Poly (aniline-co-aniline-2,5-disulfonic acid) / L-ascorbic acid / Ag@SiO₂ / polysafranin nanocomposite: synthesis, characterization and anomalous electrical behaviour

Abstract

We report the synthesis of sulfonated copolyaniline/polysafranin/L-ascorbic acid/Ag@SiO₂ fine powdered nanocomposites and investigate the influence of incorporating the dye on their conductivity. The composite was characterized via IR, UV, cyclic voltammetry (CV), electric, dielectric, SEM, TEM, TGA and DSC measurements. Microscopy images revealed intensified spherical particles that were dispersed across the entire surface, and the SiO₂/Ag particles were distributed on the surface. The XRD results exhibited peaks at many 2q values, and their interatomic spacing (d) and crystallite (grain) sizes were calculated. The thermal degradation curves exhibited an interesting model of stability. The cyclic voltammogram exhibited redox peaks identical to those of the reported analogues. The d.c. conductivity of the oligomer varied from 0.06 − 0.016 (s/cm), and that of the composite varied from 0.008 to 0.016 (s/cm). The material changed from a semiconductor to a metallic material. The observed conductivity is mainly attributed to self-doping between the sulfonate groups and the charged nitrogen atoms in the polymer chains. The frequency dependence of the permittivity, ε′, showed a marked effect on the frequency window under consideration. The permittivity, ε′, is independent of the increase in the frequency of the oligomer and the composite. This behavior supports the non-Debye dependency by confirming the occurrence of electrode polarization and space charge effects. In conclusion, the incorporation of safranin dye with a thermally stable, highly sulfonated polyaniline derivative/Ag@SO₂ nanocomposite achieved improved conductivity after heating. The d.c. conductivities are comparable to those of many commercial inorganic or organic composites, and because of their attractive electrical properties, we suggest that these materials are promising for electronic field applications.

Introduction

Due to its simple and cost-effective commercial production, polyaniline (PANI) and its derivatives are an important class of conducting polymers, and their interesting properties have made them applicable in multiple fields, such as rechargeable batteries, supercapacitors, solar cells, analytical sciences, biomedicine, conductive paints and adhesives, and environmental issues such as wastewater treatment and many other applications [1,2,3]. Tuning of their properties, such as by controlling their morphology and enhancing their conductivity, is desirable research target. Mixing such polymers with organic and/or inorganic materials has produced composites and blends that have unique properties and applications [4]. The conductivity of PANI ranges from σ ≤ 10^− 10 S cm^− 1 (undoped base form) to σ ≥ 10 S cm^− 1 (acidic doped salt form) [5]. Although acidic doping improves the polymer characteristics, it limits the heat resistance and electrical conductivity [6]. One additional problem is that acid dopants are potentially corrosive, and their use raises a risk to the environment. Self-doped polyanilines are polyaniline derivatives that bear negatively charged functional groups. The polymerization of aniline derivatives containing sulfonic (-SO₃H) groups leads to self-doped PANI. In contrast to PANI, its self-doped derivatives contain an ionizable, negatively charged functional group, which acts as an inner dopant anion bound to the polymer backbone. No anion exchange between the polymer and its surroundings occurs during oxidation or reduction. Charge compensation occurs at the expense of cation (usually a proton) exchange, which occurs much faster than the other processes and does not limit the rate of the charging (redox) process [7]. Self-doped conducting PANIs with acid moieties are considered rational alternatives because they are soluble or dispersed in organic solvents and hence can expand the utility of self-doped PANI. However, self-doped conducting polyanilines, which are soluble in organic solvents, have been little studied thus far [8]. Due to the influence of electronic and steric hinderance of the -SO₃H group, the direct synthesis of self-doped PANI has failed; however, the copolymerization of self-doped polyaniline with aniline based on the autocatalytic polymerization reaction [9] has been considered a solution. Direct polymerization of aminobenzenesulfonic acid was neither chemically nor electrochemically successful for two reasons: (i) The strong electron-withdrawing properties of the -S0₃H decreased the electron density on the amino groups, and the monomers could not be oxidized. (ii) The presence of the bulky group of -S0₃H on the phenyl rings affects the reactivity of two cationic radical monomers for head-tail coupling to form a relatively stable intermediate complex, which likely limits the polymerization process and results in a low-molecular weight polymer. PANI itself can be synthesized by chemical oxidation polymerization of aniline via oxidant radical initiators. Therefore, once the polymerization of polyaniline starts, an autocatalytic reaction takes place, and therefore, copolymerization of aniline and aminobenzenesulfonic acid can be achieved. Nevertheless, due to the influential nature of the sulfonic group, such copolymerization results in a low-molecular-weight polymer with a conductivity of only 10^− 4 S cm^− 1 [10]. The lower conductivity of the self-doped polymer is attributed to the decreased interchain diffusion of the charge carriers due to the presence of side groups, which force the chain out of planarity and result in lower crystallographic order between the chains [10].

Organic dyes have similar structural features as conducting polymers except for their low molecular weights, and they are rated as electric insulators [11]. Dyes frequently interact with conducting polymers in four fundamental ways: π–π interactions between aromatic rings, electrostatic cationic/ionic interactions, hydrogen bonding, and hydrophobic interactions [11]. Several trials have reported the influence of the presence of dyes on the morphology and conductivity of polymeric chains or films during preparation. The participation of dyes during polymer preparation significantly affects the resulting conductivity negatively or positively, as exemplified in polypyrrole and its composites [12, 13]. In the presence of a low concentration of methyl orange dye, the conductivity of polypyrrole increased, but the reverse was true when the dye concentration increased [14]. Convincing support for better morphology and conductivity enhancement of the polymer in the presence of dyes has not yet been provided in the literature; however, it is assumed that dyes assist in intermolecular charge transport in chains via their conjugated molecular structure. It is worth noting that dyes have a limited influence on the conductivity of polyaniline [15, 16]. Safranin, 3,7-diamino-2,8-dimethyl-5-phenylphenazinium chloride (1), as shown in Fig. 1, is a well-known cationic dye with various applications [17]. Chemical oxidation of dyes containing primary amino group(s) on the benzenoid ring, as occurs for safranin, is expected to produce a conducting polymer or composite from the dye [18]; however, the oxidation of safranin itself leads to a nonconducting oligomer [19]. Notably, the preparation of copolyaniline using aniline and a dye as monomers has led to products with reduced conductivity and unresolved morphology [20].

