Synthesis, characterization and non-isothermal degradation kinetics of rose Bengal end capped poly(aniline)/Cr2O3 nanocomposite

Solution polymerization of Ani was carried out in the presence of peroxydisulfate as a free radical initiator under N2 atmosphere at 0–5 °C for 2 h both in the presence and absence of Cr2O3(bulk) material under vigorous stirring condition. During the polymerization reaction, the Rose Bengal dye was added as an end capping agent. The above synthesized polymers were characterized by FTIR, UV–visible, fluorescence emission, XPS, XRD, DSC, TGA, SEM, HR-TEM, viscosity and conductivity measurements. The added Cr2O3(bulk) controlled the structure of poly(aniline) (PANI) and the same can be confirmed by FTIR spectroscopy. The Tg of Cr2O3 mediated PANI showed somewhat higher value than the pristine PANI. The XPS showed the presence of Cr3d2p3/2 and Cr3d2p1/2 peaks and this confirmed the nano-sized crystalline Cr2O3. Further, its thermal degradation was studied by non-isothermal degradation kinetics and their thermodynamic parameters were determined. The experimental data were compared with the available literature data.


Introduction
Electrically conducting polymers are familiar for its easy preparation methodology and excellent conducting properties particularly poly(aniline), (PANI) with various technological applications (Goswami et al. 2023;Azar et al. 2023;Micioiu et al. 2021).PANI is a dark green colored polymer with excellent electrical conductivity nearer to metallic regime in its doped form (Djara et al. 2020).Such an electronic material has some drawbacks such as low molecular weight, poor thermal properties, worst mechanical properties, poor processability and solubility and low optical activity.In order to increase the processability via increasing thermal stability and optical properties, the present research work was made.The focus is set on reviewing various synthesis processes available for the preparation of PANI.A novel 1D PANI nanorod was synthesized by a facile and green approach (Subalakshmi et al. 2020).
Ammonium persulfate mediated chemical synthesis of PANI was reported by Tarmizi et al. (2021).Electrochemical synthesis of PANI was studied by Nuepane et al. (2021).Fe(III) (Toshima et al. 2000), peroxosalts (Anbarasan and Gopalan 1998) and K 2 Cr 2 O 7 (Yelilarasi et al. 2009a, b) were used for the above polymerization as a chemical initiators.The processability of PANI can be increased to some extent by functionalization and copolymerization process.PANI was functionalized with acrylicacid via graft copolymerization process (Kang et al. 1997).Rhodamine6G dye was grafted onto PANI (Jangid et al. 2015).Cysteamine functionalized PANI was reported in the literature (Tao et al. 2012).Hexadecylbromide (Massoumi et al. 2012) and polysulfone (Alam et al. 2012) were grafted with PANI.Aniline and auramine O-based copolymers were reported by Ponprabhakaran and research team (Ponprabhakaran et al. 2019).Chemical grafting on phenyl ring or -NH group of PANI lead to decrease in electrical conductivity through ring flipping process.Now it requires a new technology to increase the processability of PANI without disturbing its electrical conductivity.The literature survey concludes that end capping of PANI with functional groups is a new and novel technology to increase the solubility, thermal and optical properties of PANI.Hence, this offers the novelty of the present investigation.
The thermal properties of PANI can be increased by making nanocomposites with various metal or metal oxide nanoparticles because the added nano material can occupy the space between the polymer chains as a filler.PANIs were made nanocomposite with CuO (Porselvi et al. 2014), ZrO 2 (Prasanna et al. 2016), kaolinite (Morsi et al.2019), MnO 2 (Shafiee et al. 2018), CdS (Raut et al. 2013), TiO 2 (Guo et al. 2012), V 2 O 5 (Yelilarasi et al. 2010), As 2 O 3 (Yelilarasi et al. 2009a, b), Sb 2 O 3 (Jeyakumari et al. 2009) and ZnO (Moskafei et al. 2012).By this way, the thermal stability of PANI was increased.In the present work, Cr 2 O 3 was considered because of its application as pseudo-capacitor electrode material (Ullah et al. 2015), spintronic material (Sahoo et al. 2007) with more electro-chemical applications.Cr 2 O 3 is commercially and easily available with cheaper cost.During the in situ polymerization, the Cr 2 O 3 nanoparticle was formed easily.Moreover, it has more applications in electrochemistry field.This is the first and foremost reason to select Cr 2 O 3 in this research work as a filler material.Moreover, Cr 2 O 3 -based PANI nanocomposites are rarely reported in the literature.This motivated the authors to choose the present research work.
Rose Bengal (RB) is a fluorescent xanthene dye with the λ max value of 540 nm in an aqueous medium.Under strong acidic pH, it has two functional groups: -OH and -CO 2 H.It is well known that PANI is a colored, fluorescent and water insoluble polymer.In order to increase the processability of PANI, the structure of PANI is modified with fluorescent dyes particularly with RB.RB is a fluorescent dye with OH and carboxyl functional groups.Copolymerization of RB with Ani is not possible since it has only one OH group and this leads to chain end process.When RB interacts with PANI through the -OH group, it leads to chemical grafting of PANI onto RB via free radical mechanism.If it interacts with PANI through the -CO 2 H group of RB, as a doping agent, it increases steric effect so there is restriction in doping process.Above all, there is no free position in the phenyl ring for extension of polymer for PANI chain end and acts as a chain end capping agent.RB was considered as a chain end capping agent.Apart from dyeing in textile industries, RB is used to produce reactive oxygen species in cancer treatment (Redmond et al. 2019) and also in solar cells for electron harvesting process (Sayyeed et al. 2010).Due to the difference in the experimental conditions, the RB can act as a chain end capping agent as well as a reactive oxygen species.The end capping reaction includes the role of oxidizing agent whereas the photo-sensitizing reaction includes the role of hγ.
The thermal stability of PANI was found to be less due to its low molecular weight.It is believed that the added metal oxide increases the thermal stability of PANI.When PANI was subjected to high temperature application for long period, it loses its doping agent and slowly the structure is degraded.In order to increase the thermal stability of PANI, RB with the molecular weight of 1017.6 mol/Lit was included in the PANI backbone as an end capping agent.Hence, it is necessary to study the influence of end capping agent on the thermal stability of PANI under air atmosphere.Moreover, RB has a rigid structure with fused ring systems.In order to increase the application of PANI in thermal engineering field, it was structurally modified with RB as an end capping agent.This is the key idea of the present research work.Degradation kinetics of PANI under non-isothermal condition was reported in the literature (Alves et al. 2018).Yang et al (2000) studied the thermal decomposition of PANI.Aging and thermal degradation of poly(N-methylaniline) was reported in the literature (Abthagir et al. 2004).PANI with different dispersion degree was studied under non-isothermal degradation condition (Doca et al. 2009).Amita and research team (2019) reported the E a value for the degradation of PANI/CdO nanocomposite system.The E a for the PANI/CoFe 2 O 4 nanocomposite system was determined as ~ 50 kJ/mol (Gandhi et al. 2011).The E a and thermodynamic parameters were determined for PANI/ZnS system (Bompilwar et al. 2010).The literature survey indicates that the RB end capped PANI/Cr 2 O 3 nanocomposite is not yet reported.Hence, the study is made on TD parameters of CP1 and CP2.

