1. INTRODUCTION:

Polyaniline is one of the most important materials because of its high conductivity^1,2, simple polymerization³, good stability in aqueous solutions and air, good redox reversibility⁴. Its synthesis doesn’t require any special equipment⁵. Interest in polyaniline began after the discovery that polyacetylene had a mineral conductivity in 1977. PANI as a long-known chemical substance^6,7has been used in diverse applications, such as bio-sensors, gas sensors, optoelectronics⁸. In this work, we studied some properties of polyaniline and formation reaction kinetics of the polymerization of polyaniline. The polyaniline polymerization was carried out as simple as possible⁹.

2. EXPERIMENTAL:

2.1 Used materials:

Aniline, Ammonium peroxydisulfate, sulfuric acid, water chlorine acid.

2.2 Used Equipment:

UV-Vis, Model: Optizen, OUV322, Co Mecasys, made England.

High-Performance Liquid Chromatography (HPLC), Model: Shimadzu, UFLC, SPD-M20A, made in Kyoto- Japan.

Thermo Gravimetry Analysis, TGA, the analysis was carried out at the Atomic Energy Commission in Damascus, Syria.

Electronic Scales (Sartorius Basic), Developed, manufactured tested by precise instruments Ltd., made in Switzerland.

Electric Heating of Magnetic Type, made in England.

Dryer, Model: Jsof-100, made in Korea.

Numerous laboratory glassware, made in England.

2.3 Used Method:

Preparation of polyaniline:

We weighed (5g) of aniline, and (6.25g) of ammonium peroxydisulfate dissolved in (80ml) of prepared sulfuric acid (1M), where it was added drip for 15 minutes. Then the reaction was left for an hour, stirring. The result was filtered and sodium hydroxide added to it (pH = 12), then washed with distilled water and acetone in a ratio (1: 1).

3. RESULTS AND DISCUSSION:

Polyaniline is a dark greenish-black powder. It melts at a high temperature (>300ºC) with dissociation. It is insoluble in common organic solvents such as alcohols and acetonitrile, but dissolves in dimethyl form amide DMF, and dimethyl sulfoxide DMS, and formic acid.

3.1 Infrared spectrum (FT-IR):

The polyaniline infrared spectrum showed the formation of the polymer, as in fig. 1. It is similar to the reference spectrum of polyaniline¹⁰.

Fig. 1: FT/IR spectrum of Polyaniline

3.2 Scanning Electron Microscope (SEM):

The Scanning Electron Microscopy (SEM) analysis was carried out to study the morphology of the polymer. Morphology is shown in fig. 2. The polyaniline sample shows a smooth surface and uniform nanoparticles formation with diameters of 56.4 to 90 nm.

Fig. 2: SEM images of Polyaniline

3.3 Kinetic study of the formation of polyaniline by UV-Vis:

In 1993 the optical absorption spectra of PANI were reported in the near-UV and visible regions¹². Therefore, we studied the kinetics of the polymer formation with a UV device by preparing (0.1 M) of aniline and (0.1 M) of ammonium pyrosulfate and placing them in an equal amount in the measurement cell with mixing, then we studied the kinetics of the device for (30 min) at every minute, as shown in Fig. 3.

Fig. 3: Kinetic of Polyaniline by UV-Vis

The relationship of (A∞-At) with time, and the relationship of ln(A∞-At) with time, as well as the relationship between 1/(A∞-At) and time, was drawn to study the kinetics of the reaction and the graphical representation, it was found that the zero order reaction was the best orderas shown in Fig. 4.

Fig. 4: (A_∞-At) with the time of Polyaniline; n=0

3.4 Kinetic study of the formation of polyaniline by HPLC:

HPLC has been used to determine reaction kinetics^13,14. In our study, we determined the reaction order for aniline by tracking the concentrations of the reactants, as in fig. 5.

Fig. 5: Chromatogram expressing the concentration of aniline during the polymerization reaction at: t=5 min

To know the order of the polymerization reaction for aniline, we drew [C] with time, 1/[C] with time, and ln[C] with time, so it was found that the reaction order was zero, as in Fig. 6.

Fig. 6: Aniline concentration change [C] with time during the polymerization at n=0

3.5 Kinetic and determination of the thermodynamic functions of the disintegration process using Coats-Redfern equation:

Determination of the thermodynamic function of the disintegration process of the polymer means (Ea) energy activation, (S*), entropy, (H*) Enthalpy, (G*), free Gibbs energy, they are determined from a scheme using the Coats-Redfern method^15,16. This method was checked by Johnson and Gallagher¹⁷as an integrative method assuming different orders reaction and linear comparison in each case to choose the correct order using:

Where :(α) Fraction of weight loss, (T)Temperature K, (n) Order reaction, (Z) Frequency coefficient (Arenios), (Ea) Energy activation , (R) Universal gas constant, (q) the thermal ratio (q = 0.0833 ºC).

