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Journal of Polymer Engineering

Editor-in-Chief: Grizzuti, Nino

IMPACT FACTOR 2017: 0.778

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Volume 34, Issue 5


Preparation and properties of multilayered polymer/nanodiamond composites via an in situ technique

Rozina Ashraf
  • Nanosciences and Catalysis Division, National Centre for Physics, Quaid-i-Azam University Campus, 44000, Islamabad, Pakistan
  • Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan
  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Ayesha Kausar
  • Corresponding author
  • Nanosciences and Catalysis Division, National Centre for Physics, Quaid-i-Azam University Campus, 44000, Islamabad, Pakistan
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  • Other articles by this author:
  • De Gruyter OnlineGoogle Scholar
/ Muhammad Siddiq
Published Online: 2014-03-18 | DOI: https://doi.org/10.1515/polyeng-2013-0297


Compared to conventional materials, nanocomposites of conjugated polymers are found to have excellent performance due to a larger exposed surface area. In this study, polyaniline (PANi), polypyrrole (PPy), polythiophene (PTh) and polyazopyridine (PAP)/nanodiamonds (NDs) composites were efficiently synthesized by in situ oxidative polymerization. Physical characteristics of fabricated nanocomposites were explored using Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray (EDX) spectroscopy, field emission scanning electron microscopy (FESEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). FTIR indicated layer-by-layer oxidative polymerization of various matrices on functional ND (F-ND) surfaces. FESEM revealed the fibrillar (web-like) morphology of multilayered nanocomposites having a granular arrangement of NDs. TGA of multilayered F-NDs/PAP/PANi/PTh showed 10% degradation at an enhanced temperature of 482°C compared with F-NDs/PANi/PPy/PTh (471°C). Improvement in glass transition of layered material was observed from 99°C (NDs/PANi/PPy/PTh) to 121°C (NDs/PAP/PANi/PTh). Functional filler also contributed towards the enhancement in the conductivity of NDs/PAP/PANi/PTh (5.7 S cm-1) relative to NDs/PANi/PPy/PTh (3.7 S cm-1) systems. New conducting composites are potentially important in various applications, including polymer lithium-ion batteries.

Keywords: conductivity; fibrillar morphology; multilayered nanocomposite; nanodiamonds

1 Introduction

Carbon materials [constituting fullerenes, carbon nanotubes, nanodiamonds (NDs) and graphene] have been investigated as important building blocks of modern nanoscience and nanotechnology [1]. NDs produced by detonation of carbon-containing explosives with negative oxygen balance, constituting particle sizes in a range of 2–10 nm, have attracted special research interest in recent decades [2–6]. NDs possess a fine combination of properties, such as crystallinity, hardness, chemical stability, low toxicity, non-porosity, a wide band gap, dopability, lesser particle size distribution (typically 4–6 nm) and the probability of varying the nanoparticle characteristics due to surface modification [7–9]. Owing to the above mentioned characteristics, NDs have been outstanding candidates in several advanced technological fields ranging from biomedical [10, 11], to composites [12–14], and electrochemical applications [15], such as delivery vehicles design for drug analysis and purification of proteins, genes and antibodies, luminescent ND particles used in fluorescent labelling, filler or reinforcement for nanocomposites, etc. [7, 16–19]. Polymeric materials, owing to their huge relevance in all domains of industry, have been frequently exploited for fabricating structural materials. Research efforts are still going on regarding the development of new composite materials modified by the incorporation of nanosized filler, to acquire enhanced electrical and mechanical properties. Spherical nanoparticles have been attractive nanoscale filler (nanofiller), owing to their high surface to volume ratios [20]. Generally, the small diameter or nanoscale size (5 nm on average), nearly spherical shape, exceptionally excellent mechanical and physicochemical characteristics, large surface area (300–500 m2/g), tailor able and rich surface chemistry render NDs promising materials for polymer matrix reinforcement [12, 21–24]. In the past few decades, conducting or π-conjugated polymers have received great interest among researchers owing to their superior tunable, physical, electronic, optical and mechanical characteristics, and potential applications in numerous fields such as electrochromic devices, battery applications, photovoltaics, light emitting diodes, anticorrosion coatings and organic transistors [25, 26]. Some of the well-known conducting organic polymers include polyaniline (PANi), polypyrrole (PPy), polythiophene (PTh), etc. These conducting organic polymers in combination with NDs may exhibit astounding electrical, thermal and mechanical characteristics [27–29].

