<abstract xmlns="http://www.w3.org/1999/xhtml"><p>We report the morphological and electrical study of a composite of polyvinyl alcohol (PVA) and nanotitanium dioxide (TiO<sub>2</sub>-50 nm) in conducting polymer polyaniline (PANI). The composite was synthesized using in-situ polymerization technique. The composite was characterized in terms of morphology and electrical properties using scanning electron microscopy and DC electrical conductivity (𝜎<sub>dc</sub>). We observed that the DC electrical conductivity of the composite film increased with increasing the loading of nanocomposite material from 20 % to 40 % into PVA stabilizer. The DC conductivity results showed that the molecular chain contribution of the nanocomposite material (nano-TiO<sub>2</sub>+ PANI) was the prominent carrier in the composite film made of the nanocomposite and PVA stabilizer.</p></abstract>

We report the morphological and electrical study of a composite of polyvinyl alcohol (PVA) and nanotitanium dioxide (TiO2-50 nm) in conducting polymer polyaniline (PANI). The composite was synthesized using in-situ polymerization technique. The composite was characterized in terms of morphology and electrical properties using scanning electron microscopy and DC electrical conductivity (𝜎dc). We observed that the DC electrical conductivity of the composite film increased with increasing the loading of nanocomposite material from 20 % to 40 % into PVA stabilizer. The DC conductivity results showed that the molecular chain contribution of the nanocomposite material (nano-TiO2+ PANI) was the prominent carrier in the composite film made of the nanocomposite and PVA stabilizer.


