Mechanical and electrical properties of PANI-based conductive thermosetting composites

Abstract

Polyaniline (PANI) is one of the most studied intrinsic electrically conductive polymer in recent years. In the present work, PANI-based conductive composites have been prepared using dodecylbenzenesulphonic acid (DBSA) as a dopant and divinylbenzene (DVB) as a cross-linking polymer to enhance the rigidity. In this mixture, DBSA acts as the dopant of PANI as well as the curing agent of DVB, and this means that doping and curing of the composite occur simultaneously. PANI–DBSA mixture has been prepared by physical mixing using centrifugal mixer. The ratio of PANI and DBSA has been kept constant in weight ratio 30:70. Furthermore, DVB content has been varied to prepare the different samples. It has been found that with the increase in DVB content, a good dispersion of PANI–DBSA/DVB solution is achieved. However, significant improvement in the electrical conductivity of the samples is observed with the increase in PANI content. Mechanical flexural test has been performed on the prepared samples using a three-point bending configuration. It has been found that the flexural modulus and brittleness of the prepared samples increase with the increase in the content of DVB in the composite. Morphology of the composite surfaces has been also studied and it has been found that low concentration of DVB results into a poor dispersion and hence agglomeration of PANI–DBSA in composite can be seen clearly in the images.

Keywords

Conductive composites polyaniline mechanical properties electrical conductivity

Introduction

Conductive composites always have been the center of research due to their outstanding properties and their applications; for example EMI shielding, microwave absorption and sensor materials. The change in electric and dielectric properties in the composite with application of load gives a promising area of research to fabricate more sophisticated as well as structural piezo sensors.¹ Dielectric properties of conductive composite have also attracted the attention of researcher.² The application of PANI-based composite in EMI shielding has already been established in the available literatures and hence more progress in the current technology will result into more reliable and accurate EMI shielding materials.³ Some of the promising applications of conductive composites are microwave absorption⁴ and structural super capacitor.⁵

The common fillers used to impart the conductivity to the composites are carbon nanotubes (CNTs), metal fillers, carbon black, etc. However, each one of them has its own advantages and disadvantages. For example, CNT fillers provide high electrical and mechanical properties but still the interaction between CNT and its surrounding matrix is yet to be elucidated.⁶ Metal particle-based conductive composites have been studied extensively in past, and they are known for their better thermal conductivity and wettability but higher loading is needed to achieve the desired electrical and mechanical properties in case of metal-based conductive composites.⁷ Low weight and low cost materials always inspire researchers to look for another better alternatives. Therefore, the focus of the research shifted towards the polymer-based conductive composites in past years. In last few decades, polyaniline (PANI) has gained popularity as one of the most studied intrinsic electrically conductive polymers due to its remarkable properties. High conductivity, easy synthesis, low cost, and easy availability of PANI give it upper hand over other available intrinsic electrically conductive polymers, and DBSA doped PANI also has shown better miscibility in the other organic polymers.⁸ PANI is a conductive polymer which is polymerized from inexpensive aniline monomers and therefore also cost effective. The basic structure of PANI consists of two main parts namely reduced unit and oxidized unit as shown in Figure 1. Generally, PANI is found in three different oxidized states: fully oxidized [(per)nigraniline], fully reduced [leucoemeraldine] and neutral state [emeraldine] as described in Table 1 (n and m are denoted in Figure 1).

Table 1.

Three different forms of PANI and their characteristics.

Leucoemeraldine	Emeraldine	(Per)nigraniline
White/clear & colorless (C₆H₄NH)_x	Green for the emeraldine salt, blue for the emeraldine base ([C₆H₄NH]₂[C₆H₄N]₂)_x	Blue/violet (C₆H₄N)_x
(n = 1, m = 0)	(n = m = 0.5)	(n = 0, m = 1)
Fully reduced state	Partially oxidized	Fully oxidized state