In this work, we adopted a chemical oxidative layer-by-layer polymerization route to develop functional NDs (F-NDs) and non-functional NDs (NF-NDs)-based nanocomposites using PANi, PPy, PTh and polyazopyridine (PAP) as matrices. In situ polymerization of monomers (aniline, pyrrole, thiophene and 2,6-diaminopyridine) on NDs yielded the series of NF-NDs/PANi/PPy/PTh, F-NDs/PANi/PPy/PTh, NF-NDs/PAP/PANi/PTh and F-NDs/PAP/PANi/PTh nanocomposites. To the best of our knowledge, the core shell NDs/PAP/PANi/PTh hybrids were chemically synthesized for the first time through this route. Physical characteristics of the material were explored using various suitable techniques. The idea underlying this study was to synthesize nanocomposites with good electrical conductivity, without decreasing their thermal characteristics. At present, one of the major challenges in science is to utilize more efficient, cheaper and non-contaminating energy sources. An attractive route for scientists is the use of lithium-ion battery that converts the chemical energy into electrical energy through chemical bond cleavage. Ionically conducting polymers named as polymer electrolytes, can successfully replace liquid electrolytes in rechargeable lithium-ion batteries. Similarly, conducting polymer composite electrodes can be used in rechargeable lithium ion batteries, because of the advantages in chemical synthesis, high stability towards environmental exposition, biocompatibility, reasonable conductivity, etc. [30–32].

2 Materials and methods

2.1 Materials

NDs synthesized by the detonation technique with 99% purity and size of clusters 64–120 nm were adopted. 2,6-Diaminopyridine (98%), aniline (99%), pyrrole (98%) and thiophene (>99%) were purchased from Aldrich (St. Louis, MO, USA) and kept at 0°C prior to use. Other reagents such as potassium dichromate (K2Cr2O7, 99.99%), anhydrous iron (III) 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.

2.2 Measurements

Infrared (IR) spectra were recorded using a Fourier transform infrared (FTIR) spectrometer, Model No. FTSW 300 MX, manufactured by Bio-Rad, CA, USA (4 cm-1 resolution). Field emission scanning electron microscopy (FESEM) of freeze-fractured samples was performed using JSM5910, JEOL Japan. Thermal stability was verified by a Mettler Toledo (CA, USA) TGA/SDTA 851 thermogravimetric analyzer, using 1–5 mg of the sample in an Al2O3 crucible at a heating rate of 10°C/min. Differential scanning calorimetry (DSC) was performed with a Mettler Toledo DSC 822 differential scanning calorimeter heating 5–10 mg of samples in aluminum pans at a rate of 10°C/min. X-ray diffraction (XRD) patterns were obtained at room temperature on an X-ray diffractometer (3040/60 X’pert PRO, CA, USA) using Ni-filtered Cu Kα radiation (40 kV, 30 mA). An energy dispersive X-ray (EDX) spectrometer EDX-720/800HS/900HS was also used for elemental analysis. Electrical conductivity of thin films was measured using a Keithley (CA, USA) 614 electrometer and the four-probe method.

2.3 Purification of NDs

During the synthesis, certain impurities were incorporated in ND soot, such as incombustible residues (metal and oxides, 1–8 wt%), amorphous carbon, graphite, fullerene like-carbons and some heteroatoms. These impurities must be eliminated by thermal oxidation or acid treatment (using liquid oxidants such as HNO3, H2SO4:HNO3 mixture, K2Cr2O7/H2SO4, KOH/KNO3, Na2O2, etc.) for further applications. The detonation soot was subjected to a mixture of hydrofluoric acid (40 wt%) and fuming nitric acid comprised in the ratio 25/75 wt%, to remove the metallic impurities. Further elimination of non-diamond sp2 phase was achieved through oxidation in HNO3 at 230°C and 100 atm pressure in a titanium alloy reactor [33–35].

2.4 Functionalization of NDs

An easy way to modify the properties of nanocomposites containing NDs as filler has been the attachment of the distinct functional groups on the surface of NDs, which improve the physical characteristics due to the interaction between the matrix and filler. Terminated functional groups around the diamond core are known to stimulate the possible applications for NDs. Among all types of F-NDs, more conveniently produced are the carboxylated NDs. Carboxylation was performed by treating the purified NDs with strong acids, i.e., H2SO4 and HNO3 (3:1, respectively), at room temperature with continuous stirring for 24 h. The above mixture was then poured into 200 ml hot water (70°C) and again stirred for 10 h (at room temperature). After the filtration, the residue was washed several times with deionized water and dried at 80°C for 4 h (Scheme 1).

Functionalization of nanodiamonds.
Scheme 1

Functionalization of nanodiamonds.