<div xmlns="http://www.w3.org/1999/xhtml"><sec id="j_msp-2016-0096_s_001_w2aab3b8d361b1b7b1ab1aAa"><label>1</label><div>Introduction</div><p>Conducting polymers have emerged as a separate class of materials with the development of new technologies and their applications in various fields. Among a number of conducting polymers, polyaniline (PANI) shows remarkable properties and thereby corresponding applications in sensors [<a ref-type="bibr" href="#j_msp-2016-0096_ref_002_w2aab3b8d361b1b7b1ab2ab2Aa">1</a>–<a ref-type="bibr" href="#j_msp-2016-0096_ref_005_w2aab3b8d361b1b7b1ab2ab5Aa">3</a>], solar cells <a ref-type="bibr" href="#j_msp-2016-0096_ref_004_w2aab3b8d361b1b7b1ab2ab4Aa">4</a>, corrosion coatings <a ref-type="bibr" href="#j_msp-2016-0096_ref_005_w2aab3b8d361b1b7b1ab2ab5Aa">5</a>, electronic devices <a ref-type="bibr" href="#j_msp-2016-0096_ref_006_w2aab3b8d361b1b7b1ab2ab6Aa">6</a>, field effect transistors <a ref-type="bibr" href="#j_msp-2016-0096_ref_007_w2aab3b8d361b1b7b1ab2ab7Aa">7</a>, electromagnetic shielding <a ref-type="bibr" href="#j_msp-2016-0096_ref_008_w2aab3b8d361b1b7b1ab2ab8Aa">8</a>, etc. To achieve high electrical conductivities, polyaniline can be doped with protonic acids [<a ref-type="bibr" href="#j_msp-2016-0096_ref_009_w2aab3b8d361b1b7b1ab2ab9Aa">9</a>–<a ref-type="bibr" href="#j_msp-2016-0096_ref_011_w2aab3b8d361b1b7b1ab2ac11Aa">11</a>]. But simultaneously, other factors, like its processing [<a ref-type="bibr" href="#j_msp-2016-0096_ref_012_w2aab3b8d361b1b7b1ab2ac12Aa">12</a>–<a ref-type="bibr" href="#j_msp-2016-0096_ref_014_w2aab3b8d361b1b7b1ab2ac14Aa">14</a>] and lack of solubility in common solvents, restrict its applications. The compatibility of PANI with polyvinyl alcohol (PVA) [<a ref-type="bibr" href="#j_msp-2016-0096_ref_015_w2aab3b8d361b1b7b1ab2ac15Aa">15</a>–<a ref-type="bibr" href="#j_msp-2016-0096_ref_017_w2aab3b8d361b1b7b1ab2ac17Aa">17</a>], results from molecular interactions <a ref-type="bibr" href="#j_msp-2016-0096_ref_017_w2aab3b8d361b1b7b1ab2ac17Aa">17</a> which lead to a change in its specific properties associated with solubility <a ref-type="bibr" href="#j_msp-2016-0096_ref_018_w2aab3b8d361b1b7b1ab2ac18Aa">18</a>, electrical conductivity [<a ref-type="bibr" href="#j_msp-2016-0096_ref_015_w2aab3b8d361b1b7b1ab2ac15Aa">15</a>, <a ref-type="bibr" href="#j_msp-2016-0096_ref_019_w2aab3b8d361b1b7b1ab2ac19Aa">19</a>], and mechanical properties <a ref-type="bibr" href="#j_msp-2016-0096_ref_020_w2aab3b8d361b1b7b1ab2ac20Aa">20</a>. PVA is soluble both in hot and cold water, which makes it a suitable material for the synthesis with PANI [<a ref-type="bibr" href="#j_msp-2016-0096_ref_021_w2aab3b8d361b1b7b1ab2ac21Aa">21</a>, <a ref-type="bibr" href="#j_msp-2016-0096_ref_022_w2aab3b8d361b1b7b1ab2ac22Aa">22</a>]. PVA has several advantages that make it an appropriate choice to be used for developing a composite which distinguish itself from other composites like PMMA <a ref-type="bibr" href="#j_msp-2016-0096_ref_023_w2aab3b8d361b1b7b1ab2ac23Aa">23</a> and polyethylene oxide <a ref-type="bibr" href="#j_msp-2016-0096_ref_024_w2aab3b8d361b1b7b1ab2ac24Aa">24</a>.</p><p>In the present study, we have synthesized the nanocomposite of nano-TiO<sub>2</sub> and PANI which then was loaded in polyvinyl alcohol matrix. We have studied different weight percentage loading ratios, i.e. from 20 wt.% to 40 wt.% by the morphological and electrical analysis.</p></sec><sec id="j_msp-2016-0096_s_002_w2aab3b8d361b1b7b1ab1b1Aa"><label>2</label><div>Experimental</div><sec id="j_msp-2016-0096_s_002_s_001_w2aab3b8d361b1b7b1ab1b1b2Aa"><label>2.1</label><div>Materials</div><p>The chemicals: aniline (Emerck), polyvinyl alcohol (Emerck), titanium dioxide 50 nm (SRL), HCl (Emerck), ammonium peroxodisulfate (Emerck) used in the processing of the composite were of analytical grade.