Low Temperature Curable Polyaniline/Bio-based polybenzoxazine Coated Cotton Fabric with Superior UV shielding and Oil-water Separation behaviours


 The present work detailsabout the low temperature curingbehaviour of bio-benzoxazine (C-ddm) prepared using cardanol and diaminodiphenylmethane (ddm)in the presence of polyaniline (PANI). Interestingly, it was found that the curing temperature (Tp) of the benzoxazine monomer (C-ddm) has significantly reduced to 227 ºC from 271 ºC with the incorporation of 2wt% of PANI. The plausible curing mechanism of benzoxazine ring catalysed by PANI was also discussed. Further, the resulted PANI/poly(C-ddm) matrices possessa marginally improvedvalues of glass transisition temperature and char yield. Further, PANI/poly(C-ddm) coated cotton fabric was studied for its UV shielding and surface properties. Furthermore, the UV shielding and surface behaviours of PANI/poly(C-ddm) coated cotton fabrics were explored. The PANI/poly(C-ddm) coated cotton fabric delivered UPF rating as high as 50 + and water contact angle value of162°. In addition, the PANI/poly(C-ddm) coated cotton fabric wasverified for its oil-water separation behavior. It was observed that the PANI/poly(C-ddm) coated cotton fabric offered97.5 % separation efficiency with enhanced flux value of about 6450 L/m2h flux.Thus, the obtained results suggest that the PANI/poly(C-ddm) can be used some of the industrial and environmental surface protection applications.

temperature was explored [42]. Commercially available primary amines [38] and amine-capped aniline trimer (quinoid) are chosen as catalyst to cure benzoxazines [43] by different groups. These results suggest that amine group could impart low temperature cure for benzoxazines through their catalytic role.
However, no signi cant attention has been focused on the role of polyaniline (PANI) as catalyst to cure benzoxazines. PANI is well known as conductive polymer with conjugated six membered rings holding a secondary amine (-NH-R-) and imine (-R-N = C-) linkages. Because of its easy synthesis, high conductivity, excellent stability, low-cost and high yield, great attention has been focused on the PANI towards different applications. For the rst time, it has been chosen to catalyse the curing process of benzoxazine monomers. It is expected that, similar to amine and amine capped quinoid moiety, PANI could also impart ring opening at low temperature with lower activation energy. Hence, in the present work the curing and thermal properties ofPANI/poly(C-ddm)has been studied. In addition,thevarying weight percentages ofPANI incorporated cardanol-benzoxazine(C-ddm) wascoated over cotton fabric and studied for their UV shielding and oil-water separation behaviour in detail. The data resulted from different studies are discussed and reported.
Experimental: Materials and methods: Cardanol was purchased from Satya Cashew Products, Chennai. Tamilnadu.Aniline, diaminodiphenylmethane (DDM) and formaldehyde were obtained from SRL Pvt. Ltd. India.Anhydrous sodium sulphate, ethanol, tetrahydrofuran (THF) and ethylacetate were obtained from Loba chemicals, India. Cotton fabric was procured from textile industry in Erode, Tamilnadu, India.

Preparation of Polyaniline (PANI):
Conductive polyaniline was prepared as per the reported literature [44]. 1 mL of freshly distilled aniline monomer was dissolved in 50 mL of 1 M HCl and subjected to sonication for 30 minutes. Subsequently, the homogeneous solution ofammonium per sulphate dissolved in 50 mL of hydrochloric acid (1 M) wasadded slowly and stirred for another 5 h at 0-5 °C in an ice bath. Then, the resulted green product was washed several times with 1 M HCl and dried in a hot air oven at 60 °C.
Preparation of PANI/poly(C-ddm) matrices: Cardanol and DDM benzoxazine (C-ddm) monomer was synthesized through Mannich reaction via a solvent-less method using stoichiometric quantities of (2:1:4) of cardanol, DDM, and formaldehyde as per the reported procedure [33]. Different weight percentages of PANI (0.5, 1.0, 1.5, and 2.0 wt %) were added to solution containing 5.0 g of monomer (C-ddm) dissolved in 15 ml of THF. The resulting mixtures was stirred separately for 12 h at room temperature and then poured into respective silane-coated glass plates. After evaporation of the solvent at room temperature for 8 h, the polymerization was initiated via stepwise thermal curing (120 °C, 160 °C, 200 °C, and 240 °C for 2 h respectively) to obtain the corresponding PANI/poly(C-ddm) matrices.