Synthesis of Poly(aniline) and Poly(aniline)/Cr 2 O 3 nanocomposite systems
Poly(aniline) and its nanocomposite with Cr 2 O 3(bulk) were prepared by solution polymerization technique.1 M dil.HCl was prepared with the help of DDW and used as a dopant.1 M ANI solution was prepared with the aid of 1 M HCl solution.0.10 M PDS solution was prepared in 1 M HCl solution.0.001 M RB solution was prepared in NMP medium.NMP was used as a solvent for the solution polymerization technique.The preparation procedure in brief is given below: Each 10 mL of monomer solution, initiator solution, end capping agent solution, HCl and NMP were taken in a 100 mL RBF.The concentrations of the solutions are mentioned above.The contents were mixed thoroughly under N 2 atmosphere and kept in an ice bath maintained at 0-5 °C for 3 h.The free radical polymerization of Ani was noted by change of color to green (Yelilarasi et al. 2009a, b).At the end of two hours of reaction, the contents were filtered through G4 sintered crucible and washed 3 times with acetone and then, dried at 110 °C for overnight.The dried dark green colored precipitate was collected from the crucible weighed and stored in a polythene-based cover under N 2 atmosphere to avoid further aerial oxidation reaction.This sample is named as RB end capped PANI (CP1).A similar procedure was followed for the preparation and storage of RB end capped PANI/Cr 2 O 3 nanocomposite.Here, 3% weight loading of Cr 2 O 3(bulk) was used and the resultant polymer was named as CP2.3% of bulk chromic oxide was selected based on trial-and-error method.The higher % weight loading of bulk chromic oxide powder leads to increase in viscosity of the reaction medium and resulting with decrease in % yield of polymer.The increase in viscosity of the reaction medium restricts the interaction between the monomer and initiator as well as monomer and bulk chromic oxide.Hence, 3% weight loading of bulk chromic oxide powder was chosen.
The polymerization reaction mechanism is given in Scheme 1.The present system follows the free radical mechanism that involves the initiation, propagation and termination steps.Under aqueous medium even at 0-5 °C, the PDS undergoes to cleavage to form a two similar SO 4 radicals.This is the first and foremost initiation step.The sulfate radical interacts with Ani monomer to form Ani radical cation.The interaction of two Ani monomer radical cation leads to the formation of a dimeric structure (Yelilarasi et al. 2009a, b).The propagation reaction proceeds in such a way with the formation of Ani oligomer radical cation.Similarly, the sulfate radical can interact with RB to form a RB radical.This occurs through the hydroxyl group of RB.The termination reaction occurs via the interaction between the Ani oligomer radical cation and RB radical.This leads to the formation of final end product.The primary oxidation leads to benzenoid form whereas the secondary oxidation leads to the quinonoid form of PANI.Meanwhile, the bulk Cr 2 O 3 accepts free electrons available in the reaction medium to form nano Cr 2 O 3 .The sulfate radical withdraws one electron from the amino group of Ani to form Ani radical cation with the liberation of one electron to the reaction medium.Hence, the conversion of bulk Cr 2 O 3 into nano Cr 2 O 3 involves the consumption of multiple electrons.Thus, formed nano Cr 2 O 3 attached with the imino group of PANI chain.In such a way PANI/Cr 2 O 3 nanocomposites were formed with HCl doping and RB end capping effect.The metal oxide nanoparticles are readily binding with the amino or imino group rather than the -SO 3 H or -SO 2 group or any other group.The metal oxide nanoparticle is not binding with the RB dye since it is not having any binding site on its structure.Once the Cr 2 O 3 nanoparticles are formed with more surface area, it is not acting as an oxidizing agent.Hence, it is not affecting the benzenoid to quinonoid ratio in PANI chain.The bulk Cr 2 O 3 is a mild oxidizing agent, and it may oxidize the Ani monomer or oligoaniline.The new things in the reaction steps are incorporation of RB as an end capping agent on the PANI backbone via free radical mechanism.This will enhance the thermal and optical properties of PANI due to the increase in molecular weight with resonance structure.

