Preparation , Characterization , and Thermal Degradation Studies of p-Nitrophenol-Based Copolymer

Polycondensation reactionwas employed to synthesize a new copolymer resin (p-NP-4,4-MDA-F) from p-nitrophenol (p-NP) and 4,4-methylene dianiline (4,4-MDA)with formaldehyde (F) in presence of 2Mhydrochloric acid as a catalyst at 130±1C.e resin was characterized by elemental analysis and spectral studies such as UV-visible, FT-IR, and H-NMR spectra which were used to con�ne the most probable structure of synthesized copolymer.ermal degradation pattern and kinetics have been investigated by thermogravimetric analysis.ermal degradation curve have been studied with minute detail for each degradation step. Friedman, Chang, Sharp-Wentworth, Freeman-Carroll, and Coat-Redfern methods have been implemented in order to compute the kinetic parameters, that is, thermal activation energy (EEaa), order of reaction (n), and frequency factor (z).


Introduction
Due to global applications of polymeric materials polymer science and technology have been developing rapidly and attracted much attention towards the polymer scientists.Polymers have been found to be very useful applications as adhesive, high-temperature �ame-resistant �bers, coating materials, semiconductors, catalysts, and ion-exchange resins [1][2][3][4][5][6].Varieties of researches regarding the thermal studies of polymers are emerging out to investigate their renewed applications for the betterment of mankind.Area of polymer reaction kinetics is enhanced by applying various model �tting kinetic equations in order to study its kinetic and thermodynamic aspects [7].
In this paper, we present the synthesis and characterization of phenolic straight chain copolymer (p-NP-4,4 � -MDA-F) by condensation terpolymerization reaction of pnitrophenol and 4,4 � -methylene dianiline using the linkage of formaldehyde.ermal degradation behavior was studied by TGA under nonisothermal conditions.Certain generalizations are made regarding the kinetic parameters computed by using Friedman, Chang, Sharp-Wentworth, Freeman-Carroll, and Coat-Redfern method.

Synthesis. p-NP-4,4
� -MDA-F copolymer was prepared by condensing p-nitrophenol (2.78 g, 0.2 mol) and 4,4 �methylene dianiline (1.98 g, 0.1 mol) with formaldehyde (11.25 mL, 0.3 mol) in presence of 2 M HCl as a catalyst in the molar proportion of 2 : 1 : 3 at 130 ∘ C in an oil bath for 3 hours.e temperature of electrically heated oil bath was controlled with the help of a dimmerstat.e dark reddish brown resinous solid product was immediately removed, �ltered, and repeatedly washed with cold-distilled water, dried in air and powdered with the help of an agated mortar and pestle.e product obtained was extracted with diethyl ether to remove excess of p-nitrophenol-formaldehyde copolymer which might be present along with p-NP-4,4 � -MDA-F copolymer.Dried resin sample was dissolved in 8% NaOH and regenerated using 1 : 1 HCl/water (v/v) with constant stirring and �ltered.is process was repeated twice.Resulting copolymer sample was washed with boiling water and dried in a vacuum at room temperature.Puri�ed copolymer resin was �nely ground to pass through 300 mesh size sieve and kept in a vacuum over silica gel [21].e Chemical reaction of above synthesis is given in Figure 1.

Spectral and ermal Studies.
Copolymers were subject to elemental analysis for carbon, hydrogen, and nitrogen on Perkin Elmer 2400 Elemental Analyser.UV-visible spectra were recorded in dimethylsulphoxide on Shimadzu UV-Visible double beam spectrophotometer in the range of 200-850 nm.Infrared spectra were recorded in nujol mull on Perkin-Elmer-spectrum RX-I spectrophotometer in the range of 4000-500 cm −1 . 1 H-NMR studies were performed in dimethylsulphoxide as solvent on Bruker Advance-II 400 MHz proton NMR spectrophotometer.All the analytical and spectral studies for newly synthesized copolymers were carried out at Sophisticated Analytical Instrumentation Facility (SAIF) Punjab University, Chandigarh.e nonisothermal thermogravimetric analysis of newly prepared copolymers has been carried out using Perkin Elmer, Pyris1 ermogravimetric Analyzer, in air atmosphere with a heating rate 10 ∘ C⋅min −1 in the temperature range 50-600 ∘ C. TGA was carried out at SICART, Gujrat.

Friedman Technique. We have
where,  is the conversion at time ,  is the gas constant (8.314J/mol/K), and  is the absolute temperature (K).From the slope of the linearplot of ln(1 − ) versus 1/,  can be obtained.eplot of ln(/) versus 1/ should be linear with the slope   /, from which   can be obtained.
A plot of [ln()(1 − )  ] versus 1 will yield a straight line if the order of decomposition reaction,  is selected correctly.e slope and intercept of this line will provide the (−  ) and ln() values, respectively.