As part of an ongoing project directed to research the use of PANI and its functionalized analogues as multifunctional substrates for industrial application [21,22,23,24], we recently reported the chemical synthesis and characterization of poly(aniline-co-aniline-2,5-disulfonic acid) and its composite containing L-hexuronic acid (Fig. 1) and metallic Ag/SiO₂ nanoparticles as an efficient, new, thermally stable anionic polyelectrolyte to remove safranin dye from aqueous media [25]. Uptake rates of up to 82.5% adsorption were achieved within 75 min, and the equilibrium time was 45 min. Additionally, uptake was well defined by the pseudo-second-order model with a rate constant K₂ = 0.03 g^− 1 mg^− 1 min^− 1 for 19 mg of safranin. A comparison of the safranin adsorption efficiency of the synthesized material with that of other reported materials in the same domain suggested that the composite had a high adsorption rate and capacity.

In the research described in this paper, we investigated the resynthesis of such a highly rich, self-doped polyaniline composite (Fig. 1, compound (12)) in the presence of safranin by oxidative polymerization and investigated the influence of the incorporation of the dye on achieving better conductivity. Slightly different from our reported synthesis of its analogue [25] and to ensure a better silver-doped silicacomplex sphere morphology, cetyltrimethylammonium ammonium bromide (CTAB) was used as an emulsifier. Modification of the polymer properties by incorporating safranin dye, we proposed, would help to achieve better conductivity. To our knowledge, no studies have investigated the influence of incorporating safranin dye with sulfonated PANI or polyaniline derivatives/Ag nanocomposites to achieve better conductivity.

Fig. 1

Chemical synthesis of copolymer/L-ascorbic acid/Ag@SiO₂/polysafranin nanocomposite (12)

Interestingly, according to the literature data, the oxidation of safranin itself or its copolymerized material usually impairs the material’s conductivity; however, our results described here indicated that incorporating the dye with the highly sulfonated polyaniline derivatives/Ag@SO₂ nanocomposite (Fig. 1, compound (12)) resulted in better conductivity after heating. The examined material changed from a semiconductor to a metallic material, and its DC conductivities were comparable to those of many commercial inorganic or organic composites. To understand the conduction mechanism, we also studied the electrical conductivity vs. 1000 / T (K) at various frequencies, the electric modulus characterization and the permittivity, ε′, vs. frequency.

Experimental

Materials

Commercial aniline (AlGomhoria Chemicals Co.; Egypt), 2-aminobenzene-1,4-disulfonic acid (ICI; Manchester, UK), safranin (≥ 85%; Merck, Darmstadt, Germany), ammonium persulfate (Oxford Lab Fine Chemicals, India), and tetraethyl orthosilicate (TEOS) (98%; Sigma‒Aldrich, Steinheim, Germany) were used. Cetyltrimethylammonium ammonium bromide (CTAB 98%, Sigma‒Aldrich, Steinheim, Germany) was used. Silver nitrate (99.9%, HOLPRO ANALYTICS DIVISION, Midrand, INDIA) and sodium borohydride (95%, Fluka, Switzerland) were used. The ethanol and ammonia solutions (30%) used were of analytical grade. All the chemicals were used without further purification [25]. .

Measurements

Infrared spectra (IR, KBr pellets; 3 mm thickness) were recorded on a Perkin-Elmer Infrared Spectrophotometer (FTIR 1650). All the spectra were recorded within the wavenumber range of 4000–600 cm^-1 at 25 °C. Absorption spectra were measured with a UV 500 UV–Vis spectrometer at 16 °C (rt) in DMSO with a polymer concentration of 2 mg/10 mL. Elemental analysis of the as-synthesized copolymer was performed at the Microanalytical Unit, Cairo University. Inherent viscosities (η_inh) were measured at a concentration of 0.5 g/dL in H₂SO₄ at 30 °C by using an Ubbelohde viscometer. Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses were carried out at temperatures ranging from 20 °C to 400 °C in a nitrogen atmosphere by means of a Shimadzu DTG 60 H thermal analyser. The experimental conditions involved a platinum crucible and a nitrogen atmosphere with a 30 mL/min flow rate and a heating rate of 10 C/min. Differential scanning calorimetry (DSC-TGA) analyses were carried out using an SDT-Q600-V20.5-Build-15 instrument at the microanalytical unit of Cairo University. Cyclic voltammetry (CV) was performed using an eDAQ system (www.eDAQ.com, Australia) consisting of an ER466 potentiostat connected to an e-corder that was inputted into eChem software (running on a PC using Microsoft Windows 10). The working electrode was a 3 mm diameter glassy carbon electrode, the reference electrode was Ag/AgCl, and the auxiliary electrode was a 0.25 mm diameter Pt wire. The applied potentials ranged from − 500 to + 500 mV, and the scan rate during one cycle was 100 mV s^-1. The volume of the voltammetric cell was approximately 15 ml. The polymer powder was pressed to form discs with diameters of 10 mm and thicknesses of 1 mm. Silver electrodes were deposited on both sides of the sample surface by thermal evaporation, and two copper wires were fixed on the sample using conducting silver paint. Energy-dispersive X-ray spectroscopy (EDXS) was used to observe the morphologies of the polymers by scanning electron microscopy (SEM) (JEOL-JSMIT 200, Japan) and transmission electron microscopy (TEM) (JEOL-JTM-1400 plus, Japan) at the E-Microscope Unit, Faculty of Science, Alexandria University. The samples were sonicated in deionized water for 5 min, deposited onto carbon-coated copper mesh and allowed to air dry before examination [21,22,23,24].

