Hybrid composites of short acetylated kenaf bast fiber and conducting polyaniline nanowires in epoxy resin

Abstract

This article reports the preparation and characterization of newly developed hybrid composites consisting of epoxy resin (EP) matrix, acetylated kenaf bast fiber (AKF) and conducting polyaniline (PANI) nanowires. Initially, the EP/AKF composites were prepared by varying the AKF loading (5–30 wt%). The EP/AKF displayed an optimum tensile strength at 20 wt% AKF loading which was higher than that of untreated kenaf fiber EP composites (EP/UKF). The hybrid composites of EP/AKF/PANI were then prepared by using 20 wt% AKF loading with PANI inclusions from 2 to 14 wt%. The addition of PANI into EP/AKF induced positive electrical properties without considerably sacrificing its mechanical integrity. It was found that the electrical percolation threshold of these hybrid composites was at 11 wt% of PANI loading. PANI inclusions at above the percolation loading resulted in reduction of tensile and flexural strength. Meanwhile, no significant mechanical loss was observed below the threshold. The fracture morphological analysis revealed the occurrences of PANI nanowires pull out from the matrix. The Fourier transform infrared spectroscopy showed that the PANI component still maintained its doped condition inside the EP/20AKF. Water absorption and thermal analysis indicate that the PANI incorporation induced lower water uptakes and greater thermal stability to the EP/20AKF, respectively.

Keywords

Kenaf fiber polyaniline epoxy resin electrical conductivity mechanical properties hybrid composites

Introduction

Natural fibers are suitable candidates to replace synthetic fibers for the preparation of fiber reinforced polymer composites.¹ The various advantages of natural fibers over man-made glass and carbon fibers are low cost, low density, comparable specific tensile properties, less health risk, renewability, recyclability and biodegradability.^2,3 Kenaf (Hibiscus cannabinus L., Malvaceae) is a warm season annual fiber crop. It is a fiber plant native to east-central Africa where it has been grown for several thousand years for food and fiber.⁴ Various applications had been explored to utilize the use of kenaf fiber (KF) including as adsorbent material, feed stock, insulation board, paper and as reinforcement in polymer composites.^5–9 However, certain drawback of natural fibers/polymers composites is the incompatibility between the hydrophilic natural fibers and the hydrophobic polymer matrices. This leads to undesirable properties to the composites. It is possible to chemically reduce the hydrophilicity of the inherent cellulose surface by removing the surface OH groups by means of acetylation.¹⁰ It has been shown that acetylated natural fiber had good mechanical properties and interaction with various polymer matrices.^11,12

Conducting polymers are the new class of materials used for fabrication of many electronic devices. Polyaniline (PANI) is one of the most promising candidates for industrial applications mainly for its easy preparation, low cost, excellent electrical, optical, magnetic properties and environmental stability.^13–15 PANI in its undoped state is an insulator. Doping of PANI causes the delocalization of charge along the polymer backbone thus becoming an excellent conductor. PANI’s mechanical properties are low¹⁶ and therefore they are usually used as conducting fillers in polymer matrices, mainly due to their lightweight and electronic properties. Furthermore, it has been demonstrated that PANI could induce controllable hydrophobicity¹⁷ and good biocompatibility.¹⁸ Combining PANI with epoxy resin (EP) is a good strategy to obtain thermosetting composites having good mechanical and electrical properties. The issue to address in this type of EP containing PANI composites is usually the attainment of effective percolation to achieve the conducting pathways for the composites. Percolation refers to a situation where randomly distributed conducting filler particles form percolating paths inside the insulating matrix, which leads to a sharp change in composite conductivity. Furthermore it has been reported that PANI in its nanowire form could reduce the percolation concentration.¹⁹ There are many reports in the literature that described the possibility of imparting electronic properties to EP resin by the use of PANI filler.^20–23 However, the focal point of these works is on the composites electronic properties, forgoing its mechanical properties. It has been described by some that the introduction of PANI reduces the mechanical properties of EP resin.^22,24 Tsotra and Friedrich²⁵ attempted to overcome this predicament by reinforcing the EP/PANI blend with short carbon fibers and found out that the flexural properties improved but minimally. The demand for materials having combination of wide range of desirable properties is always increasing. The development of hybrid composite (two or more filler types in a single matrix) is still in its infancy. It is generally accepted that the behavior of hybrid composites appear to be simply a weighted sum of the individual components in which there is a more favorable balance between the advantages and disadvantages inherent in any composite material. Most of the natural/synthetic fiber hybridization aims on improving mechanical properties and reducing water absorption of the composites, this is mainly by limiting the loading fraction of the natural fiber component.^10,26