2.5 Synthesis of NF-NDs/PANi/PPy/PTh

Synthesis of NF-NDs/PANi/PPy/PTh nanocomposites was achieved utilizing the oxidative in situ layer-by-layer polymerization route. In a typical synthetic procedure, 0.5 ml aniline and 50 ml of 0.1 m HCl (aqueous) were mixed and vigorously stirred for 1 h. A total of 0.1 g of NDs was then added to the above reaction mixture. The oxidation reaction was initiated by dropwise addition of 50 ml K2Cr2O7/HCl (0.5 g K2Cr2O7/0.1 m HCl) solution at 0°C for 6 h (with continuous stirring). The resultant mixture was labelled as A. Then, 0.5 ml pyrrole was mixed with 0.1 m HCl (50 ml) and stirred for 1 h. Afterwards, the resultant solution was introduced into A (in a two-neck round bottom flask) and stirred for 12 h. A solution of 1.2 g FeCl3 in 0.1 m HCl (50 ml) was added dropwise under vigorous stirring at 0°C for 6 h, and the mixture was labelled as B. In the final step, 0.5 ml thiophene was dispersed in 0.1 m HCl (50 ml), stirred magnetically for 1 h and then slowly poured to the mixture B to polymerize the thiophene monomer and stirred for 12 h. Further facilitation of polymerization was achieved by the addition of 1.2 g FeCl3 in 50 ml of 0.1 m HCl. The resultant nanocomposite was obtained by filtration following washing several times with the deionized water, and was dried at 70°C for 24 h.

2.6 Synthesis of F-NDs/PANi/PPy/PTh

Synthesis of nanocomposites of PANi/PPy/PTh with F-NDs was carried out using the same synthetic route as described in Section 2.5 (Scheme 2).

Schematic synthesis of nanodiamonds (NDs)/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh).
Scheme 2

Schematic synthesis of nanodiamonds (NDs)/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh).

2.7 Synthesis of NF-NDs/PAP/PANi/PTh

To fabricate NF-NDs/PAP/PANi/PTh nanocomposites, 1.09 g 2,6-diamino pyridine was first dissolved in H2O/HCl (13/4 ml) to obtain a 0.01 m solution. Then, 0.68 g NaNO2 was dissolved in 7 ml H2O and added to the above solution. The reaction mixture was stirred maintaining the temperature at 0°C for 3 h. Then, 0.1 g purified NDs were dispersed in the resultant mixture labelled as C. In the second step, 0.5 ml aniline was dissolved in 50 ml of 0.1 m HCl with constant stirring for 1 h. The resultant solution was introduced to the mixture C and vigorously agitated for 12 h, resulting in the adsorption of monomers on the nanofiller surface. Further, polymerization was facilitated by introducing a solution of 0.5 g K2Cr2O7 in 0.1 m aqueous HCl (50 ml). The reaction mixture was kept under constant stirring in an ice bath at 0°C (for 6 h) and labelled as D. In the final step of the reaction, 0.5 ml thiophene and 50 ml HCl (0.1 m) were mixed in a 50 ml conical flask with vigorous stirring (1 h). After completion, the above solution was added dropwise to D. A solution of FeCl3 (1.2 g) in 0.1 m HCl (50 ml) was added dropwise. The mixture was then cooled to 0°C for 6 h. The nanocomposite was obtained by filtration and after subsequent washing it was dried at 70°C for 24 h.

2.8 Synthesis of F-NDs/PAP/PANi/PTh

The preparation of NF-NDs/PAP/PANi/PTh nanocomposites using functionalized NDs was accomplished by the same synthetic procedure as in Section 2.7 (Scheme 3).

Schematic representation for the synthesis of nanodiamonds (NDs)/polyazopyridine (PAP)/polyaniline (PANi)/polypyrrole (Ppy).
Scheme 3

Schematic representation for the synthesis of nanodiamonds (NDs)/polyazopyridine (PAP)/polyaniline (PANi)/polypyrrole (Ppy).

