Materials Research Bulletin 47 (2012) 2336–2343

Polymer/organosilica nanocomposites based on polyimide with benzimidazolelinkages and reactive organoclay containing isoleucine amino acid: Synthesis,characterization and morphology properties

Shadpour Mallakpour a,b,*, Mohammad Dinari a

a Organic Polymer Chemistry Research Laboratory, Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iranb Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan 84156-83111, Islamic Republic of Iran

A R T I C L E I N F O

Article history:

Received 1 September 2011

Received in revised form 1 May 2012

Accepted 21 May 2012

Available online 30 May 2012

Keywords:

A. Composites

A. Nanostructures

B. Chemical synthesis

C. Electron microscopy

C. Thermogravimetric analysis (TGA)

A B S T R A C T

Polyimide–silica nanocomposites are attractive hybrid architectures that possess excellent mechanical,

thermal and chemical properties. But, the dispersion of inorganic domains in the polymer matrix and the

compatibility between the organic and inorganic phases are critical factors in these hybrid systems. In

this investigation, a reactive organoclay was prepared via ion exchange reaction between protonated

form of difunctional L-isoleucine amino acid as a swelling agent and Cloisite Na+ montmorillonite. Amine

functional groups of this swelling agent formed an ionic bond with the negatively charged silicates,

whereas the remaining acid functional groups were available for further interaction with polymer

chains. Then organo-soluble polyimide (PI) have been successfully synthesized from the reaction of 2-

(3,5-diaminophenyl)-benzimidazole and pyromellitic dianhydride in N,N-dimethylacetamide. Finally,

PI/organoclay nanocomposite films enclosing 1%, 3%, 5%, 7% and 10% of synthesized organoclay were

successfully prepared by an in situ polymerization reaction through thermal imidization. The

synthesized hybrid materials were subsequently characterized by Fourier transform infrared

spectroscopy, X-ray diffraction, electron microscopy, and thermogravimetric analysis techniques. The

PI/organoclay nanocomposite films have good optical transparencies and the mechanical properties

were substantially improved by the incorporation of the reactive organoclay.

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jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Rising claim for higher performance devices with smaller size,lighter weight, and better quality, make polyimide (PI)s as a uniquesubstance in response to technological applications in a variety offields such as aerospace, automobile and microelectronic [1,2].Among a large number of PIs which have been so far studied fromboth academic and industrial viewpoints, aromatic PIs are the mostreliable heat-resistant polymers, which have combined excellentproperties, i.e., high glass transition temperatures (Tg), highresistance to chemicals and radiations, relatively low dielectricconstants, and good mechanical properties [3–5]. Beyond exceptedadvantages of such materials, PIs have limitations in processabilityand solubility that caused recently significant synthetic effortsthrough the synthesis of new diamine or dianhydride monomers.Several approaches for the preparation of soluble PIs such as the

* Corresponding author at: Organic Polymer Chemistry Research Laboratory,

Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111,

Islamic Republic of Iran. Tel.: +98 311 391 3267; fax: +98 311 391 2350.

E-mail addresses: mallak@cc.iut.ac.ir, mallak777@yahoo.com,

mallakpour84@alumni.ufl.edu (S. Mallakpour).

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.05.033

incorporating of flexible bridging linkages, bulky substituents,non-coplanar conformation units, as well as pendent groups intopolymer backbone directed to significant achievement [6]. Theincorporation of bulky pendent groups produces a chain separationeffect lowering the chain packing, which increases wateraccessibility and improves solubility. Some bulky groups, suchas imidazole or benzimidazole, can form strong hydrogen bondswith water. Consequently, polymers modified with benzimidazolegroups have shown enhanced solubility, better hydrophilicity, andhigher Tgs than unmodified polymers [7–9].

