Thermal degradation kinetics and antimicrobial studies of terpolymer resins

2.5.1 Disc diffusion technique

Antimicrobial activity was tested by the filter paper disc diffusion technique involving the cultures of the selected organisms for 24 h (Devi et al., 2009). The test solutions of ASF-I, II and III terpolymers were prepared in sterile dimethyl sulfoxide solvent for the study. The compounds were tested at different concentrations ranging from 50 to l000 ppm to find out the minimum concentration of the terpolymer required to inhibit the microbial growth. Ciprofloxacin (100 μg/mL) was taken as the standard for antibacterial activity. The organisms were seeded into sterile nutrient agar medium by mixing 1 mL of inoculum with 20 mL sterile melted nutrient agar kept at 48–50 °C in a sterile petri dish. The medium was allowed to solidify first. Then the test solutions, the standard drugs as well as the blank were impregnated in whatmann filter paper discs and placed on the solidified medium in the petri dish and left undisturbed for 2 h at room temperature. The petri dishes were then incubated at 37 °C for 24 h and the zone of inhibition for the test samples, standard and the control (DMSO) was measured.

Sterile yeast nitrogen base (HI Media) with 2% agar was inoculated by a rotating swab (soaked in standard inoculum suspension) over the surface of the media. Fluconazole (100 mg/ml) was taken as the standard for antifungal activity. The test solution impregnated discs were placed on the agar and incubated at 37 °C for 18 h. The zone of inhibition was measured by measuring the minimum dimension of the zone of fungal growth around the filter paper disc.

3.1.1 FTIR spectra

The recorded FTIR spectrum of the ASF-I terpolymer resin is shown in Fig. 1. The spectrum shows a broad band at 3401 cm⁻¹ due to the ν (O–H) stretching of Ar-OH involved in the intramolecular hydrogen bonding with Ar-COOH. The strong band that appeared at 1207 cm⁻¹ is due to ν (C–N) stretching of Ar-NH₂ (Shah et al., 2006). The strong band at 1672 cm⁻¹ may be assigned to ν (C⚌O) stretching of ketonic group of acid (Riswan Ahamed et al., 2010a, b). The band that appeared at 3401 cm⁻¹ is due to ν NH-stretching of amino group (Raj et al., 2010). This band seems to be merged with a broad band of –OH group of –COOH present in the resin. The tetra substitution in the benzene ring is established by the presence of medium bands at 1100 and 803 cm⁻¹ that are assigned to ν (C–H) bending vibration (Raj et al., 2010). The weak band at 3018 cm⁻¹ is due to the ν (C–H) stretching vibration of methylene group (Shah et al., 2006).

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Figure 1

FTIR of ASF-I terpolymer.

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3.1.2 ¹H NMR spectra

¹H NMR spectrum of ASF-I terpolymer resin is shown in Fig. 2. The NMR spectrum reveals that the signals around 2.7–4.4 δ (ppm) are due to the methylenic protons of the Ar-CH₂-Ar linkage (Pretsch et al., 2000; Riswan Ahamed et al., 2011). The multiplet signals observed in the range at 6.6–7.4 δ (ppm) indicates the presence of aromatic protons (Pretsch et al., 2000; Burkanudeen et al., 2010a, b). The signal displayed at 8.35 δ (ppm) may be due to the carboxylic proton of Ar-COOH (Silverstein and Webster, 1998; Azarudeen et al., 2010). The signal in the region at 9.8 δ (ppm) is assigned to the –OH group of Ar-COOH involved in the intramolecular hydrogen bonding with proton of –OH in Ar-OH (Morrison and Boyd, 1996; Azarudeen et al., 2011). An intense signal that appeared in the region at 5.7 δ (ppm) is assigned to the –NH₂ protons of Ar-NH₂ in the terpolymer resin (Riswan Ahamed et al., 2010a, b).

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Figure 2

¹H NMR of ASF-I terpolymer.

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3.1.3 ¹³C NMR spectra

¹³C NMR spectrum of ASF-I resin is depicted in Fig. 3 and the peaks are assigned based on the literature values (Pretsch et al., 2000; Silverstein and Webster, 1998; Burkanudeen et al., 2010a, b). The ¹³C NMR spectrum shows the corresponding peaks at 112.8, 149.1, 129.5, 136.0, 130.2 and 119.1 ppm with respect to C₁–C₆ of the aromatic ring of anthranilic acid. The intense signals that appeared at 111.6, 161.0, 129.5, 135.6, 130.2 and 117.0 ppm may be attributed to C₁–C₆ of the aromatic ring of salicylic acid. Peaks around 169.7 and 171.8 ppm are due to the –C⚌O group of carboxylic acid present in both the aromatic rings of the terpolymer. The peak that appeared at 54.86 ppm may be assigned to the –CH₂ bridge between the aromatic rings in the terpolymer resin. Based on the spectral data obtained from FTIR and NMR (¹H and ¹³C), the structure of the ASF-I terpolymer resin is proposed.

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Figure 3

¹³C NMR of ASF-I terpolymer.

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3.2.1 Freeman–Carroll method

The activation energy (E_a) and the order of reaction (n) for the terpolymer degradation were calculated using the following expression proposed by Freeman–Carroll. $$\frac{\mathrm{\Delta }\log (\mathit{dw}/\mathit{dt})}{\mathrm{\Delta }\log W_r}=\frac{-E_{\text{a}}}{2.303R}〈\frac{\mathrm{\Delta }(1/T)}{\mathrm{\Delta }\log W_r}〉+n$$ where, dw/dt is the rate of change of weight with time; W_r = W_c − W, where W_c is the weight loss at the completion of reaction or at definite time and W is the total weight loss up to time t; n is the order of the reaction, T and R are the temperature and the gas constant, respectively.

