Introduction

Nanomaterials have gained tremendous interest of scientists and researchers owing to improved properties of these kinds of materials relative to the bulk materials [1, 2]. Among carbon-based nanomaterials, fullerene, carbon nanotube, nanodiamond and graphene have been widely exploited.

One of the most important forms of carbon nanostructure family is the nanodiamonds (NDs). In recent decades, nanodiamond powder (usually obtained through the detonation of carbon-based explosives) has attracted exceptional research concern [3, 4]. Nanodiamond powder has the fine combination of properties such as chemical stability, hardness, crytsallinity, dopability, and low toxicity [5, 6]. Moreover, nanodiamonds have narrow particle size distribution (typically 4–6 nm), small diameter (~5 nm), large surface area (300–500 m2/g), and superior thermal conductivity, optical and electrical properties [7–9]. Due to the above-mentioned properties, nanodiamond has vast potential as adsorbents, reinforcement of plastics and resins, colloidal suspension, microabrassive, refrigerating fluids, quantum dots, etc. Polymer-based nanocomposites have made tremendous breakthroughs in advanced functionalized materials such as rechargeable batteries, super capacitors, electronic devices, sensors and functional electrodes [10–12]. These applications have been attributed to remarkable improvement in thermal, electrical and mechanical properties of polymers by incorporation of small amount of nanofiller [13–15]. Preparation of layered nanocomposites has been an interesting research field. However, such type of nanocomposites depicts poor electrical and thermal conductivity [16, 17]. To overcome these impediments, carbon-based nanomaterials including graphite, carbon black, carbon nanotube and nanodiamonds have been used as reinforcement [18–20].

Among carbon-based nanomaterials, spherical nanoparticles have received significant interest due to their highest surface-to-volume ratio [21–23]. Consequently, nanodiamonds have been known as popular reinforcement due to nearly spherical shape, nano-size, and exceptional physiochemical and mechanical characteristics [24–27]. In this regard, π-conjugated or conducting polymers have been known as significant materials due to tunable electronic and physical, mechanical and optical characteristics. These polymers are, thus, widely applicable in batteries, electrochromic devices, anti-corrosion coatings, photovoltaics, organic transistors and light emitting diodes [28, 29]. Polyaniline, polythiophene, polypyrrole, etc. are among the most commonly used conducting organic polymers. Accordingly, these conducting organic polymers in combination with NDs exhibit remarkable mechanical, thermal and electrical characteristics.

In recent times, major research focus has been the utilization of cheaper, efficient and non-contaminating energy resources. One of the attractive ways to resolve the intrinsic problem is the use of Li-ion batteries which convert chemical energy into electrical energy through chemical bond cleavage. Ionically conducting gel polymer electrolyte (GPE) has been considered as an excellent choice for the replacement of liquid electrolyte due to flexible characteristics, good cyclability, safety, energy density, etc. [30]. New layered polymer/nanodiamond nanocomposite (electrolyte) having fine mechanical and electrical characteristics plus resistance toward environmental exposition may be used in rechargeable polymer lithium ion batteries [31].

In this report, we have opted chemical oxidative in situ polymerization route for the synthesis of functionalized (F-NDs) and non-functionalized nanodiamonds (NF-NDs)-based nanocomposites using polyaniline (PANi), polypyrrole (PPy), polythiophene (PTh) and polyazopyridine (PAP) as layered matrices. NF-NDs/PAP/PANi/PPy, F-NDs/PAP/PANi/PPy, NF-NDs/PANi/PPy/PTh and F-NDs/PANi/PPy/PTh nanocomposites were fabricated by in situ polymerization of monomers (2,6-diamino pyridine, aniline, thiophene and pyrrole) over the nanodiamond surface. To the best of our knowledge, core shell NDs/PAP/PANi/PPy was first time chemically synthesized through this route. Furthermore, physical characteristics of the prepared nanocomposites were explored using suitable techniques. The aim of this study was to synthesize nanocomposite with better electrical conductivity without decreasing the thermal properties.

