Novel chemical synthesis and characterization of copper pyrovanadate nanoparticles and its influence on the flame retardancy of polymeric nanocomposites

In this work, copper pyrovanadate (Cu3V2O7(OH)2(H2O)2) nanoparticles have been synthesized by a simple and rapid chemical precipitation method. Different copper-organic complexes were used to control the size and morphology of products. The morphology and structure of the as-synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectrum, electron dispersive X-ray spectroscopy (EDX), thermal gravimetric analysis (TGA), differential thermal analysis (DTA) and photoluminescence (PL) spectroscopy. The influence of copper pyrovanadate nanostructures on the flame retardancy of the polystyrene, poly vinyl alcohol and cellulose acetate was studied. Dispersed nanoparticles play the role of a magnetic barrier layer, which slows down product volatilization and prevents the flame and oxygen from the sample during decomposition of the polymer. Cu3V2O7(OH)2(H2O)2 is converted to Cu3V2O8 with an endothermic reaction which simultaneously releases water and decrease the temperature of the flame region.

resistance, low density and thermal insulation. However the main disadvantage of polymeric compounds is high flammability. Using of the most traditional and toxic flame retardants like halogenated and aromatic compounds are forbidden with respect to the environmental considerations. Herein, the influence of copper pyrovanadate nanostructures on the flame retardancy of the polymeric matrix nanocomposites was studied.  Table 1. Figure 2a shows XRD pattern of copper pyrovanadate sample prepared with copper-organic complex of [Cu(en) 2 ]SO 4 (sample No.1). All diffraction peaks were indexed to pure Monoclinic phase of Cu 3 (OH) 2 V 2 O 7 .nH 2 O with space group of C2/m and cell constants a = 10.6060 Å, b = 5.8740 Å, and c = 7.2130 Å (JCPDS Card No.80-1169). The crystallite diameter (D c ) of CVO nanostructures has been found to be 45 nm. EDS analysis measurement was employed to investigate the chemical composition and purity of the copper pyrovanadate nanoparticles. EDS analysis of nanoparticles (sample No. 4) is illustrated in Fig. 2b and confirms the presence of Cu, V and O in the sample. According to EDS results, atomic percentage of elements are 4.03% Copper, 15.06% Vanadium and 80.91% Oxygen. Photoluminescence (PL) spectrum of copper pyrovanadate nanoparticles were obtained at room temperature with an excitation wavelength of 375 nm, and is shown in Fig. 2c. The PL spectrum consists of one strong peak at 425 nm. Band gap of as-synthesized sample was obtained to 2.91 eV, which shows blue shift compared with Cu 3 V 2 O 7 (OH) 2 4) are about 0.0117 emug −1 and 137 Oe respectively. The behavior of copper pyrovanadate nanoparticles was changed from paramagnetic to ferromagnetic at fields of lower than 1500 Oe. The as-prepared sample shows a twofold behavior, ferromagnetic behavior in low fields and paramagnetic behavior in high fields.

Results and Discussion
The influence of different copper-organic complexes on the morphology and particle size of copper pyrovanadate samples were investigated by FESEM. Fig. 3a-d show SEM images of copper pyrovanadate samples prepared   2 ]SO 4 complex. It is observed that with increasing steric hindrance of complex, a decrease in particle size occurs. In Fig. 3e, CuSO 4 .5H 2 O was used as copper source and other reaction parameters remained unchanged. The products obtained from CuSO 4 .5H 2 O salt as a blank test lead to larger particles than those obtained from Cu-organic complexes. The bulk and agglomerated Cu 3 V 2 O 7 (OH) 2 (H 2 O) 2 products were synthesized by surfactant-free reactions 7,30,31 .
The amine ligands in Cu-organic complexes, can act as a capping agent and provide steric hindrance. These results show that using [Cu(TEPA)]SO 4 complex lead to synthesis of nano-products. It was observed that with increasing steric hindrance of complex, a decrease in particle size was appeared 32 . Results of particle size of products are shown at Table 1. The precise morphology and particle size of prepared copper pyrovanadate nanostructures with [Cu(TEPA)]SO 4 complex was elucidated by TEM. Figure 3f shows the TEM image of sample No. 4 which contains nanoparticles with size of less than 100 nm. Typical histograms of the particle diameters for the samples obtained using [Cu(en) 2 ]SO 4 and [Cu(TEPA)] SO 4 are compared in Fig. 3g, respectively. By comparing the particle size distribution of the products, Cu 3 V 2 O 7 (OH) 2 (H 2 O) 2 that was prepared using [Cu(TEPA)]SO 4 have smaller particle size distribution (30-70 nm). By using Cu-organic complexes due to the presence of amine ions in reaction medium, pH of reaction was upper than 6. Whereas, Zhang     The effect of inorganic nanostructure on the flame retardant properties of the polymers has been considered using UL-94 test. The outcomes show that CVO additives can enhance the flame retardant property of the polymeric matrices. Ex-situ products were easily obtained and made from two separated phases of copper pyrovanadate and polymeric matrices. The main challenge in the synthesis of nanocomposite is dispersion of inorganic phase in organic matrix. Hydroxyl group in copper pyrovanadate nanoparticles lead to suitable interaction with hydrophilic polymers like vinyl alcohol and cellulose acetate matrices. The results of UL-94 tests for poly styrene and cellulose acetate nanocomposites are NC and V-1 respectively (Fig. 6a-d). The worst result was obtained for PS-CVO because of incompatibility of hydrophobic polystyrene and hydrophilic CVO. The result of UL-94 tests for poly vinyl alcohol nanocomposites is V-0. HO… Cu-V-O-Cu… OH compound and other precursors as binder are compatible with polymeric matrices. The enhancement of flame retardancy of nanocomposite is due to formation of effective barrier layer of Cu-V-O that precludes flame and oxygen reaching to the nanocomposites (Fig. 6e-g).    (Fig. 7). Water vapour also dilutes flammable gases in the fire zone. In the presence of flame, magnetic nanoparticles remain together (show resistance to drop falling) and build a barrier. Cu 3 V 2 O 7 (OH) 2 (H 2 O) 2 is a hydrophilic product that show the best dispersion in water and as a results approve the best compatibility with hydrophilic PVA. Acetone as the second solvent for dispersing nanoparticles and dichloromethane has the worst compatibility with hydroxyls of the nanoparticles and as shown in SEM images agglomerated nanoparticles in the poly styrene matrix was observed.
Molar mass of Cu 3 V 2 O 7 (OH) 2 (H 2 O) 2 is 474.5 g, at the first decomposition, 2H 2 O were evaporated from compound (36 g/474.5 g = 8%). At the second step of decomposition, another OH 2 was evaporated (18 g/474 g.5 = 4%). The experimental results that were achieved from TGA analysis (8% weight loss at 450 °C and 4% weight loss at 550 °C = totally 12%) have suitable agreement with theoretical results for preparation of Cu 3 V 2 O 8 (420.5 g/474.5 g = 88%) as a residual compound (Fig. 7b). Endothermic reaction was investigated and was confirmed by differential thermal analysis (DTA) and is shown in Fig. 7c. The flame retardancy could be a source of the char consisting of mainly Cu 3 V 2 O 8 just blocking the air from reacting with the polymer. Actually flame retardancy in this work is result of synergism of barrier effect of nanostructure, release of water (3H 2 O: cooling the flame region) and endothermic decomposition (absorption the heating of flame zone) 36 . Also because of magnetic property of the nanostructures a compact and dense barrier is produced.