Preparation of silver-doped silica complex nanoparticles (4)

The synthesis of silver-doped silica complex spheres (4) was slightly modified from a method reported elsewhere [25]. In brief, a mixture of CTAB (1 g), EtOH (25 ml), NH₄OH (40 mL, 30%), and tetraethyl orthosilicate (TEOS) (2)/EtOH (10 ml/20 ml) was magnetically stirred at room temperature for 3 h, after which AgNO₃ (0.5 g) and NaBH₄ (250 mg) were added to the in situ-formed tetrahydroxyorthosilicate (3), after which the mixture was stirred for 10 h at the same reaction temperature. The crude product (4) was collected by centrifugation and worked up as reported previously [25]. The following physical data were recorded: IR (KBr pellets, υ cm^-1): 3475, 3470, 3467, 2924, 2853, 1739, 1638, 1512, 1480, 1467, 1427, 1420, 1405, 1398, 1229, 1083, 964, 798, 455, 729, 720, 711, 697, 689, 667, 558, and 537. UV‒Vis (λ_max nm): 410 nm.

In situ preparation of poly(aniline-co-aniline-2,5-disulfonic acid)/L-ascorbic acid/Ag@SiO₂/polysafranin nanocomposite (12)

The preparation of the targeted nanocomposite (12) has been reported elsewhere [25]. The synthesis of (12) in this work was performed in the presence of safranin dye (1). In brief, aniline-2,5-disulfonic acid (5) (12.65 g, 0.05 mol), aniline (7) (2.00 g, 0.0215 mol), safranin (1) (1 g, 0.0028 mol), and ascorbic acid (9) (2.00 g, 0.011 mol) were added to an aqueous 10% HCl (500 ml) solution, followed by the slow addition of ammonium persulfate (15.0 g, 0.0657 mol). A colour change pattern was clearly observed during the polymerization of the copolymer mixture (10). Without isolation, silver-doped silica (4) (0.5 g) was added to the mixture, and stirring was continued for an additional 10 h. Polymerization was stopped by the addition of CH₃OH (50 ml), and the precipitate (12) was generated as reported previously [25]. The process of in situ preparation of poly(aniline-co-aniline-2,5-disulfonic acid)/L-ascorbic acid/ Ag@SiO₂/ polysafranin nanocomposite (12) is depicted in Fig. 2. IR (KBr pellets, υ cm^-1) bands were observed at υ 3479, 3466, 3458, 3437, 3401, 3373, 3305, 3296, 3262, 3255, 3099, 2926, 2855, 1699, 1639, 1607, 1578, 1498, 1410, 1301, 1231, 1155, 1096, 1043, 1015, 881, 817, 802, 758, 704, 665, 631, 598, 587, 577, 568, 560, 539, 506, 465, 459, and 454. Calc. for C₅₈H₅₇N₁₂S₃O₁₂: (1282.3); C, 54.23; H, 4.47; N, 13.08; S, 4.99; Found: C, 54.64; H, 5.58; N, 7.65; S, 5.05.

Fig. 2

In situ preparation of poly(aniline-co-aniline-2,5-disulfonic acid)/L-ascorbic acid/Ag@SiO₂/polysafranin nanocomposite (12)

Results and discussion

Preparation of poly(anilinecoaniline-2,5disulfonic acid) (8) [26]

Direct oxidative polymerization of 2,5-aminobenzenedisulfonic acid (5) in acidic aqueous media (Fig. 1) led to the formation of the known highly water-soluble oligomer (6) [26], which was collected by evaporating the solution to dryness. The inductive and steric effects of sulfonate groups hinder the effective generation of reactive intermediates and lead to early termination of polymer growth. The copolymer (8) was chemically prepared from commercial aniline-2,5-disulfonic acid (5) and aniline (7) (10 wt % of (5)) in aqueous HCl media (pH 1.5) using 1.25x equivalent of ammonium persulfate as an oxidizing agent.

Preparation of silver-doped silica complex nanoparticles (4)

Silver-doped silica complex spheres (4) were prepared following a procedure reported elsewhere [25]. Synthesis of (4) in this work was performed in the presence of CTAB. Analysis of the IR spectrum (Fig. 3) showed that the CH₂ stretching vibrational bands of the contaminated CTAB were located at υ2924 and υ2853 cm^-l. A strong and broad band at υ3470 cm^− 1 was attributed to the vibrations of the ammonium group in CTAB. The vibrational characteristics of the other functional groups were consistent with the literature [27]. For instance, an observed broad band at υ3467 cm^-1 was attributed to the vibration of O–H bonds, confirming the presence of Si–OH and/or adsorbed water. The bands at υ1512 cm^-1 and υ1405 − 1229 cm^-1 are attributed to NO₃^– ion vibrations. The chemical composition (wt %) of Ag@SiO2 (4) was determined according to the energy-dispersive X-ray spectroscopy (EDXS) spectrum (Fig. 4) to be C, 10.64; O, 39.481; Si, 16.14; and Ag, 33.41, confirming the contamination of the sample with CTAB [25].