It is interesting to combine the advantages of environmental friendly, low cost, high specific strength of KF with the electronic properties of PANI in a polymer matrix. It has been shown earlier that the combination of such material is synergetic, provided by the use of suitable dopants and fiber treatment.²⁷ Therefore, by hybridizing KF and PANI nanowires in an EP resin, one can expect a hybrid composite having the mechanical properties of natural fiber reinforced EP including the electron conducting features of PANI. It will be shown that the advantages and characteristic of the KF and PANI will not be significantly lost by such hybridization. So far, no such attempt has been done to combine these two fillers (KF and PANI nanowires) with EP matrix. This research work reports for the first time the preparation of EP/KF/PANI hybrid composites, investigating the effect of PANI incorporation on the surface interactions, electrical, mechanical, morphological, thermal properties and percentage of water uptakes.

Experimental

Sample preparations

PANI nanowires doped with formic acid were synthesized using two face interfacial polymerization method as described elsewhere.²⁸ Briefly, the monomer, dopant and oxidizing agent used were aniline, formic acid and ammonium persulfate (Merck) of analytical grade, respectively. Untreated kenaf bast fiber (UKF) supplied by Everise Crimson (M) were pulverized and sieved to 150 µm sized short fibers. Detailed preparation method of obtaining acetylated kenaf bast fiber (AKF) was described elsewhere.²⁷ In short, the acetylation reagents used were glacial acetic acid (Merck), acetic anhydride (Sigma) and few drops of concentrated sulphuric acid. These prepared PANI nanowires and AKF were used for the preparation of EP/AKF/PANI hybrid composites. The synthesized PANI nanowires were mixed with toluene with a magnetic stirrer for 48 h to create a 5% solution of required PANI. The mixture was treated in an ultrasonic bath for 3 h to obtain a green transparent low viscosity polymer solution. In the mean time, the EP resin, DGEBA (Sigma) was mixed with AKF with a mechanical stirrer for 10 min. The PANI suspension was then transferred into the EP resin containing AKF and was then mixed under vacuum, until the whole toluene was evaporated from the premixed hybrid composite. The hardener, trifluoroboron (4-chlorobenzeneamine) (Air Products) was finally added (10 phr to EP) and mixed at room temperature. The precured hybrid composites were cast into silicone rubber molds of required dimensions and left for cure at room temperature covered with Mylar film on top. The schematic preparation of the sample is depicted in Figure 1. Following this procedure, EP/AKF/PANI hybrid composites with required AKF and PANI loading were prepared. This hybrid composites preparation was preceded by the preparation of EP/UKF and EP/AKF composites with varying fiber loading. This was done in order to investigate the effects of acetylation and to determine the optimum loading for tensile strength.

