Natural fiber reinforced conductive polymer composites as functional materials: A review

Synthetic Metals 206 (2015) 42–54

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Natural fiber reinforced conductive polymer composites as functional materials: A review Faris M. AL-Oqla a,b , S.M. Sapuan a,b, * , T. Anwer c, M. Jawaid b , M.E. Hoque d a

Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia c Department of Applied Chemistry Aligarh Muslim University, Aligarh, India d Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham Malaysia Campus, 43500 Semenyih, Selangor, Malaysia b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 6 February 2015 Received in revised form 22 April 2015 Accepted 25 April 2015 Available online xxx

Recent progress in the field of intrinsically conductive polymers (ICPs) as well as conductive polymer composites (CPCs) filled with natural fibers is reviewed here systematically. The possibilities of utilizing natural fibers as fillers for ICPs as well as CPCs to form natural fibers-conducting polymer composite materials have wide potentials in the modern industries. The unique characteristics such as electrical conductivity, mechanical strength, biodegradability and recyclability enabled them to be implemented in many novel and exciting applications including antennas, chemical sensors, tissue engineering, neural probes, biosensors, drug delivery, bio-actuators, fuel cells etc. The effects of fiber contents, fiber size, chemical treatment, temperature and moisture content on the dielectric properties of the conductive composites were reviewed. On the other hand, it was reported that relatively short natural fibers could modify the dielectric response of the polymeric matrix, but chemical treatment had negative effects on such composites and could decrease the dielectric loss factor. ã 2015 Elsevier B.V. All rights reserved.

Keywords: Intrinsically conductive polymers Biosensors Natural fibers Electroactive multifunctional Dielectric properties Functional composites

Contents 1. 2.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conductive polymer composites . . . . . . . . . . . . . . . . . . . . 2.1. Natural fiber composites . . . . . . . . . . . . . . . . . . . . Conductive polymers . . . . . . . . . . . . . . . . . . . . . . . 2.2. Dieelectric properties of conductive polymer composites Dielectric properties of natural fibers . . . . . . . . . . 3.1. 3.2. Dielectric properties of composites . . . . . . . . . . . . Effect of fiber content . . . . . . . . . . . . . . . . 3.2.1. 3.2.2. Effect of fiber size . . . . . . . . . . . . . . . . . . . Effect of chemical treatment . . . . . . . . . . 3.2.3. Effect of temperature . . . . . . . . . . . . . . . . 3.2.4. Effect of moisture . . . . . . . . . . . . . . . . . . . 3.2.5. Discussions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction * Corresponding author at: Universiti Putra Malaysia, Department of Mechanical and Manufacturing Engineering, UPM 43400 Serdang, Selangor, Malaysia. Tel.: +60 3 89466318; fax: +60 3 86567122. E-mail address: [email protected] (S.M. Sapuan). http://dx.doi.org/10.1016/j.synthmet.2015.04.014 0379-6779/ ã 2015 Elsevier B.V. All rights reserved.

Materials’ tendency to conduct electricity is generally expressed by the term of surface resistivity i.e. how they demonstrate resistance to transferring electrical charges. Polymers

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also show such tendency but the surface resistivity of thermoplastic polymers shows poor conductivity. Therefore, thermoplastic polymers are widely used as insulating wire coatings for several applications [1,2]. However, such poor conductivity can lead to undesirable consequences particularly, building -up and retaining static electrical charges, which may cause a startling shock or electrical spark. Therefore, plastics with anti-static characteristics are highly recommended for many end-use applications, while higher conductivity of plastics is desired for other applications [3–8]. In consequence, material selection process is of paramount importance to achieve successful low-cost engineering design that can fit functional requirements as well as customer satisfaction attributes for various industrial applications. This is usually carried out considering various conflicting criteria and utilizing decision making tools to to reveal the potential of new materials in expanding the possibilities of new modern sustainable applications [2,9–12]. In current technology advancement, new components require low cost-high performance materials capable of withstanding aggressive environments. Recently, the intrinsically conductive polymers (ICPs) (in which the polymer’s electronic structure is responsible for their conductivity), as well as conductive polymer composites (CPCs) (in which the addition of conductive fillers provides the conductivity) have special interest in industrial applications particularly, in organic electronic devices where ICPs have strongly contributed to the development of smart materials. CPCs are easily processed and considered to be more economic in compared to ICPs [3,13]. Such unique combination of properties (e.g. dielectric and mechanical strength) is hard to be obtained in one single material. Here, the significant value of conductive polymer composites arises in combining the electrical properties of conducting polymer with the mechanical strength of the filler and the ease of fabrication of the matrix. Such plastics that can conduct electrical charges are desired for other potential applications where the conductivity of metal is not required. Conducting polymers filled with different types of metals make composites suitable for switching devices, electromagnetic shielding of electronic equipment, static charge dissipating materials, conducting adhesives in electronics packaging, cold solders, devices for surge protection and under fill for flip chip [14–17]. Conducting polymer composites have wider technological applications such as in direction finding antennas, self-regulating heaters, chemical detecting sensors, photo thermal optical recording, tissue engineering, neural probes, biosensors, drug delivery, bio-actuators, electronic noses, chemical and electrochemical catalysts, and fuel cells [14–27].

