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Apr 5, 2013 - Shahidul Islam Bhat , Sharif Ahmad ... Asha Embrandiri , Saima Sultana , Shaikh Ziauddin Ahammad , S. Wazed Ali , T.R. Sreekrishnan...
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Castor Oil Based Hyperbranched Poly(ester amide)/Polyaniline Nanofiber Nanocomposites as Antistatic Materials Sujata Pramanik,† Jayanta Hazarika,‡ Ashok Kumar,‡ and Niranjan Karak*,† †

Advanced Polymer and Nanomaterial Laboratory, Department of Chemical Sciences, and ‡Material Research Laboratory, Department of Physics, Tezpur University, Tezpur 784028, India S Supporting Information *

ABSTRACT: Biobased hyperbranched poly(ester amide) (HBPEA)/polyaniline (PAni) nanofiber nanocomposites were prepared by incorporating the as-synthesized PAni nanofiber at varied weight percentages by an ex situ polymerization technique. Fourier transform infrared spectroscopic analyses indicated the interactions of the benzenoid−quinoid moieties of PAni with HBPEA. The transition from liquidlike to solidlike behavior of the nanocomposites with a percolation threshold at 10 wt % nanofiber content was studied using dynamic rheology. The formation of nanofibrous network within the HBPEA matrix as vouched for by TEM study and initial degradation temperature (from 277 to 307 °C) was found to be increased with the increment of nanofiber content. The evaluation of mechanical properties such as tensile strength (7.2−12.25 MPa), elongation at break (88−70%), impact resistance (>100 cm), and scratch hardness (8.5−10 kg) together with the decrease in the sheet resistance (from 107 to 105 Ω/sq) forwarded the epoxy−poly(amido amine) cured nanocomposites as prospective antistatic materials.

1. INTRODUCTION The advent of nanotechnology has great implications in revolutionizing the arena of polymer nanocomposites. The synergistic features of both the polymer and the nanomaterial such as low cost, ease of preparation, and versatility in the structure and composition forge the design of an avant-garde genre of materials.1 Polymers are inherently insulating and thus cannot dissipate the static electric charge buildup on their surfaces which may otherwise lead to attraction of dusts and electrostatic hazards like electric shocks. The control of buildup of static electric charges has been driving industries over many years. The interest in antistatic materials combined with desirable properties such as acceptable rheological behavior, adequate mechanical strength, and high thermal stability may be an apt choice for their uses in various applications. The continuous search for antistatic materials has urged on the need to modify existing polymers to meet the requirements. The conventional antistatic additives such as carbon black,2 1-ntetradecyl-3-methylimidazolium bromide,3 carbon fibers,4 and metal particles5 suffer from limitations such as high cost, blushing problems, and blooming problems together with the moisture dependency, which in turn reduces the antistatic action during the active service lifetime.6 The inclusion of appropriate conductive materials is one of the potent techniques that augments the desired electrical properties of pristine polymers and reduces the sheet resistivities to the range 10 5−10 6 Ω/sq. 7 Tremendous research in the field of conducting polymers, one area of potent antistatic materials, led to the discovery of polyaniline. The polyaniline (PAni) nanofiber carves an alcove amidst the genre of π-conjugated polymers owing to its low cost, environmental stability, and ease of doping by protonation.8,9 Also, the reduction in size of conductive materials from macro to nano results in the abatement of the percolation threshold in terms of amount © 2013 American Chemical Society

together with the enhancement of other desired properties of pristine polymers.10 The inclusion of conductive nanomaterials in an insulating polymer imparts a precipitous drop in the electrical resistivity depending on the aspect ratio and distribution of the same. Moreover, among the various aspects of nanotechnology, nanofiber technology, wherein fibrous materials are fabricated at the nanoscale regime, is an emerging field of research interest. Thus the PAni nanofiber can serve as a conductive nanomaterial to impart the improvement in the dissipation of the electrostatic charges of the pristine polymeric matrix. Oviedo et al. showed that incorporation of 30 wt % PAni−organoclay nanohybrid into the EPDM rubber/PAni− organoclay nanohybrid nanocomposite resulted in the electrical resistivity of 105 Ω/cm.7 In the context of the above, the inclusion of PAni nanofiber into hyperbranched poly(ester amide) resin (HBPEA)an eminent group of polymers11 which has occupied an unique niche in the domain of advanced polymeric materials synergistically brings remarkable improvement in the material properties over those of the pristine polymer. It is pertinent to mention here that hyperbranched polymers have captivated copious interest in recent years owing to the presence of large numbers of functionality, three-dimensional unique structural architectures, low melt and solution viscosity, high reactivity, high compatibilizing ability with others, and so on over their linear analogues. All of these characteristics have profound influence on processing and the ultimate performance of such polymers.12 In an attempt to pursue the dictates of green chemistry, castor oil (Ricinus communis) has proved to be a Received: Revised: Accepted: Published: 5700