Characterizations

The DC conductivity, σ _DC was measured on samples of circular dimension using a resistance meter (Advantest) by the four-probe method under ambient condition. The σ _DC , was calculated using L/RA, where R is the resistance, L is the thickness and A is the area of sample. Samples were stored in a desiccator filled with silica gels for approximately 24 h prior to testing to remove adsorbed moisture. Five measurements were made on each sample. The tensile test (ASTM D638) was performed using a computer-controlled Instron machine with crosshead speed of 5 mm/min at a standard laboratory atmosphere of 23℃. The width and thickness of the dumbbell shaped specimens were about 10.00 and 3.50 mm, respectively. Flexural test (ASTM D790) was performed by 3-point bending configuration at 3 mm/min under ambient condition. At least five replicates for each sample were tested. Fourier transform infrared (FTIR) spectra of the samples were obtained using a model 2000 Perkin Elmer spectrometer with a resolution of 0.4 cm⁻¹. The water absorption measurement was done according to ASTM D570. The samples were dried at 80℃ until a constant weight was achieved prior to immersion in water at 27℃. The test was performed for 40 days. Weight gains were recorded by periodic removal of the specimens from the water bath and weighing on a balance with a precision of 1 mg. The percentage of water uptake, Mt (%) was calculated using (W_w–W_d)/W_d× 100. Where W_w and W_d are the weights of the sample before and after immersion in water, respectively. Thermal analysis was performed by TGA7 Perkin Elmer Pyris. Samples were heated from 30℃ up to 550℃ with a heating rate of 10℃/min. The tensile fractured surface morphology of the hybrid composites were analyzed using scanning electron microscope (SEM), Leo Supra 50VP.

Results and discussion

Effects of KF acetylation on tensile strength of EP resin

Figure 2 depicts the tensile strength of EP/UKF and EP/AKF composites at different loading. The tensile strength of the EP composites increases with increasing filler loading. Both type of composites (untreated and acetylated) revealed an optimum value at 20 wt% loading. These increases show that the fibers are capable of acting as a stress transfer agent for the EP matrix, particularly at 20 wt% and below. It can be seen that the composites with AKF had higher strength than the composites with UKF. The changes are attributed to the acetylation of the fiber surfaces. Acetylation reduces the polar hydroxyl groups by replacing it with less polar acetyl groups, thus their interaction between the EP matrix is enhanced, resulting in higher tensile strength. Furthermore the surface of UKF contains significant amounts of surface lignin, waxy substances and impurities than AKF that prohibit favorable adhesion with the EP matrix. The schematic interaction between AKF and the EP matrix are shown in Figure 3. It can be noted that the strength of EP/AKF composites of above 20 wt% loading revealed declining trend and slight reduction compared to neat EP. This can attributed to the poor matrix wetting at high fiber volume fraction. Meanwhile the tensile strength of EP/30UKF composites drops considerably reaching reduction of 22% compared to neat EP. This decrease may due to the high fiber volume fraction plus the poor surface interaction between EP and UKF that leads to poor crosslinking reaction between the resin and the curing agent. Based on these findings, composites with 20 wt% AKF (EP/20AKF) are chosen to investigate the effect of PANI inclusions, forming EP/AKF/PANI hybrid composites.

DC conductivity and percolation threshold

Figure 4 presents the logarithmic electrical conductivity of EP/20AKF/PANI hybrid composites as function of PANI content. It shows that the conductivity increase is insignificant from 2 to 10 wt% inclusion of PANI. However, at 11 wt% loading, the conductivity jumps drastically to 3.7 × 10⁻⁸ S/cm. Below the percolation threshold (2–10 wt%), the PANI nanowires acted as fillers with no network formation and no conductive path throughout the composite. Majority of the composite’s volume at below the percolation concentration is governed by the insulating EP matrix and the micron-sized AKF. In this case, finite segregated PANI nanowires could not induce any significant electrical pathways within the polymer matrix. Thus the increment in electrical conductivity is only marginal. At 11 wt%, a 3D network of the PANI nanowires is formed. When percolation is reached, the conductivity increased abruptly because the network has created conductive pathways throughout the EP/AKF matrix. This can be shown by the illustrative diagram in the insert of Figure 4. Though the conductivity increment at percolation concentration is evident, it became stable thereafter. However, the maximum conductivity obtained is of four orders of magnitude lower than composite of EP/PANI.²⁸ This can be associated with the presence of AKF that might hinder the formation of additional conducting PANI networks with good percolation quality. It has been reported that PANI nanowires doped with formic acid achieved its percolation state at 8 wt% loading in that of similar EP matrix.²⁸ In this case, the presence of AKF dominated the volume of the matrix thus increasing its percolation concentration up to 11 wt%. It can be said that the increase in conductivity and defined percolation threshold indicates a possible way of using PANI nanowires as conducting component in natural fiber filled EP composites. Lower conductivity percolation threshold is an important factor for preparing conductive composites, because low percolation lowers the cost and minimize influences on the mechanical properties of the conductive composites.