Experimental

Materials

Nanodiamonds synthesized by detonation route with 99 % purity and size of clusters 64–120 nm were adopted. 2,6-Diaminopyridine (98 %), aniline (99 %), pyrrole (98 %), thiophene (>99 %) were purchased from Aldrich and kept at 0 °C prior to use. Various other reagents such as potassium dichromate (K2Cr2O7, 99.99 %), anhydrous iron (ΙΙΙ) chloride (FeCl3, >98 %), sulfuric acid (H2SO4, 98 %), hydrochloric acid (HCl, 37 %), nitric acid (HNO3, 70 %), sodium hydroxide (NaOH, >98 %) and sodium nitrite (NaNO2, >97 %) were also procured from Aldrich and used as received.

Measurements

Infrared (IR) spectra were recorded using a FTSW 300 MX, Bio-Rad (USA) Fourier transform infrared (FTIR) spectrometer (4 cm−1 resolution). Field emission scanning electron microscopy (FESEM) of samples was performed using a JSM5910, JEOL (Japan) microscope. Thermal stability was verified by a Mettler Toledo TGA/SDTA 851 (Switzerland) thermogravimetric analyzer using 1–5 mg of the sample in Al2O3 crucible at a heating rate of 10 °C min−1. Differential scanning calorimetry (DSC) was performed by a Mettler Toledo DSC 822 (Switzerland) differential scanning calorimeter taking 5–10 mg of samples in aluminum pans and heated at a rate of 10 C min−1. X-ray diffraction patterns were obtained at room temperature on an X-ray diffractometer (PW 3040/60 X’pert PRO, PANalytical, The Netherland) using Ni-filtered Cu Kα radiation (40 kV, 30 mA). An Energy dispersive X-ray (EDX) spectrometer (EDX-720/800HS/900HS, Shimadzu Europe, Germany) was also used for elemental analysis. Electrical conductivity was measured using a Keithley 614 electrometer (USA) and the four-probe method.

Purification of nanodiamonds

In nanodiamonds synthesis, various impurities were incorporated in nanodiamond particulates hindering their applications. Essentially, removal of these impurities for example sp 2 carbon species (graphite, amorphous carbon, fullerene like carbon and some hetero atoms) and incombustible residues (metals and oxides, 1–8 wt%) by thermal oxidation and acid treatment (using liquid oxidants such as HNO3, mixture of H2SO4 and HNO3, K2Cr2O7 in H2SO4, KOH/KNO3, Na2O2, and HNO3/H2O2 under pressure of HClO4) has been prerequisite for further appliance of nanodiamonds. To get rid of sp 2 nondiamond by product from the detonation soot, as-prepared diamond soot was subjected to thermal oxidation in conc. HNO3 at 200–250 °C and 80–100 atm pressure in a titanium alloy reactor. Further purification was performed using HCl treatment (80–100 °C) followed by repeated washing with deionized water until pH 7 was attained [32, 33].

Functionalization of nanodiamonds

Introduction of discrete functional groups on the filler surface was adopted to modify the nanodiamond characteristics. Among various functionalized nanodiamonds, carboxylated nanodiamonds have been more conveniently synthesized. Surface functionalization of purified nanodiamonds was carried out in a mixture of strong acids, i.e., H2SO4 and HNO3 (3:1, respectively) at 30 °C with continuous magnetic stirring of 24 h. The above mixture was poured into a beaker containing 200 mL hot water (70 °C) and stirred for 10 h (room temperature). After filtration, products were washed several times with deionized water and dried at 80 °C for 4 h (Scheme 1).

Scheme 1
scheme 1

Functionalization of nanodiamonds

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Synthesis of NF-NDs/PANi/PPy/PTh