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sonication using an ultrasonic processor (acoustic power density 460 W/cm2) at 60% amplitude and 0.5 cycle for a time period of 15 min. The nanocomposites were cured by using bisphenol A based epoxy resin (60:40 weight ratio of resin to epoxy) and 50 wt % poly(amido amine) hardener (with respect to epoxy resin).15 The HBPEA/PAni nanofiber nanocomposites with varying weight percentages of 5, 7.5, 10, and 12.5 wt % nanofiber were coded as HBPEAP5, HBPEAP7.5, HBPEAP10, and HBPEAP12.5, respectively. 2.3. Characterization. Fourier transform infrared spectroscopy (FTIR) was used to record FTIR spectra of HBPEA and its nanocomposites with an Impact 410 (Nicolet, USA), using KBr pellets. The mixing of the PAni nanofiber in the HBPEA matrix was aided with a standard sonotrode (tip−diameter 3 mm) in a high intensity ultrasonic processor (UP200S, Hielscher, Germany). The dispersion and surface morphology of PAni nanofibers in the nanocomposites were investigated by using a transmission electron microscope (TEM; JEOL 21X) at the operating voltage of 200 kV. The thermal degradation study was carried out in a Shimazdu TGA 50 thermal analyzer at a nitrogen flow rate of 30 mL/min and heating rate of 10 °C/ min. X-ray diffraction study was performed with a Rigaku Miniflex diffractometer with Cu Kα radiation (λ = 1.5406 Å) in the range 2−70°. The mechanical properties such as tensile strength and elongation at break were measured by a universal testing machine (UTM) of Model Z010 (Zwick, Germany), with a 10 kN load cell and at a crosshead speed of 20 mm/min. The gloss, scratch hardness, and impact resistance of the cured films were measured as per the standard methods.16 The scratch hardness test (ASTM D5178/1991) of the thermosets was performed by a scratch hardness tester (Sheen Instrument Ltd., U.K.). The front impact resistance test was carried out by using an impact tester (S. C. Dey Co., Kolkata, India) with a maximum test height of 100 cm. The gloss of the thermosets was measured using a mini glossmeter (Sheen Instrument Ltd., U.K.) at an angle of incidence of 60°. The chemical resistance test of the cured thermosets was performed as per the ASTM D 543-67 procedure in different chemical media by taking weighted amounts of samples in 100 mL of the individual chemical medium for specified periods of time.16 The rheological behavior of HBPEA and its nanocomposites were studied using a rheometer, CVO100 (Malvern, U.K.), with a 20 mm diameter parallel plate. The sheet resistance of the thin films was measured by the standard four-probe technique with a linear probe configuration (M/S Osaw Industrial Products, India), and the reported results were averaged over a set of three independent measurements.

unique feedstock and a valuable input in the midst of different nonedible vegetable oils for the synthesis of the HBPEA because of its many advantages including the presence of a very high percentage (90−95 wt %) of ricinoleic acid.13 A perusal of the literature has shown that the preparation of biobased HBPEA/PAni nanofiber nanocomposites has never been attempted, which provoked us to initiate the present study. In this study, the authors wish to report the preparation and characterization of castor oil based HBPEA/PAni nanofiber nanocomposites. The authors also attempted to analyze the sheet resistance of the nanocomposites at different nanofiber loadings of 5, 7.5, 10, and 12.5 wt % and tried to corroborate the anisotropy in the surface morphology of the nanocomposites. Further, the mechanical properties, thermal behavior, and rheological behavior of the nanocomposites were also delved into.