The curved output of a graphic  with [1/T] was a straight line.

From the slope of the straight line, the activation energy is calculated, Z is assigned from the intersection. These curves were tested using different order reaction (n), the best curve is the one that been got from the best order reaction.The entropy was calculated using¹⁸:

Where: (h) Planck constant, (K) Boltzmann constant, (Ts) The maximum temperature from TGA curve Fig.7. Free Gibbs energy and enthalpy were being calculated using:

DH*=Ea-RTs

DG*=DH*-TsDS*

Fig. 7: The Thermal dissociation (TGA) for prepared polyaniline

The first dissociation step begins at 50°C, ends at 150°C and the peak temperature is Ts=72.30ºC and the second dissociation step begins at 200°C, ends at 750°C and the peak temperature is Ts=443.53ºC.

From the first and second dissociation steps, we found that the second order reaction was the best order as in fig. 8.

Fig. 8: Log[] with the inverse absolute temperature, n=2: a)- the first step b)- the second step

We used the following algorithm to calculate the best order of reaction of prepared polymers¹⁹as in tables 1 and 2.

Table 1: Applying the values of the first dissociation step of polyaniline to the algorithm

T(ºC)	m (mg)	α	1/T (K-1)	log y	R2	0.96124	m	-1474.760561
55	8.282	0.014869	0.003047	-6.85214	n	2	i	-2.316271624
60	8.2383	0.020067	0.003002	-6.73279	Variables		Thermodynamic parameters
65	8.1987	0.024777	0.002957	-6.65208	Minimum	0	Ea(KJ)	28.23744988
70	8.1594	0.029452	0.002914	-6.58769	Maximum	2	Z	3.43055776
75	8.1208	0.034043	0.002872	-6.53528	q (ºC/min)	10	ΔH	25.36537858
80	8.0882	0.037921	0.002832	-6.49908	T (ºC)	72.3	ΔS	-2.36E+02
85	8.0534	0.04206	0.002792	-6.46443	Total weight	8.407	ΔG	106.8495136

Table 2: Applying the values of the second dissociation step of polyaniline to the algorithm

T(ºC)	m (mg)	α	1/T (K^-1)	log y	R²	0.999974	m	-3790.745056
410	5.6317	0.330118	0.001464	-5.97529	n	2	i	-0.426240613
420	5.2205	0.379029	0.001443	-5.89499	Variables		Thermodynamic parameters
430	4.8027	0.428726	0.001422	-5.81771	Minimum	0	Ea(KJ)	72.58193387
440	4.3793	0.479089	0.001402	-5.74167	Maximum	2	Z	652.4047839
450	3.9587	0.529119	0.001383	-5.66679	q (ºC/min)	10	ΔH	66.62345635
460	3.5728	0.575021	0.001364	-5.59806	T (ºC)	443.53	ΔS	-1.98E+02
470	3.1873	0.620875	0.001346	-5.52693	Total weight	8.407	ΔG	208.7526481

Where:    Ea: activation energy

                Z: coefficient of Arnos, a=1-(2RT/Ea)

From tables 6,7, we noticed that the Positive values of (DG*) indicate that the dissociation reactions were non-spontaneous. The negative values of (DS*) indicate that the polymers have a more ordered structure than the structure of the reactants. The positive values of (DH*) indicate that the dissociation reactions are (endothermic).

4. CONCLUSIONS:

Polyaniline can be prepared chemically at laboratory temperature in an acidic medium in the presence of ammonium pyrosulfate as an oxidizer. The polyaniline sample in SEM shows smooth surface and uniform nanoparticles formation with diameters of 56.4 to 90nm. We studied the polymer formation kinetics by two methods: UV-Vis and HPLC. It was found that the reaction of the polymer formation was a zero-order reaction in both of the previous two methods. On the other hand, The high Ea values for the dissociation steps indicated that the dissociation process was slow. Positive values of ΔG indicate that the dissociation reactions are not automatic, and positive values of ΔH indicate that the dissociation reactions are endothermic.

5. DATA AVAILABILITY:

The data used to support the findings of this study are available from the corresponding authors upon request.

6. CONFLICTS OF INTEREST:

The authors declare that there is no conflict of interest regarding the publication of this paper.

7. ACKNOWLEDGMENTS:

This work was supported by the “Department of Chemistry, Damascus University, Syria,”

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Received on 03.09.2020 Modified on 05.10.2020

Research J. Pharm. and Tech. 2021; 14(8):4117-4121.

DOI: 10.52711/0974-360X.2021.00713