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The mechanical and/or electrical characteristics of nonconductive materials such as polymers and composites can be modified through excedentary electric charges [28,29]. The responses of insulating materials that have dielectric properties recall the space charge physics. This new multi-physics approach is based upon the fact that when stresses of any type (electric, mechanical, radiative etc.) are applied to an insulating material they cause an injection of electric charges [30,31]. Consequently, materials with intermediate disorder can store considerable polarization energy (about 5 eV or more per trapped charges) [32–34]. Therefore, an external stress will have the ability to permit detrapping of those trapped charges causing a release of the stored polarization energy, which expresses the occurrence of dielectric breakdown, rupture or wear as catastrophic effects. It is noteworthy here that energy balances should take into account for such trapped charge as it is one of the potential energy sources. Eventually, insulating material's behavior is related to its aptitude to trap electric charges, in other words, the behavior is related to the density and energy of traps that the material have on one hand, and to its capability to diffuse such electric charges without damage on the other. Amongst different theories and mechanisms set by different scientists to explain their electronic conduction the common ones are band model, hopping and percolation mechanism [35,36]. The band model is one of the most extensively used models as it provides the basis to understand whether a particular material is conductor, semiconductor or insulator. According to molecular orbital theory, two new orbitals of high (antibonding) and low (bonding) energy are produced through overlapping of two compatible atomic orbitals when they are brought closer to each other. In real structural phenomenon there is usually a gap between the top of the occupied valence band and the unoccupied conduction band, termed as band gap. It is assumed to be essential to produce a net electron drift from valence band to conduction band to observe conduction in such material. If the band gap is of reasonable size (
1.5 eV) no electron will move from valence band to conduction band hence, no conduction will occur, and such materials will be termed as insulator. If this gap is of moderate size (1.5 eV) the movement of electrons will occur from valence to conduction band leaving a hole in the valence band, and the materials are called semi-conductors. Within this region, some materials with a very small band gap in which some electrons always get excited to conduction band by thermal means. They are called intrinsic semiconductors. The energy band diagram in polymeric materials is shown in Fig. 1. The third possible way is

Fig. 1. Energy band diagram in polymeric materials.

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that the band is partly filled without any energy gap at all. In these cases, conduction is very easy because electrons require only a small amount of energy to cause a net drift of electron. In case of biphasic or heterogeneous system in which only one phase is conducting, the conductivity depends upon concentration of conducting phase. Therefore, a sharp rise in electrical conductivity is observed at a critical concentration of conducting phase, also termed as percolation threshold. This theory successfully describes the nature of electrical conductivity in composite materials mentioned above. Conjugated polymers having alternate p-electrons display semi-conducting properties as a consequence of their low energy of optical transitions, low ionization energies and high electron affinities [36,37]. 2. Conductive polymer composites There is an emerging interest in the development and applications of electrically conductive polymeric composites particularly; the natural fiber reinforced conductive polymer composites in different aspects of industrial applications. Such interest arises from the fact that the natural fiber based polymeric composites can demonstrate combined properties such as effective insulation and high levels of desirable mechanical strength that enables it to be excellent mechanical support for field carrying conductors. These unique properties enable such composites to be used in extended areas such as terminals, connectors, printed circuit boards, switches, insulators, industrial and house hold plugs, panels etc. [38,39]. On the other hand, the dielectric properties of materials dramatically influence the conversion of the electromagnetic energy into heat [40]. Various textile fibers and fabrics such as cotton, viscose rayon, nylon lycra, polyester and wool, are currently utilized with conducting polymers for various applications such as conductive fabrics, heating devices, electromagnetic interference, super-capacitor, shielding and antimicrobial fabrics [41–43]. Fig. 2 illustrates the super-capacitor unit cell prepared using textile electrode [41]. Conducting polymer-based materials on the other hand, are promising for bio- applications like those of tissue scaffolds for the replacement or restoration of damaged or malfunctioning tissues, because a variety of tissues respond to electrical stimulation [44]. A schematic illustration of