2. MATERIALS AND METHODS 2.1. Materials. Aniline (Merck, India) was double-distilled under reduced pressure using zinc dust (S D Fine-Chem Ltd., India) and stored at 2−5 °C prior to use. Castor oil (Sigma Aldrich, India), diethanolamine (Merck, India), phthalic anhydride (Merck, India), and isophthalic acid (Sisco Research Laboratory Pvt. Ltd., India) were used after drying in vacuo at 50 °C overnight. Maleic anhydride (Merck, Germany) was used after drying in a vacuum oven at 30 °C. The bisphenol A based epoxy resin (BPA, Araldite LY 250) (epoxy equivalent 180− 190 g/equiv and density 1.16 g/cm3 at 25 °C) and poly(amido amine) hardener (HY 840) of Hindustan Ciba Geigy Ltd., Mumbai, were used as received. The solvents benzene, tetrahydrofuran (THF), and dimethylacetamide (DMAc) were distilled before use. Ammonium peroxydisulfate and hydrochloric acid (HCl) were purchased from Merck, India, and used as received. All other chemicals were of analytical grade. 2.2. Ex Situ Preparation of Castor Oil Based HBPEA/ PAni Nanofiber Nanocomposites. 2.2.1. Preparation of PAni Nanofiber. The polyaniline nanofiber doped with HCl was prepared following the interfacial polymerization technique as reported by Huang et al.14 This PAni nanofiber was washed with double-distilled water followed by methanol several times. The nanofiber was dispersed in THF (1 g/mL) prior to its incorporation into the polymer matrix. 2.2.2. Preparation of Castor Oil Based HBPEA/PAni Nanofiber Nanocomposites. The castor oil was transesterified with methanol to produce methyl ester, which was then reacted with diethanolamine in the presence of sodium methoxide to obtain the fatty amide of the oil.15 The details of the synthetic protocol of the HBPEA using the A2 + B′B2 approach have been described elsewhere.12 Briefly, the fatty amide of the oil, maleic anhydride, phthalic anhydride, and isophthalic acid served as the A2 monomers and diethanolamine served as the B′B 2 monomer for the preparation of HBPEA. The condensation reaction between A2 and B′B2 monomers in the presence of 0.8 wt % sodium methoxide, as catalyst, was carried out at 150 °C for 30 min followed by heating at 185 °C for another 1.5 h and finally at 220−225 °C for 20−25 min with continuous mechanical stirring under a nitrogen atmosphere. The dispersed PAni nanofiber in THF was added (5, 7.5, 10, and 12.5 wt %, separately) in the preformed HBPEA at 60−65 °C with constant stirring. The weight ratio of PAni nanofiber to HBPEA to THF was maintained at 5−12.5:100:4. The reaction mixture was continuously stirred for 30 min followed by

3. RESULTS AND DISCUSSION 3.1. Ex Situ Preparation of HBPEA/PAni Nanofiber Nanocomposites. The HBPEA/PAni nanofiber nanocomposites were prepared by an ex situ polymerization technique, and the protocol is shown in Scheme 1. The nanoscale dispersion of the PAni nanofiber in the HBPEA matrix led to the effective interfacial interactions of the nanofiber with the polymer matrix, which subsequently confined the nanofiber in between the polymer chains. Thus the interfacial interaction of PAni nanofiber with the HBPEA matrix helps in the improvement of the mechanical, thermal, and antistatic properties over those of the pristine polymer. 3.2. FTIR Analysis. The FTIR spectra of HBPEA, PAni nanofiber, HBPEAP5, HBPEAP7.5, HBPEAP10, and HBPEAP12.5 are shown in Figure 1. The bands of HBPEA 5701

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1498 and 1569 cm−1, respectively (Figure 1f).17 These characteristic bands of PAni nanofiber shifted to 1460 and 1556 cm−1 in the nanocomposites (Figure 1b−e), which is attributed to the π−π interactions of the same with the aromatic moieties of the pristine HBPEA. It was observed that the absorbance band of the carbonyl group of the ester moiety shifted from 1730 cm−1 to lower wavenumbers of ∼1725 cm−1 in the nanocomposites, which is due to the interaction between the carbonyl group with the nitrogenous groups of PAni. The shift of the amide band from 1632 to 1637 cm−1 may be attributed to the restriction of the vibrational motion of the carbonyl amide bond of HBPEA due to hydrogen bonding with the −NH group of PAni nanofiber. 3.3. TEM Study. Figure 2 shows the dispersion scenario of PAni nanofiber in HBPEA matrix. The HBPEA thermoset (Figure 2a) exhibited a smooth surface morphology. The prepared PAni exhibited a nanofibrillar morphology with an average diameter and length of around 30−35 nm and 300 nm, respectively (Figure S1 in the Supporting Information). An increment in the connectivity between the nanofibers within the polymer matrix was observed with the increase of PAni nanofiber content in the nanocomposites. The continuous formation of nanofibrous network was more prominent in HBPEAP10 (Figure 2d) and HBPEAP12.5 (Figure 2e), while HBPEAP12.5 showed the occurrence of agglomeration of the nanofiber in the polymer matrix. Fiji software was used to infer the preferred orientation of PAni nanofiber in the polymer

