4000 – 400 cm-1 in absorbance mode. 2.6. Scanning Electron Microscopy The Surface morphology of the prepared nanocomposites films were studied by Scanning Electron Microscopy. The SEM of blend films with different weight percentages were recorded by Hitachi Scanning Electron Microscope SU3500, Japan at 15 kV. 2.7. Positron annihilation lifetime measurements The Positron annihilation lifetime spectra of the prepared polymer nanocomposites were recorded using positron lifetime spectrometer. The o-Ps lifetime (τ3) is related to the free volume hole size by a simple relation [15], which was developed on the basis of theoretical models originally proposed by Tao for molecular liquids and later by Eldrup et al [16]. 3. Results and Discussion 3.1. XRD Studies The strongest XRD peaks as shown in Fig.1. (a) were detected corresponding to Bragg angles 24.070, 33.250 ,36.050 ,41.260 ,49.930,54.630,63.320,64.740,72.480 ,76.450 respectively. All the diffraction peaks were readily indexed to the rhombohedral phase [space group: R-3c (167)] of Cr2O3 (JCPDS card No. 84-312) with a = 4.95 Å and c = 13.56 Å. The diffracted patterns are well matched with the literature. The crystallite size was calculated by Scherrer’s formula and is found to be approximately 20-40 nm. The XRD pattern does not show impurity peaks of other unstable phase indicating the chromium oxide nanoparticles formed are of high purity. The XRD patterns of the prepared composite films indicate the shifting of the XRD peaks, towards the lower angles demonstrating the intercalated structure due to the formation of nanocomposites [17, 18]. 3.2. FTIR Fig.1. (b) FTIR spectrum indicates absorption bands at 568,635, 1633, 2923 and 3428, cm-1 for chromium oxide nanoparticles. FTIR spectrum of nanocomposites show that the characteristic absorption bands of 568 cm-1 and 635 cm-1 which corresponds to metal –oxygen stretching vibration shifts to 535cm-1 and 759cm-1 respectively. Similarly, it is observed from the Fig (3) the presence of new bands at 2464 and 2968 cm-1. The band at 2923 cm-1 corresponds to stretching vibration of C-H bonds, shifting of bands and new bands are attributed to formation of nanocomposites [19, 20].

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b

Fig. 1. (a) X-ray diffraction pattern of as-prepared Chromium oxide nanoparticles and Polymer nanocomposites; (b) FTIR spectrum of as prepared Chromium oxide nanoparticles and Polymer nanocomposites

3.3. SEM The SEM images are shown in (Fig. 2) (a) exhibit non spherical morphology for Cr2O3 nanoparticles. The SEM micrographs of nanocomposites of 2 wt%, 4wt%, 6wt% and 8 wt% are shown in Figure (b)(c)(d) &(e) whereas Fig 2 (f) shows the distribution at a higher magnification.

Fig. 2. SEM images of as prepared Chromium Oxide nanoparticles and of the PVA blended with Cr2O3 nanoparticles and NaCl with various weight percentages (a) as prepared Cr2O3 nanoparticles (b)2 wt% (c) 4 wt% (d) 6 wt% (e) 8 wt% (f) 2 wt% at higher magnification.

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3.4. Positron Annihilation Lifetime Spectroscopy PALS measurements revealed that the free volume properties are strongly affected by the amount and type of filler, the free volume size dramatically increases by increasing the filler content [21]. Fig.3. shows the plots of o-Ps lifetime (τ3), free volume (Vf). The values of o-Ps lifetime (τ3) for as received PVA sample are 1.50 ns and From the Fig.3. it is observed that the o-Ps lifetime (τ3) increases gradually as a function of Cr2O3 nanoparticles concentration and show about 360ps increase from1.50 ns to1.86ns at 4wt% of Cr2O3 nanoparticles. This indicates the increase of free volume hole size (Vf) from 54.47Å3 to 84.66 Å3 at 4 wt% of Cr2O3 nanoparticles. And then the oPs lifetime (τ3) decreases gradually as a function of Cr2O3 nanoparticles upto 8wt% which show about 92ps decrease from1.860 ns to1.768ns. This indicates the decrease of free volume hole size (Vf) from 84.66Å3 to 76.54 Å3 at 8 wt% of Cr2O3 nanoparticles. The possible explanation for this increase in free volume hole size (Vf) is as follows: in amorphous polymers, oPs are preferentially formed and localized within the free volume holes; however in semi crystalline polymers, o-Ps may be formed within the interfacial free volumes, at vacancy type defects, at the crystalline amorphous interface region [22-28] and polymer-inorganic particles interface [29, 30]. As the PALS parameters are average values, taking into the account of different dimensions of holes related to the phases and interfaces present in the materials the increase of free volume size up to 4 wt% in the PVA/ Cr2O3 nanocomposite is attributed to the formation of interface between PVA and Cr2O3 nanoparticles. The decrease in o-Ps lifetime after 6 wt% of Cr2O3 in PVA/ Cr2O3 nanocomposite is due to the effect of quenching. The formation of Cr2O3 nanoclusters during the addition of higher amount of Cr2O3 nanoparticles may quench the o-Ps lifetime [30].

Fig 3. Plots of o-Ps lifetime (τ3) and free volume (Vf) as a function of filler

4. Conclusions Using a low temperature solution combustion method the Cr2O3 nanoparticles were synthesized and PVA/ Cr2O3/NaCl blend films of different weight percentages were prepared using solvent casting method. XRD analysis confirms the formation of the Cr2O3 nanoparticles. The XRD confirms the crystal structure and of the formation of polymer nanocomposites. The SEM of Cr2O3 nanoparticles shows the non-spherical agglomerated particles. The FTIR and SEM results are in concurrence with XRD results which conclude the formation of PVA/ Cr2O3 nanocompositesThe addition of Cr2O3 nanoparticles creates the interfacial interaction between the surface of nanoparticles and polymer matrix and hence increases the o-Ps lifetime in (PVA/ Cr2O3) nanocomposites.

Acknowledgements The authors thank Department of Chemistry, MSRIT, Bangalore and Department of Physics, University of Mysore for providing the laboratory facilities, SAIF IITM for Characterization of the samples.

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