Preparation of PANI, poly(C-ddm) andPANI/poly(C-ddm) coated cotton fabric: Exactly 1 g of the C-ddm monomer was allowed to dissolve in 10 mL of THF. To the prepared solution, appropriate amount of PANI were added and sonicated for 1h. Subsequently, the pristine cotton was soaked and left for 20 minutes and then dried at 60 °C for 2 h. After complete removal of solvent, the fabric was cured at 180 °C for 5 minutes. For comparison, poly(C-ddm) and PANI were separately coated over cotton fabrics. The coating of PANI over cotton fabric was carried out during the preparation of PANI itself. While preparing PANI, the cotton fabric was soaked with aniline and sonicated in HCl medium before to the addition of oxidant APS. All other procedures are similar to preparation of PANI. The resulted PANI/cotton fabrics show dark green color as a result of coating of polyaniline network, which was washed several times with 1 M HCl and dried in a hot air oven at 60 °C. Characterization: FT-IR spectra were recorded on a Perkin Elmer 6X FT-IR spectrometer using KBr as reference. DSC measurements were recorded using Hitachi DSC7020 under N 2 purge (60 mL min −1 ) at a scanning rate of 10 °C min −1 .Thermogravimetric analysis (TGA) was performed on a Netzsch STA 409 thermo gravimetric analyzer calibrated with calcium oxalate and aluminium. About 50 mg of the samples was heated under a nitrogen atmosphere at a heating rate of 10 °C min -1 . The morphology of the PANI/poly(C-ddm), PANI, poly(C-ddm) and PANI/poly(C-ddm)coatedcotton fabrics were identi ed usingZeiss Sigma Field Emission Scanning Electron Microscope (FESEM). The water contact angle measurements of the PANI, poly(C-dm) and PANI/poly(C-ddm) coated cotton fabrics were conducted on a Data physics instrument (OCA 15, Germany) using a water drop (V = 10 μL) gently placed on the surface of the sample.UV Transmittance Analyser was used to determine UPF of the cotton fabrics in accordance with AS/NZS 4399:1996 standard. Oil-water separation studies was performed using oil-water mixture under gravitational force (i.e., without any force). The oil-water mixture was prepared using equal amount of water and petrol. Further, dichloromethane (DCM) was added to move the petrol layer down and water was once again addedto maintain the volume ratio equivalent to that of DCM. The separation e ciency and ux behaviour of the PANI/poly(C-ddm) coated cotton was calculated using equations 1 and 2 respectively.

Results And Discussion:
Spectral analysis of C-ddm: Figure 1a.and 1b illustrates the 1 H-NMR and 13 C-NMR spectra of the C-ddm monomer. The two singlets from the methylene protons of oxazine rings show signals at δ 4.5 and δ 5.5 ppm in 1 H-NMR spectrum ( Figure 1a). The terminal methyl protons of cardanol moiety appeared at δ 0.9 ppm. The signal at δ 3.9 ppm (Figure 1a) corresponds to the signal of methylene protons of ddm (Ar-CH 2 -Ar). The major signals appeared between δ 1.0 and 2.0 ppm correspond to aliphatic chain protons of cardanol moiety. The multiplet signals appeared around δ 6.5-7.2 correspond to the aryl ring protons. Figure 1b illustrates the 13 C-NMR spectrum of C-dda monomer. Two signals appeared at δ 50 and δ 80 ppm ( Figure 1b [45,46].
Curing behaviour of C-ddm and PANI/C-ddm: Before ascertaining the thermal and coating behaviour of PANI/poly(C-ddm) it is essential to study the curing nature of C-ddm monomer in the presence of PANI. The general ring opening polymerization of benzoxazine monomer is presented in Scheme 1, which normally occursat high temperature. Similarly, the curing temperature (T p ) of C-ddm monomer was observed at 271 ºC ( Figure 2). However, in the presence of PANI ( Figure 2)the polymerization occurs at T p tends to decreases. For example, in the presence of 0.5 and 1.0 wt % of PANI the T p values are observed to be 267 and 258 ºC. The T p value of Cddm further decreases and reaches the 227 ºC ( Figure 2)as the lowest T p in the presence of 2 wt. % of PANI. Addition of succeeding amount of PANI (3, 4 and 5 wt %) also results in T p value similar to that of 2 wt % PANI. This phenomenon inference that the addition of PANI beyond 2 wt. % has no signi cant contribution towards reduction in T p . The plausible mechanism corresponds to the curing of C-ddm with lower energy pathway in the presence of PANI was presented in Scheme 2. Similar to other primary and secondary amines, the PANI with continuous benzene ring linked through a secondary aminesactas a catalyst. The secondary amines nitrogen atoms of polyaniline donates their lone pair of electron to initiate the polymerization through nucleophilic attack and favours rapid formation of zwitter ion intermediate (Scheme 1), which in turn proceeds with proton transfer and assist the polymerization [42]. In the present work, curing temperature of C-ddm in the presence of 2 wt. % PANI was observed to be as low as Tp = 227 ºC, whichis considerablysigni cant in comparison to those of other amine and hydroxyl catalysts [39-41, 43, 47]In order to support the effectiveness of PANI, the curing behaviour of C-ddm in the presence of aniline as equivalent to that of PANI was added and veri ed. The observed DSC thermogram is illustrated in Figure S2. The Tp value observed for C-ddm in the presence of aniline is about 251 ºC, which is not as low as PANI.