Scheme 1. Preparative Protocol of Biobased HBPEA/PAni Nanofiber Nanocomposites

corresponding to −CO appeared at 1730 cm−1, the amide group appeared at 1632 cm−1, and hydrogen bonded −OH stretching appeared at 3422 cm−1 (Figure 1a).12 The bands for benzenoid and quinoid rings of PAni nanofiber were assigned at

Figure 1. FTIR spectra of (a) HBPEA, (b) HBPEAP5, (c) HBPEAP7.5, (d) HBPEAP10, (e) HBPEAP12.5, and (f) PAni nanofiber. 5702

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The variation of the storage modulus (G′) and loss modulus (G″) of the HBPEA/PAni nanofiber nanocomposites (Figure 3a) with the applied frequency revealed that both G′ and G″

Figure 2. TEM micrographs of (a) HBPEA, (b) HBPEAP5, (c) HBPEAP7.5, (d) HBPEAP10, and (e) HBPEAP12.5. (f) Histogram showing preferred orientation of PAni nanofibers in the HBPEA matrix.

Figure 3. Variation of (a) G′ and (b) G″ with frequency for HBPEA, HBPEAP5, HBPEAP7.5, HBPEAP10, and HBPEAP12.5.

matrix. The software computes a histogram which indicates the amount of nanofiber in a given direction in HBPEAP10. The highest peak of the histogram was fitted by a Gaussian function, taking into account its periodic nature. From this computational analysis it can be inferred that the nanocomposite with completely isotropic nanofiber content is expected to give a flat histogram, whereas the one with a preferred orientation is expected to give a histogram with a peak at that orientation. Figure 2f shows that a proportion of about 25% of the PAni nanofibers in the HBPEA matrix have a preferred orientation at an angle of 56°, with a few percent around −30°. The increasing connectivity between the PAni nanofibers in the polymer improved the charge transport, which is evident from the antistatic property study. 3.4. Rheological Behavior. The dynamic modulus reflects the elastic and viscous character of the pristine polymer and nanocomposites which is characterized by the storage modulus (G′) and the loss modulus (G″), respectively. The viscoelastic properties of the pristine polymer and nanocomposites were explored by the frequency sweep experiment. The storage modulus (G′) and loss modulus (G″) of the pristine polymer and nanocomposites were measured as a function of frequency from 1 to 10 s−1 at 25 °C under a constant stress of 20 Pa.

values increased with the increase of frequency and PAni nanofiber loading. It is pertinent to mention that G′ was higher in magnitude compared to G″ over the whole frequency region owing to the enhancement of elastic behavior of the nanocomposites on incorporation of the nanomaterial and a strong dependence of G′ on interfacial energy compared to G′. The transition from viscous liquidlike behavior, G″ > G′, of the pristine resin dominated by a polymer−polymer entangled structure, to solidlike elastic behavior, G″ < G′, in the case of the nanocomposites dictated by the amalgamation of polymer− polymer, nanofiber−nanofiber, and polymer−nanofiber interactions, indicated the formation of a continuous nanofibrous network within the polymer matrix.18 The interactions between the nanofibers became prominent with nanofiber loading, which eventually led to the formation of continuous interconnected structures within the polymer matrix. The abrupt increment in the G′ and G″ values with frequency became pronounced when the content of PAni nanofiber increased from 7.5 to 10 wt %. This may be attributed to the percolation threshold, that is, the formation of continuous PAni nanofibrous network within the polymeric matrix as evident 5703

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from the TEM images. However, the G′ and G″ values decreased beyond 10 wt %, which is due to agglomeration of the PAni nanofiber in the HBPEA matrix. In other words, HBPEAP12.5 exhibited poor dispersion of the nanofiber (as evident from the TEM study) along with the formation of discrete nanofiber-rich domains. The oscillatory experiments were carried out at an oscillatory stress of 20 Pa. The time sweeps of storage and loss modulii of the pristine resin and the nanocomposites showed a Newtonian-type behavior over a period of time from 25 to 100 s under this stress value (Figure 4).