In 250 mL round bottom flask, 0.5 mL aniline was dispersed in 50 mL aqueous HCl and stirred for 1 h. A sample of 0.1 g purified nanodiamonds were dispersed in the above mixture and again stirred for 12 h resulting in adsorption of aniline monomer on the nanodiamonds surface. To the above mixture, 50 mL K2Cr2O7/HCl solution [0.5 g K2Cr2O7 in 0.1 M aqueous HCl (50 mL)] was added dropwise at 0 °C for 6 h (labeled as A). Secondly, a mixture of 0.5 mL pyrrole and aqueous HCl (50 mL) was prepared separately and added dropwise to A and further agitated for 12 h. To initiate the polymerization of pyrrole, a solution of FeCl3 (1.2 g) in 0.1 M HCl (50 mL) was added dropwise to the above mixture at 0 °C for 6 h (labeled as B). A solution of 0.5 mL thiophene in 50 mL aqueous HCl (0.1 M) was added dropwise to B. Further stirring for 12 h was prerequisite for the adsorption of monomers. Polymerization was facilitated by the addition of a solution of FeCl3 (1.2 g) in 0.1 M HCl (50 mL) at 0 °C. The product obtained was washed with deionized water and dried at 70 °C.

Synthesis of F-NDs/PANi/PPy/PTh

In the synthesis of core shell F-NDs/PANi/PPy/PTh constituting F-NDs (functionalized nanodiamonds) the same synthetic route was opted as depicted in Scheme 2.

Scheme 2
scheme 2

Schematic synthesis of NDs/PANi/PPy/PTh nanocomposite

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Synthesis of NF-NDs/PAP/PANi/PPy

An sample of 2,6-diamino pyridine (1.09 g) was initially dissolved in H2O/HCl (13:4 mL, respectively). In a separate beaker, 0.68 g NaNO2 was dissolved in 7 mL H2O and added dropwise to the above mixture at 0 °C for 3 h (labeled as C). Amount of 0.1 g of NF-NDs was added to the mixture C and labeled as D. In the next step, 0.5 mL aniline was dissolved in 0.1 M HCl (50 mL) and added dropwise to D with continuous stirring for 12 h. The polymerization of aniline was initiated by dropwise addition of a mixture of 0.5 g K2Cr2O7 in 0.1 M HCl (50 mL) at 0 °C for 6 h (E). Afterward, a solution of 0.5 mL pyrrole in 0.1 M HCl (50 mL) was prepared and added dropwise to the E. The dropwise addition of FeCl3/HCl [1.2 g FeCl3/0.1 M HCL (50 mL)] at 0 °C for 6 h initiated the polymerization of pyrrol. The resulting nanocomposite was obtained by filtration and washed repeatedly with deionized water. Finally, the product was dried at 70 °C.

Synthesis of F-NDs/PAP/PANi/PPy

In the synthesis of F-NDs/PAP/PANi/PPy, the same procedures were adopted as illustrated in Scheme 3. The only difference was the use of functionalized nanodiamonds.

Scheme 3
scheme 3

Schematic representation for synthesis of NDs/PAP/PANi/PPy nanocomposite

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Results and discussion

Spectroscopic analysis

The structural elucidation of nanofiller and nanocomposites was carried out using FTIR (Table 1) technique. FTIR spectra of nanodiamonds (functionalized and non-functionalized) are depicted as spectra (a) and (b) in Fig. 1, respectively. The absorption bands around 3,003 and 3,001 cm−1 were found due to the aromatic protons of functionalized and non-functionalized nanodiamond particles, respectively. The carboxylic acid functionalization was confirmed by the appearance of hydroxyl and carbonyl stretching vibrations at 3,479 and 1,720 cm−1, respectively. Moreover, during acid functionalization there was the possibility of the introduction of SO3H groups on the surface of nanodiamonds. Therefore, IR bands of S=O groups was found at 1,190 cm−1 (asymmetric stretching vibration) and in the range 1,000–1,040 cm−1 (symmetric stretching vibration). Aromatic C–H stretching vibration was found at 3,019 cm−1, while secondary aromatic amine stretching and bending vibrations were observed at 3,298 and 1,596 cm−1 for NF-NDs/PANi/PPy/PTh (spectrum (c) in Fig. 1). Moreover, the absorption bands at 1,451 and 1,255 cm−1 were characteristics of thiophene ring vibration and C–N stretch, respectively. On the other hand, functionalized composite F-NDs/PANi/PPy/PTh [spectrum (d) in Fig. 1] demonstrated characteristics peaks at 3,456 and 1,718 cm−1 owing to carboxylic acid groups.