Spectral analysis of PANI, poly(C-ddm) and PANI/ poly(C-ddm) : Figure 3represents the FT-IR spectra of PANI, neat poly(C-ddm) and PANI/poly(C-ddm). Generally the benzoxazine moiety shows vibration band around 940 cm -1 .However, after thermal curing thepeak at 940 cm -1 disappears, which infers the ring opening polymerizationof benzoxazine monomer. Thus, the absence of peak at 940 cm -1 in Figure 3 con rms the occurrence of ring opening polymerization of oxazine rings and thereby con rms the formation of poly(C-ddm). Further, the appearance of peaks at 1613 cm -1 and 1502 cm -1 also ascertain the ring opening polymerization, which are corresponding to the vibration bands of trisubstituted and tetrasubstituted benzene rings of poly(C-ddm) respectively. These phenomenon are also observed for PANI/poly(C-ddm) matrices [48,49] in Figure 3, suggesting that the presence of PANI doesn't disturb the curing nature of C-ddm. In addition, the presence of PANI in the PANI/poly(C-ddm) matrices are con rmed through the appearance of bands at 1566 and 1472 cm −1 , which are corresponding to the C=C stretching vibration of the quinonoid and benzenoid rings respectively. In addition, the peak appeared at 1306 cm −1 is attributed to the C-N stretching vibration [50][51][52]. The FTIR spectrum of PANI (Figure 3) also shows a similar vibration band representing the quinonoid and benzenoid rings, which con rms the presence of PANI in poly(C-ddm) matrices. The shifting of N-H band to lower wavenumber suggeststhe existence of inter molecular hydrogen bonding between PANI and poly(C-ddm) matrices.
Morphology of poly(C-ddm) and PANI/ poly(C-ddm) The cross sectional morphology of theneat poly(C-ddm) and PANI/poly(C-ddm) matrices were investigated using FE-SEM analysis. The FESEM image of neat poly(C-ddm) presented in Figure 4a shows smooth surface corresponding to the existence of homogenous phase of cardanol polybenzoxazine. On the other hand, the morphology of PANI/poly(C-ddm) matrices shown in Figure 4b and 4c, possess rough surfaces. The appearances of rough and heterogeneous phases in the matrices con rm the presence of PANI as intercalated network. It is important to note that two different phases are observed in the SEM images with higher concentration of PANI, which are highlighted through a circle. The irregular module corresponds to the intercalated phase of PANI, whereas, the continuous module represents the phases of poly(C-ddm). Comparably, the Figure 4e, provides more irregular intercalated phase, which consequently results low temperature curing to benzoxazines.
Thermal properties: In order to ascertain the thermal stability, thermal behaviour of neat poly(C-ddm) and PANI/poly(C-ddm) matrices were studied using thermo-gravimetric analysis. The resulted thermo-grams are presented in Figure 5 and the temperatures corresponding to different percentages of degradation are also given in Table 1. It was noticed that the PANI incorporated cardanol polybenzoxazines possesses an appreciable improvement in thermal stability. Under the thermal condition, the degradation of both neat poly(C-ddm) and PANI/poly(C-ddm) matrices occurs in two steps. Initial degradation occurs below 350ºC followed by second stage degradation after 550 ºC. However, with respect to the PANI, the thermal degradation is prolonged to high temperatures. For example, 20 % weight loss of 2.0wt % PANI/poly(C-ddm) was found at 399 ºC, whereas that of neat poly(C-ddm) begins earlier at 332 ºC. This phenomenon's may be explained due to the presence of PANI network possessing continuous aromatic backbone (quinonoid and benzenoid rings) and hetero linkage (N-H), that provides stability and sustainability to the poly(Cddm) against thermal energy [53,54]. Consequently, the nal residual char yields (Table 1) of the PANI/poly(C-ddm) were increased as theconcentration of the PANI increases. Thus, 2.0wt % PANI/poly(Cddm) matrices offers highestchar residue of 10.8% at 750 ºC, when compared to that of neat poly(Cddm), which shows only 3.4 %.This con rms the presence of intercalated network of PANI within the poly(C-ddm)and provides appreciable thermal stability.