Figure 5. Variation of (a) G′ and (b) G″ with temperature for HBPEA, HBPEAP5, HBPEAP7.5, HBPEAP10, and HBPEAP12.5.

3.5. Performance Study. The performance characteristics of the epoxy−poly(amido amine) cured HBPEA/PAni nanofiber thermoset nanocomposites effectively changed with the incorporation of varying amounts of PAni nanofiber (Table 1). The epoxy−poly(amido amine) cured polymer and its nanocomposites were baked at 150 °C for specified period of time. It was found that the curing time decreased with the increase of the nanofiber content. This is attributed to the basic N-atom of PAni which acts as base and aids in the cross-linking reaction between HBPEA, epoxy, and poly(amido amine) hardener. The increase in gloss with the increase of PAni nanofiber content in the nanocomposites indicated that the cured thermosets possessed good dimensional stability together with a smooth surface texture. The increment of scratch hardness of the nanocomposite thermosets with nanofiber content is due to the enhanced synergism of strength and flexibility of the polymeric chains. The high impact resistance of the thermosets reflected optimum cross-linking together with flexibility of the long hydrocarbon fatty amide chains. The inclusion of the nanofiber into the HBPEA matrix resulted in the improvement the mechanical properties due to nanoreinforcing effect. The mechanical property of a nanomaterial-reinforced polymer depends on several parameters such as distribution and orientation, aspect ratio, domain size, shape, and degree of compatibility of the nanomaterial with the polymer matrix.20 The efficiency of transferring stress between

Figure 4. Variation of (a) G′ and (b) G″ with time for HBPEA, HBPEAP5, HBPEAP7.5, HBPEAP10, and HBPEAP12.5.

Figure 5 shows the variation of G′ and G″ for the dynamic temperature sweep experiment under isochronal condition (1 Hz) from 25 to 75 °C at a controlled oscillatory stress of 20 Pa. The decrease in the values of G′ and G″ with temperature is attributed to the increase of kinetic energy and free volume of the polymer chains with temperature which in turn decreases the entanglement density and inter- and intramolecular interactions within the polymer chains. The alignment of the PAni nanofibers along the preferred direction (parallel alignment, as evident from the XRD study) in the polymer matrix (prominent in the case of HBPEAP10 because of the formation of interconnected nanofibrous structure) along the direction of shear stress with temperature also aided in the decrease in the values of G′ and G″.19 5704

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Table 1. Coating Performances of HBPEA, HBPEAP5, HBPEAP7.5, HBPEAP10, and HBPEAP12.5 Thermosets

a

physicomechanical property

HBPEA

HBPEAP5

HBPEAP7.5

HBPEAP10

HBPEAP12.5

curing time (h) gel fraction (%) scratch hardness (kg) impact resistancea (cm) gloss at 60° tensile strength (MPa) elongation at break (%) crystallinity (%)

10 ± 0.02 77 ± 0.9 8.5 ± 0.2 100 90 ± 0.5 7.2 ± 0.3 88.12 ± 0.7 −

3.5 ± 0.02 79 ± 0.7 9 ± 0.3 100 93 ± 0.6 7.89 ± 0.5 84.73 ± 0.9 16.34

3.2 ± 0.01 79.8 ± 0.7 9.5 ± 0.2 100 93.5 ± 0.5 8.4 ± 0.5 79.52 ± 0.8 19.47

2.85 ± 0.01 81 ± 0.5 10 100 95 ± 0.4 10.5 ± 0.4 74.23 ± 0.8 25.65

2.6 ± 0.02 81.2 ± 0.6 10 100 95.2 ± 0.4 12.25 ± 0.5 70.29 ± 0.9 26.74

Maximum limit of the instrument is 100 cm.