the critical considered aspects for designing biomimetic conducting polymer-based materials are shown in Fig. 3. There are ongoing efforts to develop more biocompatible and inherently biodegradable conductive polymer types, where the functionalization of conductive polymers for a specific application is considered of paramount importance for different possible applications. Optimizing properties for the conductive polymers like conductivity, roughness, hydrophobicity, porosity and degradability in one hand, and the binding of biological molecules (that makes conductive polymers so promising for biomedical applications) on the other, can be carried out through four major chemical ways [13] (Fig. 4) which are: (1) Adsorption process: where a functionalizing agent solution is placed in contact with the polymer [45] after it has already been synthesized. The biomolecule is physically absorbed due the static interactions between the polymer matrix and the charge of the molecule [45]. (2) Entrapping the molecule inside the polymer [46]. This is usually achieved by mixing the functionalizing molecule with the monomer of the polymer, the dopant and the solvent prior to synthesis [46]. (3) By covalently bonding the molecule to the monomer of the polymer. Here, the biological molecules will be strongly bound and will not be released, thereby enhancing the long-term stability of the polymer [13]. And (4) By exploiting the very doping process that renders conductive polymers conductive. This allows the bonding of a wide range of biomolecules as long as they are charged [13,47]. The noticeable advantage of conductive polymers is their vast versatility, as the choice of dopant defines the properties of the polymer and allows its functionalization for a specific application [13,18]. The interconnection aspects in conductive polymers can be seen in Fig. 5. Therefore, the fundamental understanding of these dielectric properties is necessary for different industrial applications. 2.1. Natural fiber composites Due to the growing social, economic and ecological awareness along with government emphasis on the environmental impact and sustainability, the proper utilization of natural resources and wastes are strongly encouraged [48–51]. Consequently, the natural fiber composites (NFCs) have become valuable alternatives for

Fig. 2. Schematic illustration of the super-capacitor unit cell prepared using textile electrode, (a) over view of the unite cell, (b) cross-sectional view [41].

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Fig. 3. Schematic illustration of the critical considered aspects for designing biomimetic conducting polymer-based materials [44].

various industrial applications. In NFCs, natural fibres are used as fillers or reinforcing materials for polymer matrices [48,49,52–54]. The proper utilization of natural fibers not only resolves the waste disposal problems but also reduces environmental pollution [49,52,55,56]. NFCs are attractive from environmental point of view which enabled them to be used as an alternative to the traditional glass/carbon polymer composites [49,55,57]. They are used in various applications including packaging, furniture, automotive industries, disposable accessories, building, and insulation materials [49,57–59]. In addition, these NFCs show several advantages and superior characteristics over traditional composites due to the low cost and densities along with acceptable specific strengths and moduli [52,55,58,60] which offer the opportunity to produce light weight products. Furthermore, NFCs

are also used in producing recyclable and bio-degradable products [58,61–63]. On top of that natural fibers have several advantages over glasses such as: availability, reduced tool wear in machining, CO2 sequestration enhanced energy recovery, and reduced dermal and respiratory irritation [49,50,55,57,64]. Despite of that, natural fibers suffer from certain considerable drawbacks like poor water resistance, low durability, and poor bonding with the matrix. This weak interfacial bonding leads to undesirable characteristics of the composites and thus affects their industrial usage [56,65,66]. Therefore, different solutions have been offered to improve their compatibility and bonding such as usage of coupling agents and surface treatments via mechanical, chemical, and/or physical modifications [57,60,67–69]. Natural fibers can be classified based on their origins such as bast fibers, leaf fibers, fruit, and seed fibers as shown in Fig. 6. Wide range of natural fibers has been used to reinforce different polymer matrices. Such fibers include wood, bamboo, cotton, coir, rice straw, wheat straw, rice husk, flax, hemp, bagasse, pineapple leaf, oil palm, date palm, curaua, ramie, jowar, kenaf, doum fruit, rapeseed waste, sisal, jute etc. [63,66,70,71]. Several factors might affect the finally produced NFC and can determine their mechanical, electrical, biological characteristics. A general classifications of such factors was done by AL-Oqla and Sapuan [49] as presented in Fig. 7 that can help the selection process of natural fibers and polymers to maximize the desired properties of the NFCs. 2.2. Conductive polymers

Fig. 4. Methods of functionalizing conductive polymers: (a) physical absorption, (b) entrapping, (c) covalent bonding and (d) exploiting the doping mechanism.

Conductive polymers have wider advantages over other electroactive biomaterials (e.g. electrets, piezoelectric and photovoltaic materials) from electrical point of view [72]. They have excellent control over electrical stimulus, have a high conductivity to weight ratio, can possess very good electrical as well as optical properties, and can also contribute to making biodegradable, porous, and biocompatible products [13,73–76]. Their physical, chemical and electrical properties can be tailored for catering specific needs of their applications and considered to be one of their special advantages. This can be done by incorporating antibodies, enzymes

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Fig. 5. The interconnection aspects in conductive polymers [13].