On the other hand, further improvement of the PI propertiessuch as higher thermal stability and lower dielectric constant bythe introduction of different nanoparticles makes these materials agood choice for preparation of hybrid materials to development ofnext generation resources which act in response to the demand ofhighly very tiny electronic devices, super conductive materials, andmodern spacecrafts [10–12]. Various combinations of PIs withinorganic fillers including silica, a layered silicate such asmontmorillonite or mica, aluminum nitride, titania, bariumtitanate and carbon nano-tubes have been reported. The preparedhybrid materials show outstanding thermal stability, mechanicalstrength and physical properties [13,14].

S. Mallakpour, M. Dinari / Materials Research Bulletin 47 (2012) 2336–2343 2337

Recently PIs–silica hybrid materials have received considerableattention, because they often exhibit superior physical, mechanicaland thermal properties to conventional mineral-filled compositesor unfilled polymers. These performance improvements such asthermal stability, mechanical strength, molecular barrier, andflame resistance depend greatly on the distribution, arrangementof organoclay layered silicate and synergism between the layeredsilicate and the polymer [15–20]. In order to avoid the phaseseparation in the resulting hybrid materials and achieve a highlyhomogenous in the matrix, one approach is to control the chainlength of polymer segments, and the second one is the selection ofsuitable coupling agents and/or the use of an effective compati-bilizer [21,22]. The interaction of organoclay layered silicates withpolymers leads to intercalated or exfoliated nanocomposites (NCs).In intercalated NCs, polymer chains are introducing between thesilicate layers, generating ordered lamella with an interlayerdistance of a few nanometers. In the second class silicate layers ofabout 1 nm thick are exfoliated and dispersed in the polymermatrix [23,24].

Naturally occurring montmorillonite (MMT) exists as anioni-cally charged layers of magnesium/aluminum silicates and smallcations such as sodium or potassium located on the surface of eachplatelet which balances the negative charge of layers. In addition,MMT is hydrophilic because of the presence of strong electrostaticforces between the layers and the ability to form hydrogen bondswith water, so unmodified clays generally disperse poorly inorganic matrices. Thus, before preparation of polymer/MMT NCs,modification is generally required through ion exchange reactionbetween organic cations and inorganic cations to render hydro-philic MMT more organophilic to be compatible with polymermolecules [16,25–27]. The ease of dispersion depends on a numberof factors, including the nature of the organic cation and thedensity of cationic sites on the platelets’ surfaces. This modificationalso increase interlayer spacing of MMT, which expandedinterlayer spacing allow polymer molecules to enter and subse-quently to intercalate the silicate layers [28–30].

In this study, a series of organosoluble PI–organoclay NCmaterials are successfully prepared by an in situ polymerizationreaction through thermal imidization. For this purpose, incorpo-ration of hetroaromatic benzimidazol units into the polymerbackbone was designed to improve solubility while maintainthermal stability of polymer. For preparation of the NCs based onsynthesized PI, naturally L-isoleucine amino acid was used as aswelling agent that contains two amine and acid functional groups.After converting amine functional group of the swelling agent intoa cation (–NH3

+), this cation formed an ionic bond with thenegatively charged silicate layers of MMT. The other functionalgroup (–COOH) of the swelling agent can react with polymerschains. The physical and mechanical properties of the resultingNCs were done by various equipments.

2. Experimental

2.1. Materials

Solvents and chemicals were obtained from the AldrichChemical Co. (Milwaukee, WI, USA), Riedel-deHaen AG (Seelze,Germany), and Merck Chemical Co. (Germany). Pyromelliticdianhydride (PMDA), L-isoleucine and hydrochloric acid (HCl)were purchased from Merck Co. and was used without furtherpurification. N,N0-Dimethyformamide (DMF) and N,N-dimethyla-cetamide (DMAc) were dried over barium oxide and then weredistilled under reduced pressure. 3,5-Dinitrobenzoyl chloride, 1,2-phenylenediamine, hydrazine monohydrate, phosphorus pentox-ide (P2O5), and methanesulfonic acid (MSA) were obtained fromcommercial sources and used as-received. Cloisite Na+ was

purchased from Southern Clay Products, Gonzales, Texas (USA).The cation exchange capacity (CEC) of Cloisite Na+ is 92.6 mequiv./100 g as reported by suppliers. This compound was used withoutany further purification.