Table 2. Chemical Resistances of HBPEA, HBPEAP5, HBPEAP7.5, HBPEAP10, and HBPEAP12.5 Thermosets weight change (g) medium distilled water 5% aq HCl 10% aq NaCl 3% aq NaOH

HBPEA 2.3 3.2 1.3 1.2

× × × ×

10−2 10−2 10−2 10−2

± ± ± ±

HBPEAP5 0.09 0.05 0.08 0.07

2.1 2.6 1.0 1.2

× × × ×

10−2 10−2 10−2 10−2

± ± ± ±

HBPEAP7.5

0.07 0.05 0.06 0.05

2.1 2.4 1.1 1.1

the PAni nanofiber and HBPEA matrix is the prominent factor in improving the mechanical properties of the nanocomposites. The pristine HBPEA has a tensile strength of 7.2 MPa, while it increases from 7.89 to 10.5 MPa with the increase of the PAni incorporation into the polymer matrix from 5 to 10 wt %. However, the increment in the tensile strength even at 12.5 wt % was not so significant, which may be due to the agglomeration of the nanofibers (as evident from the TEM studies). The improvement in the mechanical properties of the nanocomposites compared to the pristine polymer is attributed to the interfacial bonding between the resin and the PAni nanofiber. The decrease of elongation at break (EB) of the thermosets with the nanofiber content is due to the decrease in molecular mobility of the polymer chains (Table 1). The good chemical resistance of the nanocomposite thermosets, particularly the alkali resistance, is attributed to the presence of amide moiety and PAni nanofiber which aid in adequate amounts of cross-linking in the thermosets (Table 2). 3.6. XRD Study. The XRD patterns of HBPEA, PAni nanofiber, HBPEAP5, HBPEA7.5, HBPEA10, and HBPEA12.5 thermosets are shown in Figure 6. The characteristic diffraction peaks of PAni nanofiber appear around 2θ = 19.2 and 26°, attributed to the (100) and (110) planes.17 These two diffraction peaks are typical of HCl-doped PAni, and are assigned to the regular spacing between phenyl rings of adjacent chains of PAni in a parallel orientation and perpendicular orientation, respectively.17 The XRD of pristine HBPEA exhibited a broad peak, which is attributed to its amorphous structure. Upon formation of nanocomposites using PAni nanofiber, an intense broad diffraction peak appeared around 19−20°. This diffraction peak of the nanocomposites was attributed to the presence or intercalation of PAni nanofiber in between the HBPEA chains. An increase in the degree of crystallinity with the increase of PAni nanofiber content is evident from Table 1. Origin 6.1 software was used to calculate the total area under the diffractogram by intregating the area from 2 to 70° of 2θ values. The intense broad diffraction peak at the 2θ value around 19− 20° of the nanocomposites was again calculated from the area under the peak using the same software by Gaussian fitting. The degree of crystallinity (k) or range of order in the HBPEA/

× × × ×

10−2 10−3 10−2 10−3

± ± ± ±

0.07 0.04 0.06 0.06

HBPEAP10 1.5 2.3 1.1 0.7

× × × ×

10−3 10−3 10−2 10−3

± ± ± ±

0.08 0.02 0.05 0.06

HBPEAP12.5 1.4 2.3 1.2 0.7

× × × ×

10−2 10−3 10−2 10−3

± ± ± ±

0.06 0.04 0.06 0.06

Figure 6. XRD diffractograms of (a) HBPEA, (b) HBPEAP5, (c) HBPEAP7.5, (d) HBPEAP10, (e) HBPEAP12.5, and (f) PAni nanofiber.

PAni nanofiber nanocomposites was thus calculated21 using the following equation: k=

area under diffraction peak × 100 (%) total area under the diffractogram

(1)

Thus, when the value of k is greater, the electrical resistance should be less as it helps to form the nanofibrous network. This is because the PAni nanofiber incorporated into the HBPEA matrix acts as a coherent electrical carrier in between polymer chains, which is also evident from the sheet resistance study. 3.7. Thermal Study. It is evident from Figure 7 that incorporation of PAni nanofiber into the HBPEA matrix resulted in the dose-dependent increment in the thermal stability of the nanocomposites. PAni nanofiber lost the adsorbed moisture around 100 °C, and the degradation of the PAni chains occurs above 390 °C.22 The thermograms of HBPEA and the HBPEA/PAni nanofiber thermoset nano5705