Table 1 FTIR data of functionalized nanodiamonds, un-functionalized nanodiamonds and nanocomposites
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Fig. 1
figure 1

FTIR spectra of a purified nanodiamonds, b functionalized nanodiamonds, c NF-NDs/PANi/PPy/PTh and d F-NDs/PANi/PPy/PTh nanocomposites

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In spectrum (a) in Fig. 2, N–H stretching and bending vibrations have been appeared at 3,324 and 1,595 cm−1, respectively, while characteristic absorption of N=N vibration is found at 1,414 cm−1 owing to the azo-based polymer in NF-NDs/PAP/PANi/PPy. Absorption peak of C–N vibration was appeared at 1,292 cm−1. Similarly, F-NDs/PAP/PANi/PPy displayed nearly identical bands at 3,354, 1,597 (N–H vibrations), 1,413 (N=N bond) and 1,298 cm−1 (C–N vibration) as shown in spectrum (b) in Fig. 2. The stretching vibrations at 1,717 and 3,498 cm−1 were attributed to the acidic carbonyl and hydroxyl functionalities, respectively. Furthermore, aromatic C–H stretching vibration appeared at 3,003 and 3,001 cm−1 for NF-NDs/PAP/PANi/PPy and F-NDs/PAP/PANi/PPy, respectively.

Fig. 2
figure 2

FTIR spectra of a NF-NDs/PAP/PANi/PPy and b F-NDs/PAP/PANi/PPy

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Elemental analysis

EDX analysis of nanodiamonds and nanocomposites

Energy dispersive X-ray (EDX) spectroscopy analysis was performed to investigate the composition of nanodiamonds (purified and functionalized) and nanocomposites. EDX spectra in Fig. 3a and b represent the composition of nanodiamonds and in Fig. 3c–f stand for the nanocomposites.

Fig. 3
figure 3

EDX spectra of a non-functionalized nanodiamonds, b functionalized nanodiamonds, c NF-NDs/PANi/PPy/PTh, d F-NDs/PANi/PPy/PTh, e NF-NDs/PAP/PANi/PPy, and f F-NDs/PAP/PANi/PPy samples

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The spectra of purified nanodiamonds (Fig. 3a) did not show the presence of any impurity. However, the minute amount of Si was due to the catalyst used in the synthetic procedure. EDX spectrum of purified nanodiamonds shows the atomic percent of C = 90 wt%, O = 9.8 wt% and Si = 0.19 wt%. As it is appeared from Fig. 3b, the functionalized nanodiamonds had higher oxygen and lower silicon content (compared with purified nanodiamonds) in addition to carbon and sulfur traces (as impurity). The atomic percentages of functionalized nanofiller were found to be C = 64.53 wt%, Si = 0.07 wt%, O = 29.86 wt%, and S = 5.55 wt%. The amount of 5.55 wt% sulfur in functionalized nanodiamond came from the sulfuric acid treatment during functionalization. High content of sulfur in the functionalized ND was due to the introduction of SO3H groups during the oxidative functionalization as also detected in FTIR results.

EDX spectrum of NF-NDs/PANi/PPy/PTh (Fig. 3c) shows atomic percentages of C = 43.56 wt%, N = 7.78 wt%, O = 35.19 wt%, Cl = 2.31 wt% and Cd = 0.38 wt%. The composition of non-functionalized nanocomposite is seemed to be dependent on the type of monomer and catalyst involved in the synthesis. Elemental composition confirmed the absence of impurities but small percentages of chlorine and cadmium were found.

Figure 3d shows the spectrum of F-NDs/PANi/PPy/PTh nanocomposite with C = 48.16 wt%, N = 41.10 wt%, O = 12.65 wt%, Cl = 7.78 wt% and Cd = 1.09 wt%. Higher oxygen and nitrogen content were detected in F-NDs/PANi/PPy/PTh nanocomposite relative to NF-NDs/PANi/PPy/PTh sample. The increase in nitrogen content for F-NDs/PAP/PANi/PTh was due to better layering of polymer on the functionalized filler. For NF-NDs/PAP/PANi/PPy nanocomposite (Fig. 3e), the composition was found as C = 60.73 wt%, O = 14.10 wt%, Si = 0.03 wt%, Cl = 2.55 wt% and Cr = 2.30 wt%. Traces of presented chlorine and chromium impurities may be due to the catalyst used during polymerization.