Fig. 3

IR spectrum of the Ag@SiO₂ nanocomposite (4)

Fig. 4

EDXS spectrum of the Ag@SiO₂ nanocomposite (4)

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the SiO₂@Ag nanocomposite (4) are shown in Fig. 5. The particles were nearly spherical in shape and well separated from each other, and the average particle size was 10 nm. Silver particles can be clearly seen embedded in the CTAB-contaminated SiO₂ matrix in the solid sample image (Fig. 4a and b). According to the TEM image of the prepared Ag@SiO₂ nanocomposite (Fig. 4c), the SiO₂ particles were light gray and semispherical with an ordered structure, and the silver nanoparticles (7–13 nm) appeared as dark gray nanoparticles, while the darkness was related to the density of the molecules.

Fig. 5

Images of the Ag@ SiO₂ nanocomposite (4): (a) SEM image scale bar = 2 nm; (b) SEM image scale bar = 500 nm and (c) TEM image

The XRD spectrum of the Ag@SiO₂ complex (4) (Fig. 6) exhibited characteristic peaks corresponding to silica and silver at 2θ values of 9.49° and 22.3° (SiO₂ and 37.9° (Ag), 44.1° (Ag), 63.9° (Ag), and 77.2° (Ag), respectively [28]. The calculated interplanar spacing values (d) obtained using Bragg’s law [29] were 0.93 nm, 0.40 nm, 0.24 nm, 0.21 nm, 0.15 nm, and 0.12 nm, while the calculated crystallite (grain) sizes obtained using the Scherrer equation [30] normal to the corresponding planes were 16.66 nm, 16.92 nm, 17.55 nm, 17.91 nm, 19.58 nm, and 21.25 nm, respectively.

Fig. 6

XRD patterns in the 2θ range of 5–80° for the SiO₂@Ag nanocomposite (4)

Preparation of the poly(aniline-co-aniline-2,5-disulfonic acid)/L-ascorbic acid/Ag@SiO₂ polysafranin nanocomposite (12)

The copolymer/L-ascorbic acid/Ag@SiO₂/polysafranin nanocomposite (12) was chemically prepared as previously reported [25] in the presence of safranin dye, where the latter was simultaneously oxidized with ammonium peroxydisulfate to actively participate in the copolymer backbone, as proposed, and/or to be converted to its corresponding oligosafranin (11) (Fig. 1 [17]), . The calculated elemental composition of the nanocomposite (12) was as follows: Calc. for C₅₈H₅₇N₁₂S₂O₁₂: (1282.3); C, 54.23; H, 4.47; N, 13.08; S, 4.99; Found: C, 54.64; H, 5.58; N, 7.65; S, 5.05, confirming the success of the nanocomposite (12) preparation. On the other hand, the chemical composition (wt %) of the sample according to the EDXS spectra (Fig. 7) of composite (12) was C, 37.86; N, 9.94; S, 1.71; O, 38.95; Si, 6.60; Ag, 3.58; and Cl, 1.36. The EDXS results clearly confirmed that a fraction of the silver particles broke away from the composite surfaces [31].

Fig. 7

EDXS spectrum of the nanocomposite (12)

Fig. 8

IR spectrum of the nanocomposite (12)

Fig. 9

UV-Vis spectrum of the nanocomposite (12)

Along with those corresponding to the metallic Ag@SiO2, the spectrum of the nanocomposite (12) exhibited IR signals (Fig. 8), vibrational bands, and additional bands assigned to the vibration modes of the NH group, aromatic benzenoid ↔ quinoid structures, and other functional groups such as C-N and O = S = O stretching modes. The vibrational peaks corresponding to ascorbic acid appeared at υ 3479 (–OH), υ 1699 (C–O stretching), υ 1410 and υ 1231 (C–O–C stretching), υ 707 and υ 598 (–OH out-of-plane deformation). Peaks corresponding to the safranin dye appeared at υ 3466 (N-H)_asym, υ 3373 (N-H)_sym, 3305 (N-H…H-bonded), υ 3099 (C = C), υ 1639 (NH₂ scissoring), υ 1607 (C = C), υ 1498 (C = C), υ 1410 (ring aromatic stretching), and υ 802 (aromatic deformation). The UV‒vis spectrum of the nanocomposite (12) (Fig. 9) displayed absorption bands at λ 275 nm (π–π* transition), λ 355 nm, λ530 nm (π–π* transition) [32], and λ 655 nm, and their bandgap energies were 4.51 eV, 3.70 eV, 2.34 eV, and 1.89 eV, respectively. The bandgap energies were calculated from the equation ΔE = hc/λ,, where ΔE is the bandgap energy (eV), h = 6.625 × 10^-34 JS, c = 3 × 10⁸ m/s, and λ is the wavelength. Notably, the absorption spectrum of safranin showed its characteristic peak at λ 520 nm due to the n-π* transition [33]; thus, the observed slightly redshifted band at λ 530 nm in the nanocomposite (7) suggested an interaction with safranin.

SEM images of the copolymer/SiO₂@Ag nanocomposite (12) are shown in Fig. 10. As shown in Fig. 9 (A), the sample microstructure included particles that were almost spherical in shape and had a uniform size, with an average of ∼20 nm. TEM analysis (Fig. 9 (B)) revealed an aggregate of mixed light/dark gray semispherical particles with an average diameter of 20 nm due to CTAB contamination.

Fig. 10

(A) SEM and (B) TEM images of the nanocomposite (12)

In addition to the main amorphous organic copolymer content, the XRD spectrum of the nanocomposite (12) (Fig. 11) exhibited characteristic peaks of amorphous silica and crystalline silver at 2q values of 9.34°, 10.67°, 13.43°, 14.56°, 15.25°, 15.71°, 17.29°, 18.81°, 22.90° (SiO₂), 25.21°, 27.58°, 31.98° (Ag), 43.57°, 45.94° (Ag), 54.61°, 57.32°, 76.43° (Ag), and 77.86° (Ag).