2.2. Characterization techniques

Fourier transform infrared spectroscopy (FT-IR) spectra of thehybrid films were recorded with a Jasco-680 (Japan) spectrometerat a resolution of 4 cm�1 and they were scanned at wavenumberrange of 400–4000 cm�1. Vibration bands were reported aswavenumber (cm�1). Thin films of NCs were made by evaporatingsolvent at 80 8C and used for FT-IR analysis. FT-IR spectra of CloisiteNa+, organoclay and PI were also collected by making their pelletsin KBr as a medium. The band intensities are assigned as weak (w),medium (m), shoulder (sh), strong (s), and broad (br).

The intercalation of polymers into the galleries of organoclaywas confirmed via X-ray diffraction (XRD) microscopy. The XRDanalyses were performed via X-ray diffractometer (Bruker, D8Advance) with Cu Ka characteristic radiation (wavelengthk = 0.154 nm at 45 kV, 100 mA, and with a step size of 0.028 inthe range of 2u = 1.2–108. XRD analysis was carried out to measurethe change in interlayer spacing of clay based on Bragg’s law.

Proton nuclear magnetic resonance (1H NMR) spectra wererecorded on Bruker Avance 400 MHz spectrometer operatingpolymer solution in dimethylsulfoxide (DMSO-d6). The protonresonances were designated as singlet (s) and multiplet (m).

Inherent viscosities were measured by a standard procedureusing a Cannon-Fenske routine viscometer (Germany) at theconcentration of 0.5 g/dl at 25 8C.

Thermogravimetry analysis (TGA) is performed with a STA503win TA at a heating rate of 10 8C/min from 25 8C to 800 8C undernitrogen atmosphere.

The dispersion morphology of the nanoparticles on PI matrixwas observed using field emission scanning electron microscopy(FE-SEM) [HITACHI; S-4160]. Transmission electron microscopy(TEM) images were obtained using a Philips CM 120 microscopewith an accelerating voltage of 100 kV.

UV–vis absorption of neat PI and PI/organoclay NCs wasmeasured by UV-Vis spectrometer, JASCO V-750 in the spectralrange between 200 and 800 nm.

Tensile properties of the NC films were measured according toDIN procedure 53455 having a crosshead speed of 5 mm min�1

using Zwick 1446-60.

2.3. Organic modification of MMT

Accurately weighed 2 g of 325-mesh (<120 mm) screened finepowdered Cloisite Na+ MMT with cation exchange capacity of92.6 mequiv./100 g was slowly and carefully dissolved in theprotonated L-isoleucine amino acid solution, which was preparedby dispersion of L-isoleucine amino acid with stoichiometricamount of concentrated HCl in 100 ml of deionized water andheating at 80 8C for 3 h. This reaction mixture was further heatedand maintained at 60 8C with vigorous agitation for 6 h. The masswas cooled down to 40 8C and vacuum filtered. The solid wet cakewas washed with deionized water in a large beaker with rapidstirring for 1 h and then it was dried in a vacuum oven at 80 8C for24 h [31].

2.4. Synthesis of 2-(3,5-diaminophenyl)-benzimidazole

At first 2-(3,5-dinitrophenyl)-benzimidazole 3 was preparedfrom 3,5-dinitrobenzoyl chloride and 1,2-phenylenediamine usingMSA and P2O5 to yield 70% of this compound. Than 2-(3,5-diaminophenyl)-benzimidazole (DABI) was prepared by reduction

Fig. 1. Flow diagram for preparation of the PI/organoclay hybrid NC films.

S. Mallakpour, M. Dinari / Materials Research Bulletin 47 (2012) 2336–23432338

of dinitro precursor using palladium on activated carbon (Pd/C)and hydrazine monohydrate in refluxed ethanol (yield: 77%; m.p.242–243 8C) [7,8,32].