Characterizations
The binding energy of the polymer samples was determined using Thermo Scientific, Theta Probe (UK) instrument.The electrical conductivity value of polymer sample was determined using Keithley Four Probe conductivity meter (India).The UV-visible spectrum of the polymer samples was measured with the help of Shimadzu 3600 NIR (Japan) instrument from 200 to 800 nm in DDW.The TGA was done with the help of Universal V4.4A TA Instruments (USA) under air condition at the heating rate of 10 K min −1 .The FTIR spectrum was recorded from 400 to 4000 cm −1 for the polymer samples using Shimadzu-8400 S (Japan) instrument.The Elico SL 174 (India) instrument was used for the fluorescence emission spectrum measurement in the range of 350 to 700 nm.The surface morphology was recorded with the help of SEM, 6300 JEOL (USA) instrument.The HR-TEM images were recorded with the aid of JEM 2100 (Japan) instrument.XRD was recorded on Brucker K 8600 (USA) instrument from 10 to 80° at the scanning rate of 5°/min.The DSC of polymer samples are determined (Universal V4.4A TA Instruments (USA) under N 2 atmosphere at the heating rate of 10 K min −1 ).

Non-isothermal degradation kinetics
The ultimate aim of the present research work is determination of energy of activation (E a ) for the degradation PANI and its nanocomposite.For this purpose, Flynn-Wall-Ozawa (FWO) method, Auggis-Bennet method and Kissinger methods were followed (Amita et al. 2019).The thermodynamic parameters also determined and tabulated.

FWO method
The equation used for the determination of E a by FWO method is given below: where R is gas constant (8.314), β is heating rate ( o C/min), T is degradation temperature ( o C), and E a is apparent activation energy (kJ/mol).In this method, plot of ln(β) versus 1/T was made and from the slope value, the E a value was calculated. (1)

Auggis-Bennet model
The Auggis and Bennett equation is given below to find out the E a value of PANI degradation.
where T d is the degradation temperature exactly determined from the DTA curve.The E a value can be obtained from the slope of the plot ln(β/T d ) versus 1/ T d .A is pre-exponential factor.