EDX spectrum of F-NDs/PAP/PANi/PPy nanocomposite (Fig. 3f) shows C = 50.26 wt%, O = 34.39 wt%, N = 29.46 wt%, Cl = 5.93 wt% and Cr = 0.24 wt%. Oxygen and chlorine atomic percentages were found to be higher in functionalized nanodiamond nanocomposite. The significant amount of nitrogen observed in this nanocomposite sample can be due to the better layering of PAP. Moreover, traces of chromium as impurity were detected.

Morphological investigation

Morphology of nanodiamonds

FESEM micrographs of nanodiamonds (Fig. 4a–d) depict the typical aggregates of purified nanodiamonds due to significant interaction among nano-sized particles and the presence of micro and nano-sized grains/nanodiamond crystals. Steady re-nucleation or polynucleation of initial nuclei resulted in the columnar morphology of microcrystalline diamond. Nanocrystalline diamonds were formed when the newly formed growth center suppressed the growth of former crystals. Nanocrystallites with smooth granular surface resembling spherical particles were thus observed.

Fig. 4
figure 4

FESEM micrographs of a non-functionalized nanodiamonds at magnification ×5,000; b non-functionalized nanodiamonds at magnification ×25,000; c functionalized nanodiamonds at magnification ×25,000; d functionalized nanodiamonds at magnification ×50,000

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Figure 4a and b shows larger and smaller aggregates of non-functionalized nanodiamonds, while the micrographs of functionalized nanodiamonds are presented in Fig. 4c and d, correspondingly. Morphological characterization confirmed that the nanodiamond particles possessed 90–120 nm diameters depicting their high aspect ratio. Functionalization busted the microcrystalline diamond aggregates into smaller one thus leading to homogenous dispersibility of functionalized NDs. Functionalization also created significant number of active sites (defects) on the surface without altering nanodiamond structural chemistry. Micrograph at higher resolution (Fig. 4c) depicts even dispersion, smooth and clear diamond shape of nanoparticulates.

Morphology of NDs/PAP/PANi/PTh nanocomposites

Figure 5a, b shows irregular morphology of nanocomposite comprising of non-functionalized nanodiamond particulates embedded in polymer aggregates. Morphologies of F-NDs/PANi/PPy/PTh nanocomposites, i.e., based on functionalized nanodiamonds and conducting polymers are depicted in Fig. 5c–f. Functionalized nanodiamonds having high specific surface area provided large number of sorption sites for the adsorption of monomers. Therefore, homogenous layered polymerization was facilitated due to the enhanced interaction between the external surface of functionalized nanodiamonds and the matrix. Consequently, F-NDs/PANi/PPy/PTh depicted exclusive morphology of tightly woven interconnected fibrous network (polymer) with embedded nanodiamonds (Fig. 5d, e). Besides, the nanodiamonds homogenously coated with conducting polymers formed granular arrangements on the fibrous matrix network. Such a morphological profile may offer a pathway for the transport of ions and solvent molecules within the nanocomposite and may lead to increased electrochemical characteristics.

Fig. 5
figure 5

FESEM micrographs of a NF-NDs/PANi/PPy/PTh at magnification ×20,000; b NF-NDs/PANi/PPy/PTh at magnification ×30,000; c F-NDs/PANi/PPy/PTh at magnification ×5,000; d F-NDs/PANi/PPy/PTh at magnification ×20,000; e F-NDs/PANi/PPy/PTh at magnification ×30,000; f F-NDs/PANi/PPy/PTh at magnification ×55,000

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Morphology of NDs/PAP/PANi/PPy nanocomposites

Figure 6a, b shows agglomerated and globular morphology for NF-NDs/PAP/PANi/PPy nanocomposite. Polymer agglomerates can be observed in the micrographs due to uneven adsorption of monomers over non-functionalized nanodiamond surface. Figure 6b depicts the complete covering of nanodiamonds with polymer aggregated due to non-homogenous polymerization. FESEM images of F-NDs/PAP/PANi/PPy (Fig. 6c–f) are somewhat different. The micrographs reveal comparatively even dispersion of polymer-coated nanodiamonds in F-NDs/PAP/PANi/PPy. Due to consistent polymerization of monomers on functionalized NDs surface, the diameter of nanofillers (Fig. 6c, d) was found to be larger relative to that of pure nanodiamonds.