</p></sec><sec id="j_msp-2016-0096_s_002_s_002_w2aab3b8d361b1b7b1ab1b1b3Aa"><label>2.2</label><div>Synthesis of composite</div><p>The synthesis of nanocomposite material (TiO<sub>2</sub>/PANI) was carried out in a 250 mL round-bottom flask equipped with a stirring assembly. First, 1 g of TiO<sub>2</sub> nanoparticles (size of 50 nm) were dispersed in 50 mL of 1N HCl solution under ultrasonic vibrations to reduce the aggregation of TiO<sub>2</sub> nanoparticles and then 2 mL aniline were added dropwise into the solution under constant stirring. After one hour, a solution of 4.9984 g of ammonium peroxodisulphate ((NH<sub>4</sub>)<sub>2</sub>S<sub>2</sub>O<sub>8</sub>) and 1N HCl were added dropwise into the aniline and TiO<sub>2</sub> solution. Polymerization was carried out at a temperature of 0 °C to 5 °C with controlled stirring for 5 h. Dark green TiO<sub>2</sub>/PANI composite was then filtered and washed with distilled water several times. So, the core shell nanocomposite of (TiO<sub>2</sub>/PANI) was synthesized without any surfactant. The PVA solution was prepared by dissolving PVA powder in 300 mL of distilled water at temperature of 80 °C to 90 °C under continuous stirring for 3 hours. Core shell nanocomposite (TiO<sub>2</sub>/PANI) was then added into the PVA solution at different weight percentage ratios 20 wt.% to 40 wt.% at room temperature and the mixture was continuously stirred for 10 hours. The blended solution was allowed to dry in a Petri dish under ambient temperature for four to five days. The obtained sheets of the nanocomposite materials were used in such form for characterization.</p></sec><sec id="j_msp-2016-0096_s_002_s_003_w2aab3b8d361b1b7b1ab1b1b4Aa"><label>2.3</label><div>Characterization</div><p>The morphology of the nanocomposite materials was studied using a scanning electron microscope (SEM Model: 6610 LV). The DC electrical conductivity of the composite film was investigated for different compositions of nanocomposite (TiO<sub>2</sub>/PANI) materials with a Keithley 6487 unit. For electrical study silver was used as electrode material. All measurements were performsp at room temperature.</p></sec></sec><sec id="j_msp-2016-0096_s_003_w2aab3b8d361b1b7b1ab1b2Aa"><label>3</label><div>Results and discussion</div><p><a ref-type="fig" href="#j_msp-2016-0096_fig_001_w2aab3b8d361b1b7b1ab1b2b3Aa">Fig. 1</a> shows the SEM micrographs (magnification of × 1000 to × 3000 at 5 kV) for the core-shell TiO<sub>2</sub>/PANI nanocomposite material. The SEM micrographs show that the TiO<sub>2</sub> nanopar-ticles are fully covered with polyaniline. This could be due to the formation of the core shell structure of TiO<sub>2</sub>/PANI i.e. the formation of PANI shell on the surface of TiO<sub>2</sub> nanoparticles which caused repulsion forces between the nanoparticles and prevented their agglomeration [<a ref-type="bibr" href="#j_msp-2016-0096_ref_025_w2aab3b8d361b1b7b1ab2ac25Aa">25</a>, <a ref-type="bibr" href="#j_msp-2016-0096_ref_026_w2aab3b8d361b1b7b1ab2ac26Aa">26</a>]<a ref-type="fig" href="#j_msp-2016-0096_fig_002_w2aab3b8d361b1b7b1ab1b2b4Aa">Fig. 2</a>shows the SEM micrographs of a 40 wt.% TiO<sub>2</sub>/PANI nanocomposite loaded to PVA.The micrographs reveal that on addition of core-shell TiO<sub>2</sub>/PANI nanocomposite to PVA, a homogeneously distributed fibrous type structure has been obtained. PVA as stabilizer helped also in developing this fibrous structure due to cross linking of PANI and PVA [<a ref-type="bibr" href="#j_msp-2016-0096_ref_027_w2aab3b8d361b1b7b1ab2ac27Aa">27</a>]. Similar results were also reported by Somani et al. [<a ref-type="bibr" href="#j_msp-2016-0096_ref_028_w2aab3b8d361b1b7b1ab2ac28Aa">28</a>] for the surface of TiO<sub>2</sub> when PANI content was high.</p><figure id="j_msp-2016-0096_fig_001_w2aab3b8d361b1b7b1ab1b2b3Aa" position="float" fig-type="figure"><h2></h2><figCaption><p>SEM micrographs of core-shell TiO<sub>2</sub>/PANI nanocomposite at two magnifications.</p></figCaption><img xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j_msp-2016-0096_fig_001.jpg" src="https://sciendo-parsed.s3.eu-central-1.amazonaws.com/647250b8215d2f6c89dc3e1a/j_msp-2016-0096_fig_001.jpg?X-Amz-Algorithm=AWS4-HMAC-SHA256&amp;X-Amz-Date=20240723T053848Z&amp;X-Amz-SignedHeaders=host&amp;X-Amz-Expires=18000&amp;X-Amz-Credential=AKIA6AP2G7AKP25APDM2%2F20240723%2Feu-central-1%2Fs3%2Faws4_request&amp;X-Amz-Signature=349a467d2c9f998da89573127fb9364ff5a5bdf20e17e382a38583585db7d589" class="mw-100"/></figure><figure id="j_msp-2016-0096_fig_002_w2aab3b8d361b1b7b1ab1b2b4Aa" position="float" fig-type="figure"><h2></h2><figCaption><p>SEM micrographs of core-shell TiO<sub>2</sub>/PANI nanocomposite loaded in PVA stablizer</p></figCaption><img xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j_msp-2016-0096_fig_002.jpg" src="https://sciendo-parsed.s3.eu-central-1.amazonaws.com/647250b8215d2f6c89dc3e1a/j_msp-2016-0096_fig_002.jpg?X-Amz-Algorithm=AWS4-HMAC-SHA256&amp;X-Amz-Date=20240723T053848Z&amp;X-Amz-SignedHeaders=host&amp;X-Amz-Expires=17999&amp;X-Amz-Credential=AKIA6AP2G7AKP25APDM2%2F20240723%2Feu-central-1%2Fs3%2Faws4_request&amp;X-Amz-Signature=d3dfa220aab97aa6e7dc1e3cd226ce1d6d0c9461a3ac2560cdbfe832dacd4f32" class="mw-100"/></figure><p>The DC electrical conductivity of the composite film for different compositions of nanocomposite (TiO<sub>2</sub>/PANI) materials loaded in PVA materials was investigated with a Keithley 6487 unit in which probe distance was maintained at 1 mm and film thickness was 0.745 mm. <a ref-type="fig" href="#j_msp-2016-0096_fig_003_w2aab3b8d361b1b7b1ab1b2b6Aa">Fig. 3</a> shows the variation of DC electrical conductivity (log𝜎) with increasing doping concentration of core-shell TiO<sub>2</sub>/PANI nanocomposite to PVA stabilizer at room temperature in the voltage range of 10 V to 100 V. The variation of conductivity over this voltage range clearly shows that with the increase of core-shell TiO<sub>2</sub>/PANI nanocomposite loading to PVA, the conductivity increases with increasing voltage. It can be observed that at room temperature the DC conductivity of core-shell TiO<sub>2</sub>/PANI nanocomposite loaded PVA varies from 1.54 × 10<sup>-12</sup> S/cm to 3.21 × 10<sup>-7</sup> S/cm with increasing content of the nanocomposite from 20 wt.% to 40 wt.%. This increase in conductivity may be attributed to the increased amount of TiO<sub>2</sub>/PANI which maximizes the number of carriers. The highest number of carriers can further be attained by the delocalization effect of doping process and the formation of polarons or bipolarons in the composite structure, thereby increasing the conductivity of nanocomposite [<a ref-type="bibr" href="#j_msp-2016-0096_ref_029_w2aab3b8d361b1b7b1ab2ac29Aa">29</a>–<a ref-type="bibr" href="#j_msp-2016-0096_ref_031_w2aab3b8d361b1b7b1ab2ac31Aa">31</a>]. The increase in conductivity may also be explained on the basis of good correlation (discussed later) between PANI and TiO<sub>2</sub> [<a ref-type="bibr" href="#j_msp-2016-0096_ref_034_w2aab3b8d361b1b7b1ab2ac34Aa">34</a>] and thereby lower resistance and faster response which can be offered by the nanocomposite (TiO<sub>2</sub>/PANI) for the transport of electrons between conduction band of TiO<sub>2</sub> to lower the orbitals of polyaniline [<a ref-type="bibr" href="#j_msp-2016-0096_ref_035_w2aab3b8d361b1b7b1ab2ac35Aa">35</a>].</p><figure id="j_msp-2016-0096_fig_003_w2aab3b8d361b1b7b1ab1b2b6Aa" position="float" fig-type="figure"><h2></h2><figCaption><p>Variation of log𝜎 with voltage for different composite film.</p></figCaption><img xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j_msp-2016-0096_fig_003.jpg" src="https://sciendo-parsed.s3.eu-central-1.amazonaws.com/647250b8215d2f6c89dc3e1a/j_msp-2016-0096_fig_003.jpg?X-Amz-Algorithm=AWS4-HMAC-SHA256&amp;X-Amz-Date=20240723T053848Z&amp;X-Amz-SignedHeaders=host&amp;X-Amz-Expires=18000&amp;X-Amz-Credential=AKIA6AP2G7AKP25APDM2%2F20240723%2Feu-central-1%2Fs3%2Faws4_request&amp;X-Amz-Signature=d2cc23c387e4c281c315b92be98e3bf69964a51af69da5a287961795a7cecea1" class="mw-100"/></figure><p><a ref-type="fig" href="#j_msp-2016-0096_fig_004_w2aab3b8d361b1b7b1ab1b2b9Aa">Fig. 4</a> shows a comparison of our results for DC conductivity (𝜎) of core-shell TiO<sub>2</sub>/PANI nanocomposite loaded in PVA from 20 wt.% to 40 wt.% with (i) PANI-HCl loaded in PVA stabilizer where PANI volume percentage varies from 20 % to 40 % [<a ref-type="bibr" href="#j_msp-2016-0096_ref_033_w2aab3b8d361b1b7b1ab2ac33Aa">33</a>] and (ii) PANI-SnO<sub>2</sub> hybrid nanocomposite where SnO<sub>2</sub> nanoparticles increases from 20 % to 40 % [<a ref-type="bibr" href="#j_msp-2016-0096_ref_032_w2aab3b8d361b1b7b1ab2ac32Aa">32</a>]. We have further compared the correlation coefficient from our results with PANI-PVA [<a ref-type="bibr" href="#j_msp-2016-0096_ref_033_w2aab3b8d361b1b7b1ab2ac33Aa">33</a>], PANI-SnO<sub>2</sub> nanohybrid material [<a ref-type="bibr" href="#j_msp-2016-0096_ref_032_w2aab3b8d361b1b7b1ab2ac32Aa">32</a>] and nano-TiO<sub>2</sub>+ PANI [<a ref-type="bibr" href="#j_msp-2016-0096_ref_036_w2aab3b8d361b1b7b1ab2ac36Aa">36</a>]. The values of the correlation coefficients have been given in <a ref-type="table" href="#j_msp-2016-0096_tab_001_w2aab3b8d361b1b7b1ab1b2b8Ab2">Table 1</a>. The correlation coefficient shows that the synthesized PANI was deposited on the surface of nano-TiO<sub>2</sub> material and formsp encapsulated structure [<a ref-type="bibr" href="#j_msp-2016-0096_ref_036_w2aab3b8d361b1b7b1ab2ac36Aa">36</a>]. This encapsulated nanocomposite material (nano-TiO<sub>2</sub>+ PANI) when loaded to PVA stabilizer made a conductive nanocomposite which showed a good correlation coefficient, i.e. 0.997, and forecasted correlation coefficient of 0.999 (<a ref-type="table" href="#j_msp-2016-0096_tab_001_w2aab3b8d361b1b7b1ab1b2b8Ab2">Table 1</a>). The DC conductivity of PANI/PCC [<a ref-type="bibr" href="#j_msp-2016-0096_ref_035_w2aab3b8d361b1b7b1ab2ac35Aa">35</a>] was 1.03 × 10<sup>-5</sup> S/cm for anticorrosive performance on mild steel surfaces. Our results of DC conductivity are also in agreement with the reported ones and the reported nanocomposite loaded in PVA matrix might be explored for anticorrosive applications on steel surfaces.</p><table-wrap position="float" orientation="portrait" id="j_msp-2016-0096_tab_001_w2aab3b8d361b1b7b1ab1b2b8Ab2"><label>Table 1</label><caption><p>Comparison of correlation coefficient for core-shell TiO<sub>2</sub>/PANI nanocomposite loaded to PVA (this work) with PANI and PVA [<a ref-type="bibr" href="#j_msp-2016-0096_ref_033_w2aab3b8d361b1b7b1ab2ac33Aa">33</a>], PANI-SnO<sub>2</sub> nanohybrid material [<a ref-type="bibr" href="#j_msp-2016-0096_ref_032_w2aab3b8d361b1b7b1ab2ac32Aa">32</a>], nano-TiO<sub>2</sub>+ PANI [<a ref-type="bibr" href="#j_msp-2016-0096_ref_036_w2aab3b8d361b1b7b1ab2ac36Aa">36</a>].</p></caption><table rules="groups"><thead><tr><th align="center">Sample</th><th align="center">Correlation coefficient</th><th align="center">Forecasted correlation coefficient</th></tr></thead><tbody><tr><td align="center">This work</td><td align="center">0.997</td><td align="center">0.999</td></tr><tr><td align="center">[<a ref-type="bibr" href="#j_msp-2016-0096_ref_033_w2aab3b8d361b1b7b1ab2ac33Aa">33</a>]</td><td align="center">0.914</td><td align="center">0.914</td></tr><tr><td align="center">[<a ref-type="bibr" href="#j_msp-2016-0096_ref_004_w2aab3b8d361b1b7b1ab2ab4Aa">4</a>]</td><td align="center">0.999</td><td align="center">0.999</td></tr><tr><td align="center">[<a ref-type="bibr" href="#j_msp-2016-0096_ref_003_w2aab3b8d361b1b7b1ab2ab3Aa">3</a>]</td><td align="center">0.99998</td><td align="center">0.99998</td></tr></tbody></table></table-wrap><figure id="j_msp-2016-0096_fig_004_w2aab3b8d361b1b7b1ab1b2b9Aa" position="float" fig-type="figure"><h2></h2><figCaption><p>Comparison of DC conductivities of different composite materials.</p></figCaption><img xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/j_msp-2016-0096_fig_004.jpg" src="https://sciendo-parsed.s3.eu-central-1.amazonaws.com/647250b8215d2f6c89dc3e1a/j_msp-2016-0096_fig_004.jpg?X-Amz-Algorithm=AWS4-HMAC-SHA256&amp;X-Amz-Date=20240723T053848Z&amp;X-Amz-SignedHeaders=host&amp;X-Amz-Expires=18000&amp;X-Amz-Credential=AKIA6AP2G7AKP25APDM2%2F20240723%2Feu-central-1%2Fs3%2Faws4_request&amp;X-Amz-Signature=01f130ded402bf08bd09b49b89de84e81a5e26124f56fdf71d1d6d1a428d37de" class="mw-100"/></figure></sec><sec id="j_msp-2016-0096_s_004_w2aab3b8d361b1b7b1ab1b3Aa"><label>4</label><div>Conclusions</div><p>We have synthesized the core-shell nanocomposite of nanotitanium dioxide in polyaniline. The flexible and conductive films have been prepared with blending of different percentages of core-shell TiO<sub>2</sub>/PANI composite in water soluble polymer PVA. We have observed regular shape and fibrous structure of PANI-TiO<sub>2</sub>-PVA composite film. The DC electrical conductivity of this composite material was found to increase from 1.54 × 10<sup>-12</sup> S/cm to 3.21 × 10<sup>-7</sup> S/cm with increasing amount of the nanocomposite (from 20 wt.% to 40 wt.%). Looking at the electrical results and comparing them with other materials of similar kind, these compositions may be explored for electrical, electronics-related and anticorrosive applications.</p></sec></div>