3 Results and discussion

3.1 Spectroscopic analysis

The tabulated FTIR data (Table 1) established the structure of respective multilayered nanocomposites. Figure 1A and 1B depict the FTIR spectra of F-NDs and NF-NDs. The aromatic protons appeared around 3003 cm-1 and 3001 cm-1 for F-NDs NF-NDs, respectively. Both spectra displayed the characteristic C=C stretching vibration in the range 1500–1600 cm-1. The functional groups that were generated on the nanodiamond surface in the process of the detonation and further chemical cleaning of the detonation soot appeared at 1000–1500 cm-1. Moreover, Figure 1B shows absorption bands at 3479 cm-1 and 1720 cm-1 corresponding to the hydroxyl and carbonyl groups, respectively, of carboxylic acid after functionalization. NF-NDs/PANi/PPy/PTh displayed typical aromatic C–H stretching vibrations at 3019 cm-1, while secondary amine stretching and bending vibration appeared at 3298 cm-1 and 1596 cm-1. A thiophene ring vibration was found at 1451 cm-1 and a C–N vibration at 1255 cm-1. Similarly, F-NDs/PANi/PPy/PTh exhibited secondary amine stretching and bending vibration at 3291 cm-1 and 1597 cm-1, respectively. In addition, Figure 1D shows absorption bands at 3456 cm-1 and 1718 cm-1 corresponding to carboxylic acid groups. While studying Figure 2A, N–H vibrations were observed at 3324 cm-1 and 1598 cm-1 for NF-NDs/PAP/PANi/PTh. A thiophene ring vibration was detected at 1471 cm-1 and a C–N vibration was found at 1293 cm-1. Due to the incorporation of an azo-based polymer, an N=N bond appeared at 1415 cm-1. Similarly, peaks were observed at 3329 cm-1, 1599 cm-1, 1461 cm-1, 1414 cm-1 and 1294 cm-1 for F-NDs/PAP/PANi/PTh in Figure 2B. Peaks at 1719 cm-1 and 3449 cm-1 were also observed, due to acidic carbonyl and hydroxyl functionality. Aromatic C–H protons appeared around 3012 cm-1 and 3009 cm-1 in NF-NDs/PAP/PANi/PTh and F-NDs/PAP/PANi/PTh, respectively. A layer-by-layer polymerization mechanism can be somewhat understood by FTIR analysis. Figure 3A shows the spectrum of NF-NDs singly layered with an azo-polymer. Aromatic C–H stretching vibration appeared at 3020 cm-1, while a pyridine ring vibration was found at 1447 cm-1. In addition, an N=N stretching vibration emerged at 1511 cm-1. By contrast, NF-NDs singly layered with PANi (Figure 3B) indicated secondary amine stretching and bending vibrations at 3399 cm-1 and 1599 cm-1 while aromatic C–H stretch was found at 3012 cm-1. An FTIR spectrum of NF-NDs/PAP/PANi (Figure 3C) depicts secondary amine stretching and bending vibration at lower wavenumbers 3215 cm-1 and 1587 cm-1 due to hydrogen bonding and interaction between the N–H of layered PANi and nitrogen in the PAP structure. Another indication of this bonding was the disappearance of the pyridine ring vibration at 1447 cm-1. Moreover, a lowering of N=N stretching vibration was observed 1417 cm-1.

Table 1

Fourier transform infrared spectroscopy (FTIR) data of nanocomposites (wavenumber cm-1).

Fourier transform infrared (FTIR) spectra of: (A) purified nanodiamonds (NDs); (B) functional NDs (F-NDs); (C) non-functional NDs (NF-NDs)/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh); (D) F-NDs/PANi/PPy/PTh.
Figure 1

Fourier transform infrared (FTIR) spectra of: (A) purified nanodiamonds (NDs); (B) functional NDs (F-NDs); (C) non-functional NDs (NF-NDs)/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh); (D) F-NDs/PANi/PPy/PTh.

Fourier transform infrared (FTIR) spectra of: (A) non-functional nanodiamonds (NF-NDs)/polyazopyridine (PAP)/polyaniline (PANi)/polythiophene (PTh); (B) functional-NDs (F-NDs)/PAP/PANi/PTh.
Figure 2

Fourier transform infrared (FTIR) spectra of: (A) non-functional nanodiamonds (NF-NDs)/polyazopyridine (PAP)/polyaniline (PANi)/polythiophene (PTh); (B) functional-NDs (F-NDs)/PAP/PANi/PTh.

Fourier transform infrared (FTIR) spectra of: (A) first layered non-functional nanodiamonds (NF-NDs)/polyazopyridine (PAP) nanocomposite; (B) first layered NF-NDs/polyaniline (PANi) nanocomposite; (C) NF-NDs/PAP/PANi nanocomposite.
Figure 3

Fourier transform infrared (FTIR) spectra of: (A) first layered non-functional nanodiamonds (NF-NDs)/polyazopyridine (PAP) nanocomposite; (B) first layered NF-NDs/polyaniline (PANi) nanocomposite; (C) NF-NDs/PAP/PANi nanocomposite.