2.5. Synthesis of reference PAAs and PI

Into a 100-ml three-neck round-bottom flask equipped with amechanical stirrer, nitrogen inlet, and drying tube containingcalcium chloride were placed DABI (2.00 g, 8.93 mmol) and DMAc(10 ml). The solution was stirred until the diamine 4 completelydissolved. Then PMDA with the same molar ratio of diamine wasadded into the solution in four times within 1 h. The viscosityincreased quickly over 2 h. The mixture was stirred under nitrogenat room temperature (R.T.) for another 8 h. The resulting yellow-brown poly(amic acid) (PAA) solution was clear and viscous. Thesolution was subsequently used to prepare thin films forcharacterization. The inherent viscosity of the synthesized PAAwas 0.57 dl/g and the yield was 85%.

FT-IR (KBr, cm�1): 2500–3500 (m, br), 1662 (m), 1554 (s), 1486(w), 1422 (m), 1240 (m), 1072 (br), 1013 (w), 864 (m), 800 (w), 764(m), 650 (w), 618 (m), and 560 (w).

The neat PI film as reference was prepared by casting the PAAsolution onto a glass plate. After the film was dried at R.T. for 2 h, itwas heated at 50, 80, 120, 200 and 250 8C for 1 h each, and at 300 8Cfor 2 h, to obtain yellow colored transparent films.

FT-IR (KBr, cm�1): 3378 (m), 1776 (m), 1723 (s), 1600 (m), 1534(w), 1457 (m), 1440 (m), 1376 (s), 1320 (s), 1276 (w), 1228 (w),1097 (s), 1010 (m), 936 (w), 830 (s), 742 (m), 722 (s), 677 (m), 618(m), and 565 (w).

2.6. In situ polymerization of PI/organoclay NCs by thermal

imidization

PAA/organoclay mixture and PI/organoclay NCs were preparedby in situ polymerization, as shown by the flow charts in Fig. 1. Anappropriate amount of organophilic clay was introduced into 3 g ofDMAc under magnetically stirring for 24 h at R.T. (solution A),1 mmol of diamine DABI was introduced into 1.5 g of DMAc undermagnetically stirring for 10 min at R.T. (solution B). And solution Bwas introduced into solution A by stirring 24 h at R.T., 1 mmol ofPMDA was added into 1.5 g of DMAc under magnetically stirring atR.T. (solution C). After that, solution C was added into solutionsA + B by stirring for 24 h at R.T. The resulting yellowish PAA/organoclay solution was viscous and clear.

Imidization of PAA/organoclay for the synthesis of PI/organo-clay hybrides was carried out by putting the samples in an air-circulation oven for 2 h at R.T. and then at 100, 150, 200, and 250 8Cfor 1 h, respectively, and then at 300 8C for 2 h. Then the yellowcolored transparent samples were removed from the plate glassand characterized with different techniques.

3. Results and discussion

3.1. Preparation of organoclay

The organoclay chosen for dispersion in the PI needed enoughthermal stability to withstand the high temperature used to curethe PI. This requirement led to the selection of thermally stablemolecules for surface modification of the clay. In this study,positively charged of L-isoleucine amino acid swelling agent ischosen to replace the chemically synthesized surfactant formodification of Cloisite Na+. These phenomena causes enlarge inthe d-spacing which facilitates the entry of the host polymermolecules into the organoclay gallery and they are morehydrophobic than Cloisite Na+ for further applications.

3.2. Preparation of benzimidazole diamine

DABI 4 was prepared with a two-step procedure outlined inScheme 1. In the first step, the dinitro 3 was obtained by the directcondensation of 1,2-phenylenediamine and 3,5-dinitrobenzoylchloride with a mixture of P2O5/MSA as the convenient dehydrat-ing agent and solvent [33]. In the second step, nitro groups werereduced to the corresponding amino groups with hydrazinehydrate as the reducing agent and palladium on activated carbonas the catalyst. By this procedure, pure DABI was attained afterrecrystallization [32].