The emeraldine form of PANI can be rendered conductive by doping with a strong functional protonic acid H⁺(M⁻R), where H⁺M⁻ is a protonic group and R is an organic group, e.g. dodecylbenzenesulphonic acid (DBSA), hydrochloric acid or camphorsulphonic acid (CSA), to result into a conductive emeraldine salt (ES).⁹ Heeger¹⁰ and Cao et al.¹¹ reported the use of functional protonic acids to both doping of non-conductive emeraldine base and, simultaneously as surfactants in common organic solvents. The protons react with the imine nitrogen of PANI, rendering the Emeraldine Base (ES) to conductive salt. Simultaneously, R functional group enhances the solubility with common organic polymers. The PANI–DBSA blend is usually mixed with different insulating polymers like polystyrene (PS), polyamides and epoxy resin to prepare conductive polymer composites.^12,13 Long et al.¹⁴ prepared a conductive composite film by using two different types of dopant namely, camphor sulfonic acid (PANI-CSA) and dodecylbenzenesulphonic acid (PANI–DBSA) and compared their electrical and thermal conductivities. Jeon et al.¹⁵ used polycarbonate (PC) as a matrix to prepare PANI–DBSA/PC composites by using emulsion polymerization in which DBSA acts as both surfactant and dopant. They reported the electrical conductivity of 10⁻²S/cm of PANI–DBSA/PC composite with 13 wt% of PANI. Other than solution blending technique, Zilberman et al.¹⁶ used melt processing technique to prepare conductive blends of thermally DBSA-doped PANI with thermoplastic polymers. They prepared PANI–DBSA/PS, PANI–DBSA/PE(polyethylene) and PANI–DBSA/Co-PA(polyamide copolymer) blends by melt processing. It was found that with the same ratio of PANI–DBSA to matrix polymer, the electrical conductivity varied with different polymer matrix. Haba et al.¹⁷ reported new PANI–DBSA/polymer blending via aqueous dispersion. They obtained PANI–DBSA/polymer blends by mixing an aqueous PANI–DBSA dispersion with an aqueous emulsion of the matrix polymer, followed by water evaporation. They reported the electrical conductivity of the composite at a very low PANI–DBSA content (0.5 wt.%). They achieved highest conductivity up to 10⁻² to 10⁻³S/cm at 4.5 wt.% of PANI–DBSA. The morphology of the thermally doped PANI–DBSA mixture (aniline monomer unit:DBSA = 1:1 mol ratio) mixture was studied and explained by Goto et al.¹⁸

Different doping phenomena¹⁹ have been studied and compared in past. Similarly, the mechanical properties of PANI-based composites with different reinforcements such as CNT and natural rubber^20,21 also have been investigated by the researchers. Different techniques have been used in past to fabricate of composite films which exhibit a good balance of electrical conductivity, mechanical properties and processability as explained above. Few more examples are emulsion polymerization,¹⁵ casting from solution of Intrinsic conductive polymers (ICPs) and insulator polymer matrixes using solvents,²² oxidative polymerization of absorbed PANI onto an insulating matrix²³ and mechanical mixing.¹⁹ Generally, in all of the above techniques, conductive composites are prepared by two distinct steps, i.e. doping (strong acid like DBSA, CSA as dopants) and curing (using curing agents). However, this paper reports a simplified one-step method to prepare PANI-based conductive composites, where doping of PANI and curing of the composite take place simultaneously and hence save time and efforts.

As per the authors’ knowledge, divinylbenzene (DVB) has been used for the first time as a polymer matrix with PANI–DBSA mixture to prepare a conductive polymer composite. Physical mixing techniques such as centrifugal mixing have been used to prepare all mixtures. DBSA acts both as a dopant and a curing agent, and therefore doping of PANI–DBSA and curing of the composite takes place simultaneously as shown in Figure 2. When DBSA reacts with DVB polymers a cationic polymerization takes place to form a stable and high cross-linking network.²⁴ Furthermore, the PANI–DBSA mixture and DVB are mixed together using centrifugal mixer and then further cast molded to prepare composite samples. And finally, the electrical conductivity, mechanical properties and morphology of the prepared composites have been investigated.

Experimental

Sample preparation

PANI (supplied by Regulus Co. Ltd., Tokyo, Japan) was kept in an oven for 2 h at 40℃ to eliminate moisture content. Dried PANI and DBSA (supplied by Kanto Chemical Co. Inc., Tokyo, Japan) have been mixed to form a PANI–DBSA mixture by physical mixing in the ratio of 30:70 by weight percentage which is equivalent to the molar ratio of 1:0.69 of PANI:DBSA. This weight ratio was selected by considering the ideal molar ratio for emeraldine salt (1:0.5) and processability of PANI–DBSA. This mixture was further mixed with DVB (supplied by Sigma-Aldrich Co., St. Louis, USA) at room temperature to form a homogenous mixture. The ratio of PANI:DBSA has been kept constant while the DVB content has been varied to prepare different samples. The speed of centrifugal mixer has been kept constant at 2000 rpm. However, the mixing time has been monitored carefully to avoid curing during mixing itself. Different weight percentage of DVB (30%, 50% and 70%) with corresponding PANI content (21%, 15% and 9%) have been used to prepare different samples. The mixture of PANI–DBSA and DVB was further poured into a mold (mold size 50 mm × 50mm × 2mm) and cured using a Hot-press machine (Toyoseiki. Mini test press.10). Samples were first put in the Hot-press machine at 60℃ for 15 min to eliminate all volatile content and avoid voids in the final sample then at 120℃ for 2 h for curing. Samples of different dimensions were cut for various measurements (electrical and mechanical properties).