3.2 EDX of NDs and nanocomposites

EDX spectroscopy was conducted to reveal the composition of NDs and nanocomposites. Figure 4 represents the EDX analysis of NDs (Figure 4A and 4B) and nanocomposites (Figure 4C–F). An EDX spectrum of the purified NDs confirmed the absence of impurities, but traces of Si were attributed to the catalyst used in the synthesis of NDs. For purified NDs, elemental analysis confirmed that C=90.01%, O=9.80% and Si=0.19%. Compared with the purified NDs, functionalized NDs contained higher levels of oxygen and relatively lower levels of silicon (than purified NDs), carbon and some extent of sulfur as an impurity. Sulfur, as an impurity, was observed due to the acid treatment involved in the functionalization process and could be removed by excessive washing with highly deionized water. Consequently, the atomic percent of the functionalized nanofiller was found as C=64.53%, O=29.86%, Si=0.07% and S=5.55%. Figure 4C describes the elemental analysis of NF-NDs/PANi/PPy/PTh while that of F-NDs/PANi/PPy/PTh is given in Figure 4D. The spectrum of NF-NDs/PANi/PPy/PTh revealed that the nanocomposite was comprised of carbon, nitrogen, oxygen, chlorine and cadmium. The composition was found to be C=43.56%, O=7.78%, N=35.19%, Cl=2.31% and Cd=0.38%. The composition values were strongly dependent upon the type of the monomer and catalyst involved in the polymerization. For example, the nitrogen content of the nanocomposite was estimated due to the monomers (aniline and pyrrole), whereas chlorine and cadmium were present as impurities. An EDX spectrum of the nanocomposite (F-NDs/PANi/PPy/PTh) shows that carbon, nitrogen, oxygen, chlorine and cadmium were the constituting elements. The atomic percent was found to be C=48.16%, N=41.10%, O=12.65%, Cl=7.78% and Cd=1.09%. It is noteworthy that relative to NF-NDs/PANi/PPy/PTh, the nanocomposite having F-NDs (F-NDs/PANi/PPy/PTh) had relatively higher levels of oxygen due to the incorporation of functional filler. Increased nitrogen content of the nanocomposite was attributed to better layering of polymers. NF-NDs/PAP/PANi/PTh analysis confirmed the presence of carbon, nitrogen, oxygen and chlorine. Elemental analysis showed that C=52.86%, N=35.24%, O=5.68% and Cl=6.23%. Compared with NF-NDs/PAP/PANi/PTh, the nanocomposite F-NDs/PAP/PANi/PTh contains relatively increased content of oxygen and nitrogen. Consequently, the atomic percent was estimated as C=50.52%, N=37.49%, O=9.87% and Cl=2.12%.

Energy dispersive X-ray (EDX) spectra of: (A) non-functional nanodiamonds (NF-NDs); (B) functional NDs (F-NDs); (C) NF-NDs/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh); (D) F-NDs/PANi/PPy/PTh; (E) NF-NDs/polyazopyridine (PAP)/PANi/PTh; (F) F-NDs/PAP/PANi/PTh.
Figure 4

Energy dispersive X-ray (EDX) spectra of: (A) non-functional nanodiamonds (NF-NDs); (B) functional NDs (F-NDs); (C) NF-NDs/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh); (D) F-NDs/PANi/PPy/PTh; (E) NF-NDs/polyazopyridine (PAP)/PANi/PTh; (F) F-NDs/PAP/PANi/PTh.

3.3 Morphological investigation

3.3.1 Morphology of NDs

The morphology of purified and functionalized NDs was investigated using FESEM. Figure 5A–D show the SEM images of the NDs. The purified but NF-NDs appeared in the form of aggregates, which was due to higher interactions between nanosized particulates and the presence of nanosized and microsized grains. These grains are usually assigned the name ND crystals. Microcrystalline diamonds appeared to form columnar morphology due to constant polynucleation or renucleation process of the initial nuclei. A newly formed growth center inhibits the further growth of diamond crystal, thus resulting in nanocrystalline diamond (NCD). In diamond clusters (NCD), nanocrystallites were obtained with granular surfaces and ball-like particles. Larger aggregates are visible in Figure 5A, while smaller aggregates appear in Figure 5B. A typical morphology of F-NDs is presented in the SEM images of F-NDs at lower and higher resolution (Figure 5C and 5D). Purified but NF-NDs were functionalized by acid treatment. SEM micrographs also confirmed the diameter of NDs about 90–120 nm, which indicated their high aspect ratio. Functionalization created the maximum number of active sites on the surface without altering the structure and broke down the microcrystalline diamonds to smaller sizes, leading to a better dispersion of F-NDs compared with NF-NDs. At higher resolution (Figure 5D), F-NDs were seen to be evenly dispersed and nanoparticles were smooth with a near diamond shape.

Field emission scanning electron microscopy (FESEM) images of: (A) non-functional nanodiamonds (NF-NDs) at 5 μm; (B) NF-NDs at 1 μm; (C) functional NDs (F-NDs) at 1 μm; (D) F-NDs at 0.5 μm.
Figure 5

Field emission scanning electron microscopy (FESEM) images of: (A) non-functional nanodiamonds (NF-NDs) at 5 μm; (B) NF-NDs at 1 μm; (C) functional NDs (F-NDs) at 1 μm; (D) F-NDs at 0.5 μm.

3.3.2 Morphology of NDs/PANi/PPy/PTh

The morphology of NF-NDs/PANi/PPy/PTh nanocomposite is presented in Figure 6. The NF-NDs/PANi/PPy/PTh sample has irregular morphology having polymer aggregates (Figure 6A and 6B). Figure 6A (lower resolution) does not reveal the dispersion of NDs in the matrix and only polymer junks were seen. However, Figure 6B indicated the dispersion of polymer-coated NDs in the PANi/PPy/PTh nanocomposite. The F-NDs/PANi/PPy/PTh composite with fibrillar like morphology was synthesized by in situ layer-by-layer polymerization. The morphology of F-NDs/PANi/PPy/PTh was examined as displayed in Figure 6C–F. Modification of ND surface facilitated the covalent interaction between the polymer matrix and functional groups on the external surface. The micrographs show the formation of randomly arranged polymer fibers without any network formation. Polymer-coated NDs were found to be embedded on the matrix fibers without any preferred orientation. This morphology may provide an excellent pathway for ions and solvent molecules transport within F-NDs/PANi/PPy/PTh material that may lead to enhanced electrochemical characteristics.