3.3. Preparation of the PAI/organoclay NC films

The extent of improvement in the NC properties depended onthe state of dispersion of the silicate layers of the organoclay. Apartfrom the chemical structure and the length of the organic moiety inthe organoclay, the processing conditions determined the state ofdispersion. Several approaches were examined by Delozier et al. forPI-clay NCs [25]. According to their findings, the best results wereobtained using an in situ polymerization technique, wherein PAAwas synthesized in the presence of organoclay, which was laterthermally imidized using a solution-casting technique. We usedthis technique to prepare different NC film samples (Scheme 1).

Scheme 1. Preparation of benzimidazole containing aromatic diamine 4 and thermally stable PI/organoclay NCs.

S. Mallakpour, M. Dinari / Materials Research Bulletin 47 (2012) 2336–2343 2339

3.4. Characterization

3.4.1. FT-IR and 1H NMR spectra

The existence of the characteristic peaks for NH2 functions andthe absence of the original peaks arising from the NO2 groups in thecorresponding dinitro provided the successful formation of theDABI. Absorption of amine NH2 and NH benzimidazole bondsappeared around 3394 and 3323 cm�1 and the peak at 1627 cm�1

confirms the presence of NH deformation. Two absorption bands at1577 and 1484 cm�1, were characteristic peaks for aromatic rings.The benzimidazole groups gave bands at 1484 cm�1, 1590 cm�1

and a shoulder at 1630 cm�1 (Fig. 2).Typical FT-IR spectra of PAA, PI, organoclay and PI/organoclay

NC5% were also shown in Fig. 2. In the spectrum of PAA, the

Fig. 2. FT-IR spectra of the diamine 4, PAA, PI, organoclay and PI/organoclay NC5%.

characteristic absorption bands of the amic acid appeared near3466 (N–H and O–H stretching), 1722 (acid, C55O stretching), 1662(amide, C55O stretching), and 1554 cm�1 (N–H bending). Afterthermal imidization of PAA, the peaks observed for PAAdisappeared, and instead a series of new bands corresponding toPI were observed. In the spectrum of PI, the characteristicabsorption bands of the imide ring appeared near 1776 (asym-metric C55O stretching), 1723 (symmetric C55O stretching), 1376(C–N stretching), and 722 cm�1 (imide ring deformation). Further-more, the representative FT-IR spectra of the organoclay and PI/organoclay NC5% are shown in Fig. 2. Characteristic vibrationbands of organoclay were shown at 3630 cm�1 (O–H), 2963 cm�1

(C–H), 1700 cm�1 (C55O), 1040 cm�1 (Si–O), 525 cm�1 (Al–O), and468 cm�1 (Mg–O). In the spectrum of the NC5%, the presence of apeak at 1048 cm�1 corresponding to the Si–O and peaks at 400–600 cm�1 for Al–O and MgO indicated the incorporation oforganoclay in the PI matrix. From these figures, it can be concludedthat the NC films not only have characteristic neat PI bands, butalso have characteristic peaks for organoclay.

In the 1H NMR spectra of DABI, appearances of the N-H protonsof benzimidazole group at 12.62 ppm as broad singlet peaks,indicate presence of this group. The absorption of aromatic protons

Fig. 3. 1H NMR (500 MHz) spectrum of the diamine 4 in DMSO-d6 at R.T.

Fig. 4. XRD of the Cloisite Na+, organoclay and different PI/organoclay NCs.

S. Mallakpour, M. Dinari / Materials Research Bulletin 47 (2012) 2336–23432340

appeared in the range of 5.96–7.50 ppm. The proton of the aminegroups appeared as broad singlet peaks at 4.91 ppm (Fig. 3).