and other biological moieties [13,73,76,77]. Moreover, such useful properties of conductive polymers can be controlled, and altered even after synthesis through stimulation (e.g. utilizing different means like electricity, light, pH etc.) [78–80]. Table 1 presents a list of conductive polymers and their abbreviations whereas, the structure of polyaniline, repeating units of polythiophene, and some conductive polymers are demonstrated in Fig. 8. In recent years, their potential applications in functional papers as well as packaging industries have drawn special attention. Several studies confirmed dire need of using such conductive polymers in electrical applications. Coated paper with conducting properties can be used to make anti-static and electro-magnetic shielding papers, anti-bacterial papers, novel wall coverings and electrical resistive heating papers [82–84]. Johnston et al. [84] prepared conducting paper utilizing natural fibers and conductive polymers, where unbleached bagasse and/or rice straw fibers were infused into polyaniline (PANi). Differential scanning calorimeter (DSC), Fourier transform infrared (FTIR), spectroscopy thermal gravimetric analysis (TGA) etc. were used to characterize such produced composites, whereas scanning electron microscope (SEM) was used to investigate themorphology. Results showed that increased conductivity was obtained with the increase of PANi in the composite. However, the breaking length, tear factor and burst factor decreased with the increase of PANi, and such effects were more obvious in bagasse-based composites. In addition, the cure characteristics, thermal and microwave properties, DC conductivity as well as mechanical properties of both natural rubber/polypyrrole and natural rubber/polypyrrole/polypyrrolecoated short nylon fiber composite were studied by Pramila Devi et al. [4]. It was noticed that the DC conductivity of the natural rubber/polypyrrole composite was enhanced only at very high polypyrrole loading and the maximum conductivity of 8.3  104 S/cm was achieved at 100 phr loading. Results also showed that the composite’s thermal stability was increased with

loading of polypyrrole and polypyrrole coated fiber, whereas dielectric constant as high as 55.5 was obtained for 100 phr polypyrrole loaded sample at 3.98 GHz frequency. The conducting composites showed substantial improvement in dielectric heating coefficient as well as skin depth and absorption coefficient. Moreover, conductive papers of graphite/carbon/cellulose fiber composites with low production cost, good mechanical properties and tunable electrical conductivity were produced by Jabbour et al. [5]. It was observed that UV absorbance was increased with increasing the Carboxymethyl cellulose. Furthermore, efficient conductive nano-filler pathways were made through agglomerates or dispersed nano-filler [85]. It was also concluded that resistivity dramatically differed with the dispersion of carbon nano-fiber in polycarbonate that were controlled by sonication conditions. Also, conductive polymer with silk fiber bundle was successfully utilized in making string-shaped electrodes [86], where electro- conductive polyelectrolyte, poly(3,4-ethylenedioxythiophene) -poly(styrenesulfonate) (PEDOT-PSS) and silk thread were combined in electrochemical manner to develop the electrodes. The polymer composite was shown to have a conductivity of 0.00117 S/cm. The addition of glycerol to the PEDOT-PSS silk thread was able to improve the conductivity to 0.102 S/cm. It was also reported that such biocompatible electrodes can be implemented in the field of biomedical as well as health promotions. 3. Dieelectric properties of conductive polymer composites Several researches investigated the efficiency and appropriateness of the natural fibers as fillers for conductive polymers. Such works investigated the effects of several parameters like the fiber type, fiber content, fiber length, humidity and water absorption, temperature, and their chemical treatments with suitable agents to improve desired electrical characteristics. These electrical characteristics include DC conductivity,

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Fig. 6. Classifications of natural fibers [71].

capacitance, dielectric constant, electric charging phenomenon, surface current curves, relaxation phenomenon, electrical resistivity, volume and surface conductivities, dielectric loss, dissipation and loss factors and so on.

Fig. 7. General classification levels of criteria that affect the selection of NFC [49].

3.1. Dielectric properties of natural fibers The dielectric characteristics of a composite material depend upon the polarize-ability of its constituents (i.e. matrix and fillers) and are contributed mainly by atomic, interfacial, electronic polarizations and dipoles [87–90]. The material's ability to become polarized and to store charge when subjected to an external electric field is called dielectric constant (e0 ) [91], whereas the loss tangent (tand) or dissipation factor is the ratio of the dissipated electrical power in a material to the total power circulating in the circuit. In other words, it is the measure of the electrical energy which can be converted into heat in an insulator [91]. The loss factor (e0 ) is usually used to express the losses in industrial energy transmission and distribution and can be defined as the average power factor over a given period of time. There were little studies about the dielectric properties of the natural fibers. The dielectric properties of particular natural fibers in audio frequency range were studied by Bora et al. [92], in that study, the DC conductivity of ramie, jute, and cotton fibers were investigated. Both the capacitance and the dielectric constant of these fibers were studied in the audio frequency range from 1 kHz to 20 kHz under thermochemical conditions. The dielectric constant was found to be 5.18 for ramie, 4.46 for jute and 4.23 for cotton at moderate temperatures. Results also demonstrated that such electrical values of the fibers decreased with the increase of frequency. On the other hand, The dielectric properties of oil palm biomass as well as biochar were investigated at varying frequency in the range 0.2–10 GHz [40]. It was found that the dielectric constants of both oil palm and oil pal shells were inversely proportional to the