Field emission scanning electron microscopy (FESEM) images of: (A) non-functional nanodiamonds (NF-NDs)/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh) at 1 μm; (B) NF-NDs/PANi/PPy/PTh at 0.5 μm; (C) functional NDs (F-NDs)/PANi/PPy/PTh at 5 μm; (D) F-NDs/PANi/PPy/PTh at 1 μm; (E) F-NDs/PANi/PPy/PTh at 0.5 μm; (F) F-NDs/PANi/PPy/PTh at 0.2 μm.
Figure 6

Field emission scanning electron microscopy (FESEM) images of: (A) non-functional nanodiamonds (NF-NDs)/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh) at 1 μm; (B) NF-NDs/PANi/PPy/PTh at 0.5 μm; (C) functional NDs (F-NDs)/PANi/PPy/PTh at 5 μm; (D) F-NDs/PANi/PPy/PTh at 1 μm; (E) F-NDs/PANi/PPy/PTh at 0.5 μm; (F) F-NDs/PANi/PPy/PTh at 0.2 μm.

3.3.3 Morphology of NDs/PAP/PANi/PTh

FESEM micrographs of NF-NDs/PAP/PANi/PTh constituting three conducting polymers are shown in Figure 7A and 7B. Both the higher and lower resolution micrographs show the irregular morphology of nanocomposite constituting polymer junks. Furthermore, the NDs were embedded in the polymer matrix, as seen in the micrograph at a higher resolution. The micrographs confirmed the transformation of bare NDs to the polymer-coated NDs. The F-NDs/PAP/PANi/PTh composite based on chemically conducting polymers and F-NDs was also carefully investigated through FESEM. Figure 7C–F depicted the representative micrographs of the F-NDs/PAP/PANi/PTh nanocomposite fabricated by the in situ polymerization technique. The smaller size of NDs constituting a high surface area provided a better way of adsorption to the monomers and consequently led to polymerization and uniformly polymer-coated NDs. Moreover, the external surface groups of F-NDs were also a better choice for homogenous polymerization. The nanocomposite was composed of an interwoven fibrous network and polymer-coated NDs were found to be embedded on the matrix fibers. In other words, there was a granular arrangement of NDs on the polymer fibers. Hence, the morphology of conducting polymer-coated nanocomposites elaborated the complete merging of NDs in the polymer matrix.

Field emission scanning electron microscopy (FESEM) images of: (A) non-functional nanodiamonds (NF-NDs)/polyazopyridine (PAP)/polyaniline (PANi)/polythiophene (PTh) at 0.5 μm; (B) NF-NDs/PAP/PANi/PTh at 0.2 μm; (C) functional NDs (F-NDs)/PAP/PANi/PTh at 1 μm; (D) F-NDs/PAP/PANi/PTh at 0.5 μm; (E) F-NDs/PAP/PANi/PTh at 0.2 μm; (F) F-NDs/PAP/PANi/PTh at 0.2 μm.
Figure 7

Field emission scanning electron microscopy (FESEM) images of: (A) non-functional nanodiamonds (NF-NDs)/polyazopyridine (PAP)/polyaniline (PANi)/polythiophene (PTh) at 0.5 μm; (B) NF-NDs/PAP/PANi/PTh at 0.2 μm; (C) functional NDs (F-NDs)/PAP/PANi/PTh at 1 μm; (D) F-NDs/PAP/PANi/PTh at 0.5 μm; (E) F-NDs/PAP/PANi/PTh at 0.2 μm; (F) F-NDs/PAP/PANi/PTh at 0.2 μm.