3.4.2. X-ray diffraction

Fig. 4 shows the XRD patterns of clay and NC samples. ForCloisite Na+ clay, the diffraction pattern shows a distinct peak at2u = 7.568 corresponding to 11.70 A, which is assigned to the 001lattice spacing of MMT. After modification with protonated L-isoleucine amino acid, 2u decreased to 6.108, i.e., the d spacingincreased to 14.50 A for organoclay. The greater spacing in theorganoclay, when compared to Cloisite Na+ clay, would help thesilicate layers to disperse easily. In NC samples, no distinct peakwas observed at lower organoclay concentrations (up to 5%),whereas for 7 and 10% organoclay concentrations, a distinct peak atabout 2u = 3.228 (d = 27.4 A) was observed. The peak observed in2u = 7.568 corresponding to the basal spacing of organoclay hasdisappeared in the NCs 1%, 3% and 5%, suggesting the disorder andloss of structure regularity of the clay layers. Thus, the clay tactoidsare considered to be exfoliated and the 0.96-nm thick clay layersare dispersed at the molecular level into PI. However, with theincrease of the organoclay loading more than 5%, the hybrids showa slight peak at 2u = 3.228 that was corresponds to a d spacing of2.74 nm suggesting that a small part of organoclay is not dispersedin the molecular level. It also suggested a degree of intercalation ofPI into the organoclay galleries.

3.4.3. Morphology studies

3.4.3.1. FE-SEM. The morphological images of the Cloisite Na+ andorganoclay were studied by SEM technique in our previous study[31]. Before modification, Cloisite Na+ shows massive, aggregatedmorphology and in some instances, there are some bulky flakes.After modification, organoclay has more fragments of smaller sizeand they are formed with irregular shapes. The morphologicalimages of the pure PI, NC3% and NC10% were studied by FE-SEM(Fig. 5). Pure PI exhibited rigid fracture deformation morphology(Fig. 5a and b). The FE-SEM micrographs of the NC5% show that theorganoclay platelets were uniformly distributed without agglom-eration at low organoclay content (Fig. 5c and d). When theorganoclay was increased to 10%, agglomeration of clay particleswas observed as indicated by the white circle in the FE-SEMmicrograph (Fig. 5e and f). Silica domains can be clearly observedwith good dispersion to the organic polymer matrix.

3.4.3.2. TEM. TEM provides direct and unambiguous qualitativeevidence for the formation of a true nanoscale composite. Structuresof the organo-modified clay and PI/organoclay NCs were directlyinvestigated by means of TEM (Fig. 6). According to our previousstudy, the layer-structure images of untreated Cloisite Na+ cannot beobserved with TEM [34]. Contrasting to unmodified Cloisite Na+,organoclay modified with protonated isoleucine amino acid showclear layer structures. Fig. 6a–c shows the TEM micrographs of the PI/organoclay NC5% with different magnification (100 nm, 70 nm and50 nm) in which the brighter region represents the polymer matrixwhile the dark narrow stripes represent the stacked and intercalatedstacked nanoparticles. Due to the potential interaction between thecarboxylic acid groups of the modifier with imide and benzimidazolegroups of PI, the TEM micrographs of NC5% reveal well-exfoliatedstructures. This suggests that a greater interaction force between PIand organic modifiers are required for formation of exfoliatedstructures. Both exfoliated silicate lamellae and intercalated stacksare visible, consistent with the XRD and FE-SEM results. The majorityof the particles observed were exfoliated silicate-lamella stacks.

3.4.4. Thermal properties

The TGA curves for Cloisite Na+ and organoclay of L-isoleucineamino acid were reported in our previous study [31]. The resultsshow that the decomposition of Cloisite Na+ occurs in two steps:one before 100 8C was attributed to the desorption of water fromthe interlayer space, another around 660 8C due to dehydroxyla-tion of the layers and proceeds till around 700 8C. The presence oforganic cations in organoclay increases the number of decomposi-tion steps and it was shown that the decomposition of anorganoclay takes place in four steps. At 800 8C, the residue fororganoclay is up to 87%.