TiO<sub>2</sub>/PANI nanocomposite loaded in PVA for anticorrosive applications

Department of Mechanical Engineering, Invertis University, Bareilly,

Middle East College, Knowledge Oasis Muscat, Muscat,

School of Chemical Technology, G.G.S. Indraprastha University, New Delhi,

Department of Physics and Materials Science, Jaypee University of Information Technology, Waknaghat, Solan, H.P. (173234),

Materials Science-Poland

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

TiO/PANI nanocomposite loaded in PVA for anticorrosive applications

We report the morphological and electrical study of a composite of polyvinyl alcohol (PVA) and nanotitanium dioxide (TiO2-50 nm) in conducting polymer...

TiO₂/PANI nanocomposite loaded in PVA for anticorrosive applications

Publié en ligne: 17 nov. 2016

Pages: 721 - 725

Reçu: 15 déc. 2015

Accepté: 06 août 2016

DOI: https://doi.org/10.1515/msp-2016-0096

Mots clés
DC conductivity, nanocomposite, in situ polymerization, corrosion, PVA

© Wroclaw University of Technology

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

TiO2/PANI nanocomposite loaded in PVA for anticorrosive applications

Publié en ligne: 17 nov. 2016

Pages: 721 - 725

Reçu: 15 déc. 2015

Accepté: 06 août 2016

DOI: https://doi.org/10.1515/msp-2016-0096

Mots clésDC conductivity, nanocomposite, in situ polymerization, corrosion, PVA

© Wroclaw University of Technology

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License.

TiO₂/PANI nanocomposite loaded in PVA for anticorrosive applications

Mots clés
DC conductivity, nanocomposite, in situ polymerization, corrosion, PVA