Synthesis and characterization of functionalized polyvinylidene fluoride ( <scp>PVDF)</scp> and the high temperature catalytic activity of <scp> PVDF‐ g ‐MAH </scp> / <scp> V 2 O 5 </scp> nanocomposite toward transesterification reaction

Functionalization of polyvinylidene fluoride (PVDF) was carried out by two different methodology namely, aza‐Michael addition and free radical reaction from the dehydrofluorinated PVDF. Functionalization agents like 2MI, CR, and indole were grafted onto KOH treated PVDF via aza‐Michael addition reaction whereas MAH and MI were grafted onto the same via free radical reaction. The functionalized PVDF was characterized by FTIR, DSC, TGA, WCA measurement, SEM, and EDX. Further, the MAH grafted PVDF was doped with V2O5 nanoparticles to form a catalytic membrane. Transesterification reaction (TER) was done using the PVDF‐g‐MAH/V2O5 membrane but ended with negative results due to the low residential time. Hence, it was done in a conventional technique in the presence of pieces of membrane material as a template. The product with the red shift in the absorbance spectrum confirmed the TER.

Keywords: characterization; composite membrane; electron microscopy; synthesis; thermal properties

• Abbreviations
• CA cinnamic acid
• CR Congo red
• DCP dicumyl peroxide
• DSC differential scanning calorimetry
• EDX energy dispersive spectrum
• EY eosin Y
• FRR free radical reaction
• FTIR Fourier transform infrared
• GMA glycidyl methacrylate
• KOH potassium hydroxide
• MAH maleic anhydride
• MMA methyl methacrylate
• MO metaloxide
• NaBH 4 sodium borohydride
• NIR near infrared
• PAA poly(acrylic acid)
• PEI polyethyleneimine
• PEO polyethyleneoxide
• PI polyimide
• PSF polysulfone
• PVA poly(vinyl alcohol)
• PVDF polyvinylidene fluoride
• SEM scanning electron microscope
• T c crystallization temperature
• T d degradation temperature
• TE transesterification
• TER transesterification reaction
• TGA thermogravimetric analysis
• T m melting temperature
• WCA water contact angle

INTRODUCTION

In the 21st century, nearly 1 billion people lack of accessing safe drinking water and this is one of the great challenges globally. Due to water pollution, the quality of environment and the health of the human beings are highly affected. Various techniques are adopted to purify the water like adsorption, flocculation, distillation, and so on. Even then, the purified water is not up to the expected quality due to the presence of microbes and other water soluble toxic materials. In order to outwit the above said problems, membrane filtration is used. Various polymeric materials are used as a membrane like polyimide (PI),[1] polysulfone (PSF),[2] and polyvinylidene fluoride (PVDF)[3] which are expensive. These membrane materials have low half‐life period because of absence of ionizable group, hydrophobic character, and absence of antimicrobial functional groups. In the present research work the main target is to increase the application of PVDF membrane via functionalization. Hence, it is necessary to know the previous work done on the functionalization of PVDF. Poly(ε‐caprolactone) modified PVDF exhibited good antifouling properties.[4] PEI/tannin modified PVDF was reported with high hydrophilicity.[5] PAA modified PVDF was used for the oil/water separation.[6] GMA was grafted onto PVDF by γ‐irradiation technique.[7] Hydroxyl functionalized PVDF was reported in the literature.[8] CA was grafted onto PVDF microporous membrane via free radical grating method.[9] Ce(IV) initiated chemical grafting of N,N‐methylene bisacrylamide onto PVDF was reported by Shi et al.[10] PVDF with –OH, –NH2, and –CO2H functionalities were done by Schulz et al.[11] PEI grafted PVDF for pulse cleaning was reported by Ma et al.[12] Similarly, PVDF was functionalized with protein.[13] dimethylaminopropyl methacrylamide,[14] quaternary ammonium compounds,[15] heparin,[16] melamine,[17] PEO,[18] MAH,[19] and hydroxyl ethyl acrylates.[20] On going through the literature, it was found that functionalization of PVDF was reported by various techniques but not with aza‐Michael addition (AMA) technique and the free radical (FR) grafting was rarely reported. In the present research work, the KOH treated PVDF (PVDF2) was functionalized by AMA technique and FR grafting technique and the same was characterized by various analytical techniques.

The process of exchanging the alkyl or aromatic group of an ester with the alkyl or aromatic group of an alcohol in the presence of a catalyst in organic chemistry is known as TER. Various catalyst systems are used for TER. TE of palm oil was carried out using dolomite as a catalyst.[21] In 2018, epoxidized soybean oil was transesterified in the presence of NaOH catalyst.[22] Quicklime catalyzed TE of canola oil was reported in the literature.[23] Lani et al. utilized CaO/silica as a catalyst for the TER.[24] TiO2 catalyzed TER of jatropha oil was done by Chen et al.[25] Mussel shell was used as a catalyst for the TE of soybean oil.[26] NaOH mediated TE of sunflower oil was reported in the year 2018.[27] Microwave catalyzed TE of palm oil was studied.[28] Enzymatic TE of jatropha oil was studied by Kumari et al.[29] Lipase catalyzed TE of cellulose polyurethane was reported.[30] Cu(I) complexes was used as TE catalyst.[31] Base catalyzed TE of cooking oil was thoroughly studied by Cetinkaya et al.[32] Similarly, SnO2[33] and LiBr[34] were used as a catalyst for the TER. In 2020, Thamizhlarasan et al. used zinc acetate as a TE catalyst for the preparation of PET.[35] Zn cluster catalyzed TE of various methylester was carried out by Iwasaki et al.[36] The literature survey indicates that PVDF‐g‐MAH/V2O5 catalyzed TE with simultaneous esterification report is not available in the past literature. This motivated the authors to do the present research work. The present catalyst system acted as a catalyst for both TE and simultaneous esterification reaction. This is the novelty of the present research work.