Further, the DSC thermograms of neat poly(C-ddm) and PANI/poly(C-ddm) matrices were analysed in order to ascertain their molecular chain mobility with respect to temperature [(glass transition temperature (T g )]. Figure 6 (Table 1). This phenomenon could be attributed to the (i) inherent exible network of PANI as well as (ii) intercalation of PANI in between the polybenzoxazine matrices [55].In general, polybenzoxazine are widely used as coating material to avoid corrosion due to their superior hydrophobicity [56,57]. Hence, in the present work to verify the real time utility of PANI/poly(C-ddm) the samples were coated over cotton fabric and subsequently studied for UV shielding and surface properties along with oil-water separation behaviour. Hence, the 2.0 wt% PANI/poly(C-ddm) was used as coating material for cotton fabric and studied for its surface properties and radiation shielding behavior in comparison with neat cotton, PANI, and poly(C-ddm) coated cotton.
Spectral and morphology of treated cotton fabrics: Figure. 7 illustrates the FTIR spectra of the PANI, poly(c-ddm) and PANI/poly(C-ddm) coated cotton fabric in comparison with pristine cotton. In case of pristine cotton, the presence of polar carboxylic acid functional group and C-O-C linkage on the surface gives rise to vibration bands at 1712 cm -1 and 1246 cm -1 respectively. Since the fabric was treated with sodium hydroxide, the primary hydroxyl groups might have been oxidized to the carboxylic group [58]. After coating with PANI, due to the presence of only 2 wt % coating, the respective bands corresponding to quininoid and benzenoid rings of polyaniline usually occurred at 1571 and 1476 cm −1 shows very weak signals. On the other hand, after coating with poly(Cddm), the fabrics no distinct peaks for benzoxazines, which suggest that the ring-opening reactions of oxazine groups [15]. However, the peaks observed at 2917 cm -1 and 2844 cm -1 correspond to the asymmetric and symmetric stretching vibrations of a methylene group (-CH 2 -) respectively of long alkyl side chains of cardanol moiety [31].
The morphology of cotton fabrics coated with poly(C-ddm), PANI and PANI/Poly(C-ddm) obtained from FE-SEM are compared with that of pristine fabric and the images are presented in Figure 8. The pristine fabric delivers smooth surface of cellulose brous before coating (Figure 8a-b). After coating the fabric shows rough surfaces due to the presence of supramolecular assembly of both poly(C-ddm) and PANI. Figure 8c-d shows the presence of new bril morphology at the interfaces of warp and weft cellulose bers, which con rms the coating of poly(C-ddm) over cotton fabric. Figure 8e-f shows the morphology of PANI coated fabric with uniform rough texture, which is due to nucleation and growth of conductive PANI layer. PANI/Poly(C-ddm) coated cotton fabrics contributes to both rough and bril morphology [ Figure  8(g-h)]. The formation of hierarchically rough structured surface is highly desirable to enhance the water repelling behavior [26]. Thus, the rough surfaces formation aided by the coating of PANI/Poly(C-ddm) could create hydrophobic behavior of cotton fabric.