Sharp-Wentworth Technique.
We have where,  = rate of change of fraction of weight with change in temperature;  is linear heating rate, ;  is the fraction of polymer decomposed at time .us, a linear plot of log(()(1 − )) versus 1 is obtained whose slope gives the value of   and  may be evaluated from the intercept.e linear relationship con�rmed that the assumed order is correct.

Freeman and Carroll
Technique.We have

Results and Discussion
3.1.Spectral Studies of p-NP-4,4 � -MDA-F Copolymer 3.1.1.Elemental Analysis.e yield of resin was found to be 80%.Composition of copolymer obtained on the basis of elemental analysis data and was found to be in good correlation to that of calculated values as follows (Table 1).

UV-Visible Spectra.
UV-visible spectrum of p-NP-4,4 � -MDA-F copolymer sample in pure DMSO was recorded in the wavelength region 200-850 nm which is shown in Figure 2. e spectrum displayed two characteristic bands at wavelengths 250 and 330 nm.e former and less intense band may be due to    * transitions [28,29] observed at 250 nm indicating the presence of biphenyl methane moiety.Hypsochromic shi observed in biphenyl may be due to introduction of -CH 2 group between two phenyl groups which destroys the conjugation.
Later and more intense band observed at 330 nm may be accounted for    * transition might be due to chromophoric (-NO 2 ) group.e auxochromic substituents (-OH) interact with  electron of the benzene ring.is interaction stabilizes  * state and thus lowers the energy as a result bathochromic shi is caused.e auxochromic in�uence is more pronounced when an electron donating and electron attracting groups are para to each other.is is called complementary substitution.So a strong bathochromic shi is observed due to charge transfer absorption related to the contribution of following polar resonance structures.

FT-IR Spectra.
e FTIR-spectrum of p-NP-4,4 � -MDA-F copolymer is represented in Figure 3 and the data is reported in Table 2. Broad band appeared at 3451.0 cm −1 , which may be assigned to the stretching vibration of the phenolic -OH groups exhibiting intermolecular hydrogen bonding [30,31].e presence of a weak peak at 2924.4 cm −1 describes the -NH-in 4,4 � -methylene dianiline moiety which might be present in copolymeric chain [31].A sharp and weak peak obtained at 2855.0 cm −1 indicates the presence of stretching vibrations of methylene group (-CH 2 -) in the copolymer chain [30].A medium band, displayed between T 2: IR frequencies and 1 H-NMR spectral data of p-NP-4,4 � -MDA-F copolymer.
IR Wavenumber (cm −1 ) Assignment  1650.0-1660.2cm −1 , may be due to stretching vibration of >C=C< in aromatics.Broad and strong bands displayed at 1498.9 cm −1 for asymmetric and 1338.8 cm −1 for symmetric vibrations con�rm the presence of nitro group in the polymer chain [31].C-O stretch in phenol is represented at 1217.2 cm −1 .e presence of methylene bridges (-CH 2 -) in the polymeric chain can be accounted by the presence of bands at 1460 cm −1 , 1345-1350 cm −1 , and 750-770 cm −1 for bending, wagging, and rocking vibrations, respectively [32,33].e presence of tetrasubstitution of aromatic ring [31] is recognized from the weak band appearing at 850. 3.1.4. 1 H-NMR Spectra. 1 H-NMR spectral data is given in Table 2 and spectrum is presented in Figure 4. Spectra reveal different patterns of peaks, since each of them possesses a set of protons having different proton environment.A signi�cant down�eld in chemical shi of proton of phenolic -OH group, observed at  10.7 ppm, is due to intermediate proton exchange reaction of phenolic -OH group [32,34].A weak singlet is observed at  7.9 ppm and is due to meta protons of phenol.In 4,4 � -methylene dianiline moiety, the doublet observed in the region  6.8-6.9 and  8.0 ppm is attributed to protons ortho to -NH and protons ortho to-CH 2 , respectively.A broad singlet observed at  3.5 ppm may be  3.1.5.ermogravimetric Analysis.ermal degradation behavior and kinetic data of copolymer are recorded in Table 3 and thermogram is shown in Figure 5. e resin p-NP-4,4 � -MDA-F exhibits three stages of decomposition.
When temperature was raised to 130 ∘ C, it showed the weight loss about 3.48% and is corresponding to the moisture entrapped in molecule or water of crystallization associated with this copolymer [15,35].is is in agreement with the weight loss calculated theoretically which is about 3.39%.Second step is slow decomposition in temperature range 130-270 ∘ C, corresponding to 27.30% mass loss which may be attributed to loss of two hydroxyl and nitro groups against calculated value of 27.16% loss present per repeat unit of polymer.e third step of decomposition starts from temperature 270 to 600 ∘ C, corresponding to 61.64% loss of degraded moiety (Figure 5) against calculated weight loss of 61.13%.