The results of the TGA for pure PI and PI hybrid films with 3%,5%, 7% and 10% of organoclay are shown in Fig. 7 and Table 1 innitrogen atmospheres. All the NCs were found to have single stepdecomposition and be stable up to 440 8C. The 10% weight losstemperatures of the NCs were higher than those of the pristine PI.The higher thermal stability of NCs when compared to the pure PIwas attributed to the formation of char-like material as a result ofthe decomposition of organoclay and this material acted both as abarrier to mass transport of the decomposed products to thesurface of the decomposing polymer and as a thermal barrier toprevent additional exposure of the polymer to the flame. Thepercent char yield at 800 8C increased with an increase oforganoclay content in the NCs, which indicated that the dispersionof organoclay in the PI matrix was uniform. Char yield can beapplied as decisive factor for estimated limiting oxygen index (LOI)of the polymers based on Van Krevelen and Hoftyzer equation [35].

LOI ¼ 17:5 þ 0:4CR; where CR ¼ char yield

The NC3%, NC5%, NC7% and NC10% have LOI values in rang of42–46% which were calculated from their char yield at 800 8C. Onthe basis of LOI values, all NCs can be classified as self-extinguishing materials (Table 1). From these data it is unambigu-ous that neat PI and it is NCs are thermally stable owing toexistence of various linkages such as imide and benzimidazolesgroups in polymer backbones.

3.4.5. Optical transparency

When single layers of layered silicates are dispersed in apolymer matrix, the resulting NC is optically clear in visible light.The films prepared with the solvent casting process are almostcolorless, and their levels of transparency are increases with theorganoclay content. Moreover, since the hybrid films have phasedomains smaller than the wavelengths of visible light, the

Fig. 5. FE-SEM images of the (a and b) pure PI, (c and d) PI/organoclay NC5%, and (e and f) PI/organoclay NC10%.

S. Mallakpour, M. Dinari / Materials Research Bulletin 47 (2012) 2336–2343 2341

materials are transparent even for clay loadings up to 5 wt.%. Fig. 8shows the UV–visible transmission spectra of pure PAI and NCmaterials with 3, 5, 7 and 10 wt.% of organoclay. The spectra ofNC3% and NC5% were slightly affected by the presence of theorganoclay and retained the high transparency, indicating thatintercalated NC materials might exist at low clay contents.Furthermore, NC7% and NC10% had a lower transparency resultingfrom agglomeration of the organoclay particles. The translucenciesof the NC films come from the nanoscale dispersion of theorganoclay particles in the matrix polymer.

Fig. 6. TEM images of PI/organoclay NC

3.4.6. Mechanical properties

The amount of organoclay added to PI significantly affects theobserved mechanical properties. The stress–strain curves of purePI and NC materials with 3, 5 and 10 wt.% of organoclay are shownin Fig. 9. The strength and modulus values were found to beenhanced with respect to those of PI for filler contents up to acritical content, with inferior values above that content. It appearsthat there is a critical amount of filler beyond which the filler’sreinforcing capacity diminishes. The tensile strength of the neat PImatrix is 75 MPa. The tensile strength of the NCs with 3, 5 and

10% with different magnifications.

Fig. 7. TGA thermograms pure PI and different PI/organoclay NCs.

Fig. 8. UV–vis of the pure PI and different NCs.

Fig. 9. Representative stress–strain curves for the neat PI, NC3%, NC5% and NC10%.

S. Mallakpour, M. Dinari / Materials Research Bulletin 47 (2012) 2336–23432342

10 wt.% of organoclay are 87, 102 and 96 MPa, respectively. Themaximum stress at break was found to increase initially withincrease in organoclay content, and at 5 wt.% organoclay showed amaximum value of 102 MPa relative to the 75 of the neat PI. Thisdecrease in the ultimate tensile strength is mainly owing to theagglomeration of filler particles over critical organoclay content.The general tendency for improvement in the stress level isincreased by the addition of organoclay which play the role ofreinforcement. Another reason for the enhancements in the tensilemodulus of NCs is the strong interaction between the PI matrix andorganoclay via formation of hydrogen and chemical bonding [21].From these results, it is deduced that the reinforcing effect of theorganoclay is very marked. As the organoclay content in thepolymer increases, the stress level gradually increases but at thesame time the strain of the NCs decreased.