Fig. 11

XRD pattern of the nanocomposite (12) in the 2θ range of 5–80°

A broad peak observed between 9.34° and 18.81° 2θ degrees is associated with the amorphous nanocomposite (12). Interestingly, the characteristic 2θ values due to van der Waals distances between the masses of the phenylene rings of the parallel chains in the main organic copolymer in (12) were 25.21° and 27.58° [32]. These values corresponding to two of the peaks may be influenced by increased polymer chain separation due to side chain steric hindrance [10]. The calculated interatomic spacing values (d) corresponding to the abovementioned successive peaks are 0.95 nm, 0.83 nm, 0.66 nm, 0.56 nm, 0.51 nm, 0.47 nm, 0.39 nm, 0.35 nm, 0.32 nm, 0.28 nm, 0.21 nm, 0.20 nm, 0.20 nm, 0.17 nm, 0.16 nm, and 0.12 nm, while their calculated crystallite (grain) sizes are 16.66 nm, 16.67 mm, 16.72 nm, 16.74 nm, 16.75 nm, 16.76 nm, 16.79 nm, 16.83 nm, 19.94 nm, 17.01 nm, 17.09 nm, 17.27 nm, 17.88 nm, 18.03 nm, 18.68 nm, 18.92 nm, 21.13, and 21.35 nm, respectively.

Figure 12 shows the thermogravimetric analysis (TGA), differential thermal analysis (DTA), and differential scanning calorimetry (DSC) curves of the copolymer/L-ascorbic acid/Ag@SiO₂/polysafranin nanocomposite (12). The TGA curve of the nanocomposite (12) exhibited successive weight losses at 161 °C (–0.137%), 404 °C (-4.82%) and 530 °C (-8.96%). The DTG spectrum exhibited three endothermic peaks at 170 °C, 383 °C (CTAB removal) [27], and 532 °C, leaving 86.07% of its weight as a residue. The DSC curve shows a weak endothermic peak at 148 °C (energy 127.27 J/g) and a weak broad endothermic peak centred at 380 °C up to 600 °C due to elimination of side chain substituents and subsequent morphological changes. In particular, the DSC spectrum did not display an exothermic degradation peak. This is ascribed to the slow crystallization, and the consequent evolved heat flow over a temperature range was lost in the baseline. This conclusion was also demonstrated by the XRD results for the less ordered and less intense crystalline pattern.

Fig. 12

TGA (A), DTG (B) and DSC (C) analyses of the nanocomposite (12)

The cyclic voltammograms (Fig. 13) of the copolymer/L-ascorbic/Ag@SiO₂/polysafranin nanocomposite (12) exhibited four cathodic redox peaks at -5.76 µA/-2.06 V, -0.32.72 µA/-0.04 V, -67.48 µA/-0.11 V, and − 61.23 µA/-0.346 V; correspondingly, four anodic redox peaks were observed at 0.84 µA/0.41 V, 26.06 µA/0.246 V, 61.54 µA/0.10 V, and 55.79 µA/-0.194 V, respectively. The observed redox peaks at 0.11 V and 0.10 V correspond to safranin. The electrochemical behavior of the composite (12) was in accordance with that of a reported analog [34].

Fig. 13

Cyclic voltammogram (50 mV/s) of the nanocomposite (12) in DMSO

Electrical conductivity is a critical feature or parameter that provides crucial details on the characteristics of electrical dynamics and transport phenomena in noncrystalline materials. The ac conductivities of our samples at various frequencies (Fig. 14) follow Jonscher’s power law (1) [35]:

$${\sigma _{(\omega )}} \, = \, {\sigma _{\rm{dc}}} \, + \, {\rm{A}}{\omega ^{\rm{s}}}$$

(1)

where σ_(ω) is the total conductivity, σ_(dc) is the dc conductivity, ω = 2πf, f is the applied field frequency, s is the constant exponent factor, which takes values between 0< s<1, and A is a temperature-dependent parameter that controls the polarizability strength.

Figure 14 shows the ac electrical conductivity versus frequency from 10 to 2 × 10⁷ (Hz) in the 303 K to 393 K temperature range for neat organic poly(aniline-co-aniline-2,5-disulfonic acid (8) [25]. Figure 14 (a), poly(aniline-co-aniline-2,5-disulfonic acid)/L-ascorbic acid/Ag@SiO₂/polysafranin nanocomposite (12), Fig. 14 (b) and poly(aniline-2,5-disulfonic acid) (6), Fig. 14 (c). The ac conductivity, as shown in Fig. 14, tended to be frequency independent for both the neat polymer (8) [25] and its composite (12). It is well known that different disordered conductive materials, such as polymers and semiconductors, exhibit similar responses to an applied electric field. Their behaviour is described by the real part, σ′(ω), of the complex electrical conductivity, which is the ac conductivity. At low frequencies, random diffusion of the charge carriers via activated hopping gives rise to a frequency-independent conductivity. The real part of the complex conductivity in the low-frequency regime and in the absence of electrode polarization effects is given by a similar empirical equation, σ′(ω) = σ_dc[1+ (ω/ω₀)ⁿ], where n is the same constant exponent factor. The characteristic frequency, ω_ο, corresponds to the onset of the ac conductivity and is a characteristic hopping frequency of those ions contributing to the conductivity. At frequency ω_ο; σ′(ω₀) = σ_d. The total conductivity σ_(ω) and the real part of the complex conductivity in the low-frequency regime σ′(ω) are equivalent when σ_dc = Aω_oⁿ, where the last relation gives the relationship between dc and ac conductivity. In the intermediate plateau region, the conductivity is almost constant and frequency independent, which is called the dc conductivity σ_dc [36]. An increase in temperature increases the mobility (µ) and density (n) of charge carriers, and the dc conductivity is the product of the charge carrier density and mobility according to the following formula: (σ_dc = q nµ). This behaviour, we suggest, was because the applied field’s frequency was high enough to synchronize with the hopping frequency. It can be concluded that incorporating safranin dye into highly sulfonated polyaniline derivatives/Ag@SO₂ nanocomposites achieved better conductivity than did the pure polysafranine analogue [19]. The dc conductivity of the organic oligomer (8) [25] varied from 0.06 to 0.016 (s/cm) and that of the composite (12) from 0.008 to 0.016 (s/cm) with increasing temperature to 363 K, after which both decreased at 393 K. These findings revealed that when the temperature increased to 363 K, the examined material changed from a semiconductor to a metallic material. As shown in Fig. 10, σ’_ac exhibits one anomaly at 363 K corresponding to the phase transitions found by DSC (Fig. 12). This phenomenon could be due to the superposition of many mechanisms, such as the reorientation of the side groups and the increase in the mobility of charge carriers. This phase transition is accompanied by a rapid decrease in σ’_ac at > 363 K. It is more than likely that this effect camouflages the expected relaxation process [37]. In particular, the dc conductivities found for the organic oligomer (8) and its composite (12) were comparable to those of many commercial inorganic or organic composites. For example, the reported dc conductivity of the known manganite compound La_0.62Eu_0.05Ba_0.33Mn_0.85Fe_0.15O₃ was 0.001 (s/cm) [36], while those of the vinyl chloride/vinyl acetate/graphene composite and polystyrene/graphene sheet composite were 0.01 (s/cm) and 1.0 (s/cm), respectively [38].