Currently, a metal or MO nanoparticle loaded PVDF plays an important role in the catalysis field because of its high selectivity of the reaction and low separation cost. Co NP loaded PVDF membrane exhibited 100% reduction efficiently toward NiP reduction.[37] A Fe/Pd bimetallic nanoparticle loaded PVDF/PVA membrane was used for the preparation of biphenyl from dichorobenzene.[38] TiO2 coated PVDF membrane was used for the decolorization of rose Bengal‐19 dye[39] and methylene blue dye.[40] Prussian blue loaded PVDF membrane reactor was used for the degradation of organic dyes.[41] Catalytic oxidation of benzyl alcohol was done with the help of TiO2 loaded PVDF membrane.[42] The PVDF membrane modified with TiO2/Fe3+ was used for the photodegradation of orange II dye.[43] PVDF/SiO2@Ag nanocomposite membrane was used for the catalytic reduction of nitrophenol.[44] The Fe3+/PVDF/PMMA membrane was used for the degradation of orange IV dye.[45] Amino acid derivatives were synthesized with the help of Cd‐MoF@PVDF membrane.[46] For rhodamine degradation, SnO2–TiO2/PVDF membrane was utilized.[47] PVDF/MoS2 nanosheet was used for the degradation of oxytetracycline.[48] While referring the literature, PVDF based membrane was not used in the high temperature reaction particularly in the TER. This motivated the authors to do the present research work.

Recently, polymer nanocomposite based membrane plays a vital role in water purification process because of its weightless, antifouling activity with high mechanical strength. In 2022, Asiri et al.[49] reported the cellulose acetate/ZnO nanocomposite membrane. Krishnan et al.[50] reviewed the polymer nanocomposite membrane for desalination application. Poly(sulfone)/Zn–Fe layered double hydroxide based nanocomposite membrane was reported in the literature for water purification purpose.[51] Poly(sulfone)/MXene nanocomposite membrane was used for the filtration of dye solutions.[52] Polyamide/CNT nanocomposite membrane was used for ethanol purification.[53] Polyethersulfone (PES)/Fe3O4 nanocomposite membrane was prepared toward fuel cell application.[54] PES/TiO2 nanocomposite membrane was used for gas separation.[55] There is no literature report on PVDF‐g‐MAH/V2O5 nanocomposite membrane for catalytic application. Simultaneous TE and esterification was noticed while using PVDF‐g‐MAH/V2O5 nanocomposite membrane as a catalyst. This is the novelty of the present research work. TE of MMA with EY dye was done in slightly acidic pH in the presence and absence of catalyst. Above all, this methodology opened a new route for the synthesis of NIR dye from an economically cheaper conventional dye.

EXPERIMENTAL

Materials

Polyvinylidene fluoride (PVDF, Mw: 180,000 g/mol, PVDF1), 2‐methyl imidazole (2MI), and indole were purchased from Sigma Aldrich, The United States. Congo red (CR) and eosin Y (EY) dyes were purchased from Merck, The United States. Potassium hydroxide (KOH), acetone, and N‐methyl pyrrolidone (NMP) were purchased from Alfa Aesar, The United States. Maleic anhydride (MAH), maleimide (MI), dicumyl peroxide (DCP), and sodium borohydride (NaBH4) were purchased from Baker chemicals, Taiwan. V2O5 bulk powder was purchased from Nice chemicals, India. Double distilled water (DDW) was used for solution preparation work.

Synthesis of V 2 O 5 nanoparticles

The V2O5 nanoparticle was prepared by chemical reduction method. One gram of V2O5 powder was dispersed in 50 ml DDW under ultrasonication. Then, 2.0 g (in excess) NaBH4 was added at a time. The evolution of brisk effervescence confirmed the reduction of V2O5 bulk powder into V2O5 nanoparticles. The reaction was allowed to continue for 2 h under mild stirring condition. After 2 h of reaction the contents were filtered and washed five times with acetone. Then, it was subjected to air drying overnight. The dried product was stored in a polythene bag under nitrogen atmosphere to avoid further aerial oxidation reaction.

Functionalization of PVDF

The main aim of the present research work is to increase the application of PVDF via functionalization reaction. PVDF1 was functionalized by two different methodologies, aza‐Michael addition reaction (AMAR) and free radical reaction (FRR). For this purpose, a C=C should be created on PVDF1 backbone. In order to get C=C on the PVDF1 backbone, 10 g of PVDF1 sample was dissolved in 250 ml NMP at 85°C for 1 h in the presence of 2 g KOH. During the heating process, 1 mole of HF was removed from the PVDF1 backbone.[56] Finally, it was treated with 1000 ml of acetone to precipitate. The precipitate was dried at 100°C for 2 h to afford PVDF2. This is used as a starting material for the functionalization process (95% yield).