Fig. 6
figure 6

FESEM micrographs of a NF-NDs/PAP/PANi/PPy at magnification ×5,000; b NF-NDs/PAP/PANi/PPy at magnification ×25,000; c F-NDs/PAP/PANi/PPy at magnification ×5,000; d F-NDs/PAP/PANi/PPy at magnification ×20,000; e F-NDs/PAP/PANi/PPy at magnification ×30,000; f F-NDs/PAP/PANi/PPy at magnification ×55,000

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Thermal analysis

DSC and TGA profiles of nanocomposites are showed in Figs. 7, 8, 9, 10 and their corresponding data were summarized in Table 2. The glass transition temperatures (taken as middle point of change in slope of base line in DSC thermograms) were found to be 99 and 105 °C for NF-NDs/PANi/PPy/PTh and F-NDs/PANi/PPy/PTh, respectively (Fig. 7). On the other hand, NF-NDs/PAP/PANi/PPy and F-NDs/PAP/PANi/PPy showed higher Tg’s of 109 and 118 °C, respectively (Fig. 8). The increase in glass transition temperature was due to the incorporation of azopolymer in multi-layered nanocomposites resulting in the enhanced chain rigidity and restricted the segmental mobility. The chain rigidity was also attributed to better interaction between the filler and polymer chains increasing the glass transition temperature. In this way, the thermal stability of new nanocomposites was enhanced relative to the reported materials [34].

Fig. 7
figure 7

DSC thermograms of NDs/PANi/PPy/PTh at heating rate of 10 °C min−1 in N2

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Fig. 8
figure 8

DSC thermograms of NDs/PAP/PANi/PPy at heating rate of 10 °C min−1 in N2

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Fig. 9
figure 9

TGA thermograms of NDs/PANi/PPy/PTh at heating rate of 10 °C min−1 in N2

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Fig. 10
figure 10

TGA thermograms of NDs/PAP/PANi/PPy at heating rate of 10 °C min−1 in N2

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Table 2 Thermal analyses data of nanocomposites
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Thermal stability was also investigated in terms of the initial degradation temperature (T 0), 10 % weight loss (T 10), maximum decomposition temperature (T max) and char residue at 600 °C using TGA. According to the literature, degradation of NDs usually starts at 100 °C and continue up to 550 °C. When NDs were heated up to 600 °C, only 5 % of weight loss was generally observed due to the oxidation of graphite and amorphous carbon (sp 3 phase) content. A total weight loss of 11.5 % was observed at 900 °C, above which all oxygenated groups were usually released [34, 35].

Figure 9 shows a comparison between the thermal stability of NF-NDs/PAP/PANi/PPy and F-NDs/PAP/PANi/PPy nanocomposites. Thermogram of NF-NDs/PAP/PANi/PPy showed single stage decomposition starting at 444 °C and continuing up to 550 °C. Major weight loss was observed in the range of 444–559 °C. At this stage, polymer backbone was completely broken down and heavier products were disintegrated into smaller fragments and gaseous by products. NF-NDs/PAP/PANi/PPy exhibited char yield of 58 % at 600 °C. On the other hand, F-NDs/PAP/PANi/PPy showed T 0 of 459 °C, T 10 at 489 °C, T max around 567 °C and char yield of 59 %. NDs/PANi/PPy/PTh nanocomposite also showed single stage decomposition (Fig. 10). NF-NDs/PANi/PPy/PTh had T 0 of 433 °C, T 10 at 459 °C and T max at 549 °C. In contrast, F-NDs/PANi/PPy/PTh exhibited higher thermal degradation temperature as T 0 = 436 °C, T 10 = 471 °C and T max = 555 °C. On the whole, F-NDs/PANi/PPy/PTh and F-NDs/PAP/PANi/PPy based on functionalized nanodiamonds showed higher values in thermal properties and char residue.

