Enhanced electrical conductivity of polypyrrole/polypyrrole coated short nylon fiber/natural rubber composites prepared by in situ polymerization in latex
Highlights
► PPy/PPy coated fiber/NR composites were prepared by in situ polymerization in latex. ► DC conductivity 6.25 × 10−2 S/cm was attained for the composite. ► PPy declined mechanical properties of NR while PPy coated fiber enhanced. ► The conducting composites exhibited high dielectric permittivity and AC conductivity. ► Skin depth and dielectric heating coefficient of the composites decreased substantially.
Introduction
During the last decade, the focus on the use of intrinsically conductive polymers (ICPs) in organic electronic devices has led to the development of a totally new class of smart materials. Electronic devices working at high operating frequencies, such as fast computers and cellular phones, require materials that combine good dielectric properties with both mechanical strength and ease of processing. The unique combination of dielectric and mechanical properties is hard to achieve in an one-component material. Here comes the significance of conducting polymer/insulating polymer blends and composites which combine the electrical properties of conducting polymer with mechanical properties and processability of the matrix.
Polypyrrole (PPy) is one of the most stable known conducting polymers and also one of the easiest to synthesize [1]. It has already been used as an electrode for rechargeable batteries [2], as electromagnetic shield to electronic equipment [3], in printed circuit boards [4], [5] and many other diversified applications. The processability, flexibility and strength of PPy can be can be improved by forming polypyrrole composites or blends with suitable, commercially available polymers like natural and synthetic rubbers, polyolefins, polystyrene, polycaprolactone, polyethylene glycol, and polyvinyl acetate. Polypyrrole composites can be prepared directly by mechanical mixing. However, due to thermal aging of PPy during processing at elevated temperatures, the resultant composites usually have very low conductivity [6], [7]. Electrical polymerization, on the other hand, can be used to prepare conducting composite films with good mechanical properties and high conductivity. However, it is not suitable for large scale industrialization, because only thin films of pristine host polymers are used, which are only as large as the size of the electrode area of the products. Polypyrrole composites can also be produced in situ by chemically oxidative polymerization. There are two in situ polymerization methods to obtain PPy composites. One is to form a substrate film mixed with oxidant first, followed by adding the monomer to the film. This method is adopted for obtaining PPy coated conducting fibers and fabrics. The second method is to polymerize pyrrole by means of an oxidant in the presence of an insulating polymer matrix, followed by precipitation, drying and moulding into various shapes.
Electrically conductive composites were prepared via in situ chemical oxidative polymerization of the pyrrole monomer in polystyrene (PS) and zinc neutralized sulfonated polystyrene (Zn-SPS) films under super critical carbon dioxide (SC-CO2) conditions by Gang et al. [8]. Due to the strong swelling effect of SC-CO2, the pyrrole monomer was efficiently incorporated into and well dispersed in the matrix, thus leading to relatively higher conductivity after the polymerization. Zoppi and De Paoli [9] obtained semi-interpenetrating polymer networks of PPy/ethylene-propylene-diene rubber (EPDM) via oxidation polymerization of pyrrole using two methods. In the first method EPDM containing CuCl2 powder and dicumyl peroxide were obtained by mechanical mixing and crosslinking by heating and exposed to pyrrole vapors. In the second method crosslinked EPDM was swollen in a FeCl3/tetrahydrofuran solution and exposed to pyrrole vapors. Conductivity of the composites obtained by the second route (10−5 S/cm) was higher than for the first route. Omastova et al. [10] prepared PPy composites with polyethylene, polypropylene or poly(methyl methacrylate) by a chemical modification method, resulting in a network-like structure of PPy embedded in the insulating polymer matrix. Ruckerstein and Yang [11] synthesized a stable conductive PPy/poly(methyl methacrylate) (PMMA) latex composite through first preparing PMMA emulsion using sodium dodecyl sulfate as a stabilizer, followed by in situ polymerization of pyrrole. Xie et al. [12] dealt with two kinds of conducting PPy composites, namely, chlorinated polyethylene (CPE)/PPy and natural rubber/PPy composites, prepared by in situ oxidation polymerization of pyrrole in the presence of CPE suspension or natural rubber latex, using ferric chloride as oxidant. Preparation conditions, characterization and properties of the composites were studied.
Conductive elastomeric composites are widely used for different applications such as electrostatic charge dissipation [13], touch control switches and electromagnetic interference shielding [14], [15]. Eventhough the processability and mechanical properties of polypyrrole are enhanced by the fabrication of the conducting composites, the composites never attain the strength of the host polymer matrix as there is deterioration of properties of polymer by the incorporation of PPy. It is well established that mechanical properties of rubber composites can be greatly improved by adding short fibers [16]. Hence conducting fibers used as additives can impart good mechanical properties along with desirable electrical properties. Elastomeric conducting composites of polyaniline coated short nylon fiber with chloroprene rubber was prepared and studied by Chandran and Kutty [17]. In our earlier work, we synthesized polypyrrole and polypyrrole coated short nylon fiber which were then used to prepare elastomeric composites based on natural rubber by dry rubber compounding in two roll mill [18]. It was found that DC conductivity of NR/PPy composites was enhanced only at very high PPy loading and the mechanical properties of NR were declined by PPy loading which was compensated by the addition of PPy coated fiber. Maximum conductivity attained in the case of the prepared composite, NR/PPy/F-PPy composite was 3.6 × 10−5 S/cm. The aim of the present work is to fabricate conducting elastomer composites with enhanced electrical conductivity along with good mechanical properties by in situ polymerization method. PPy/NR blends are prepared by in situ polymerization of pyrrole in NR latex. PPy/F-PPy/NR composites are prepared by carrying out the polymerization of pyrrole in NR latex in presence of short nylon fiber so that a uniform coating of PPy is formed on the fiber. The new method is expected to produce elastomer composites with enhanced DC conductivity, high dielectric properties and good mechanical properties.
Section snippets
Materials
Nylon-6 fiber was obtained from SRF Ltd., Chennai, India. Pyrrole and p-toluene sulphonic acid were supplied by Spectrochem Pvt. Ltd., Mumbai. Anhydrous Iron(III) chloride was obtained from Merck Specialities Pvt. Ltd., Mumbai. Fresh natural rubber latex and vulcastab VL were supplied by Rubber Research Institute of India, kottayam, Kerala. Zinc oxide, stearic acid, tetramethylthiuramdisulphide, mercaptobenzothiazyl disulphide and sulfur used were of commercial grade. Pyrrole monomer was
Cure characteristics
Fig. 1 shows the cure curves of the composites. The nature of the cure curves is different for the series, which indicates that PPy and F-PPy interact differently with the matrix.
The cure parameters of the composites are presented in Table 2. Cure time (T90,) represents the time for achieving 90% of the maximum torque. A sudden increase in cure time is observed with the incorporation of PPy which then becomes constant. For LNPFp series cure time decreases substantially and tends to level off at
Conclusion
The cure characteristics, DC conductivity, mechanical, thermal and microwave properties of natural rubber/polypyrrole and natural rubber/polypyrrole/polypyrrole-coated short nylon fiber composites prepared by in situ polymerization in NR latex were investigated. In NR-PPy composites PPy retards the cure reaction while in NR/PPy/PPy coated fiber composites, addition of fiber accelerates the cure reaction substantially. PPy and fiber form a homogenous dispersion in NR matrix as is evident from
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