Functionalization by AMAR was done by taking 1.0 g PVDF2 which was dissolved in 25 ml NMP at 85°C for 1 h. After the complete dissolution, 1.0 g of CR or 2MI or indole was added under vigorous stirring. Now, AMAR started from the C=C double bond of PVDF2 with the NH group of 2MI or CR or indole.[57] The addition reaction was allowed for further 1 h. Finally, it was added with 250 ml acetone. The precipitate was dried at 100°C for overnight to give PVDF3, PVDF4, or PVDF5, respectively (Scheme 1). The dried mass was stored in a polythene cover under nitrogen atmosphere. The reaction is illustrated in Scheme 1.

Functionalization by FRR was carried out by taking 1.0 g of PVDF2 which was dissolved in 25 ml NMP solvent at 85°C for 1 h. Then, 1.0 g MAH or MI was added followed by the addition of 0.50 g of DCP initiator. The addition reaction was carried out under nitrogen atmosphere for further 1 h.[19] At the end of the reaction, excess amount (200 ml) of acetone was added. The precipitated sample was dried and subjected to further analytical characterization to give PVDF6 (Scheme 1). Till then, the samples were stored in a polythene cover under nitrogen atmosphere. The reaction is illustrated in Scheme 1.

Preparation of PVDF‐ g ‐MAH/V 2 O 5 nanocomposite (PVDF8) and PVDF‐ g ‐MAH/V 2 O 5 nanocompos...

The following procedure was followed to prepare the nanocomposite membrane. The PVDF‐g‐MAH sample (for, e.g., 1.0 g) was dissolved in 20 ml NMP and 0.20 g of V2O5 nanoparticle was added. The contents were allowed to stir for further 1 h under nitrogen atmosphere to avoid further aerial oxidation reaction. Then, 200 ml acetone was added to precipitate. The precipitate was dried for overnight, stored in a polythene bag with the name PVDF8. In order to prepare membrane, phase inversion method was adopted. For this purpose, 18% solution of PVDF‐g‐MAH/V2O5 (PVDF8) nanocomposite system was prepared in NMP solvent. Thus, prepared viscous solution was poured on a glass plate to spread uniformly. Then, the thickness of the membrane was controlled by a doctor blade. It was allowed for 2 min for crystallization and the polymer sample was spread on the glass plate was immersed in a water bath for 4 h. During this period, the NMP solvent was diffused into aqueous medium. The membrane is taken out from the water bath, washed well with acetone, dried at room temperature for 48 h and named as PVDF‐g‐MAH/V2O5 nanocomposite membrane. The resultant product is a typical catalytic membrane.

Catalytic TER

PVDF‐g‐MAH/V2O5 nanocomposite membrane was used for the catalytic TER. For TER, two different methodologies were tried as mentioned in Scheme 2A,B. First, the catalytic TER was carried out through Scheme 2A. One gram of EY dye was dissolved in 100 ml DDW and maintained at 85°C for 10 min. Separately, 2 ml of quinol stabilized MMA (to avoid thermal polymerization) was added and maintained at 85°C. The EY dye solution and MMA suspension were mixed with few drops of 1 M HCl for 10 min and then passed through the PVDF‐g‐MAH/V2O5 membrane.[37] While applying vacuum, the filtrates were emerged out from the membrane and collected in a conical flask. The filtrate was tested by UV–visible spectrophotometer for every 10 min. Even after 10 filtration process, there was no change in the UV–visible spectrum. This confirmed the absence of TER due to the low residential time during the membrane filtration process. Hence, TER conducted via membrane filtration process was a failure one, so that the authors used an alternate methodology.

As mentioned in Scheme 2, the alternate method (Scheme 2B) was used to carry out the catalytic TER. Required amount of dye solution and MMA suspension were taken in a 250 ml round bottomed flask (RBF). Three drops of 1 M HCl was added under vigorous stirring. Now, the catalytic membrane was cut into a 2 × 2 cm2 sized pieces and added to the RBF. The TER was carried out at 85°C for 12 h. In an aqueous medium, the PVDF‐g‐MAH/V2O5 catalytic membrane was not soluble. When the contents were heated, the reactants were intercalated into PVDF membrane containing V2O5 nanoparticles. Hence, the membrane is acting as a template. Finally, membrane pieces were removed from the reaction medium and washed with water. The washings were added with bulk solution. Two milliliters of aliquots were taken out and subjected to UV–visible spectral measurement. Then the contents were dried at 110°C for 6 h. A pale yellow colored product was formed. It was washed with acetone for three–five times to remove the unreacted MMA. Then, the dye grafted MMA was dried at room temperature for overnight. The dried mass was dye grafted MMA and subjected to further characterization.