Mechanical test

The tensile strength of EP/20AKF/PANI with increasing amount of PANI is shown in Figure 5. The composite shows slight reduction (8% drop) with the inclusion of 2 wt% PANI. This drop is caused by the brittle nature of PANI that acts as non-reinforcing fillers due to the significant difference in polarity and crystallinity between PANI and EP. Though one might expect a substantial reduction once PANI is introduced into the composite, this is avoidable due to the favorable interaction of PANI dopant moieties (formate ion) with the EP and AKF components.^27,28 The interactions between the three components are discussed in the FTIR spectroscopy. Meanwhile, further addition up to 8 wt% PANI revealed minimal changes in the tensile strength. At low PANI loading, the mechanical properties of the hybrid composites are preserved by the high strength of EP/AKF. However, the presence of 10–14 wt% PANI in the composite leads to a progressive fall in the tensile strength values. This behavior could be due to two main reasons: (1) the large presence of PANI that might retard the curing reactions of the EP resin and (2) disrupts the load transfer process from the EP to the reinforcement components (AKF). On the other hand, hybrid composite of 14 wt% PANI displays a loss in tensile strength of about 35% compared to EP/20AKF. This is related to the highly percolated PANI within the matrix. The flexural properties of the composites are presented in Figure 6. Both, flexural strength and modulus of the hybrid composites exhibit similar trend with the tensile deformation mode. The decrease at high loading is owed to the percolated PANI components that reduce the EP/AKF resistance to shearing. In this case, the utilization of AKF provide sufficient mechanical support to the hybrid composites revealing tensile and flexural strength values of several magnitudes higher than EP/PANI composites without any mechanical reinforcements.^22,24,28

FTIR spectroscopy

Spectrum of neat EP (Figure 7(a)) is the characteristic of cured bisphenol-A epoxy. FTIR spectrum of PANI (Figure 7(b)) exhibits the presence of benzoid and quinoid ring vibration at 1530 and 1575 cm⁻¹, respectively. These indicate the oxidation state of the conducting salt of PANI. The strong peak at 1140 cm⁻¹ is the characteristic peak of PANI conductivity and is a measure of the degree of delocalization of electrons.²⁹ The weak and small peak at 3230 cm⁻¹ is assigned to the N–H stretching mode. EP/20UKF in Figure 7(c) exhibits the OH band of free cellulose that of kenaf at 3400 cm⁻¹. Peak of 1734 cm⁻¹ (as shown by the black arrow) represent the acetyl group of hemicellulose and ester linkage of lignin. EP/20AKF sample (Figure 7(d)) shows decreased in 3400 cm⁻¹ and increased of 1734 cm⁻¹ peak intensities. This signifies that the acetylation of fibers had occurred.²⁷ Hybrid composite with 4 wt% PANI (Figure 7(e)) revealed that the 1140 cm⁻¹ peak still exists, which essentially indicates that the PANI nanowires still keeps the doped conducting condition in the mechanically reinforced matrix. This condition is vital to ensure that the incorporated PANI nanowires do not deprotonate either by AKF or hardener. Schematic of the PANI deprotonation by base is presented in insert of Figure 7. Hybrid composites of 12 wt% PANI (Figure 7(f)) exhibits similar pattern to the 4 wt% PANI composites except for the existence of the 3230 cm⁻¹ peak and more distinct 1140 cm⁻¹ peak. The slight emergence of N–H stretching might suggest that charge-transfer activity between the conducting PANI chains can occur, resulting in an appearance in the N–H stretching.¹⁵ The more pronounced 1140 cm⁻¹ peak is attributed to the presence of more conducting PANI nanowires (above the percolation threshold) inside the matrix. Figure 8 depicts the interaction between the three components. It has been described previously^27,28 that the interactions between: (1) PANI-AKF is by the π interactions of the formate ions and acetyl group of AKF, (2) AKF-EP is by the methyl groups affinity plus the polarity between the carbonyl and the EP chain and (3) EP-PANI is by the charge-transfer and relatively small formate ions that facilitate mixing between the two.

