Thermal Degradation Studies of Terpolymer Derived from 2-Aminothiophenol , Hexamethylenediamine , and Formaldehyde

Terpolymer (2-ATPHMDAF-I) has been synthesized by the condensation of 2-aminothiophenol and hexamethylenediamine with formaldehyde in the presence of 2M hydrochloric acid as a catalyst with 1 : 1 : 2 molar proportion of reacting monomers. The structure of newly synthesized terpolymer has been elucidated and confirmed on the basis of elemental analysis and various spectral techniques, that is, UV-visible, FT-IR, and H-NMR spectroscopy. Number averagemolecular weight (Mn) has been determined by conductometric titration in nonaqueousmedium.The viscositymeasurements in dimethyl sulfoxide (DMSO) have been carried out to ascertain the characteristic functions and constants.The studies have been further extended to nonisothermal thermogravimetric analysis for determination of their mode of decomposition and relative thermal stability. Activation energy (E a ), order of reaction (n), and frequency factor (z) were calculated by Friedman, Chang, Sharp-Wentworth and Freeman-Carroll methods. Activation energy calculated by Friedman andChangmethods are in close agreementwith each otherwhile the results obtained fromFreemanCarroll and Sharp-Wentworth’s methods are found to be in a similar order.


Introduction
Study of thermal analysis comprises a group of techniques in which a physical property of substance is measured as a function of temperature when the substance is subjected to a controlled temperature program.Thermally stable terpolymers have recently become boon to polymer chemist due to their superior and high performance utility.Since the modern history of thermogravimetry, thermal degradation of polymers and the study of their kinetics have been at the center of thermal analysis.Many researchers tried to improve the thermal stability at elevated temperature by changing the monomer composition in polymer synthesis.
The thermal degradation study of terpolymer has become a subject of recent interest, being an important property which primarily decides thermal stability and processability.A wide variety of thermally stable polymers have been synthesized and studied their thermogravimetric property and finds many applications such as ion-exchangers [1][2][3][4][5], semiconductors [6], high dielectric constant for energy storage capacitors [7], packaging, adhesives and coatings in electrical sensors, activators, catalysts and thermally stable materials [8][9][10].Phenolic resins are known for their wide applications in various areas because of their thermal stability, easy availability, cost effectiveness, and some of their excellent properties [11].
In our earlier communications [3, 12, 13, 15-17, 19, 20, 22], various studies on synthesis, characterization, and thermogravimetric analysis of some new polymeric resins have been reported.Hence, in the present investigation, it has been planned to study the nonisothermal thermogravimetric analysis of terpolymer derived from 2-aminothiophenol, hexamethylenediamine, and formaldehyde which has not been reported so far in literature.

Starting Materials.
2-Aminothiophenol and hexamethylenediamine used in the present investigation of analytical grade purity were purchased from Acros Chemicals, Belgium, and Aldrich, USA, respectively.Formaldehyde (37%) was purchased from S.D. Fine Chemicals, India.All the used solvents like N,N-dimethylformamide, dimethyl sulfoxide, tetrahydrofuran, acetone, and diethyl ether were procured from Merck, India.

Synthesis. 2-ATPHMDAF-I terpolymer was prepared by condensing 2-aminothiophenol and hexamethylenediamine
with formaldehyde in the presence of 2 M hydrochloric acid as a catalyst in 1 : 1 : 2 molar proportions at temperature 150 ∘ C in an oil bath for about 6 hrs.The yellowish white colored solid product was obtained.The product obtained was extracted with diethyl ether to remove the excess of 2-aminothiophenol-formaldehyde copolymer which might be present along with 2-ATPHMDAF-I terpolymer.The chemical reaction of above synthesis has been depicted in Figure 1.

Characterization of Terpolymer.
Newly synthesized and purified terpolymer was subjected to elemental analysis for carbon, hydrogen, nitrogen, and sulphur on Elementar Vario EL-III Elemental Analyzer and ultraviolet-visible spectra of terpolymer in dimethyl sulfoxide (DMSO) solvent recorded on Varian Carry 5000 UV-Vis spectrophotometer in the range from 200 to 800 nm at Sophisticated Test and Instrumentation Center Cochin, University of Science and Technology, Cochin. 1 H-NMR study was performed in DMSO as solvent on Bruker Advance-II 400 NMR spectrophotometer.Infrared spectrum was recorded in nujol mull on Perkin Elmer Spectrum RX-I spectrophotometer in the range from 4000 to 400 cm −1 at Sophisticated Analytical Instrumentation Facility (SAIF), Punjab University, Chandigarh.The nonisothermal thermogravimetric analysis of newly prepared terpolymer has been carried out using Perkin Elmer Diamond 3-II thermogravimetric analyzer, in air atmosphere with a heating rate of 10 ∘ C min −1 in the temperature range from 40 to 1000 ∘ C at Vishveshwarya National Institute of Technology (VNIT), Nagpur.