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Table 1 Conductive polymers with abbreviations [13,81]. Polypyrrole (PPy) Polyaniline (PANI) Poly(3,4-ethylenedioxythiophene) (PEDT, PEDOT) Polythiophene (PTh) Polythiophene-vinylene (PTh-V) Poly(2,5-thienylenevinylene) (PTV) Poly(3-alkylthiophene) (PAT) Poly(p-phenylene) (PPP) Poly-p-phenylene-sulphide (PPS) Poly(p-phenylenevinylene) (PPV) Poly(p-phenylene-terephthalamide) (PPTA) Polyacetylene (PAc) Poly(isothianaphthene) (PITN) Poly(a-naphthylamine) (PNA) Polyazulene (PAZ) Polyfuran (PFu) Polyisoprene (PIP) Polybutadiene (PBD) Poly(3-octylthiophnene-3-methylthiophene) (POTMT) Poly(p-phenylene-terephthalamide) (PPTA)

frequency, whereas the loss factors had direct proportional relationship with the frequency. It was also reported that oil palm biomass demonstrated poor microwave absorbing properties. It was investigated that the dielectric constant of oil palm fibers was in the range of 7.76–8.31 [93]. Fig. 9 shows the dielectric properties of some known solids [40]. 3.2. Dielectric properties of composites Various studies investigated the dielectric properties of composites made from natural fibers with numerous polymers including the conductive ones. The electrical resistivity of the chicken feather fiber (CFF)/Epoxy composites was found to be two to four orders of magnitudes higher than resistivity of E-glass fiber

composites [94]. It was also found that composites with hybrid (CFF-E-glass/Epoxy) fibers usually had a low value of dielectric constant. On the other hand, highly conductive polymers with silk fibroin composite fibers were fabricated via in-situ polymerization by Xia and Lu [95]. It was reported that polypyrrole/silk fibroin, polyaniline/silk fibroin, and poly3,4-ethylene-dioxythiophene/silk fibroin composite fibers exhibited varied conductivity in the range of 3.8–4.2  101, 0.9–1.2  102 and 4.9–5.2  103 S cm1, respectively. It was also shown that these composites demonstrated better electrical and thermal characteristics and may have potential applications as novel functional materials in textile and biological areas. The electrical resistivity of composites based on polypropylene/coconut fibers composites was also studied by Gelfuso et al. [96].They aimed to investigate the electrical properties of low cost and eco-friendly composites to improve their implementation in the industrial applications. On the other hand, the electrical conductivity of composites based on epoxy resin with polyaniline-DBSA fillers was studied and analyzed by W. Jia et al. [97] They have utilized all of conductive filler PANI-DBSA in form of powder and paste in matrix polymer bisphenol, hydride hardener and epoxy resin as well as accelerator to form the composite. Results demonstrated that a conductivity of the order 103 was achieved at high filler content. Moreover, Wang et al. [98] had introduced the percolation theory and its concepts through carrying out experiments to study the effect of moisture absorption on the electrical conductivity in natural fiber plastic composites. It was reported that the dry natural fiber reinforced polymer composite has no measurable electrical conductivity. But after water submersion, electrical conductivity for the natural fiber composite was successfully achieved. Furthermore, the dielectric properties of date palm fiber/epoxy composite was studied where three relaxation processes were identified [99]. These process were mainly the a mode relaxation, relaxation process due to carrier charges diffusion for high temperature above glass transition and low frequencies, and

Fig. 8. The structure of polyaniline in (a), and the repeating units of polythiophene and some conductive polymers in (b) [13].

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Fig. 9. Dielectric properties of some solids.

interfacial or Maxwell–Wagner–Sillars relaxation [99–101]. The isothermal runs of the loss factor versus frequency for the date palm/epoxy composite are illustrated in Fig. 10. Similarly, conductive nanocomposite utilizing polypyrrole/ dextrin was synthesized as a biodegradable material [102]. The

nano-composites’ conductivity was investigated by four probe method and analyzed for antioxidant activity using 2,2-diphenyl1-picrylhydrazyl assay (DPPH). It was demonstrated that both the conductivity and the antioxidant activity were increased by increasing the polypyrrole in the matrix. Analysis for antibacterial

Fig. 10. The isothermal runs of the loss factor (e0 0 ) versus frequency for the date palm /epoxy composite [99].

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activity against gram-positive and gram-negative bacteria was also performed. It was indicated that the nano-composites were considerably effective against all such studied bacteria. It was also reported that composites were biodegradable under natural environment in the range of 30.18–74.52% degradation. Besides, the electrical conductivity effects of electrodeposited copper powder content filled in lignocellulose matrix were investigated by Pavlovi c et al. [103]. The conductivity measurements were shown to have S-shaped dependency with percolation transition from non-conductive to conductive region. The percolation threshold concentration was achieved at 14.4% (v/v) volume fraction of copper. 3.2.1. Effect of fiber content The effect of fiber content in natural fiber composites was studied by a number of researchers to investigate its effect on the dielectric properties of such produced composites. Sreekumar et al. [104] studied the electrical properties of sisal/polyester composites fabricated by resin transfer molding (RTM). Tests were performed at the frequencies ranging from 5 Hz to 300 kHz and at the temperatures ranging from 25  C to 160  C, where a sinusoidal voltage was applied to provide an alternating electric field. It was found that all the characteristics like dielectric constant, conductivity, dissipation factor and loss factor increased with fiber content for the entire range of frequencies. These values were high for the composites with fiber content of 50 vol.%. Likewise, the volume resistivity varied with fiber loading at lower frequency but merged together at higher frequency. The dielectric constant values were found to be increased when temperature increased followed by a decrease after the glass transition temperature. This variation also depended upon the fiber content. The variation of dielectric constant of sisal/polyester composites as function of fiber content and frequency at 30  C is demonstrated in Fig. 11. In another study, Paul and Thomas [105] compared the electric characteristics of natural fiber reinforced/LDPE composites with glass/LDPE and carbon black/LDPE composites. It was reported that the small change of dielectric constant of glass/LDPE composite occurred with increasing frequency and fiber content compared to coir and sisal fiber/LDPE composites. It was also reported to be due to low interfacial polarization. Authors also reported that electrical conductivity of hydrophobic LDPE can be improved by mixing it with hydrophilic lignocellulosic fibers and conductive carbon black. Moreover, Cabral et al. [106] reported a change in the dependence of dielectric properties of short jute fiber reinforced polypropylene composites as fiber loading changes at critical fiber