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nanofiber, a noticeable decrement of resistivity of approximately 2 orders of magnitude (7.37 × 105 Ω/sq) relative to the pristine polymer (2.94 × 107 Ω/sq) occurred, indicating formation of the percolation threshold. A further increase in the concentration of PAni nanofiber (12.5 wt %) in the polymer matrix did not significantly augment the decrease in the sheet resistance (9.21 × 105 Ω/sq) of the nanocomposite. Thus, the electrical percolation threshold at 10 wt % indicated the formation of conductive nanofibrous networks in the polymer matrix. The electrical conduction in these nanocomposites can be described by the formation of a conductive network of PAni nanofiber within the HBPEA matrix.4 Consequently, a decrease in the electrical resistivity with increasing content of PAni nanofiber is observed. Also the interaction between the polymer matrix and the PAni nanofiber is influential in effecting the percolation threshold of the nanocomposites. Mamunya et al. reported that higher interaction of the polymer with the nanomaterial means that the wettability and dispersion of the nanomaterial will be higher. Consequently, a higher percolation threshold results due to lowering in the clustering of the nanomaterial, which is required for conductivity.23

Figure 7. TGA thermograms of HBPEA, PAni nanofiber, HBPEAP5, HBPEAP7.5, HBPEAP10, and HBPEAP12.5.

composites exhibited a two-step degradation pattern with the increment in the thermal stability on inclusion of the PAni nanofiber. While the pristine HBPEA is stable up to 277 °C,12 the thermal stability increased to 307 °C in 12.5 wt % nanofiber containing nanocomposites. The enhancement of the thermal stability of the nanocomposites over the pristine HBPEA is attributed to the presence of PAni nanofiber which acts as a physical cross-linker improving the interfacial interaction between the PAni nanofiber and the polymer matrix and thereby obstructing the movement of the polymer chains which in turn increased the thermal stability. 3.8. Antistatic Property of the Nanocomposites. The variations of sheet resistance of the pristine and nanocomposite systems with PAni nanofiber content are shown in Figure 8.

4. CONCLUSIONS PAni nanofiber complemented by the templating facet of a hyperbranched framework has a profound influence on the mechanical, thermal, and antistatic properties of nanocomposites. A FTIR study showed the interaction of PAni nanofiber with HBPEA matrix. A TEM study indicated the formation of a nanofibrous network with the increase of PAni nanofiber content in the polymer matrix, and the occurrence of agglomeration above 10 wt % loading of the nanofiber. A dynamic rheological behavior study showed the transition from viscous liquidlike behavior of the pristine resin to solidlike elastic behavior upon the formation of the nanocomposites. The increase in connectivity between the conducting PAni nanofiber in the HBPEA matrix resulted in a decrease in the sheet resistance values. The noticeable rheological behavior, adequate mechanical strength, high thermal stability, and desirable sheet resistance values put forward the epoxy cured thermosets as potential biobased antistatic materials.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1, TEM micrograph of PAni nanofiber. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91 3712-267009 or +913712-267009. Fax: 91-3712-267006. Figure 8. Variation of sheet resistance with different weight percentages of PAni nanofiber content in the HBPEA matrix.

Notes

The incorporation of the PAni nanofiber with varying weight percentages from 5 to 12.5 led to the formation of electrically conductive networks (as evident from the magnitude of sheet resistances of the nanocomposites). The PAni nanofiber has a sheet resistance of the order of 104, while the inclusion of 5 and 7.5 wt % PAni nanofiber in the HBPEA matrix caused a change of approximately 1 order of magnitude (106) over the pristine thermoset (107). This decrease in the electrical resistivity of the nanocomposites is not large, but at 10 wt % loading of PAni

ACKNOWLEDGMENTS The authors express their gratitude and thanks to the research project assistance given by DRL, India, through Grant DRL/ 1047/TC, dated March 2, 2011, SAP (UGC), India, through Grant F.3-30/2009(SAP-II), and FIST program-2009 (DST), India, through Grant SR/FST/CSI-203/209/1 dated May 6, 2010. The authors are thankful to Mr. Rocktotpal Konwarh for his valuable help in the computational analysis using Fiji software in the TEM study.

The authors declare no competing financial interest.

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liquid-liquid interfacial polymerization method. Chem. Chem. Technol. 2011, 5, 55. (23) Mamunya, E. P.; Davidenko, V. V.; Lebedev, E. V. Effect of polymer-filler interface interactions on percolation conductivity of thermoplastics filled with carbon black. Compos. Interfaces 1997, 4, 169.

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dx.doi.org/10.1021/ie4002603 | Ind. Eng. Chem. Res. 2013, 52, 5700−5707