Kissinger model
The Kissinger equation for the determination of E a value is given below: The E a can be obtained from the slope of the plot of ln

Determination of TD parameters
The TD parameters can be determined from the following equations as given below (Cincovic et al. 2013): where A (min −1 )-frequency factor, e-Neper number (2.7183), T max -peak temperature, χ-transmission coefficient, k-rate constant, h-Plank's constant, k B -Boltzmann constant, ΔG ≠ , ΔS ≠ andΔH ≠ are the changes in Gibb's free energy, entropy and enthalpy, respectively, for the activated complex formation.

Results and discussion
Determination of % yield is very important to analyze the role of an oxidizing agent during the polymerization reaction.PANI (CP1) is a simple and steric effect free polymer.In the present work, RB end capped PANI was prepared by using peroxydisulfate as an oxidizing agent with the yield of 95.2% (Table 1).Addition of 3% weight of Cr 2 O 3 under the same experimental conditions, the CP2 led to the yield of 97.3%.In the presence of metal oxide (Cr 2 O 3 ) nanoparticle, the % of yield of PANI increased slightly.This can be explained as follows.During the in situ polymerization, the bulk Cr 2 O 3 is reduced into its nano-sized Cr 2 O 3 .The Cr 2 O 3 nanoparticle has high surface area, resulting in surface catalytic effect.As a result, more number of Ani monomer was oxidized to Ani radical cation.In this situation, the added oxidizing agent also helped to form Ani radical cation from Ani monomer.Now these two combined effects led to a higher % yield of PANI in the case of CP2.
The Rp gives an idea on the formation of polymer.The CP1 formed at the rate of 2.83 × 10 -4 mol/lit/sec.But the CP2 system exhibited the Rp value of 2.90 × 10 -4 mol/ lit/sec.This explains the mild catalytic effect of Cr 2 O 3 nanoparticle as discussed above.The Cr 2 O 3 nanoparticle formed during the in situ polymerization has more surface area.As the size is small, the Cr 2 O 3 nanoparticles act as a filler.Due to the filling effect, the thermal, optical and electrical properties of PANI are improved and discussed in the forth coming sessions.Once the Rp of Ani is increased automatically, the molecular weight of PANI will be increased with the increase in resonance effect.This will ultimately increase the thermal and optical properties of PANI.
The functional groups present in the RB end capped PANI backbone, CP1 is characterized by FTIR spectroscopy (Fig. 1a), and it contains quinonoid (1582 cm −1 ), benzenoid (1488 cm −1 ), C-N stretching (1304 cm −1 ), aromatic stretching (817, 688 cm −1 ), N-C stretching (1027 cm −1 ), C-H out of plane bending vibration (752 cm −1 ) and chloride ion stretching (616 cm −1 ).This is in accordance with our earlier report (Yelilarasi et al. 2009a, b). Figure 1b represents the FTIR spectrum of CP2 system.This system too presents the above discussed peaks with one new peak which is seen due to Cr-O stretching (591 cm −1 ) (Ponprabhakarann et al. 2019).This confirmed the PANI/Cr 2 O 3 nanocomposite formation.One important point to be noted here is the benzenoid form in Fig. 1a which appears as a doublet due to the presence of PANI and RB.But in Fig. 1b, it appeared as a single sharp peak due to the added Cr 2 O 3 .It means, the added Cr 2 O 3 controls the structure of PANI.Moreover, during the polymerization of Ani, emeraldine and pernigraniline structure were formed due to the homogenization by the bulk Cr 2 O 3 , a mild oxidizing agent.In this work, a bulk sized Cr 2 O 3 was added.During the polymerization, it gains electrons from the reaction medium and reduced to its nano-size.This is further confirmed by XPS and HR-TEM techniques.Electrons are released to the reaction medium because of oxidation of Ani by persulfate radical and bulk Cr 2 O 3 .The advantage of the present research work is synthesis of PANI with simultaneous formation of Cr 2 O 3 nanoparticle.Above all, the PANI was formed with unique structure, i.e., the PANI backbone was formed by benzenoid and quinonoid forms.But in the absence of Cr 2 O 3 benzenoid structure of pernigraniline and emeraldine salts were formed, this is confirmed by the stereo selective reaction of Cr 2 O 3 with increase in electrical conductivity.Thus, the present methodology offers more advantage to the conducting polymer field.By using the FTIR spectrum software, the absorbance peak  The HCl doped PANI is a dark green colored polymer with the λ max value of 600 nm (Amita et al. 2019) with a broad peak corresponding to the conducting form of PANI.In the case of RB end capped PANI (Fig. 2a), a sharp peak appeared with the λ max value of 550 nm (Roy et al. 2008).Here, the PANI chains are chemically grafted with RB through its steric free -OH group.The same chemical reaction occurred in the presence of Cr 2 O 3 , shifted the peak to higher λ max value of 565 nm (Fig. 2b).The red shift is responsible for the association of Cr 2 O 3 with the ether oxygen atom of RB which leads to the increase in resonance stabilization.Generally, the nano-sized metal oxide degrades the structure of dye through