Characterizations

The Perkin Elmer, 100 FTIR spectrophotometer instruments was used to take FTIR spectrum. Two milligrams of polymer sample was mixed with 200 mg of spectral grade KBr and made into a disc under 7 tons of pressure. The Rigaku Thermoplus‐TG8120 instrument was used for the TGA measurement under air atmosphere at the heating rate of 10°C/min. DSC‐Q20, TA instrument was used for the measurement of Tg and Tm for the polymer samples under N2 atmosphere at 10°C/min heating rate. The second heating scan was considered here. The contact angles of polymer samples were determined by VCA 2500, Taiwan instrument. The surface morphology (SEM) and EDX of the polymer samples were measured by JSM 6300, Jeol product, SEM instrument. The binding energy and % elements were determined from XPS Thermo Scientific‐Theta Probe‐Al radiation. The Bruker AVIII 500 MHz NMR instrument was used to measure the 1H‐NMR for the polymer sample in CDCl3 solvent. The fluorescence emission spectral measurement was carried out with the help of an instrument Elico SL 172, India. The high resolution transmission electron microscope (HRTEM) was measured using JEM‐200 CX, USA, transmission electron microscope instrument. The UV–visible spectrum was recorded on UV‐1800 Shimadzu spectrophotometer, Japan.

From the FTIR RI, the % grafting onto C=C was calculated as follows using a standard literature[58]:

$$\%\text{Grafting}=\frac{\text{AGRI}\frac{\mathrm{C}=\mathrm{C}}{\mathrm{C}-\mathrm{H}}}{\text{BGRI}\frac{\mathrm{C}=\mathrm{C}}{\mathrm{C}-\mathrm{H}}}\times 100.$$

where, AG is after grafting; BG is before grafting; and RI is the relative intensity.

From the UV–visible absorbance spectrum, the rate of TER (RTER) was calculated as follows[59]:

2 $$R_{\mathrm{TER}}=\frac{\text{Absorbance}}{V\times t\times M}\times 1000.$$

where, V is the total reaction volume, t is time, and M is molecular weight of EY dye and MMA.

RESULTS AND DISCUSSION

Characterization of functionalized PVDF

The functional groups present in the PVDF before and after the structural modification reaction was confirmed by FTIR spectrum. Figure 1a indicates the FTIR spectrum of pristine PVDF (PVDF1) system. The system showed peaks at 3020, 2961, 1404, 1051, 747, and 495 cm−1 corresponding to C–H asymmetric, C–H symmetric, C–H bending vibration, C–F stretching, C–H out of plane bending vibration and fluoride ion stretching respectively.[60] Figure 1b indicates the FTIR spectrum of PVDF2 system. Here, a new peak appeared around 1600 cm−1 explained the presence of C=C. During the KOH treatment dehydrofluorination occurred. By the way of producing C=C, the reactivity of PVDF is accelerated. Now, the functionalization of PVDF was carried out in two different methodologies namely Aza‐Michael addition reaction (AMAR) and free radical reaction (FRR). The FTIR spectrum of functionalized PVDF3 is given in Figure 1c. In this system, the C=C double bond was functionalized through AMAR. The new C–N stretching peak appeared at 1346 cm−1 as a small hump corresponding to the 2MI. Even after the functionalization reaction, a small hump appeared at 1596 cm−1 due to the C=C double bond. Figure 1d indicates the FTIR spectrum of CR functionalized PVDF (PVDF4) system through AMAR. A small hump appeared at 1333 cm−1 (C–N stretching), 656, 768 cm−1 (aromatic stretching) and 1292 cm−1 (SO2 stretching) were seen corresponding to CR. Still a hump at 1616 cm−1 appeared due to the C=C stretching. The indole grafted PVDF (PVDF5) system via AMAR produced three new peaks (Figure 1e) corresponding to C–N stretching (1323 cm−1), C–N–C stretching (1061 cm−1) and aromatic stretching (757, 676 cm_SP_−1_sp_). These stretchings are derived from indole. The FTIR spectrum of MAH grafted PVDF (PVDF6) system (Figure 1f) via FRR exhibited some new peaks at 1854 and 1731 cm−1 corresponding to anhydride stretching and C=O stretching of MAH.[19] Figure 1g confirmed the FTIR spectrum of MI grafted PVDF (PVDF7) system via FRR. The imide stretching (1606 cm−1) and C–N–C stretching (1070 cm−1) confirmed the chemical grafting of MI onto PVDF2 via FRR. The main intension is to find out the % grafting. Table 1 shows the FTIR relative intensity (RI) of different functional groups grafted PVDF and % grafting onto C=C. The FTIR RI of C=C was quantitatively calculated from the FTIR spectrum using IR solution software.[62] Among the systems, the PVDF3 system exhibited 98.8% grafted PVDF2 with respect to C=C. This high % grafting is due to smaller size of 2MI. At the same time, the PVDF5 system yielded the lowest % grafting among the AMAR dye to the absence of chain extension group. The bulky indole group is directly grafted onto C=C and lead to more steric structure. Among the FRR, the MAH produced the highest grafting of 97.6% due to the vigorous polymerization nature of MAH. The steric effect is less here due to smaller size of MAH. In overall comparison, the PVDF3 system produced the highest grafting onto C=C of PVDF2.