When we determine the above curing process, the thermal stability of PANI–DBSA at 120℃ was investigated using UV–Vis–NIR analyzer (U-4100, HITACHI High Technologies Corporation, Tokyo, Japan). PANI–DBSA with 50% toluene mixture and PANI–DBSA/DVB (50 wt% DVB) mixture were subjected to heat treatment at 120℃ for 10 min, 1 h and 2 h, respectively.

Table 2 shows the relation between PANI, DBSA and DVB content in each of the composite samples. At higher DVB content, the doping and curing of composite take place very slowly which defies the purpose of a single-step, fast manufacturing of the composite due to the unavailability of the necessary protons to start the polymerization of the DVB, On the contrary, lower DVB content results into fast curing without proper dispersion. Three samples for electrical conductivity measurement and three samples for mechanical properties measurement have been used and their mean value has been reported in this text. Same samples have been used for morphological study.

Table 2.

Relation between PANI, DBSA and DVB content in each composite.

Composite sample type	PANI		DBSA		DVB
Composite sample type	(wt%)	(mol)	(wt%)	(mol)	(wt%)	(mol)
1	9	1	21	0.69	70	5.8
2	15	1	35	0.69	50	2.5
3	21	1	49	0.69	30	1.1

Note: The molar ratios have been normalized for comparison.

Measurements

The electrical conductivity (DC measurement) of the samples was measured using LCR meter (3522-50 LCR HiTESTER, Hioki E.E. Corporation, Ueda, Japan) by four-probe method. DOTITE conductive adhesive paste (supplied by Fujikura Kasei Co. Ltd. Tokyo, Japan) and aluminum tape have been used to measure the conductivity. DOTITE was applied and aluminum tape was attached to the perpendicular sides of the measuring direction of the sample, i.e. thickness direction or longitudinal direction. DOTITE paste was dried completely by keeping it in a furnace at 70℃ for 45 min and then cooled to the room temperature before measuring the electrical conductivity.

The flexural modulus of the samples were measured using Universal Testing Machine (Instron-5582) by three-point bending method, using a load cell of the range of 5 kN and the crosshead speed of 1 mm/min. The radii of loading nose and supports had been kept at 5 mm. The total length, width and thickness of samples taken as 50 mm, 10 mm and 2 mm, respectively, while the span distance is taken as 40 mm.²⁵ The morphology of the samples has been studied using the optical microscope (Eclipse L150, Nikon Corporation, Tokyo, Japan).

Results and discussions

UV–vis–NIR spectra

The UV–Vis–NIR spectra of PANI–DBSA mixture and PANI–DBSA/DVB (with 50% DVB) mixture have been shown in Figure 3(a) and (b), respectively. The peaks at around 600 nm due to the π_B→ π_Q excitation absorption in the quinoid imine ring is one of the characteristic peaks of undoped PANI, i.e. Emeraldine base (EB), dissolved in N-Methyl-2-pyrrolidone (NMP). The characteristic peak for doped PANI, i.e Emeraldine salt (ES), is at around 400 and 800 nm and free carrier tail after 1000 nm.²⁶ The peak at 800 nm can be assigned to the formation of polaron band. The characteristic peak of doped PANI can be seen just after 10 min of heating and still present till 2 h as shown in Figure 3(a) and (b). It can be seen from the plots that there is very little change in the UV–Vis–NIR spectra until 2 h at 120℃. Absorbance increases in the 1000–2500 nm range which indicates the doping of PANI and its extended conformation, after heat treatment starts. Therefore, we can infer that PANI–DBSA is thermally stable in PANI–DBSA mixture and PANI–DBSA/DVB composites at 120℃.

Electrical properties

The electrical conductivity of the PANI–DBSA/DVB composite samples were measured and analyzed as a function of PANI content. The effect of PANI content on the electrical conductivity of the PANI–DBSA/DVB composite has been shown in Figure 4. It can be clearly seen from the figure that the PANI–DBSA/DVB composite with 15 wt% of PANI exhibits a conductivity value of 0.27 S/cm, which is one order high as compared to other conductive composites mentioned in the literature. Heeger¹⁰ reported the conductivity of order 10⁻²S/cm of PANI–DBSA/PC composite with PANI 13 wt.%. Similarly, Cao et al.¹¹ also reported the conductivity in the order of 10⁻⁴–10⁻³S/cm of PS/PANI–DBSA composite with 20 wt.% and 30 wt.% of PANI–DBSA, which is also very less as compared to the conductive composites reported in this paper. The value of conductivity of PANI–DBSA/DVB conductive composites with 9 wt.% and more PANI content has been found to be higher than many semiconductors (in the order of 0.001–0.01 S/cm).