Fig. 11. Variation of dielectric constant of sisal/polyester composites as function of fiber content and frequency at 30  C [104].

Fig. 12. Variation of reciprocal of dielectric constant as a function of fiber content (vol.%) at different frequencies [91].

content (about 30%). Authors also mentioned that the conventional preparation methods (like extrusion, injection molding, internal mixer etc.) of the natural fiber composites affected the dielectric characteristics due to high shear forces exerted on the fibers. A variation in reciprocal of dielectric constant as a function of fiber content at different frequencies is demonstrated in Fig. 12. Zhan et al. [94] also reported that the dielectric constant of chicken feather fiber/epoxy composites decreased with fiber contents. 3.2.2. Effect of fiber size Several works focused on the effect of the natural fiber size on the dielectric properties of thus produced composites. Kechaou et al. [107] studied the motion, trapping and detrapping phenomena of the electric charges in unsatured polyester and epoxy composites with short natural fibers of Alfa type using different coupling agents. Authors evaluated the charges induced by the injection of electrons via scanning electron microscopy (SEM) mirror effect method coupled with the measurement of the induced current. Results demonstrated that the fiber–matrix interfaces could allow electric charges to diffuse and to delocalize the polarization energy which could delay the damage of the composite. On the other hand, the study showed that improper sizing could cause trapping of electric charges along these same interfaces with as a consequence, a localization of the polarization energy. Same study also demonstrated that the optimum composite could be obtained for one sizing to help having a strong fiber–matrix adhesion and making the electric charges to flow easily along the interface. Moreover, authors concluded that a good composite behavior could be achieved using strong interfaces able to play two roles: (A) transferring of mechanical load. (B) enabling the flow of the electric charges along these interfaces without trapping [107]. 3.2.3. Effect of chemical treatment Different chemical treatments like KMnO4, toluene di-isocyanate (TDI), maleic anhydride modified polypropylene (MAPP), and stearic acid (ST) were applied to jute yarn/polypropylene composites [91] to enhance the interfacial adhesion between the matrix and the filler. It was noted that the untreated sample showed highest dielectric constant values compared to the treated ones. It was also reported that all used chemical treatments decreased the dielectric constant as well as the loss factor. The

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MAPP treated composites showed the least dielectric constant value. The researchers related that effect to the ability of the treatment that decreased the hydrophilic nature of jute yarns by reducing the moisture absorption. This in turn, caused a reduction in the orientational polarization that led to lower dielectric constant and loss factor values. Besides, the chemical treatment was able to reduce the number of voids and other irregularities that led to decrease the water absorption and hence the dielectric constant [91]. In another study, chemical modification utilizing a biodegradable zein coating for flax reinforced polypropylene (PP) composites was investigated by John et al. [31]. They studied the effect of chemical modification as well as fiber loading on the composites particularly, their thermo-physical and dielectric properties. Also the dielectric constant of the composites was found to be higher than that of polypropylene. It was also shown that the reinforcement of flax fillers in the polypropylene increased the relative dielectric permittivity. Besides, the composites of banana, hemp, and agave (both treated with maleic anhydride and untreated cases) with high density polyethylene (HDPE) resin were separately studded for both surface and volume resistivity with different fiber loading conditions [108]. It was reported that the surface resistivity decreases, whereas volume resistivity increases with an increase in fiber content in the composites. The effect of chemical treatment on the volume resistivity of different ratios of natural fibers/HDPE is shown in Fig. 13. In addition, it was also demonstrated that chemical modification of the fibers decreased both thermal conductivity and diffusivity due to the increase in the matrix interfacial adhesion. The effects of fiber treatment, fiber size and fiber loading on the physical and dielectric properties of oil palm fiber/linear low density polyethylene compression molded composites were investigated by Shinoj et al. [93]. They were able to predict both the density and the dielectric constant of the fiber and composite utilizing different models. They found the dielectric constant of the oil palm fibers to be in the range of 7.76–8.31 whereas, that of the composite was in the range of 3.22–6.73. It was also reported that Alkali treatment was able to reduce the first degradation temperature of the composite to 297.1  C. In addition, jute fabric/polypropylene composites treated with red dye solutions (0.1–1%, w/w) for different soaking times were investigated regarding the dielectric properties by Zaman et al. [109]. It was found that both the dielectric constant and loss tangent of the treated and untreated composites increased with increasing temperature up to the transition temperature then decreased