1 TABLEDetermination of % grafting onto C=C via FTIR RI method

System	Code	RI[C=C]	RI[C–H]	RI[C=C/C–H]	% grafting onto C=C
PVDF	PVDF1	—	—	—	—
PVDF‐KOH	PVDF2	0.0395	0.0220	1.7954	—
PVDF‐g‐2MI	PVDF3	0.0760	3.5370	0.0214	98.8
PVDF‐g‐CR	PVDF4	0.1280	3.9310	0.1374	92.3
PVDF‐g‐Indole	PVDF5	0.1044	0.1148	0.9094	49.3
PVDF‐g‐MAH	PVDF6	0.0140	0.3360	0.0416	92.6
PVDF‐g‐MI	PVDF7	0.0960	1.5140	0.0634	96.5

The melt transition and cold transition temperatures of PVDF before and after functionalization were determined from DSC study. The data is given in Table 2. Figure 2A(a–g) indicates the DSC heating scan of PVDF before and after functionalization process. The PVDF1 system showed the Tm of 168°C. The literature report showed the Tm of PVDF at 163.8°C.[19] When compared with the literature, the present system yielded somewhat higher Tm value which may be due to the difference in Mw of PVDF. Among the systems considered, the PVDF4 system exhibited the highest Tm value of 169.9°C (Table 2). Even though, the bulky size of the CR dye,[61] due to the presence of pendant –NH2 group, the steric effect was reduced to some extent. The PVDF5 system exhibited the lowest Tm value of 154.6°C due to the less steric effect when compared with PVDF4 system. When compared with other systems, the present PVDF5 system experienced less steric effect. Figure 2B(a–g) indicates the DSC cooling curves of different systems (Table 2). The PVDF1 system showed the Tc value of 121.7°C. According to the literature report, the Tc of β‐phase crystallites of PVDF was found to be 140°C.[63] The difference in Tc can be explained on the basis of cooling rate. The present research work exhibited lower Tc value when compared with the literature value.[63] After functionalization, the Tc peak becomes more sharpened and shifted to slightly higher temperature. Among the systems, the PVDF5 system exhibited the highest Tc value of 136.1°C. This is due to the rigid nature of indole. This study indicates that the grafted group not only increased the Tc value of PVDF but also as a nucleating agent. This is the positive point of the present investigation.

2 TABLEDSC, TGA and WCA data of PVDF systems

Code	Tm	Tc	Td	WCA
(°C)	(°C)	(°C)	(o)
PVDF1	168.2	121.7	459	99.7
PVDF2	167.3	134.4	452	88.6
PVDF3	158.1	120.7	431	54.7
PVDF4	169.9	132.7	473	61.3
PVDF5	154.6	123.4	442	76.8
PVDF6	164.0	132.7	460	96.4
PVDF7	163.2	136.1	400	64.1
PVDF8	163.5	135.2	451	90.2

1 Abbreviations: Tc, crystallization temperature; Td, degradation temperature; Tm, melt transition temperature; WCA, water contact angle.

The thermal stability of PVDF before and after functionalization was determined by TGA. Figure 2C(a–g),D(a–g) indicates the TGA and DTA scans of different functionalized PVDF systems. Table 2 showed the Td values of different PVDF systems. Figure 2C(a) indicates the TGA scan of PVDF1 system with two step degradation process. The major weight loss around 450°C is due to the removal of HF from the PVDF backbone. The next major weight loss around 626°C is associated with the degradation of –HC=CF– backbone. The literature showed the Td of PVDF as 440°C.[64] When compared with the literature report, the present research work yielded somewhat good result. The other systems showed one more degradation step between 100°C and 210°C due to the removal of NMP solvent. Among the systems considered, the PVDF4 system produced the highest Td of 473°C due to the stiffness aromatic grafted side chain CR dye. Thus, the chemical functionalization altered the thermal stability of PVDF.

The surface morphology of PVDF1 system is given in Figure 3A(a) with cage like structure (i.e.) availability of more number of empty spaces.[19] The SEM image of PVDF2 system is given in Figure 3A(b). Here, one can see the spheres with the size of ~3 μm, fused together. The SEM image of PVDF3 system is given in Figure 3A(c). Here, also spheres with the size of ~1 μm are seen in more number. After grafting with 2MI, the size of the PVDF spheres reduced. The size of the voids also reduced. Figure 3A(d) confirmed the SEM image of PVDF4 system. Here the spherical shape of the PVDF was completely crushed. Here and there one can see bright spots due to the random grafting of CR onto PVDF2 backbone. The SEM image of PVDF5 system is shown in Figure 3A(e). Here, number of microvoids is seen with cage like structure. The surface of PVDF5 becomes fluorescent. The SEM of MAH grafted PVDF system is given in Figure 3A(f) with PVDF pattern. The spherical form of PVDF was compressed. The appearance of white fiber confirmed the random grafting of MAH onto PVDF2 backbone. The length of the fiber like portion is different and this confirmed the oligomerization or polymerization of MAH since MAH contains C=C and the FRR was carried out in the presence of DCP as an initiator. Figure 3A(g) represents the SEM image of MI grafted PVDF with scissor‐like structure. The spherical shape of PVDF was compressed to ~1 μm but with lesser in number. This confirmed the chemical grafting of MI onto PVDF2 backbone.