Equilibrium water absorption

Figure 9 shows that the water absorption of EP reaches saturation after 33 days with water uptake of about 2.4%. This indicates that the absorption reaches quasi-equilibrium condition.³⁰ In EP, water molecules couple strongly with hydrophilic functional groups such as hydroxyl. Composite of EP/20UKF shows water uptake of about 3.7% upon immersion of 40 days. This is attributable to the hydrophilic characteristic of the surface of natural fiber provided by the hydroxyl groups that interact with water through hydrogen bonding.³¹ Furthermore, impurities on the UKF surface contribute to the water absorption of the composite. It can be seen that the EP/20AKF composite reduces the water uptake compared to EP/20UKF. The acetylation process replaces the hydroxyl groups with acetyl groups, thus leaving fewer accessible hydroxyl groups on the fiber and resulting in lower equilibrium water uptakes. Addition of 4 wt% PANI seems to have similar water uptake trend to that of neat EP. This behavior can be due to the hydrophobic features of PANI.¹⁷ Meanwhile, EP/20AKF/12PANI revealed reduction in equilibrium water uptakes compared to EP. This indicates that the surface of the AKF and the EP matrix had been dominated by the percolated hydrophobic PANI. These findings indicate that PANI inclusions in the composite could reduce the degree of moisture absorption of EP/AKF which in turn will preserve their functional, structural and mechanical properties.

TGA analysis

TGA thermograms of cured EP, EP/KF and hybrid composites are presented in Figure 10. Neat EP (curve a) exhibits the features of cured EP that underwent thermal degradation at 380℃. Both the TGA curves of EP/20UKF and EP/20AKF (curve b and c, respectively) show first stage of weight loss around 110℃, which correspond to water evaporation. The weight loss between 250℃ and 300℃ is attributed to degradation of the KF. The third degradation at around 350℃ is attributed to the degradation of the EP matrix. This shows that the reinforcement of KF decreases the thermal stability of the EP composite. However, the composites with AKF show higher stability than composites with UKF. This is mainly due to the removal of easily hydrolyzed low thermal stability impurities contained in the UKF. The hybrid composites (4 and 12 wt% PANI inclusion) tend to maintain the stability of the EP/20AKF at higher temperature. Higher thermal stability of PANI²⁷ than the natural fiber provide increased thermal barrier to the EP matrix as opposed to the EP/KF composites without PANI inclusions. This shows that the inclusion of conductive PANI increases the thermal stability of EP/20AKF and does not significantly affect the thermal stability of neat EP.

SEM

Figure 11(a) shows the tensile fractured surface of EP/20UKF. The matrix surface exhibit the typical fractured surface of cured EP. It can be seen that there exists a small gap between fiber and matrix which means that the adhesion between UKF and EP matrix is relatively poor. The fractured surface of the EP/20AKF (Figure 11(b)) revealed much smaller fiber–matrix gap with traces of the EP matrix on the AKF surface. This is an indication that the adhesion is good and favorable for load transfer. It is also noticed that the fiber failed by tearing but no interfacial failure is observed. The fractured surface of EP/20AKF/4PANI (Figure 11(c)) revealed presence of small PANI particles on the AKF and EP matrix surfaces. This suggests that PANI nanowire pull out occurred due to the relatively poor interaction with the EP matrix. This is in agreement with the reduction in mechanical properties as discussed earlier. Though the PANI is synthesized in its nanowire state, the pull out causes the nanowires to agglomerate and form particles of less than 1 µm. On the other hand, it can be seen that the interaction between the UKF and the EP matrix is still intact. It can be said that at low PANI loading (before the percolation threshold) the influence of PANI on the failure mechanism of the hybrid composite is only by its pull out from the matrix, without causing any undesirable effects on the AKF-EP matrix bonding. Meanwhile hybrid composite at 12 wt% PANI (Figure 11(d)) revealed larger PANI particles including the distinct large gap between the AKF and matrix. The increased in particle size is related to the percolated PANI nanowires network within the matrix which increases the probability of the PANI nanowires to agglomerate during the failure process. Additionally, these particles may result in fiber–matrix debonding as seen by the gap between the AKF and EP which leads to reduction in mechanical properties. However, no AKF pull out was observed. Indicating that the AKF still act as a stress transfer medium to compensate the matrix mechanical properties deterioration caused by the PANI pull out.