the photography degradation reaction.But in this case, such a reaction was suppressed due to reaction temperature (0-5 °C).Hence, the nanocomposite formation enhanced the optical properties of RB end capped PANI.It is necessary to find out the direct band gap (BG) energy of the present research work because it contains a colored polymer conjugated with a RB dye and semi-conducting Cr 2 O 3 .From the UV-visible data, Tauc's plot was made and indicated in Fig. 2c for CP1 with the band gap value of 2.16 eV.Table 1 showed the data.Similar plot was made for CP2 (Fig. 2d) system and calculated the band gap value as 2.09 eV (Table 1).It was found that after the nanocomposite formation, the BG energy was found to be decreased.This confirmed the chemical grafting of RB with PANI backbone, i.e., chain end capping effect.The decrease in BG energy confirmed the existence of chemical interaction between -NH group of PANI and Cr 2 O 3 nanoparticles.Even though the PANI is a dark green colored one, its emission activity is not good when compared to the pristine dyes.In order to improve the emission behavior of PANI, the present research work was made.The fluorescence emission spectrum (FES) of CP1 is given in Fig. 3a and the data is given in Table 1.The spectrum showed one emission peak at 557 nm with the intensity of 574 cps (Baynakutan et al. 2020).The CP2 system showed an emission peak at 579 nm (Fig. 3b) with an intensity of 863 cps.In comparison, the emission intensity and wavelength were increased in the The surface morphology (SEM image) of CP1 is given in Fig. 4a with dried sky like morphology with micro voids.The edge of the polymer particles is white, and this confirmed the fluorescent nature of the RB end capped PANI.The sizes of the particles varied in length from 150 to 800 nm. Figure 4b represents the SEM image of CP2 system.The arrow marked portion confirmed the cage like structure.The size of Cr 2 O 3 nanoparticle was determined as ~ 50 nm.Further, the size of the same will be confirmed by HR-TEM technique.Here, the entire image is white, which implies that the Cr 2 O 3 nanoparticles enhanced the fluorescent nature of PANI.Hence, the SEM study proved the fluorescence enhancing activity of Cr 2 O 3 nanoparticles.
The size and shape of the Cr 2 O 3 present in CP2 system are given in Fig. 5.The rhombohedral structure of Cr 2 O 3 nanoparticle is shown by the arrow mark in Fig. 5a. Figure 5b shows the presence of various crystal planes and confirmed the crystalline structure of Cr 2 O 3 nanoparticles.Figure 5c, d confirmed the presence of perfectly ordered region of Cr 2 O 3 .The circled portion in Fig. 5d denotes the lattice ordered structure propagated on the edge of the grains (Ohyma et al. 1989).In Fig. 5e, the arrow mark confirmed the presence of lattice planes.Figure 5f shows the SAED pattern of Cr 2 O 3 present in the CP2 system.The SAED declared the presence of ( 012), ( 104), ( 113) and ( 024) crystal planes in Cr 2 O 3 nanoparticles (Galvan et al. 2014).Hence, the HR-TEM confirmed the presence of Cr 2 O 3 in crystalline form with rhombohedral shape and with clear edge.
The crystalline nature of polymer samples was confirmed by XRD. Figure 6a indicates the XRD of CP1 system.It is well known that the PANI is an amorphous powder.The end capping agent influenced the crystallinity of PANI.The diffractogram showed the peaks at 11.3° (002), 15.7 o (030), 20.9 o (010), 24.5 o (102) and 26.9 o (200).Appearance of these peaks confirmed the PANI formation due to the repetition of benzenoid and quinonoid rings (Mostafei et al. 2009).The other peaks are corresponding to the end capping agent.The XRD of CP2 system is shown in Fig. 6b.Here, also, the above said peaks corresponding to RB and PANI are seen.Apart from these peaks, some new peaks are also seen corresponding to Cr 2 O 3 nanoparticles.New peaks at 24.7°(012), 33.4 o (104), 35.9 o (110), 40.6 o (113), 50.6 o (024), 55.0 o (110), 63.5 o {a small hump, (214)} and 65.1° {a small hump, (300)} can be seen.Appearance of these peaks confirmed the rhmbohedral structure of α-Cr 2 O 3 (Ullah et al. 2015).At the same time, the crystalline peaks of α-Cr 2 O 3 were suppressed.Hence, the XRD results further supported the HR-TEM and SAED reports.
The intrinsic viscosity (IV) measurement gives an idea about the molecular weight of the polymer chain.The IV values of CP1 and CP2 are determined as 0.51 and 0.48 dL/g, respectively, and the data are given in Table 1.It was found that the CP2 system showed lower IV value due to the increase in viscosity of the reaction medium while adding bulk Cr 2 O 3 during the solution polymerization of Ani.Moreover, once the tetramer is formed immediately, it will be precipitated from the reaction medium and further increase in polymer chain length is restricted.This depends on the nature of solvent and monomer considered for polymerization.If the solute-solvent interaction is dominant than solute-solute interaction, there is a chance for the increase in IV value of PANI.In the present work, NMP was used as a reaction medium.Hence, the chain length of PANI was slightly increased.This leads to increase in IV value of the polymer.This is further supported by TGA result.