Experimental
Materials and Physical Measurements. CuSO 4 .5H 2 O, NH 4 VO 3 , ethylenediamine (en), propylenediamine (pn), triethylenetetramine (TETA) and tetraethylenepentamine (TEPA) were purchased from Merck Company. All of the chemicals were used as received without further purifications. For characterization of the products, X-ray diffraction (XRD) patterns were recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-filtered Cu Ka radiation. Scanning electron microscopy (SEM) images were obtained on Philips XL-30ESEM. Transmission electron microscopy (TEM) image was obtained on a Philips EM208 transmission electron microscope with an accelerating voltage of 200 kV. Fourier transform infrared (FT-IR) spectra were recorded on Shimadzu Varian 4300 spectrophotometer in KBr pellets. The magnetic properties of the samples were detected at room temperature using a vibrating sample magnetometer (VSM, Meghnatis Kavir Kashan Co., Kashan, Iran). Room temperature photoluminescence (PL) was studied on a Perkin Elmer (LS 55) fluorescence spectrophotometer. Synthesis of Cu-organic complexes. An aqueous solution including 1 mol of CuSO 4 .5H 2 O in 50 mL of distilled water was added to a stoichiometric amount of ethylenediamine in 50 mL of distilled water under magnetic stirring. The mixture was stirred and heated (80 °C) for 5 h. The blue obtained precipitate was centrifuged, washed with ethanol and distilled water and dried at 50 °C. Other complexes were prepared via mentioned method.
Synthesis of pure Cu 3 V2O 7 (OH) 2 (H2O) 2 nanoparticles. First, 0.5 g of Cu-organic complex was dissolved into deionized water and was added to aqueous solution of NH 4 VO 3 with a molar ratio of Cu:V = 3:2. After that, the above solution was heated at 100 °C and stirred for 1-2 h. The black precipitate was dried at 80 °C under vacuum for 2 h. In Scheme. 1, schematic diagram of formation of Cu 3 V 2 O 7 (OH) 2 (H 2 O) 2 nanoparticles is depicted. The preparation conditions for synthesis Cu 3 V 2 O 7 (OH) 2 (H 2 O) 2 nanoparticles have been illustrated in Table 1.
Preparation of ex-situ nanocomposites. 5 g of polymer was dissolved in 10 mL of solvent (25 °C) and then Cu 3 V 2 O 7 (OH) 2 (H 2 O) 2 nanoparticles (1 g) was dispersed in 5 mL of solvent with ultrasonic waves (20 min, 60 W). Next, the dispersion of copper pyrovanadate was added slowly to the polymer solution. Solvent for poly styrene (PS), poly vinyl alcohol(PVA) and cellulose acetate (CA) are dichloromethane, water and acetone respectively. The solution was mixed under stirring for 6 h. For preparation of samples for UL-94 test after stirring, the product was casted on a template with dimension 130 × 13 mm and after about 48 h of solvent evaporation; the nanocomposite was placed in the vacuum oven for another 5 h for removal of residual traces of solvent. The final sheets for the test are 130 × 13 × 1.6 mm in dimension.
In UL-94 test a bar shape specimen of plastic 130 × 13 × 1.6 mm is positioned vertically and held from the top. A Bunsen burner flame is applied to the specimen twice (10 s each). A V-0 classification is given to material that is extinguished in less than 10 s after any flame application, drips of particles allowed as long as they are not inflamed. A V-1 classification is received by a sample with maximum combustion time < 30 s, drips of particles allowed as long as they are not inflamed. The sample is classified V-2 if it satisfies the combustion time criteria of V-1, but flaming drips are allowed. Materials are ranked as N.C. in UL-94 tests when the maximum total flaming time is above 50 s. The sample is classified HB when slow burning on a horizontal specimen; burning rate <76 mm/min [28,29].