3.4 Thermal analysis

The thermal stability of NDs/PANi/PPy/PTh and NDs/PAP/PANi/PTh nanocomposites was studied using thermogravimetric analysis (TGA) and DSC. The thermal stability of nanocomposites was studied at different heating rate, i.e., 10 and 20°C/min. TGA thermograms scanned at 10°C/min are given in Figure 8 and data is presented in Table 2. Both the NF-NDs and F-NDs/PANi/PPy/PTh showed single-stage decomposition starting at approximately 431°C and the major weight loss occurred between 450°C and 550°C. For NF-NDs/PANi/PPy/PTh, the TGA curve showed initial decomposition T0 at 433°C, 10% gravimetric loss T10 459°C and the decomposition continued up to 549°C. By contrast, F-NDs/PANi/PPy/PTh showed higher values in thermal degradation. F-NDs/PANi/PPy/PTh had T0 of 436°C, T10 471°C and Tmax up to 555°C. F-NDs/PAP/PANi/PTh had a higher heat stability among these hybrids prepared with T0 461°C, T10 482°C and Tmax was up to 571°C. Thus, the addition of F-NDs significantly increased the heat stability of the material. Similarly, F-NDs-based hybrids had higher char yield, F-NDs/PANi/PPy/PTh 57% and F-NDs/PAP/PANi/PTh 64%, at 600°C compared with NF-NDs material. TGA results on ND heated to 900°C showed that the ND started to lose weight from 100°C onward and the weight loss was linear up to 550°C. However, a weight loss of just 3% occurred at 550°C which was not so large. Above 550°C, the weight loss rate of ND changed abruptly and became more intense. Above this temperature, all of the oxygenated groups were evolved and the total weight loss at 900°C was about 11.5% [36]. Increasing the heating rate influenced the thermal degradation temperature of these materials considerably. Consequently, the TGA scans at a higher heating rate, i.e., 20°C/min, shifted the thermal degradation temperature of all the nanocomposites to the higher values as shown in Table 2. Figure 9 represents DSC thermograms of hybrids containing NF-NDs and F-NDs. F-NDs/PANi/PPy/PTh demonstrated a higher Tg value of 105°C relative to that of NF-NDs/PANi/PPy/PTh (99°C). Thermal transition of NF-NDs/PAP/PANi/PTh was recorded as 119°C. An obvious shift in Tg occurred to the right for F-NDs/PAP/PANi/PTh with a Tg of 121°C. Similarly, the glass transition was positively affected by changing the heating rate and Tgs shifted to higher values at 20°C/min. Thermal data revealed that the addition of azo-polymer to the multilayered nanocomposites had a positive effect on the thermal properties, due to increasing rigidity of the polymer backbone. A typical DSC trace of PANi signifies an endothermic transition starting at approximately 50°C and centered at approximately 120°C, due to the evaporation of water, HCl molecules or other small molecules trapped inside the polymer or bound to the polymer backbone [37]. The exothermic transition was also observed at 240°C, which can be attributed to a series of chemical reactions like bond scission followed by new chemical bond formation. Bond cleavage is usually endothermic and bond formation exothermic, with the overall reaction being an exothermic process. Bond cleavage shortens the conjugation length, resulting in an increase in the free radical concentration and a decrease in the conductivity of the doped polymer. PPy generally shows a broad endothermic dip at about 74°C, which may be a bond cleavage reaction including the release of small molecules [38]. Doping of a conducting polymer such as PTh may decrease the Tg, indicating that the dopant acts as a plasticizer. Therefore, the thermal stability may be found to decrease due to doping. Literature results of thermal analysis have demonstrated that after doping, the conjugated polymers by FeCl3 increase the glass transition. The increase in Tg has shown that there was no plasticization effect by dopant molecules [39]. In this work, addition of doping substances into a polymer may result in an increase in the effectiveness of intermolecular/interchain interactions in the polymer due to layer-by-layer polymerization. Therefore, layer-by-layer doping and polymerization increase the glass transition and no release of dopants as exo/endotherm was observed in DSC traces. The results indicated increased thermal stability of new nanocomposites. On the whole, new materials were thermally more stable relative to reported nanodiamond hybrids [40].

Thermogravimetric analysis (TGA) thermograms of nanodiamonds (NDs)/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh) and NDs/polyazopyridine (PAP)/PPy/PTh at a heating rate of 10°C/min in N2.
Figure 8

Thermogravimetric analysis (TGA) thermograms of nanodiamonds (NDs)/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh) and NDs/polyazopyridine (PAP)/PPy/PTh at a heating rate of 10°C/min in N2.

Table 2

Thermal analyses data of nanocomposites.

Differential scanning calorimetry (DSC) thermograms of nanodiamonds (NDs)/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh) and NDs/polyazopyridine (PAP)/PPy/PTh at a heating rate of 10°C/min in N2.
Figure 9

Differential scanning calorimetry (DSC) thermograms of nanodiamonds (NDs)/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh) and NDs/polyazopyridine (PAP)/PPy/PTh at a heating rate of 10°C/min in N2.