It is evident from the plot that with the increase of the wt% of PANI in both the composites, there is significant increase in the electrical conductivity. However, it should be noted that the dispersion quality of the PANI–DBSA mixture in DVB is also a very important factor to have consistent electrical conductivity results.

Mechanical properties

The mechanical properties of the composite have been determined using 3-point flexural testing. Figure 5(a) and (b) shows the load–deflection curve and flexural modulus, respectively. It is observed from the results that the composite with the composition of 30 wt % of DVB and 21 wt % of PANI, shows a flexural modulus of 0.54 GPa which increases up to 1.794 GPa with the composition of 70 wt% of DVB and 9 wt% of PANI. The value of flexural modulus of the composite with 70 wt% of DVB is quite comparable to flexural modulus value of pure epoxy resin and other polyamides. These results show that a composite with good electrical and mechanical properties can be synthesized by adding conductive PANI–DBSA mixture to the DVB using a one-step synthesis method. It is further evident that the mechanical properties improve significantly with the increase of DVB content in the composite. PANI interacts with DBSA to form a conductive emeraldine salt, and DBSA also provides proton to DVB for the initiation of cationic polymerization.² The high strength at high concentration of DVB can be assigned to the higher crosslinking density. Highly dense polymerized DVB accounts for the high brittleness and stiffness of the composite. Maximum bending stress and maximum bending strain of the PANI–DBSA/DVB composite are shown in Table 3. The brittle fracture of the composite is also dominant in the composites with higher concentration of DVB.

Table 3.

Maximum bending stress and strain of the PANI–DBSA/DVB composite samples with different DVB content.

DVB wt. % in composite	Maximum bending stress (MPa)	Maximum bending strain (%)
30	9.17 (0.84)	1.73 (0.37)
50	18.09 (1.77)	1.60 (0.08)
70	26.73 (3.88)	1.59 (0.32)

Note: Parentheses indicate the standard deviations.

A good conductive composite with good mechanical and processability properties is the demand of future conductive composite industries. This paper gives an initial start for the structural applications of conductive composites.

It is also noted that the composites with a low DVB content are little flexible and highly conductive. Hence, new applications of the flexible conductive composites can be sought out in future.

Morphology of the composite samples

The surface of PANI–DBSA/DVB composites has been studied under optical microscope. In Figure 6, the microscopic surface views of the composites with 30, 50 and 70 wt% of DVB content have been compared. For all samples, the centrifugation speed has been kept at 2000 rpm and the mixing time has been varied according to the need. At higher content of DVB, a more homogenous surface has been obtained due to proper dispersion of PANI–DBSA mixture in DVB. On the other hand, a low concentration of DVB results into a poor dispersion and hence agglomeration of PANI–DBSA in composite can be seen clearly in the images. Figure 6(c) shows a better dispersion compared to Figure 6(a) and (b). The phases are not so distinct at higher DVB concentration. It can be seen from Figure 6 (c) that higher DVB concentration results into a better dispersion of PANI–DBSA into DVB as compared to lower concentration as shown in Figure 6(a) and (b).

It has been further observed that the surface properties of the composites improve significantly with the increase of DVB content in the composite. The electrical and mechanical properties can be well understood with the help of morphology. Pure PANI–DBSA mixture has conductivity around 10 S/cm while DVB is an insulator. Therefore, PANI–DBSA agglomerates account for the high conductivity at low DVB concentration and highly dense polymerized DVB accounts for the better mechanical properties in composites with high DVB content.

Concluding remarks

PANI-based conductive composites are prepared using Dodecylbenzenesulphonic acid (DBSA) as a dopant and Divinylbenzene (DVB) as a cross-linking polymer to enhance the rigidity. Single-step composite preparation technique has been demonstrated where DBSA acts as the dopant of PANI as well as the curing agent of DVB. It has been shown that doping and curing of the composite occurs simultaneously in this technique. Morphology of the samples has also been studied using an optical microscope. It has been shown that with the increase in DVB content, a good dispersion of PANI–DBSA/DVB solution can be achieved. The agglomeration of PANI–DBSA results in a low content of DVB. Furthermore, the electrical conductivity of the samples has been determined using a four-point probe method. Significant increment in the electrical conductivity of the samples has been observed with the increase in PANI content. Mechanical flexural test has been performed to demonstrate the mechanical properties of the composite. It has been found that the flexural modulus and brittleness of the prepared samples increase with the increase in the content of DVB.

From the results, it can be inferred that the PANI–DBSA/DVB conductive composite of the desired electrical and mechanical properties can be prepared by altering the content of PANI, DBSA and DVB.

Footnotes

Conflict of interest

None declared.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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