The EDX spectrum of PVDF1 system is shown in Figure 3B(a). The spectrum showed C and F peaks.[65] Here, the F/C ratio was calculated as 0.9895 (Table 3). But after KOH treatment, the F/C ratio was reduced to 0.9421. This confirmed the decrease in F concentration. The TGA result indicated that while heating the PVDF dehydrofluorination was followed by the degradation of PVDF structure. In such a way the EDX result supported the TGA result. During the KOH treatment, 4.79% of F was removed from the PVDF1 backbone. The PVDF3 system showed the F/C ratio of 0.8051 with 18.63% F loss (Table 3). Similarly, the CR and indole grafted PVDF showed the F/C ratios of 0.7755 and 0.7797, respectively. The loss of F was found to be 21.62% and 21.20%, respectively, for PVDF4 and PVDF5 systems. In comparison among the AMA grafted PVDF, the PVDF3 system showed the highest loss of F. While coming to the FRR, the MAH grafted PVDF showed the F/C ratio of 0.6712 with the 32.17% F loss. In the case of MI grafted PVDF (PVDF7), the F/C ratio was determined as 0.6062 with 38.74% F loss. Among the FRR, the MI grafted PVDF (PVDF7) exhibited the highest F loss. In overall comparison, the FR grafted PVDF showed the highest F loss. This confirmed that during the grafting reaction onto PVDF2 some amount of F was removed as HF. When PVC or PVA was heated to 85°C in a suitable solvent, HCl or H2O was removed from the respective polymer backbone. Hence, this is in accordance with literature report.[66]

3 TABLEEDX data of PVDF systems

Code	% atomic weight C	% atomic weight F	F/C	% loss of F
PVDF1	49.67	49.15	0.9895	—
PVDF2	48.56	45.75	0.9421	4.79
PVDF3	53.78	41.69	0.8051	18.63
PVDF4	52.21	40.49	0.7755	21.62
PVDF5	54.07	42.16	0.7797	21.20
PVDF6	57.15	38.36	0.6712	32.17
PVDF7	58.20	35.28	0.6062	38.74

Measurement of WCA also confirmed the grafting reaction onto PVDF2 backbone. The PVDF1 exhibited the WCA of 99.7° (Table 2). The PVDF nanofiber membrane exhibited the WCA of 91.2°.[67] This indicates that the WCA of PVDF depends on the size and shape of the material. The WCA of PVDF2 was slightly reduced to 88.6°. The C=C made the backbone more rigid but with less voids. As a result the WCA value is slightly reduced. After grafting reaction, the WCA was drastically reduced. In the case of PVDF6 system, the WCA was found to be 96.4° due to the anhydride grafting but with random grafting reaction. Among the systems grafted, the MI grafted PVDF (PVDF7) showed the WCA of 64.1°. This is due to the presence of O=C–N–C=O like structure. It may be in oligomeric form or in polymeric form. Since, the MI contains C=C, hence it is possible to form an oligomer or polymer. The decrease in WCA confirmed the chemical grating onto PVDF backbone.

Characterization of PVDF‐ g ‐MAH/V 2 O 5 (PVDF8) nanocomposite

Among the systems considered, the authors are very much interested in MAH grafted PVDF system because of easy preparation by FRR. Moreover, during the course of the water filtration process, the anhydride group was hydrolyzed to acid groups and this leads to decrease in WCA. This is an added advantage of the present research work. Hence, further, analytical characterizations are important. Figure 4A,B indicates the FTIR spectrum of PVDF and PVDF‐g‐MAH systems respectively. The FTIR peaks of the same were already discussed. For the sake of comparison, the FTIR spectrum of PVDF1 and PVDF7 systems are given. Figure 4C indicates the FTIR spectrum of PVDF‐g‐MAH/V2O5 (PVDF8) nanocomposite system. The spectrum showed some new peaks. Now, the anhydride peak appeared at 1810 cm−1. The PVDF‐g‐MAH system showed the MAH stretching at 1854 cm−1.[19] The blue shifting in the anhydride peak can be explained as follows. The added V2O5 nanoparticles are encapsulated by the anhydride group of PVDF‐g‐MAH system. Moreover, the intensity of the C=O group was suppressed. This confirmed the chemical interaction between V2O5 nanoparticles and anhydride group of PVDF‐g‐MAH. The C=C stretching appeared at 1634 cm−1. One more new peak appeared at 464 cm−1 is responsible for the MO stretching of V2O5.[68] The difference in the wavenumber is due to the high electronegativity of fluorine atom.

The 1H‐NMR spectrum of PVDF8 system taken in CDCl3 solvent at room temperature is given in Figure 4D. The –CH2 proton of PVDF appeared at 1.62 ppm. The –CH peak of PVDF appeared at 1.26 ppm.[69] The –CH peak of MAH grafted onto –CH center of PVDF appeared at 2.0 and 2.39 ppm.[70] The –CH peak of MAH grafted onto –CF center appeared at 2.91 and 3.41 ppm. A small hump appeared at 0.80 ppm is associated with the –CH3 proton signal of DCP initiator since the grafting reaction occurred through the free radical reaction onto PVDF2 backbone. Hence, the 1H‐NMR spectrum confirmed the MAH grafting onto PVDF2 backbone.

After the incorporation of V2O5 nanoparticles onto PVDF‐g‐MAH system, the thermal properties like DSC and TGA were tested. Figure 5(curves—a and b) indicates the DSC thermogram of PVDF1 and PVDF6 systems, respectively. These two systems were already discussed. For the sake of comparison, it was included here. Figure 5 (curve c) indicates the DSC thermogram of PVDF8 system. The Tm was reported at 163.5°C (Table 2) which is lower than that of the Tm of the PVDF1 system.[63] The Tc of PVDF8 system was noted at 135.2°C which is higher than that of the Tc of the PVDF1 system. The decrease in Tm and increase in Tc confirmed the existence of chemical interaction between anhydride group and V2O5 nanoparticles. Here, the V2O5 nanoparticles act not only as a filler but also as a nucleating agent.