The electrical conductivity values of CP1 and CP2 systems are given in Table 1.The CP2 system exhibited higher electrical conductivity value of 6.82 × 10 -2 S/cm.This is due to the smaller size of Cr 2 O 3 nanoparticle.The increase in electrical conductivity is not only due to increase in resonance stabilization of PANI structure but also due to the surface catalytic effect offered by the Cr 2 O 3 nanoparticles.During the in situ polymerization reaction, the bulk Cr 2 O 3 is converted into nano-sized Cr 2 O 3 simultaneously.The nanoparticle has more surface area due to nano-size.This might accelerate the Rp reaction and increase the polymer chain length.But the case is reverse here.The solution polymerization of Ani was started with the addition of PDS and bulk Cr 2 O 3 .This restricts the interaction between monomer and initiator molecules and resulting with decrease in IV of PANI.The increase in electrical conductivity value indirectly indicates the chain length of PANI is increased with the increase in resonance effect.Once the PANI chain length is increased, automatically the IV of PANI is also increased.If the molecular weight of PANI is increased the thermal properties like T g and T d (Table 1) will also be increased.This proved that the increase in electrical conductivity will indirectly enhance the other thermal and optical properties.
The main aim of the present research work is to increase the processability of PANI via increase in solubility of the same.Hence, the solubility of PANI in its doped form was tested in different polar and non-polar solvents.The RB end capped PANI is soluble in DMSO, DMF, HCO 2 H, THF, NMP, CHCl 3 and EtOH.In DDW, it is soluble only after long ultrasonication.Now the solubility of PANI in its HCl doped form is increased via end capping process.
The T g of PANI synthesized both in the presence and absence of Cr 2 O 3 was determined and compared.The CP1 system showed the T g value of 59.4 °C (Fig. 8a) (Table 1) whereas the CP2 system showed the T g at 64.8 °C (Fig. 8b) (Table 1).The CP2 system showed the higher T g as a first order transition due to the presence of Cr 2 O 3 .The induced crystallinity by the RB and Cr 2 O 3 are responsible for this higher T g .The DSC study proved that the T g of PANI was increased by the addition of Cr 2 O 3 .When compared with the literature (Bhadra and Khastgir 2009), the present system yielded excellent T g value.It means the thermal property of the PANI was enhanced by the added Cr 2 O 3 .
The TGA thermogram of RB end capped PANI (CP1) is shown in Fig. 9a with three step degradation process.The minor weight loss around 115 °C is due to the removal of moisture and HCl dopant.The major degradation occurred at 458 °C is due to the degradation of PANI backbone.A minor weight loss step around 521 °C is ascribed to the degradation of rigid phenyl structure of RB dye.Table 1 showed the TGA data.Above 750 °C 15.5% weight remained as residue.When compared with Kumar et al (2020) report, the present system exhibited higher T d value.Figure 9a-e showed the TGA thermograms of PANI carried out at five different heating rates under atmospheric oxygen condition.The DTA thermograms of CP1 are given in Fig. 9f-j.It was found that while increasing the heating rates the T d of CP1 increased slowly.This can be explained on the basis of fast scanning process.In order to find out the energy of activation (Ea) for CP1 backbone, three different methods were adopted.The first method includes the plot of 1000/T d Vs ln (β) (Fig. 9k) which is known as Flyn Wall Ozawa method.The second method includes the plot of 1000/T d Vs ln (β/T d ) (Fig. 9l) also known as Auggis-Bennett method, and the third method includes the plot of 1000/T d Vs ln (β/T d 2 ) (Fig. 9m), also known as Kissinger method.From the slopes of the plots, the average Ea value was calculated as 87.28 kJ/mol (Table 2) for step 2. In such a way, the Ea value for the degradation of RB dye (step 3) was determined by plotting 1000/T d Vs ln β (Fig. 9n), 1000/T d Vs ln (β/T d ) (Fig. 9o) and 1000/T d Vs ln (β/T d 2 ) (Fig. 9p).The average Ea value was determined as 107.28 kJ/mol (Table 2).In comparison, step 3 consumed more amount of thermal energy than step 2 for its degradation.This is due to more rigid structure of RB dye.When compared with the Amita and research team's report (Amita et al. 2019), the present system yielded lower Ea value due to the presence of RB end capping agent.