XRD analysis

Final characteristics of the materials synthesized were studied using XRD analysis in the region 2θ = 5°–50° at room temperature. The XRD scans of nanodiamonds and nanocomposites are presented in Fig. 11. The purified nanodiamond displayed peaks with 2θ values of 42.01°, 43.13° and 48.99° due to three-dimensional detonation nanodiamond structures. The diffractogram of functionalized nanodiamonds showed sharp peak at 2θ = 43.46° depicting that the nanodiamond particulates were functionalized without altering the nanofiller crystalline structure. The characteristic peaks of NF-NDs/PANi/PPy/PTh were observed around 2θ = 43.47°. While F-NDs/PANi/PPy/PTh nanocomposite depicted diffraction peaks at 25.02° and 43.86°. The pattern of NF-NDs/PAP/PANi/PPy revealed three peaks at 2θ = 17.22°, 25.08° and 27.72°. Whereas, a broad peak appeared at 2θ = 26.92° for F-NDs/PAP/PANi/PPy. For pure PANi, diffraction peaks have been observed at 2θ = 20° and 25° due to the periodically arranged parallel and perpendicular polymer chain structures [36].

Fig. 11
figure 11

XRD patterns of a purified nanodiamonds, b functionalized nanodiamonds, c NF- NDs/PANi/PPy/PTh, d F-NDs/PANi/PPy/PTh, e NF-NDs/PAP/PANi/PPy, and f F-NDs/PAP/PANi/PPy samples

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Electrical conductivity

Table 3 represents the electrical conductivity of heteroaromatic NDs/PAP/PANi/PPy nanocomposites with functionalized and non-functionalized filler. Multi-layered azo-, pyridine- and thiophene moieties were found to enhance the conductivity of nanocomposites. The conductivity of NF-NDs/PAP/PANi/PPy was 3.8 Scm−1 and was improved with functionalized filler in F-NDs/PAP/PANi/PPy as 5.4 Scm−1. Whereas, NF-NDs/PANi/PPy/PTh and F-NDs/PANi/PPy/PTh nanocomposites had relatively lower values of 2.9 and 3.7 Scm−1, respectively. Moreover, the increase in conductivity in this system was less significant.

Table 3 Conductivity measurements of nanocomposites
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Nanodiamonds, having high surface area and tunable surface structure, doped with conducting polymers showed significant enhancement in the electrical properties owing to the synergetic effects of layered conducting matrices and filler. Functionalized filler was found to enhance the electrical conductivity proficiently relative to non-functionalized filler due to better interaction with the matrices. Morphology comprising of fibrous matrix network with embedded functionalized nanodiamonds also assisted pathways for the transportation of electrons in functionalized nanocomposites. In the literature, nanodiamonds have also been known to increase the electrical properties of the final composite material [37].

Conclusion

In this exploration, we have discussed the rational control of structural, thermal, morphological and electrical properties of nanocomposites through the surface modification of nanodiamonds as (a) introduction of functional groups and (b) in situ doping of various matrices over the filler surface. New layered polyaniline, polypyrrole, polythiophene and polyazopyridine nanocomposites filled with nanodiamond were obtained through facile synthetic strategy (in situ route). In particular, four types of nanocomposites were studied, i.e., non-functionalized and functionalized ND-based layered PAP/PANi/PPy and non-functionalized and functionalized ND-based layered PANi/PPy/PTh nanocomposites. Thermal and electrical properties of the nanodiamond composites comprising of nanodiamonds dispersed on polymeric fibers were considered. The addition of functionalized nanodiamonds to multi-polymer matrix enhanced the thermal stability and render the nanocomposite heat resistant at low filler concentration in contrast to NF-NDs containing systems. Functionalized nanodiamond-based hybrids also showed slight enhancement in the glass transition temperature of nanocomposites relative to non-functionalized systems. Newly prepared high-performance nanocomposites may be potentially applicable in Li-ion battery components, microelectronics and several energy-related industries.