3.5 XRD analysis

Final characteristics of the synthesized materials were strongly connected to the crystalline structure, which was evaluated by XRD studies. Herein, the 2θ scan presents (Figure 10) the typical XRD pattern of NDs and composites. Purified ND particles show diffraction peaks at 43.13° (equivalent to an interplanar spacing of 2.05 Å). This characteristic diffraction peak was attributed to the (111) plane of diamond [41]. Similarly, the XRD pattern of functionalized NDs revealed a sharp peak at 2θ=43.46°, corresponding to an interplanar spacing of 2.05 Å and the (111) plane of diamond. By contrast, XRD measurement of the NF-NDs/PANi/PPy/PTh nanocomposite depicted the characteristic peak 2θ=43.47°, while for the F-NDs/PANi/PPY/PTh nanocomposite, peaks appeared at 25.02° and 43.86°. In these nanocomposites, diffraction peaks at 43.47° and 43.86° were obviously due to the interplanar spacing of nanofiller (∼2.05 Å), while a peak at 25.02° for F-NDs/PANi/PPY/PTh was the typical diffraction pattern of polymeric chains. These reflections indicated that NDs were dispersed in PANi, PPy and PTh chains with some crystalline phase. In the case of pure PANI, the peaks are generally found to appear at 20.26° and 25.32° [42]. XRD spectra of the new layered polymer structure acquired all the diffraction peaks present in the pure polymer spectrum in the range 2θ=25–26° (corresponding to the reflections of the polymer structure). For the core shell NF-NDs/PAP/PANi/PTh, diffraction peaks were visible around 2θ=25.215° and 43.59°. Similarly, the XRD pattern of F-NDs/PAP/PANi/PTh composite had peaks centered at 26.48° and 43.74°. Here again, 2θ values of 43.59° and 43.74° clearly indicated the presence of a cubic diamond structure. In nanocomposite materials, 2θ around 43° indicated that the synthesis process was able to insert the nanodiamond particles in the polymeric network, without interfering with the growth of the conducting polymer that still retains its crystalline structure.

X-ray diffraction (XRD) pattern of: (A) purified nanodiamonds (NDs); (B) functional NDs (F-NDs); (C) non-functional NDs (NF-NDs)/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh); (D) functional NDs (F-NDs)/PANi/PPy/PTh; (E) NF-NDs/polyazopyridine (PAP)/PANi/PTh; (F) F-NDs/PAP/PANi/PTh.
Figure 10

X-ray diffraction (XRD) pattern of: (A) purified nanodiamonds (NDs); (B) functional NDs (F-NDs); (C) non-functional NDs (NF-NDs)/polyaniline (PANi)/polypyrrole (PPy)/polythiophene (PTh); (D) functional NDs (F-NDs)/PANi/PPy/PTh; (E) NF-NDs/polyazopyridine (PAP)/PANi/PTh; (F) F-NDs/PAP/PANi/PTh.

3.6 Electrical conductivity

Table 3 represents the electrical conductivity of heteroaromatic NDs/PAP/PANi/PTh nanocomposites with functional and non-functional filler. Multilayered azo-, pyridine- and thiophene moieties were found to enhance the conductivity of nanocomposites. The conductivity of NF-NDs/PAP/PANi/PTh was 4.1 S cm-1 and was improved with functional filler in F-NDs/PAP/PANi/PTh as 5.7 S cm-1. NF-NDs/PANi/PPy/PTh and F-NDs/PANi/PPy/PTh had relatively lower values of 2.9 S cm-1 and 3.7 S cm-1, respectively. Moreover, the increase in conductivity in this system was not significant. The effect of temperature on the conductivity values of the nanocomposites was also studied (Table 4). Figure 11 shows the enhancement in electrical conductivity with temperature for F-NDs/PAP/PANi/PTh. Among all of the prepared nanocomposites, the effects were much pronounced in the case of functional filler/PAP/PANi/PTh, owing to the development of a better conducting network.

Table 3

Conductivity measurement of nanocomposites.

Table 4

Conductivity measurement of nanocomposites versus temperature.

Effect of temperature on the conductivity of nanocomposites.
Figure 11

Effect of temperature on the conductivity of nanocomposites.

4 Conclusions

We fabricated multilayered nanocomposites of PANI, PPy, PTh and PAP and NDs. Pre-treatment of NDs, i.e., acidic functionalization without damaging the structural properties of filler, was carried out. Afterwards, the incorporation of functional and non-functional NDs in the matrix, by means of in situ polymerization, led to a high performance of the multilayered polymer/NDs materials. FTIR revealed the successful fabrication of hybrids via a layer-by-layer oxidative method. FESEM micrographs provided evidence that the homogeneous dispersibility and discrete interfacial bonding was achieved inside multilayered nanocomposites using modified NDs. The incorporation of F-NDs into the matrix led to obvious increments in the thermal and electrical properties as compared to NF-NDs-based hybrids. In the future, the present material may be of great value for polymer Li-ion battery components, as well as microelectronics and energy-related industries.


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About the article

Corresponding author: Ayesha Kausar, Nanosciences and Catalysis Division, National Centre for Physics, Quaid-i-Azam University Campus, 44000, Islamabad, Pakistan, e-mail:

Received: 2013-11-14

Accepted: 2014-02-15

Published Online: 2014-03-18

Published in Print: 2014-07-01

Citation Information: Journal of Polymer Engineering, Volume 34, Issue 5, Pages 415–429, ISSN (Online) 2191-0340, ISSN (Print) 0334-6447, DOI: https://doi.org/10.1515/polyeng-2013-0297.

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