The TGA thermogram of PVDF8 system is shown in Figure 5(curve f) with four step degradation process. The first two weight loss step below 210°C is due to the removal of moisture and NMP solvent from PVDF8 backbone. The dehydrofluorination was observed at 451°C (Table 2).[64] The degradation of PVDF backbone was noticed at 546°C. The present system exhibited lower Td than the PVDF6 and PVDF1 systems due to the presence of NMP solvent. For the sake of comparison, the TGA thermogram of PVDF1 and PVDF6 systems is included as Figure 5 (curves—d and e) respectively.

The surface morphology of PVDF8 system is given in Figure 6C with more number of micro spheres with the size of <1 μm. The cross section of the same was given in Figure 6F. Here, one can see spherical voids instead of finger[19] like voids. But the SEM of PVDF1 appeared as a cage like structure (Figure 6A). The cross section of the PVDF1 system is given in Figure 6D with micro porous structure. The SEM image of PVDF6 system is shown in Figure 6B. In overall comparison, the PVDF8 system exhibited an entirely different morphology and cross section. Figure 6G indicates the SEM image of PVDF8 system with higher magnification. This confirmed that the surface of the PVDF was encapsulated by the V2O5 nanoparticles. The size of V2O5 nanoparticle was calculated as <100 nm. Figure 6H represents the EDX of PVDF8 system with %C, F, V, and O as 47.4%, 46.91%, 4.02%, and 2.03%, respectively. The WCA of PVDF8 system (Figure 6I) was found to be 90.2°. Even after the addition of V2O5 nanoparticle the WCA was not reduced to hydrophilic region and this confirmed the hydrophobic nature of V2O5 nanoparticles. This is an advantage for the aqueous phase catalytic reaction.

The XPS of PVDF8 system is given in Figure 7A with C1s, V2p, Ols, and F1s peaks appeared at 285.8, 515.7,[71] 530.7, and 686.09 eV[72] respectively. The appearance of V2p peak confirmed the incorporation of V2O5 nanoparticles in PVDF8 backbone. The HR‐TEM image of PVDF8 system is given in Figure 7B with the size of <20 nm. The circled and arrow marked portion indicated the same. The report received from HR‐TEM is more accurate than the SEM. Particularly, the determination of nanoparticle size because their basic principles are different.

Catalytic TER

As mentioned in the experimental part, the TER was done by two different methodologies. The FTIR spectrum of transesterified product (Scheme 3, green color, Product) prepared in the presence of catalyst is shown in Figure 8A. The spectrum showed –OH stretching at 3445 cm−1 due to the EY dye. This may be arised from the –OH group or CO2H group. The C–H symmetric and asymmetric stretching of EY appeared at 2842 and 2912 cm−1, respectively. The aliphatic C–H symmetric and asymmetric stretchings noted at 2944 and 2997 cm−1, respectively. The aliphatic C–H stretchings are derived from MMA. The C=O (1744 cm−1)[66] and C=C stretching (1642 cm−1)[66] are associated with MMA. The C–O–C stretching due to the transesterified product appeared at 1141 cm−1. The aromatic stretchings appeared at 874 and 688 cm−1 (due to EY dye). The C–H out of plane bending vibration appeared at 752 cm−1 due to the MMA. The bromide stretching noted at 544 cm−1. The appearance of C–O–C stretching confirmed the formation of transesterified product. Figure 8B indicates the FES of TE product when it was excited at 472 nm. The emission spectrum showed one peak at 567.5 nm. The interesting point to be noted here is the shape and intensity of the emission peak. The broadening of the emission peak is due to the existence of different chromophoric groups nearer to each other. It reveals that the material can emit energy in a wide spectrum with a single excitation at lower wavelength. This type of material is very much useful in biomedical engineering field, particularly in cancer treatment.