Fig. 14

AC electrical conductivity versus frequency (10 to 2 × 10⁷ Hz) in the temperature range (303 K to 393 K) for the (a) organic copolymer (8), (b) copolymer/L-ascorbic/Ag@SiO₂/ploysafranin nanocomposite (12) and (c) poly(disulfonated aniline) (6)

The ac conductivity of the organic oligomer (6) increased with increasing frequency, indicating the regular performance of the semiconductor (Fig. 14c). Two distinct trends are observed: at low and high frequencies caused by the electrode in the former case and by the increase in the ionic conductivity in the latter case. The effect of charge carriers travelling over shorter distances increases the ac conductivity as the frequency increases. The exponent factor, s, can be obtained by plotting lnσ_(ω) against ln_ω, as shown in Fig. 15, where s is the slope of the obtained line. The dc conductivity, A parameter and the values of s power in the temperature range 303 K to 393 K were σ’_ac 1.2 × 10^− 7 (s/cm), A; -9.89, s; 0.36 at 303 K; σ’_ac 1.2 × 10^− 7 (s/cm), -10.38, s; 0.42 at 333 K; σ’_ac 1.2 × 10^− 7 (s/cm), -10.53, s; 0.425 at 363 K; and σ’_ac 1.2 × 10^− 7 (s/cm), -10.39, s; 0.426 at 393 K, respectively. It is observed that S tends to increase with increasing temperature. This behaviour is due to nonoverlapping small polaron tunnelling [36, 37]. This variation as a function of temperature is consistent with a thermally triggered process.

Fig. 15

The variation in lnσ_(ω) against ln_ω for poly(disulfonated aniline) (6)

The observed ac conductivity of the organic oligomer (6) (Fig. 14c) can be attributed mainly to the effect of self-doping between the sulfonate side groups and the positively charged nitrogen atoms in the polymer chains [34], as depicted in Fig. 16.

Fig. 16

Proposed self-doping effect of the poly (disulfonated aniline) (6)

The dependence of the electrical conductivity on the grain and grain boundaries of the oligomers (6), (8) and the composite (12) in the 303–393 K temperature range and their temperature dependence could be fitted to the usual Arrhenius Eq. (2) (Fig. 17).

$$\sigma ={\sigma }_{o}exp\left({-E}_{a\sigma }/KT\right)$$

(2)

where E_aϭ is the activation energy.

The Arrhenius equation can be written in a nonexponential form and is often more convenient to use and interpret graphically; see Eq. (3).

$${\rm{ln}}{\sigma _{{\rm{ac}}}}\, = \,{\rm{ln}}{\sigma _0}\, - \,{{\rm{E}}_{\rm{a}}}\sigma /{\rm{KT}}$$

(3)

Figure 17 shows the graphs of Ln_ϭac versus 1000/T (K) at various frequencies for organic poly(aniline-co-aniline-2,5-disulfonic acid (8) [25] (Fig. 17 (a)), the copolymer/L-ascorbic acid/Ag@SiO₂/polysafranin nanocomposite (12) (Fig. 17 (b)) and poly(aniline-2,5-disulfonic acid) (Fig. 17 (c)). The electrical conductivity tended to increase with increasing temperature within the 303–393 K range, indicating semiconducting behaviour for all the compositions studied. As shown in the figure, there were two distinct regions with two different slopes. The thermal activation energies were calculated from the slopes of the two regions. The values of the activation energy E_a1 from 303 to 333 K and E_a2 from 333 to 393 K are tabulated in Table 1. The two distinct conduction regions correspond to two different conduction mechanisms: one can be attributed to limited grain boundary scattering, and the second is referred to as variable range hopping. Nearly all the carriers are inhibited by grain boundary effects at low temperatures. The thermal energy of the charge carriers increases with temperature, facilitating them to easily hop over obstacles caused by imperfections accumulating at grain boundaries.