Figure 10a-e confirms the TGA thermogram of CP2 system with three steps degradation process (Table 1).The minor weight loss taken place around 130 and 215 °C are due to the removal of HCl dopant and NMP solvent, respectively.The major weight loss occurred at 396 °C.Above 750 °C, 42.5% weight remained as residue.In comparison, the degradation temperature (T d ) of PANI was deceased in the presence of Cr 2 O 3 and this can be explained as follows.(i) The added Cr 2 O 3(bulk) during the solution polymerization of ANI increased the viscosity and reduced the interaction between ANI radical cations, (ii) the interaction between persulfate radicals and ANI monomer was restricted, (iii) at 0-5 °C, the decomposition rate of persulfate in the presence of Cr 2 O 3 may be delayed.As a result, the molecular weight of resultant PANI chains decreased.At the same time the % weight residue remained above 750 °C rose due to the added Cr 2 O 3 .In overall comparison, the CP2 system exhibited a lower T d due to the molecular weight of PANI.The decrease in molecular weight of PANI was confirmed by IV measurements.Chemical polymerization of aniline, in the presence of bulk Cr 2 O 3 , was carried out.The present system also exhibited three step degradation processes.As usual, the de-doping process was noticed below 200 °C.The PANI backbone degradation was noticed at 395 °C whereas the RB structure degradation happened at 439 °C.The TGA thermograms of CP2 system at various heating rates are shown in Fig. 10a-e.The DTA thermograms are shown in Fig. 10f-j.It was found that while increasing the heating rates the T d of PANI are proportionally increased.In order to find out the Ea for the degradation of PANI back  bone, the following plots were made for step 2. Plots of 1000/T d Vs ln β (Fig. 10k), 1000/T d Vs ln (β/T d ) (Fig. 10l) and 1000/T d Vs ln (β/T d 2 ) (Fig. 10m) were made and noted as a straight line with decreasing trend.By linear fitting method, a tangent was made.Now the slopes of the plots were noted, and the average Ea value was calculated as 77.15 kJ/mol.The Ea value for step 3 of CP2 was determined by using the above-mentioned plots.Plots of 1000/T d Vs ln β (Fig. 10n), 1000/T d Vs ln (β/T d ) (Fig. 10o) and 1000/T d Vs ln (β/T d 2 ) (Fig. 10p) were made and noted as a straight line with decreasing trend.The average Ea value was calculated as 85.06 kJ/mol for the degradation of rigid RB structure.When compared with step 2, the step 3 consumed more amount of thermal energy.This is due to the rigid structure of RB dye.In overall comparison, the PANI/Cr 2 O 3 nanocomposite system consumed lower amount of thermal energy for the PANI backbone degradation.Under atmospheric air condition, the Cr 2 O 3 nanoparticles can act as an oxidizing agent and catalyst for the degradation of PANI backbone which is continuously enhanced through the surface catalytic effect of Cr 2 O 3 nanoparticles.While using different formulae for the calculation of Ea, the final value is also varied but within the limit.When the T d is squared in the denominator, the Ea  value will be slightly lowered.Anyhow, the three methods used for the determination of Ea produced the values within 2% error only.Determination of TD parameter value for both CP1 and CP2 systems is very important to analyze the degradation mechanism.During the degradation reaction under air atmosphere at different heating rates, the TD parameters such as entropy (S), enthalpy(H) and free energy(G) of the products are varied.It is well known that PANI and its nanocomposite with Cr 2 O 3 were prepared by solution polymerization method via free radical mechanism.The synthesis involves an exothermic reaction.Various TD parameters were determined and tabulated in Table 3.The thermodynamic parameter values for CP1-stage-I were calculated at various heating rates.The average ∆S value for the degradation of step 2 was calculated as − 242 J/K.mol.The average ∆H and ∆G values were calculated as − 6144 J/mol and 175,022 J/mol, respectively.From the TGA curves, the thermodynamic parameters for step 3 of CP1 system were also determined and compared.The average ∆S value was calculated as − 244 J/K.mol (Table 3).The average ∆H value was calculated as − 6692 J/mol and the average ∆G value was calculated as 190,268 J/mol, respectively (Table 3).Bompilwar et al (2010) reported that the H 2 SO 4 doped PANI exhibited the ΔS and ΔG values of − 43.16 J/ mol/K and 24.4 kJ/mol, respectively.Table 3 indicates the TD parameter values of CP2 system which was carried out at five different heating rates.The average ∆S value for the PANI backbone (stage-2) degradation in presence of Cr 2 O 3 nanoparticle was calculated as − 241 J/K.mol.When compared to the neat polymer system, the present system is having almost the equal value.The average ∆H value for the degradation of step 2 was calculated as − 5995 J/mol.The ∆H value is somewhat lower when compared with the neat polymer system.While coming to ∆G value for the degradation of step 2, it was calculated as 169,104 J/mol.This value is somewhat higher when compared to neat polymer system.The average ∆S, ∆H and ∆G value for the stage-3 of CP2 system was calculated as − 333 J/K.mol, − 5638.9J/mol and 220,395 J/mol, respectively (Table 3).In overall comparison, the ∆G value is higher for CP2 system.This explains the filling effect offered by Cr 2 O 3 nanoparticle.The present system exhibited similar values.The PANI nanoparticle exhibited high ΔG, ΔH and ΔS values (Ebrahimi et al. 2018).By analyzing these results, one can come to a conclusion that the TD parameter depends on the size, shape and nature of the nanoparticle.