The transesterified product (Scheme 3, final product is mentioned in green color, Product) was further confirmed by UV–visible spectrum. The UV–visible spectrum of neat EY appeared at 517 nm.[73] The UV–visible spectrum of neat EY is given in Figure 9A with the λmax value of 517 nm. The TER carried out in the absence of catalyst is given in Figure 9B with two absorbance peaks at 502 and 543 nm corresponding to dimeric and monomeric structure of EY. The red shift in the peak is explained on the basis of TE between the –OH group of EY and ester group of MMA units. The UV–visible spectrum of TER carried out in the presence of membrane catalyst (PVDF8) is shown in Figure 9C with three absorbance peaks. The first important peak appeared at 562.7 nm due to the TE product formed between the –OH group of EY and ester group of MMA. The second absorbance peak appeared at 667.4 nm corresponding to the esterified and transesterified product (i.e.) the –CO2H group of transesterified EY reacts with CH3OH to form an ester which on further transesterification with another one –OH group of EY. Then, the second acid group of EY reacts with CH3OH. Then it reacted with another one –OH group of EY to form the final product. The UV–visible spectrum of final product is given in Figure 9C with the λmax value of 744.9 nm. This is nearer to the NIR region. This indicates that by the way of TER one can prepare a dye with the absorbance in the NIR region. This methodology is economically cheaper and environmentally green. A dye with absorbance in the NIR region has an excellent biomedical application. The UV–visible spectrum concluded that TE of EY in the presence of a catalyst yielded excellent result than in its absence. From the UV–visible spectrum it is possible to find out the direct band gap value by using the Tauc's plot. The Tauc's plot for neat EY is given in Figure 9D with the band gap value of 2.29 eV.[74] The TE product prepared in the absence of membrane catalyst has the band gap value of 2.02 eV (Figure 9E). When compared with the neat EY the TE product showed slightly lower band gap value. The Tauc's plot for TE product prepared in the presence of membrane catalyst is given in Figure 9F with two band gap value at 1.99 and 1.2 eV. Here, the appearance of two band gap value is due to the presence of mixture of products as mentioned in the UV–visible spectrum. The decrease in band gap value of dye increased its application in biomedical engineering and solar cell fields. Based on the UV–visible absorbance spectrum, a simple comparative study was made by calculating the rate of reaction using Equation (1). In the absence of catalyst, the rate of product formation was found to be 8.48 × 10−7 mol/L/s. In the presence of catalyst, three different products were formed. The rate of formation of product (1) was determined as 4.07 × 10−7 mol/L/s. Similarly, the rate of formation of product (2) and (3) were calculated as 1.70 × 10−7 mol/L/s and 0.9259 × 10−7 mol/L/s, respectively. In comparison, in the absence of membrane catalyst, the rate of reaction was very high but with one single product. In the presence of catalyst three different products were formed with lower rates but with NIR absorbance.

TER mechanism

Generally, the TER is catalyzed by acid or by a nanomaterial.[75] The simple acid catalyzed reaction involves the one step TER between the –OH group of EY dye and ester group of MMA. This is confirmed by UV–visible spectral measurement by noting the red shift in the absorbance peak. The TER was carried out under the same experimental conditions in the presence of 2 × 2 cm2 membrane catalyst pieces. Now, the acid and V2O5 nanoparticles combinedly play together toward transesterification reaction. Various possible TER is mentioned in Scheme 3. The acid catalyzed reaction leads to simple one step TER. The V2O5 nanoparticles provide more surface area. The PVDF8 membrane catalyst is acting as a template. Through the porous structure of PVDF, the reactants were intercalated where the V2O5 nanoparticles mediate the TER. This leads to the formation of NIR dye. The authors believe that oligomerization has taken place during the TER. Further investigation is going on in our lab.

CONCLUSIONS

Functionalization of PVDF was done via AMA and FRR using PVDF2 system. The FTIR‐RI study declared the 98.8% grafting of 2MI onto PVDF2. The PVDF6 system exhibited a peak at 1854 cm−1 in the FTIR spectrum, corresponding to the anhydride stretching. The PVDF4 system showed the highest Tm of 169.9 °C due to the bulky and rigid nature of CR. The PVDF7 system exhibited the highest Tc value of 136.1°C and this confirmed the nucleating activity of MI. In the case of TGA, the PVDF4 system showed the highest dehydrofluorination temperature of 493°C due to the bulky size of CR. The PVDF3 system showed the WCA of 54.7° due to the smaller in size with the hydrophilic nature of azole group. The SEM showed that the PVDF1 exhibited more number of microvoids but after grafting reaction the numbers of voids reduced. The EDX report indicated that the PVDF7 system showed the lowest F/C ratio of 0.6062 while coming from PVDF1 system and this confirmed the dehydrofluorination during the grafting reaction from the –CH2–CF2– structure. A peak at 464 cm−1 in the FTIR spectrum confirmed the V2O5 nanoparticle doped PVDF6 system. The 1H‐NMR spectrum showed the vinyl protons signal from 2.0 to 2.4 ppm. The PVDF8 system showed the Tm, Tc, and Td values of 163.5°C, 135.2°C, and 451°C, respectively. The Tc value confirmed the nucleating effect of V2O5 nanoparticle. The EDX report showed 2.03% doping of V2O5 nanoparticles. The WCA of PVDF8 system was found to be 90.2° due to the hydrophobic nature of V2O5 nanoparticles. The SEM image showed the deposition of spherical shaped V2O5 on the surface of the PVDF6 system with the size of <20 nm. The XPS showed the V2p at 515.7 eV and confirmed the dispersion of nanosized V2O5 on PVDF6 backbone. The membrane based catalyst was not working well due to the low residential time. The FTIR spectrum of the transesterified product showed a peak at 1632 cm−1 due to the C=C structure of MMA. The broadening of the FES confirmed the presence of various chromophoric groups in the transesterified product. The UV–visible spectrum showed a peak at 744.9 nm with the band gap value of 1.24 eV. The PVDF8 system templated catalysis reaction showed the yield of 9.23% NIR dye from the conventional EY dye through TER. Thus, the main target of the present research work was achieved. Based on the results obtained from the present investigation, it was decided that our research team will focus on the synthesis of NIR dyes using membrane as a template.

ACKNOWLEDGMENT

We express our sincere thanks to Dr. N. Sundararajan, Associate Professor, Department of English, KCET, Madurai for his valuable help during this manuscript preparation work.

DATA AVAILABILITY STATEMENT

There is no supporting data available for the present research work.

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By Anbarasan Thamizhlarasan; Ramamoorthi Vignesh; Ramasamy Anbarasan and Kuo‐Lun Tung

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