Hence, a plot of Δlog(dw/dt)/ΔlogW_r versus Δ(1/T)/ΔlogW_r gives a slope of −E_a/2.303R with an intercept equal to n on the y-axis where x = 0.

3.2.2 Sharp–Wentworth method

The following expression given by Sharp–Wentworth was used to determine the activation energy (E_a), entropy change (ΔS) and free energy change (ΔF). $$\frac{\log (\mathit{dc}/\mathit{dt})}{(1-C)}=\frac{\log {\alpha }^{\prime }}{\beta }=\frac{E_{\text{a}}}{2.303\mathit{RT}}$$ where, β is the linear heating rate. The graph of log dc/dt/(1 − C) versus 1000/T gives a slope of −E_a/2.303R with an intercept on the y axis where x = 0.

A representative thermal activation energy plot of Sharp–Wentworth and Freeman–Carroll method for the ASF-I terpolymer resin is shown in Figs. 5 and 6, respectively. Kinetic parameters such as entropy change (ΔS), free energy change (ΔF), apparent entropy (S^∗) and frequency factor (Z) were calculated based on the thermal activation energy (E_a) using the expressions shown in Table 2. The thermal stabilities of the terpolymer resins were predicted to be high due to the low activation energy of the degradation process. The linear structure of the resin further proves the higher thermal stability of the resin.

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Figure 5

Sharp–Wentworth plot of ASF-I terpolymer.

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Figure 6

Freeman–Carroll plot of ASF-I terpolymer.

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Using the Freeman–Carroll and Sharp–Wentworth methods, the kinetic parameters were calculated and presented in Table 3. The values obtained for all the terpolymer resins suggest that the degradation takes place in the same mode for ASF-I, II and III. From the results, it is obvious that the activation energy values calculated from FC and SW are in good agreement with each other for all terpolymers. The low frequency factor value obtained predicts that the degradation reaction is slow (Singru and Gurnule, 2010; Masram et al., 2010). This is further supported by the negative value of the entropy change. However, a few points do not fall on the straight line in the graphs (Figs. 5 and 6) which show that the reaction does not obey the exact first kinetics.

3.2.3 Phadnis–Deshpande method

Phadnis–Deshpande method is a mathematical tool for the confirmation of the mechanism of a solid state reaction from thermo-analytical curves obtained isothermally or non-isothermally. In general, the correlation coefficient of the plot for different mechanism functions was regarded as a standard procedure for determining the reaction mechanism (Liu et al., 2010). One of the expressions is, $$g^{\prime }(\alpha )=-\frac{E_{\text{a}}}{\mathit{RT}}$$ where, g′(α) is the integral function of conversion α. The functions of g′(α) frequently used are listed in Table 4. The activation energies calculated for ASF terpolymers according to Phadnis–Deshpande method are presented in Table 5. It is identify that the value of activation energy (22.59 kJ/mol) of ASF-I calculated by the above expression suits very well the value of reaction mechanism 5 proposed by Phadnis–Deshpande (Phadnis and Deshpande, 1983). This is comparable to E_a calculated by FC and SW methods. Hence, the ASF-I terpolymer degradation kinetic model follows a nucleation and nuclei growth (Avrami-Erofeev nuclei growth) mechanism.

3.3.1 Antibacterial activity

An admirable result was obtained for ASF terpolymer resins against E. coli bacterium which is markedly higher than the standard. E. coli, a gram negative rod-shaped bacterium affects the urinary tracts in humans. Klebsiella is a genus of non-motile, gram-negative bacterium that causes a wide range of disease states, notably pneumonia, septicemia and soft tissue infections. The results showed a good activity against Klebsiella. S. aureus, a gram-positive and spherical bacterium leads to life-threatening diseases like pneumonia, osteomyelitis, endocarditis and toxic shock syndrome. From the experiment, the resins were found to have moderate activity against S. aureus species. P. aeruginosa is a common bacterium which can cause diseases like pulmonary tract and urinary tract burns and wounds in animals and human beings for which the ASF resins had moderate activity.

3.3.2 Antifungal activity

The antifungal activities of the ASF terpolymer resins also provide appealing results against the chosen fungal strains. The terpolymers possess a moderate activity against A. flavus, a mold type fungal strain which may invade arteries of the lung or brain to cause infections and also produces a toxin. A. niger is one of the most common causes for otomycosis. The resins found to have moderate activity for A. niger. Penicillium sp. cause infection in humans and the resulting disease is known generically as penicilliosis. The resins had moderate activity against Penicillium sp. C. albicans is a diploid fungus and a causal agent of opportunistic oral and genital infections in humans and also emerged as an important cause of morbidity and mortality in immuno compromised patients. The ASF-I terpolymer resin exhibits a very good activity against C. albicans over the standard. C. neoformans is an encapsulated yeast-like fungus that can live in both plants and animals cause lung infection. The resins had good activity against this fungal strain. The resins showed good activity against Mucor species, a filamentous fungus which causes septic arthritis, renal infections and pulmonary infections. The antimicrobial activity of the terpolymer resins may be due to the presence of –NH₂ and –OH groups. It is perceived that the factors, such as solubility, conductivity, dipole moment and cell permeability may also contribute to the increased antimicrobial activity against the chosen microbes (Bagihalli et al., 2009; Azarudeen et al., in press).