Conclusions
From the present research work, the important results are collected and given here as conclusion.The Cr 2 O 3 mediated solution polymerization decreases the % yield of PANI due to the increase in viscosity of the reaction medium.During the solution polymerization reaction, the bulk sized Cr 2 O 3 was converted into nano-sized Cr 2 O 3 simultaneously.The benzenoid form appeared around 1500 cm −1 in the FTIR spectrum was enhanced by the Cr 2 O 3 nanoparticles.Both the absorbance and emission spectra showed enhanced results with red shifting in their peaks due to the chain end capping effect.The band gap value of PANI was decreased slightly while adding Cr 2 O 3(bulk) .Increase in T g value was noticed for PANI due to the influence of Cr 2 O 3 nanoparticle; at the same time, the IV was found to be reduced due to the high viscosity of the reaction medium by the added bulk Cr 2 O 3 .The XRD results declared the crystalline nature of PANI/ Cr 2 O 3 nanocomposite and further evidenced by SAED pattern.The SEM image showed the fluorescent surface with cage like morphology.The HR-TEM confirmed the rhombohedral geometry of Cr 2 O 3 nanoparticle.The Cr 2 O 3 mediated synthesis of PANI exhibited Cr3d2p 3/2 and Cr3d2p 1/2 peaks in the XPS, and this confirmed the formation of PANI/Cr 2 O 3 nanocomposite.Due to the presence of Cr 2 O 3 nanoparticle, the electrical conductivity of PANI was increased slightly.While increasing the heating rate, the T d of RB end capped PANI also increased due to the fast scanning.Higher thermal energy was consumed by PANI system for its degradation.The ΔS value was higher for CP2 system due to the catalytic degradation effect offered by the Cr 2 O 3 nanoparticle.Next, our research team is going to test the supercapacitance activity of PANI and its nanocomposite with Cr 2 O 3 nanoparticle.

Fig. 1
Fig. 1 FTIR spectrum of a CP1 and b CP2 systems This leads to the conclusion of both the chemical structure of CP1 and CP2 are different.In 2014,Abdullah et al (2014) reported the band gap energy of Cr 2 O 3 as 3.2 eV.The present research work yielded lower band gap value and exhibited excellent results.In the present research work, the added bulk Cr 2 O 3 slightly shifted the λ max value, and this confirmed the existence of chemical interaction between Cr 2 O 3 nanoparticle and PANI chains.The -NH group of PANI interacted with the Cr 2 O 3 nanoparticle.

Fig. 4
Fig. 4 SEM image of a CP1 and b CP2 systems

Fig. 6
Fig. 6 XRD of a CP1 and b CP2 systems Fig. 7 XPS of a CP1 and b CP2 systems

Fig. 8
Fig. 8 DSC thermogram of a CP1 and b CP2 systems