Table 1 Values of activation energy for the organic copolymer (8) [25] and copolymer/L-ascorbic acid/Ag@SiO₂/polysafranin nanocomposite (12)

Generally, the relation [σ’_ac (ω,T) = Aω^s(ω,T)] represents the electrical conductivity described by the Jonscher power law model, and s(T) represents the power exponent, which depends on the temperature satisfying the condition 0 ≤ s(T) ≤ 1. In the case of polymers, relaxation process analysis through this formalism is not applicable, especially at low frequencies and high temperatures, because the dc conductivity phenomenon hides any relaxation process [37]. To avoid such obstacles, the electric modulus formalism was defined as the inverse of the relative complex permittivity ε*(ω,T), as calculated as given in Eq. (4) [39].

$$M\ast\, = \,1/\varepsilon\ast \,(\omega ,{\rm{T}})\, = \,{M^{\prime}}(\omega ,{\rm{T}})\, + \,{jM^{\prime \prime}}(\omega ,{\rm{T}})$$

(4)

where M’ and M’’ are the real and imaginary parts of the complex modulus and are defined by Eqs. (5) and (6), respectively [37].

$${M^{\prime}}\, = \,{\varepsilon ^{\prime}}\,(\omega ,\,{\rm{T}})\,/\,{\varepsilon ^{\prime 2}}\,(\omega ,\,{\rm{T}})\, + \,{\varepsilon ^{\prime \prime 2}}\,(\omega ,\,{\rm{T}})$$

(5)

$${M^{\prime \prime}}\, = \, - {\varepsilon ^{\prime \prime}}\,(\omega ,\,{\rm{T}})\,/\,{\varepsilon ^{\prime 2}}(\omega ,\,{\rm{T}})\, + \,{\varepsilon ^{\prime \prime 2}}(\omega ,\,{\rm{T}})$$

(6)

ε’ (ω, T) and ε’‘(ω, T) are the real and imaginary parts of the complex permittivity, which represent the storage and the subsequent losses of energy, respectively, during every electric field cycle.

To study the electrical relaxation behaviour of conducting materials, electrical modulus characterization is commonly used. The complex electric modulus is measured by the inverse of the complex permittivity [37, 40]. The electric modulus analysis eliminates unwanted capacitance effects caused by electrode contacts and provides a good picture of dc conduction and dipole relaxation. Figure 17 shows the plots of the real M^’ of the electric modulus versus frequency for the organic poly(aniline-co-aniline-2,5-disulfonic acid (8) [25] composite (Fig. 17 (a)), copolymer/L-ascorbic acid/Ag@SiO₂/polysafranin nanocomposite (12) (Fig. 17 (b)) and poly(aniline-2,5-disulfonic acid) (6) (Fig. 17 (c)). As the frequency increased for the various temperatures, the real part of the electric modulus exhibited an increase in value but decreased for increasing T up to 363 K and then increased. The anomaly M’ at 363 K corresponded to the phase transitions found by the DSC (Fig. 12). This phenomenon could be attributed to the reorientation of the side groups and the increase in the mobility of charge carriers. The phase transition is accompanied by a rapid decrease in M’ at temperatures > 363 K. Furthermore, for each temperature, M^’ reaches its highest value at high frequencies, indicating a relaxation process [37, 41]. The charge carriers were movable over great distances at frequencies below the maximum limit. This may correspond to a conduction phenomenon caused by the short-range mobility of charge carriers [38].

Fig. 17

Plots of Ln_ϭac vs. 1000/T (K) at various frequencies for the (a) organic copolymer (8), (b) copolymer/L-ascorbic acid/Ag@SiO₂/polysafranin nanocomposite (12) and (c) poly(disulfonated aniline) (6)

The complex dielectric function, ε^∗(ν), and the complex conductivity function, σ^∗(ν), were both evaluated via broadband dielectric spectroscopy (BDS). According to Eqs. (7) and (8), these factors are linked to each other [42].

$${\varepsilon }^{\ast}\left(\omega \right)={\varepsilon }^{{\prime }}\left(\omega \right)-i{{\varepsilon }^{\prime \prime }}\left(\omega \right)$$

(7)

$${\sigma}^{\ast}\left(\omega \right)=i{\varepsilon }_{0}\omega {\varepsilon }^{\ast}\left(\omega \right)$$

(8)

implying that ${\sigma }^{{\prime }}={\varepsilon }_{o}\omega {\varepsilon }^{\prime \prime }$, ${\sigma }^{{\prime }{\prime }}={\varepsilon }_{o}\omega {\varepsilon }^{{\prime }}$, (${{\varepsilon }}_{\text{o}}$is the vacuum permittivity, and ${\omega }(=2{\pi }{\varepsilon })$ is the radial frequency).

Like with any other kind of spectroscopy, M^’ spectroscopy involves the use of a fingerprint of the investigated sample. It is distinguished from the other kinds of methods by its wide range of frequencies. This divides the dielectric spectrum into three sectors: molecular dynamics at the microscopic level; charge carrier mobility, which is proven in conductivity mechanisms; and the accumulation of charge carriers at the interfaces at the boundaries between different phases of the multicomponent composites and at the electrode/dielectric material interface. The measured permittivities, ε’, of the prepared samples are graphically plotted against frequency in Fig. 18. The frequency dependence of the permittivity, ε′, showed a marked effect on the frequency window under consideration. However, the figure does not show the marked effect of temperature on the permittivity since all the curves collapse together. The permittivity was essentially independent of the frequency increase for the oligomer (8) and composite (12). As shown in Fig. 19, ε’ exhibited one anomaly at a temperature of 363 K corresponding to the phase transitions found by DSC (Fig. 12). The variations in ε’ increase with increasing temperature, with a peak at 363 K. The phase transition was accompanied by a rapid decrease in ε’ at temperatures > 363 K. More than likely, this effect camouflaged the expected relaxation process.

Fig. 18

Plots of the real M’ of the electric modulus versus frequency for the (a) organic copolymer (8), (b) copolymer/L-ascorbic acid/Ag@SiO₂/polysafranin nanocomposite (12) and (c) poly(disulfonated aniline) (6)

The permittivity, ε’, of poly(disulfonated aniline) (6) decreased with increasing frequency (Fig. 19c). Two prominent dynamics of the real part of the complex permittivity, ε′ can be seen here as represented against the frequency. In the frequency range (0.1 Hz – 10 kHz), the graph displays a gradual reduction in magnitude. The high permittivity at low frequency is due to electrode polarization. The decrease in ε′ at high frequencies (from 10 kHz up to 20 MHz) was due to the dipoles not having enough time to coincide in motion with the frequency of the applied electric continuous field [42].