Kinetics of ermal Decomposition by
Friedman, Chang, Sharp-Wentworth, and Freeman-Carroll Techniques.e decomposition is due to pyrolysis of straight-chain-linked structure of copolymer due to which it does not takes much more time to attain the thermal equilibrium as well as at 600 ∘ C degradation process occurs upto �nal level leaving behind the remaining moiety.e thermal stability of p-NP-4,4 � -MDA-F copolymer is concluded to be higher, may be due to the stronger intermolecular hydrogen bonding present in polymer structure because of water of crystallization which would be more difficult to break and hence more resistant to higher temperature [36] or it may be due to possibility of an almost linear structure of copolymer chain which gives stability to polymer chain [37].
In general, the water of hydration may be considered as crystal water.In the present case removal of one water molecule is complete and is probably crystal water or moisture entrapped in the molecule.By applying the thermogravimetric data to �ve thermal degradation kinetic techniques (viz.Friedman, Chang, Sharp-Wentworth, Freeman-Carroll and Coat-Redfern), it shows three different degradation steps corresponding to loss of respective groups.Kinetic parameters for p-NP-4,4 � -MDA-F copolymer have been calculated using Friedman method (1).Activation energy (  ) has been obtained from the plot between ln( versus (1) (Figure 6) and order of reaction () from the plot between ln(1− versus (1) (Figure 7).Values of ln () are calculated at each temperature region from (1) with the help of   and .calculate   and ln( of respective degradation reaction for best �tted value of  (from Friedman equation), which corresponds to correct reaction order for each respective thermal decomposition step.Also kinetic parameters for different thermal degradation zones have been calculated by Sharp-Wentworth method (3).  and ln () values are calculated from the plot between log((   versus  with best �tted values of n for each respective degradation reaction (Figure 9).
Similarly using the Freeman-Carroll technique (4) kinetic parameters has been calculated.Figure 10  intercept for each step are computed from (4), which is equal to (  ) and  respectively.Kinetic parameters have been evaluated by using Coat Redfern (5) method by plotting ln[((    ( 2 ( versus () for p-NP-4,4 � -MDA-F, which results in straight line of slope [  ] for correct value of  (Figure 11).e results of kinetic parameters of above-mentioned methods have been incorporated in Table 4.
A plot of percentage mass loss versus temperature is shown in Figure 5 for a representative p-NP-4,4 � -MDA-F copolymer.From the TG curves, the thermoanalytical data and decomposition temperature have been determined for different stages as given in Table 3. is kinetic analysis should be a starting point to obtain the useful information on the behavior of samples.Fairly comparable results in the kinetic parameters, that is,   , , and ln( that are obtained by Friedman and Chang may be due to analogy in mathematical model.Also fairly similar results with slight variations obtained by Sharp-Wentworth and Freeman-Carroll methods, but Coat-Redfern method, show different observations.From the above discussion, it is therefore concluded that for each technique, the values of kinetic parameters depend on calculation technique used as well as degrading species at a particular step.Total calculations obtained from different kinetic models demonstrated that the numerical value of kinetic parameters depends on the mathematical model used to analyze the experimental data and level of degradation [20].Due to complex phenomena of polymer degradation process in nonisothermal thermogravimetry, the computed kinetic parameters are in fact only parameters of given mathematical equation which has the form of kinetic rate equation and which is used to �t the weight loss curves accompanying the thermal degradation of polymers in nonisothermal conditions.Low values of frequency factor revealed that decomposition reaction of copolymer may be slow and no other possible reason can be given [38,39].As a consequence these kinetic parameters are �ctive from the point of view of chemical kinetics.
By using above-mentioned techniques fairly similar results are obtained which represents versatility and great utility of thermal degradation mathematical equations in thermal studies and attempts are developing to implement the model free kinetic equations.
Synthesis of targeted copolymer (p-NP-4,4 � -MDA-F) has been con�rmed which is supported by the results obtained by the elemental analysis and spectral data.ermogram has shown three degradation stages, �rst indicating degradation of water molecule, second shows removal of two (-OH) and (-NO 2 ) groups, and third step represents two side benzene rings with methylene groups.Friedman and Chang methods show nearly similar values of kinetic parameters may be due to resemblance in mathematical model, whereas results obtained from Freeman-Carroll and Sharp-Wentworth are in well agreement with each other, but Coat-Redfern method shows different observations.e values of kinetic parameters are signi�cantly controlled by level of degradation and calculation technique used to analyze the experimental data.
,  is rate of change of weight with time.  is   − ;   is Weight loss at the completion of reaction;  is Total weight loss upto time.  is Energy of activation;  is order of reaction.eΔlog() and Δlog   values are taken at regular intervals of 1.