Analytical and Physicochemical Studies.
The number average molecular weight (Mn) was determined by conductometric titration in nonaqueous medium such as dimethyl sulfoxide (DMSO) using ethanolic KOH as a titrant.Conductometric titration in nonaqueous media has been proved to be a simple yet effective method used by earlier research workers [13,23,24] for the determination of number average molecular weight of phenol-formaldehyde resins [25].
The intrinsic viscosity of newly synthesized terpolymer has been evaluated using Ubbelohde viscometer [26,27] fabricated in our research laboratory at different concentrations ranging from 0.3 to 0.05% of terpolymer in DMSO at 30 ∘ C. Intrinsic viscosity [] was calculated from relevant plots of Huggins equation (1) [28] and Kraemer's equation ( 2) [29].Consider ln where   is the relative viscosity, Huggins' constant, and  2 is Kraemer's constant.

Thermal Studies 2.5.1. Theoretical Consideration.
Thermogram was interpreted and analyzed to obtain information about the % weight loss at different temperatures which gives information about sample composition, product formed after heating, and kinetic parameters.Kinetics parameters have been determined using Friedman [30], Chang [31], Sharp-Wentworth [32], and Freeman-Carroll [33] methods as follows.
Friedman's technique is as follows: 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 linear plot of ln(1 − ) versus 1/,  can be obtained.The plot of ln(/) versus 1/ should be linear with the slope   /, from which   can be obtained.
Chang's technique is as follows: A plot of [ln(/)/(1 − )  ] versus 1/ will yield a straight line if the order of decomposition reaction, n, is selected correctly.The slope and intercept of this line will provide the (−  /) and ln() values, respectively.
Sharp-Wentworth's technique is as follows: where / is the rate of change of fraction of weight with change in temperature;  is linear heating rate, /;  is the fraction of polymer decomposed at time .Thus, a linear plot of log((/)/(1 − )) versus 1/ is obtained whose slope gives the value of   and  may be evaluated from the intercept.The linear relationship confirmed that the assumed order is correct.
Freeman-Carroll's technique is as follows: where / is the rate of change of weight with time,   =   − ,   is the weight loss at the completion of reaction;  is the total weight loss up to time,   is the energy of activation, and  is the order of reaction.

Results and Discussion
3.1.Elemental Analysis.The results of elemental analysis are shown in Table 1 used to assign empirical formula and empirical weight for 2-ATPHMDAF-I terpolymer.Composition of terpolymer was assigned on the basis of elemental analysis and was found to be in good agreement with that of calculated values.
The number average molecular weight (Mn) of this terpolymer has been determined by conductometric titration method in nonaqueous medium using standard potassium hydroxide (0.05 M) in absolute ethanol as a titrant.The specific conductance was plotted against milliequivalents of ethanolic KOH required for neutralization of 100 g of each terpolymer.There are several breaks before the complete neutralization of all phenolic thiol groups.The first break in the plot was the smallest break and it is assumed that this corresponds to a stage in titration when an average of one phenolic thiol group of each chain was neutralized.From the plot, the first and final breaks were noted.The average degree of polymerization (Dp) and hence the number average molecular weight (Mn) of terpolymer have been determined using the following formula: Total milliequivalents of base required for complete neutralisation Milliequivalents of base required for smallest interval , (Mn) = (Dp) × Repeat unit weight.(7) It is observed that the molecular weight of terpolymer increases with the increase in hexamethylenediamine content.This observation is in agreement with the trend observed by earlier researchers [17].Conductometric titration curve of terpolymer was shown in Figure 2 and the results are presented in Table 2.The intrinsic viscosity [] was determined by the corresponding linear plots (Figure 3).Huggins and Kraemer's  constants were determined by ( 1) and ( 2).According to the above relations, the plots of  sp / and ln  rel / against  were linear with slopes of  1 and  2 , respectively.By extrapolating linear plot to zero concentration, intercepts on the viscosity function axis give [] value in both plots.The calculated values of the constants  1 and  2 (Table 2) in most cases satisfy the relation  1 +  2 = 0.5 favorably.It was observed that terpolymer having higher (Mn) shows higher value of [].

UV-Visible Spectra.
The UV-visible spectrum is depicted in Figure 4 for 2-ATPHMDAF-I terpolymer sample in pure DMSO recorded in the region from 200 to 800 nm.The spectrum exhibits two absorption maxima in the regions 280-300 and 300-330 nm.These observed positions for the absorption bands have different intensities.The more intense band is due to  →  * allowed transition of conjugation in aromatic ring which is shifted from the basic value 269 nm to 280-300 nm and the medium band at 300-330 nm is due to n →  * forbidden transition of −SH group.The more intense band is due to the fact that bathochromic shift may be due to the combining effect of conjugation and the phenolic −SH and −NH 2 groups.Both bathochromic and hyperchromic effects of conjugation are shown by chromophore (aromatic ring) due to auxochromic substituents −SH and −NH 2 groups in terpolymer.This observation is in good agreement with the proposed most probable structure of above terpolymer.