Fig. 19

Measured permittivity versus frequency of the (a) organic copolymer (8), (b) copolymer/Ag@SiO₂/polysafranin nanocomposite (12) and

Conclusions

In this work, we report for the first time the synthesis of poly(aniline-co-aniline-2,5-disulfonic acid)) in a composite containing safranin, L-ascorbic acid, and metallic Ag/SiO₂ nanoparticles and investigated the influence of incorporating the dye on the morphology and conductivity. The incorporation of L-ascorbic acid had dual effects: as an oxidant and a reactant. Polyanone (aniline-co-aniline-2,5-disulfonic acid) was chemically prepared in a low-pH 1.5 aqueous HCl media using 1.25x equivalent of ammonium persulfate oxidant. The silver-doped silica complex spheres were prepared by the first hydrolysis of ethanol-diluted TEOS in the presence of CTAB followed by the subsequent addition of silver nitrate and NaBH₄. Targeted poly(aniline-co-aniline-2,5-disulfonic acid)/polysafranin/L-ascorbic acid/Ag@SiO₂ nanocomposites were chemically prepared at RT by stirring a mixture of Ag@SiO₂ nanoparticles, aniline-2,5-disulfonic acid, aniline (10 weight%), safranin, and L-ascorbic acid using 1.25x equivalents of ammonium persulfate oxidant. The elemental composition of the nanocomposite indicated the contribution of (1:3) disulfonated aniline/aniline units to the resulting nanocomposite. The polymeric composite was characterized via IR and UV spectroscopic techniques, cyclic voltammetry, electrical conductivity, and dielectric measurements. SEM, TEM, TGA, and DSC measurements were also performed for additional analysis. The electronic spectrum showed absorption bands attributed to the quinoid ring transition, charge transfer from the HOMO of the benzenoid ring to the LUMO of the quinoid ring and a characteristic safranin absorption peak. EDXS analyses further confirmed not only the contamination of the composite with CTAB traces but also the successful synthesis of the copolymer/SiO₂@Ag nanocomposite. SEM and TEM images of the SiO₂@Ag nanospheres revealed well-separated spherical particles with an average size of 10 nm. The surface morphology of the organic copolymer was marked by the presence of hemispherical, well-separated particles with an average size of 21.5 nm. SEM and TEM images of the targeted nanocomposite revealed intensified spherical particles that were dispersed over almost the entire surface, and the SiO₂/Ag particles were distributed on the composite surface. The DSC curve of these compounds showed a weak endothermic peak at 148 °C (energy 127.27 J/g) and a weak broad endothermic peak centred at 380 °C up to 600 °C due to substituent elimination and subsequent morphological changes. The DSC curve showed a weak endothermic peak at 148 °C (energy 127.27 J/g) and a weak broad endothermic peak centred at 380 °C up to 600 °C due to substituent elimination and subsequent morphological changes. The DSC spectrum did not display an exothermic degradation peak, indicating slow crystallization. The XRD spectrum exhibited peaks corresponding to amorphous silica and crystalline silver at many 2q values, and their interatomic spacing (d) and crystallite (grain) sizes were calculated. The thermal degradation curve of the organic copolymer or the composite exhibited an interesting model of the stability of the polymer, and their subsequent weight loss was completed in four steps, leaving nearly 50% of their weight as a remaining residue. The cyclic voltammogram of the nanocomposite exhibited redox peaks similar to those reported for the aniline/orthanilic acid copolymer. The observed redox peaks at 0.11 V and 0.10 V correspond to safranin.

Because the applied field frequency is high enough to synchronize with the hopping frequency, the ac conductivity tends to be frequency independent for both the copolymer and its composite, where the value of electrical conductivity equals only the d.c conductivity. The d.c. conductivity of the organic oligomer varied from 0.06 − 0.016 (s/cm) and that of the composite from 0.008 to 0.016 (s/cm) with increasing temperature up to 363 K and then decreased at 393 K. The examined material changed from a semiconductor to a metallic material when the temperature was increased up to 363 K. The dc conductivity exhibited one anomaly at a temperature of 363 K corresponding to the phase transitions found by DSC analysis. This phenomenon could be attributed to reorientation of the side groups and an increase in the mobility of the charge carriers. This phase transition was accompanied by a rapid decrease in ac conductivity at this temperature (> 363 K), most likely because of the expected relaxation process. The d.c. conductivities of the examined materials are comparable to those of many commercial inorganic or organic composites. Notably, the conductivity of poly(aniline-2,5-disulfonic acid) itself proportionally increased with frequency, indicating the regular performance of the semiconductor. The observed low conductivity is mainly attributed to self-doping between the sulfonate side groups and the positively charged nitrogen atoms in the polymer chains. The electric conductivity/temperature correlation is attributed to two distinct conduction regions corresponding to two different conduction mechanisms, i.e., grain boundary scattering limited and variable range hopping. The frequency dependence of the permittivity, ε′, showed a marked effect on the frequency window under consideration. The permittivity is independent of the frequency increase for the oligomer and the composite. The permittivity of poly(aniline-2,5-disulfonic acid) decreased with increasing frequency. This behavior supports the non-Debye dependency by confirming the occurrence of electrode polarization and space charge effects. In conclusion, when dyes participate in the preparation of conducting polymers, they significantly affect both their morphology or conductivity and both. Incorporating safranin dye with thermally stable, highly sulfonated polyaniline derivatives/Ag@SO₂ nanocomposites achieved better conductivity after heating.