UV Shielding property: The UV-visible transmittance spectra of pristine cotton, poly(C-ddm)/cotton PANI/cotton and PANI/Poly(C-ddm)/cotton fabrics are presented in Figure 9. The mean UPF values and UPF rating observed are also presented in Table 2. The UPF value (10.98) of pristine cotton presented in Table 2 and UV transmittance spectra (Figure 9) possesses the poor UV shielding behavior. However, after coating with PANI and poly(C-ddm) over the cotton fabrics, the UV shielding behavior was enhanced to an appreciable extent(Table2). Thus, the UPF values of PANI and poly(C-ddm) coated cotton fabrics are observed to be 22.7 and 1966 respectively. Further, the cotton fabric with both PANI and poly(C-ddm) delivers UPF value as 2000. The UPF value of poly(C-ddm)/cotton shows about an increase of 179 fold, whereas that of poly(C-ddm)/PANI/cotton shows an enhancement of about 183 fold. These phenomenon suggest that the polybenzoxazines layer present over the cellulosic ber render complete UV protection. In addition to superhydrophobicity, the synergistic effect contributed by both PANI and poly(Cddm) also provides constant protection for both UV-A and UV-B regions (Figure 9). It was well known that the PANI and polybenzoxazines are stable against UV radiations. The presence of three dimensional poly(C-ddm) matrices with continuous carbon network protects the fabric cellulose structure from UV radiations. In addition, the quinonoid and benzenoid rings of PANI also possessthe strong UV absorption behavior due to their C=C skeleton and thereby provides the e cient UV shielding effect. It is reported that the energy of C=C bonds is about 335 kJ which is approximately equal to the energy of UV photons [58]. Thus, synergistic effect of both poly(C-ddm) and polyaniline contributes to the UV shielding behavior, which are in equivalent to those of high cost graphene, metal oxide coated cotton fabric [59][60][61].
Water contact angle: The values of water contact angle (WCA) of the pristine cotton, poly(C-ddm)/cotton PANI/cotton and PANI/poly(C-ddm)/cotton fabrics are presented in Figure 10. The WCA of pristine fabric, poly(Cddm)/cotton, PANI/cotton and PANI/Poly(C-ddm)/cotton fabrics are 0°, 132°, 124°, and 162° respectively. This result infers that the cotton fabrics coated with PANI/poly(C-ddm) contributes to enhanced superhydrophobic behaviour due to the synergistic effect. It is well known that the aliphatic chain moiety could favour hydrophobic behaviour due to their non-polar nature [62]. As discussed from the SEM, the PANI/poly(C-ddm) coated fabric surfaces possesses protuberance and air beneath textured surfaces and in turn stack the water droplet to be sited on the top without contacting the fabric surface [63]. Thus, the reduced interfacial interaction creates the state of Cassie-Baxter attraction between the fabric and water surface and delivers enhanced values of contact angle.
Oil-water separation behaviour of PANI/poly(C-ddm)/cotton: Further, to evaluate the real time utility of PANI/poly(C-ddm)/cotton fabricwas used to separate oil and water. Figure 10a-d illustrates the petrol-water separation behaviour using PANI/poly(C-ddm)/cotton fabrics as membrane. Figure 11a presents the prepared petrol-water mixture, in which due to low density the petrol layer exist above the water layer. In order to bring it down and make contact with the PANI/poly(C-ddm)/cotton fabrics DCM was added. After the addition of DCM, the petrol layer moves down and exist in the bottom of water layer (Figure 11b). Further, upon transferring the petrol-water mixture in DCM to the separating ask (Figure 11c), the petrol dissolved in DCM penetrates through the PANI/poly(C-ddm)/cotton fabrics immediately and leaving behind the water layer. Finally, Figure 11d illustrates separated layers water over the fabric and petrol in DCM in collection ask kept at the bottom. This phenomenon demonstrates the superhydrophobic and oleophilicity nature of PANI/poly(Cddm)/cotton fabric. Further, the separation e ciency and ux behavior of the PANI/poly(C-ddm)/cotton fabrics for subsequent 10 cycles were evaluated using equations 1 and 2. The resulted separation e ciency and ux values are presented in Figure. 12a-b. The values of separation e ciency and ux were observed to be about 97.5 and 6425 L/m 2 /h respectively.

Conclusion
In the present work, an attempt has been made to assess the thermal and surface properties of polyaniline (PANI) incorporated cardanol-diaminodiphenylmethane based polybenzoxazine [poly(c-ddm)] materials. The curing process of cardanol based benzoxazine was reduced to low temperature 227°C from 271 °C, with the incorporation of 2wt % PANI. The curing mechanism with the presence of PANI was also discussed. The incorporation of PANI enhances the value of glass transition temperature and thermal stabilityaccording the weight percentage.Further, the water contact angle studies suggest that the coating of PANI and poly(C-ddm)favours superhydrophobic surface (162°)and good oil-water separation (97.5 %). Superior UV shielding throughout the UVA and UVB region renders excellent UPF rating as 50+. Data resulted from different studies suggestthat the developed PANI incorporated polybenzoxazine matrices coated cotton fabric could be effectively used in some of the industrial and environmental surface protection applications. Figure 9 UV shielding behavior of pristine cotton, poly(C-ddm)/cotton, PANI/cotton and PANI/poly(C-ddm)/cotton fabrics.