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after that to became almost constant. Similarly, coconut coir/ polypropylene composites were investigated for their mechanical and electrical properties after different chemical modifications were applied by Lai et al. [110]. Alkali, permanganate, and stearic acid treatments were used in the study. The dielectric property of the composites was almost the same for all the chemical treatments but significantly increased with fiber loading. The maximum value of dielectric constant was found to be 2.56 at 25% fiber loading. Besides, the effect of addition of porous additives on dielectric constant of sisal/polypropylene was investigated by Sharma and Chand [111]. Composites having sisal fiber with cylindrical pores as well as cenospheres having spherical pores, were shown to have low dielectric constant. Also new relation concerning porosity was proposed to predict the dielectric constant of polypropylene based composite. The dielectric constant was increased and AC conductivity was improved by fiber loading. 3.2.4. Effect of temperature Although the composite’s net polarizibilty depends upon several factors like atomic, interfacial, electronic and orientational polarisability which contribute to the material’s dielectric constant. Only orientational polarisability depends directly on temperature [91]. In natural fiber reinforced composites both orientational and interfacial polarisability are the main responsible factors for the net polarisability. Therefore, the effect of temperature is much important in determining the dielectric properties of the natural fiber composites. It was reported that very high values of dielectric constant were achieved when the temperature of jute/ polypropylene composites was raised to around 60  C, and then the values started to decrease [91]. Zaman et al. [109] reported that the dielectric constant and loss tangent of jute fabric/polypropylene composite increased with the increase of temperature reaching a maximum value at high temperature. It was also reported that only some of the molecules in the composite could have the required energy to align themselves with the applied electric field at room temperature (at around 30  C), whereas in the range of 30–60  C, the whole remaining molecules could get the required potential energies to get themselves aligned with the applied field. Therefore, high net orientational polarization occurred in this range of temperature causing an increase in dielectric constant values [91]. On the other hand, increasing the dielectric constant values with increasing temperature was related to the fact that the mobility of water

Fig. 13. Volume resistivity of different ratios of natural fiber/HDPE composites based on treatment with and without of maleic anhydride [108].

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that of glass fiber based composites. They also noticed that the permittivity decreased when the frequency increased. Their results also showed that removing sizing from fibers by washing with hot water could increase the effective coefficient diffusion. Moreover, in the frequency range of 200 Hz–1 MHz, a non-linear behavior was noticed in the changes of the permittivity with water absorption. It was also reported by George et al. [91] that reducing the moisture content of the composites usually led to the reduction in orientational polarization that could cause low dielectric constant values. 4. Discussions

Fig. 14. Effect of temperature on the dielectric constant of different jute/ polypropylene composites at 5000 Hz frequency [91].

dipoles (as a result of moisture absorption) could increase with temperature increase ultimately resulting in increase of the orientational polarization and dielectric constant. Moreover, the fact that values of dielectric constant were reduced at high temperature was related to the high levels of molecular vibrations along the random thermal motion that could lead to less closely aligned molecules to the applied electric field [91]. In addition, the effect of temperature on the dielectric spectrum of sisal/ polypropylene composites was studied by Sharma and Chand 2013 [111] at the frequency ranging from 1 to 10 kHz. The dielectric constant, dissipation factor and AC conductivity of the composites were investigated. It was found that all of these properties were increased with increasing temperature at different frequencies. The effect of temperature on the dielectric constant of different jute/polypropylene composites is shown in Fig. 14, while Fig. 15 represents the effect of temperature on the orientational polarization of a system. 3.2.5. Effect of moisture Much little studies were found in the literature investigating the effects of moisture and water uptake on the dielectric properties of the conducting polymer composites with natural fiber fillers. Fraga et al. [112] monitored the water uptake utilizing gravimetric and dielectric measurements in composites with glass, jute and washed jute fibers. Authors reported that the dielectric constant of composites used jute fiber as fillers was higher than