Conclusions

Novel hybrid composites made of EP, AKF and conducting PANI nanowires were successfully developed having favorable properties and synergizing effects. Tensile strength of EP/AKF was better than EP/UKF and achieved optimum values at 20 wt% AKF loading. Furthermore the interaction between EP and AKF is good, indicated by the reduction of hydroxyl groups and more intense 1734 cm⁻¹ peak of FTIR spectra. Composites of EP/20AKF were used for the hybridization with the PANI nanowires. It was shown that the percolation of PANI nanowires inside the EP/20AKF occurred at 11 wt% PANI loading. This essentially demonstrated the possibility of inducing electrical properties towards natural fiber reinforced EP composites. Mechanical properties of the hybrid composites were not considerably affected by the presence of PANI at below percolation concentration but showed reduction after the percolation has been reached. However, the tensile strength and flexural values of these hybrid composites are higher than that of EP/PANI from reported studies. It was shown that at below the percolation concentration, PANI pull out occurred but nonetheless the EP–AKF interaction was retained. Meanwhile, failure of the composites at above the percolation was influenced by the highly significant PANI pull out which led to reduction in EP–AKF interaction. The water absorption tests showed that the hybrid composites had better water absorption profiles than that of EP, EP/UKF and EP/AKF. This reduction in moisture uptake indicates a positive effect, which makes PANI a suitable hygroscopic component. TGA results revealed that the inclusions of PANI in the EP/20AKF increased its thermal stability. FTIR spectroscopy of the hybrid composites showed that the electron delocalization peak of PANI was preserved. This newly developed hybrid composites show promise for multiple applications, which require low conducting particulate content with good mechanical integrity.

Footnotes

Funding

The financial support provided by the Universiti Teknologi Malaysia through RUG grant is gratefully acknowledged.

Conflict of interest

None declared.

References

Summerscales

Dissanayake

Virk

. A review of bast fibres and their composites. Part 2 – Composites. Composites Part A 2010; 41(10): 1336–1344.

Gibson

. A review on recent research on mechanics of multifunctional composite materials and structures. Compos Struct 2010; 92(12): 2793–2810.

Singleton

ACN

Baillie

Beaumont

PWR

. On the mechanical properties, deformation and fracture of a natural fibre/recycled polymer composite. Composites Part B 2003; 34(6): 519–526.

Mohanty

Misra

Hinrichsen

. Biofibres, biodegradable polymers and biocomposites: an overview. Macromol Mater Eng 2000; 277(1): 1–24.

Özturk

. Effect of fiber loading on the mechanical properties of kenaf and fiberfrax fiber-reinforced phenol-formaldehyde composites. J Compos Mater 2010; 44(19): 2265–2288.

Ibraheem

Ali

Khalina

. Development of green insulation boards from kenaf fibres and polyurethane. Polym Plast Technol Eng 2011; 50(6): 613–621.

Symington

Banks

West

. Tensile testing of cellulose based natural fibers for structural composite applications. J Compos Mater 2009; 43(9): 1083–1108.

Ang

Lee

. Assessment of Pb uptake, translocation and immobilization in kenaf (Hibiscus cannabinus L.) for phytoremediation of sand tailings. J Environ Sci 2008; 20(11): 1341–1347.

Supri

Tan

Ismail

. Effect of poly(methyl methacrylate) modified water hyacinth fiber on properties of low density polyethylene/natural rubber/water hyacinth fiber composites. Polym Plast Technol Eng 2011; 50(9): 898–906.

10.

Mishra

Mohanty

Drzal

. Studies on mechanical performance of biofibre/glass reinforced polyester hybrid composites. Compos Sci Technol 2003; 63(10): 1377–1385.

11.

Ismail

Abdullah

Bakar

. Influence of acetylation on the tensile properties, water absorption and thermal stability of (high-density polyethylene)/(soya powder)/(kenaf core) composites. J Vinyl Additive Technol 2011; 17(2): 132–137.