FT-IR Spectra.
The FT-IR spectrum of 2-ATPHMDAF-I terpolymer depicted in Figure 5 and spectral data are   tabulated in Table 3.A broad and strong band appeared at 3434 cm −1 which may be assigned due to the >NH stretching (sec.amine).A medium and sharp band displayed at 3024 cm −1 may be assigned due to the stretching vibration of aromatic C-H group.A weak band at 2600 cm −1 indicates the presence of a −SH (phenolic thiol) group.A sharp band appearing in the region of 1650-1700 cm −1 may be due to bending vibration of >NH (sec.amine) group.A medium band appearing at 1482 cm −1 indicates the presence of >C=C< (aromatic) group.Aromatic C-N stretch appeared in the region of 1236 cm −1 which shows medium band.The presence of tetrasubstituted benzene skeleton shows weak and medium peaks at 807 cm −1 and 1013 cm −1 .The methylenic bridge shows broad and medium peaks for bending, wagging, and rocking vibration appearing at 1467, 1345, and 729 cm −1 .

3.4.
1 H-NMR Spectra. 1 H-NMR spectral data are incorporated in Table 4 and spectrum is presented in Figure 6.Spectrum reveals different patterns of peaks, since each of them possesses a set of protons having different proton environment.The significant singlet signal appearing at the region of  8.05 ppm is due to metaproton of Ar-H.A singlet observed at  7.2 ppm is due to proton of Ar-SH (thiophenol).Amino proton of -CH 2 -NH-CH 2 -linkage gives singlet that is observed at  6.7 ppm.Amino proton of aromatic amine gives singlet at  6.5 ppm.Methylenic proton of -NH-CH 2 -CH 2 -linkage gives triplet at  4.1 ppm.Singlet is observed at  2.5 ppm due to methylenic proton of Ar-CH 2 -NH-linkage.The quintet at  1.5 ppm may be attributed to methylenic proton of -CH 2 -CH 2 -CH 2 -linkage.
3.5.Thermogravimetric Analysis.2-ATPHMDAF-I terpolymer was subjected to thermogravimetric analysis and the data was used to assess the degradation pattern.Thermal degradation behavior of synthesized terpolymer has been incorporated in Table 5 and decomposition pattern is shown in Figure 7 in temperature range from 40 to 1000 ∘ C. Decomposition pattern of terpolymer thermogram shows four decomposition steps in which the loss of water molecule (6.12% expected and 6.36% calculated) has been observed in the first step up to 130 ∘ C. Second decomposition step starts from 130 to 230 ∘ C which represents the degradation of −SH and −NH 2 groups attached to aromatic nucleus (23.52% expected and 23.67% calculated).The third decomposition step starts from 230 to 320 ∘ C, corresponding to 59.66% mass loss of aromatic nucleus with two methylene groups against calculated 59.71%.The fourth decomposition step starts from 320 to 990 ∘ C which corresponds to loss of complete hexamethylenediamine moiety (98.67% expected and 100% calculated).Consequently, no residue may be assigned after complete degradation.

Kinetics of Thermal Decomposition by the Friedman, Chang, Sharp and Wentworth, and Freeman-Carroll Methods.
The observed thermal stability of 2-ATPHMDAF-I terpolymer may be due to the stronger intermolecular hydrogen bonding present in polymer which may be attributed due to the presence of water of crystallization resulting in the resistance to higher temperature.By applying the thermogravimetric data to four thermal degradation kinetic methods, that is, the Friedman, Chang, Sharp and Wentworth, and Freeman-Carroll methods, it shows four different degradation steps corresponding to loss of respective groups.The thermoanalytical data has been determined for different stages as given in Table 6.This kinetic analysis should be   a starting point to obtain the useful information on the behavior of sample.
To obtain the relative thermal stability of terpolymer, the methods described by Friedman, Chang, Sharp and Wentworth, and Freeman-Carroll were adopted.From the results, it is concluded that the values of kinetic parameters show good agreement with each other in Friedman and Chang's methods and some different results are obtained in Sharp and Wentworth and Freeman-Carroll's methods as shown in Table 6.
Fairly comparable results of kinetic parameters, namely,   , , and , are obtained for each degradation step by Friedman and Chang method and may be due to analogy in mathematical model.Also, results obtained by Sharp and Wentworth and Freeman-Carroll methods are in good agreement with each other with slight variations between the results; it is concluded that the values of kinetic parameters depend on kinetic methods used as well as degrading species at a particular step.Total calculations obtained from different kinetic methods demonstrated that the numerical value of  kinetic parameters depends on the mathematical models used to analyze the experimental data and level of degradation.By using the above mentioned techniques, variations in the results are obtained which represents versatility and great utility of thermal degradation of mathematical kinetics equations in thermogravimetry.However, it is difficult to draw any unique conclusion regarding the decomposition mechanism.
From the thermogravimetric analysis, kinetic plots of the terpolymer have been shown in Figures 8, 9  calculate energy of activation, order reaction, and frequency factor.By using the above mentioned methods, fairly similar results are obtained in Friedman and Chang's methods and slight variations are obtained in between Sharp and Wentworth and Freeman-Carroll method which are found to be in good agreement with each other.From the point of view of chemical kinetics, 2-ATPHMDAF-I is thermally stable.