There are promising potentials of utilizing and implementing ICPs as well as CPCs in different modern technologies and industrial applications. Several methods could be implemented to utilize the electrical properties of CPCs in various applications. The maximum conductivity of CPCs with natural fiber fillers was observed to be lower than that of the intrinsic conductive fillers due to the contact resistance between them. However, enhancement in the conductivity of CPCs still remains a challenge to be achieved. It was shown that both the dielectric constant and dielectric dissipation of the natural fiber reinforced polymer composites decreased with frequency, and increased with temperature. It could be deduced that increasing the frequency at fixed temperature could decrease the dielectric loss factor of these composites, whereas increasing temperature at lower frequency values could increase the dielectric loss factor. It was also observed that chemical treatment of the natural fibers could decrease the dielectric loss factor of the composites. Moreover, both the dielectric constant and loss tangent increased with increasing temperature for a certain limit then decreased in high temperature due to the high levels of molecular vibrations along random motion that could lead to less closely aligned molecules to the applied electric field. These dielectric properties of the conductive composites depend upon various factors like the composition, chemical structure, physical texture and the morphology of the composites. Therefore, extra investigations regarding the dielectric properties such as dielectric constant, volume resistivity, loss factor and conductivity are still required for the successful applications of the natural fiber reinforced conducting polymer composite materials. In most commercial compositions, additives used for conductive polymer composites tend to increase the mechanical properties like stiffness, tensile strength, impact and elongation. This means, the addition of natural fibers to the intrinsic conductive polymer to produce a conductive composite not only enhances the behavior of the composite in terms of transferring the mechanical loads but also eases the flow of electric charges along the interfaces where

Fig. 15. Effect of temperature on the orientational polarization of a system [91].

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the charges can defuse through the interfaces without trapping. It can be noticed that short natural fibers can enhance the electrical charge flow more efficiently than long ones and thus enhances the dielectric properties of the conductive polymers. 5. Conclusions There are wide possibilities in the future for utilizing and implementing the ICPs as well as the CPCs in different modern technologies and industrial applications. However, enhancement in the conductivity of CPCs still remains a challenge to be solved. Moreover, chemical treatments of the natural fibers can decrease the dielectric loss factor of the composites. Extra investigations regarding the dielectric properties such as dielectric constant, volume resistivity, loss factor and conductivity are still required for the successful applications of the natural fiber reinforced conducting polymer composite materials. In most commercial compositions, additives used for conductive polymer composites tend to increase the mechanical properties like stiffness, tensile strength, impact and elongation. That is; the addition of natural fibers to the intrinsic conductive polymer to produce a conductive composite, not only enhances the behavior of the composite in terms of transferring the mechanical loads, but also to ease the flow of electric charges along the interfaces where the charges can defuse through the interfaces without trapping. Technically speaking, the nature and sizing of fibers were critical in determining the dielectric properties of the natural fiber conductive composites. In addition, chemical treatments of fibers could increase the interfacial adhesion in natural fiber conductive composites, but a decrease in the dielectric constant of the composites could be observed. Overall, the use of natural fibers with conducting polymers to develop new materials aligned to different new-tech applications like biosensors, drug delivery, bioactuators, fuel cells, anti-bacterial packaging and so on has highly promising potential. However, theoretical modeling that should be able to predict the value of dielectric constant of different natural fibers with wide range of conducting polymers is still challenging in this particular field of technology. Acknowledgements Would like to thank Universiti Putra Malaysia for granting Prof S.M. Sapuan his sabbatical leave for the period of October 2013– June 2014 to have some scholarly attachment to the Faculty of Engineering, Universiti Malaya, Kuala Lumpur. Would also like to thank Professor Faiz Mohammad, Aligarh Muslim University, India for his generous discussion and suggestions in preparing this work. References [1] V.K. Thakur, M.K. Thakur, P. Raghavan, M.R. Kessler, ACS Sustainable Chem. Eng. 2 (2014) 1072–1092. [2] F.M. AL-Oqla, S.M. Sapuan, M. Ishak, A. Nuraini, Int. J. Polym. Anal. Charact. 20 (2015) 191–205. [3] H. Deng, L. Lin, M. Ji, S. Zhang, M. Yang, Q. Fu, Prog. Polym. Sci. (2013) . [4] D. Pramila Devi, P. Bipinbal, T. Jabin, S.K.N. Kutty, Mater. Des. 43 (2013) 337– 347. [5] L. Jabbour, D. Chaussy, B. Eyraud, D. Beneventi, Compos. Sci. Technol. 72 (2012) 616–623. [6] V.K. Thakur, M.K. Thakur, R.K. Gupta, Int. J. Biol. Macromol. 62 (2013) 44–51. [7] V.K. Thakur, M.K. Thakur, R.K. Gupta, Carbohydr. Polym. 97 (2013) 18–25. [8] V. Thakur, A. Singha, M. Thakur, Int. J. Polym. Mater. Polym. Biomater. 63 (2014) 17–22. [9] F.M. AL-Oqla, M.T. Hayajneh, Design Challenge Conference: Managing Creativity, Innovation, and Entrepreneurship, Amman, Jordan,, 2007. [10] M.I. Al-Widyan, F.M. AL-Oqla, Build. Simul. 7 (2014) 537–545. [11] M.I. Al-Widyan, F.M. AL-Oqla, Int. J. Eng. Res. Appl. 1 (2011) 1610–1622. [12] F. Dweiri, F.M. Al-Oqla, Int. J. Comput. Appl. Technol. 26 (2006) 182–189. [13] R. Balint, N.J. Cassidy, S.H. Cartmell, Acta Biomater. 10 (2014) 2341–2353.

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