12.

Bledzki

Mamun

Lucka-Gabor

. The effects of acetylation on properties of flax fibre and its polypropylene composites. Exp Polym Lett 2008; 2(6): 413–422.

13.

Mahmoud

. A novel photodiode made of hybrid organic/inorganic nanocomposite. J Phys D 2009; 42(15): 155502–155502.

14.

Ebrahim

. Fabrication of schottky diode based on Zn electrode and polyaniline doped with 2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt. J Polym Res 2009; 16(5): 481–487.

15.

Razak

SIA

Ahmad

Zein

SHS

. MnO₂-filled multiwalled carbon nanotube/polyaniline nanocomposites with enhanced interfacial interaction and electronic properties. Scripta Mater 2009; 61(6): 592–595.

16.

Valentova

Stejskal

. Mechanical properties of polyaniline. Synth Metals 2010; 160(7): 832–834.

17.

Zhu

Zhang

Zheng

. Stable, superhydrophobic, and conductive polyaniline/polystyrene films for corrosive environments. Adv Funct Mater 2006; 16(4): 568–574.

18.

Kim

Hobbs

Wang

. Biocompatible composites of polyaniline nanofibers and collagen. Synth Metals 2009; 159(13): 1313–1318.

19.

Jia

Wang

. Electrically conductive epoxy resin composites containing polyaniline with different morphologies. Mater Sci Eng A 2007; 448(1): 356–360.

20.

Jia

Tchoudakov

Segal

. Electrically conductive composites based on epoxy resin with polyaniline-DBSA fillers. Synth Metals 2003; 132(3): 269–278.

21.

Liu

Lee

. Electrical properties of well dispersed nanopolyaniline/epoxy hybrids prepared using an absorption-transferring process. J Phys Chem C 2008; 112(41): 15956–15960.

22.

Soares

Celestino

Magioli

. Synthesis of conductive adhesives based on epoxy resin and polyaniline-DBSA using the in situ polymerization and physical mixing procedures. Synth Metals 2010; 160(17): 1981–1986.

23.

Jang

Bae

Lee

. Synthesis and characterization of polyaniline nanorods as curing agent and nanofiller for epoxy matrix composite. Polymer 2005; 46(11): 3677–3684.

24.

Tsotra

Friedrich

. Thermal, mechanical, and electrical properties of epoxy resin/polyaniline-dodecylbenzenesulfonic acid blends. Synth Metals 2004; 143(2): 237–242.

25.

Tsotra

Friedrich

. Short carbon fiber reinforced epoxy resin/polyaniline blends: their electrical and mechanical properties. Compos Sci Technol 2004; 64(15): 2385–2391.

26.

Rodrigues

LPS

Silva

Aquino

EMF

. Effect of accelerated environmental aging on mechanical behavior of curaua/glass hybrid composite. J Compos Mater 2012; 46(17): 2055–2064.

27.

Razak SIA, Rahman WAWA, Hashim S, et al. Enhanced interfacial interaction and electronic properties of novel conducting kenaf/polyaniline biofibers. Polym Plast Technol Eng 2013; 52(1): 51–57.

28.

Razak SIA, Rahman WAWA and Yahya MY. Electrically conductive nanocomposites of epoxy/polyaniline nanowires doped with formic acid: effect of loading on the conduction and mechanical properties. NANO 2012; 7(5): 1250039.

29.

Razak

SIA

Zein

SHS

Ahmad

. MnO₂-filled multiwalled carbon nanotube/polyaniline nanocomposites: effect of loading on the conduction properties and its percolation threshold. NANO 2011; 6(1): 81–91.

30.

Popineau

Rondeau-Mouro

Sulpice-Gaillet

. Free/bound water absorption in an epoxy adhesive. Polymer 2005; 46(24): 10733–10740.

31.

Najafi

Tajvidi

Chaharmahli

. Long-term water uptake behavior of lignocellulosic-high density polyethylene composites. J Appl Polym Sci 2006; 102(4): 3907–3911.