Table 1Thermal properties of the neat PI and different NC films.

Organoclay content (wt.%) T10a Char yield (%)b LOIc

0 421 58 40.7

3 440 61 41.9

5 449 64 43.1

7 461 67 43.3

10 467 71 45.9

a Temperature at which 10% weight loss was recorded by TGA at heating rate of

10 8C min�1 in a N2 atmosphere.b Weight percent of the material left undecomposed after TGA at maximum

temperature 800 8C in a N2 atmosphere.c Limiting oxygen index (LOI) evaluating at char yield at 800 8C.

4. Conclusions

In this study at first a processable PI was prepared based onaromatic imide and pendant heterocycles benzimidazole moietiesin the side chain. From the chemical point of view, theincorporation of benzimidazoles group into the backbone ofpolymer systems results in versatile polymers with interestingproperties such as thermal stability and good solubility. Difunc-tionalized L-isoleucine swelling agent was used for organomodi-fication of Cloisite Na+. The amine function of modifier formed anionic bond with negatively charged silicates and free acidic groupis available for further reaction with imide and benzimidazolegroups of the PI. PI–organoclay NC hybrids with differentorganoclay contents ranging from 1 to 10 wt.% with good dispersedsilicate layers in the PI matrix have been successfully synthesizedby an in situ polymerization. XRD, TEM and FE-SEM resultsdemonstrated that NCs containing low organoclay content werefound to have an exfoliated structure whereas at higher clayconcentration, agglomeration was observed leading to intercalatedstructures. Dispersed organoclay platelets into PI matrix werefound to increase the thermal stability such as the enhancement ofthermal decomposition temperature and char yield of PI/organo-clay NCs based on the TGA studies. Moreover, the original reactiveswelling agent becomes a part of the polymer molecules, and itmakes these NCs more stable thermally and stronger mechanically.According to mechanical properties, as the organoclay content inthe polymer increases, the stress level gradually increases but atthe same time the strain of the NCs decreased.

Acknowledgments

We wish to express our gratitude to the Research AffairsDivision, Isfahan University of Technology (IUT), Isfahan, for partialfinancial support. Further financial support from National EliteFoundation (NEF), Iran Nanotechnology Initiative Council (INIC)and Center of Excellency in Sensors and Green Chemistry Research(IUT) is gratefully acknowledged.

References

[1] S. Andre, F. Guida-Pietrasanta, A. Rousseau, B. Boutevin, G. Caporiccio, J. Polym.Sci. A: Polym. Chem. 43 (2005) 2237–2247.

[2] T. Nguyen, X. Wang, J. Power Sources 195 (2010) 1024–1030.[3] J. Ishii, N. Shimizu, N. Ishihara, Y. Ikeda, N. Sensui, T. Matano, M. Hasegawa, Eur.

Polym. J. 46 (2010) 69–80.[4] F. Zhai, X. Guo, J. Fang, H. Xu, J. Membr. Sci. 296 (2007) 102–109.

S. Mallakpour, M. Dinari / Materials Research Bulletin 47 (2012) 2336–2343 2343

[5] H.W. Wang, R.X. Dong, C.L. Liu, H.Y. Chang, J. Appl. Polym. Sci. 104 (2007) 318–324.

[6] Y.H. Yu, J.M. Yeh, S.J. Liou, C.L. Chen, D.J. Liaw, H.Y. Lu, J. Appl. Polym. Sci. 92 (2004)3573–3582.

[7] V. Ayala, E.M. Maya, J.M. Garcia, J.G. De La Campa, A.E. Lozano, J. De Abajo, J. Polym.Sci. A: Polym. Chem. 43 (2005) 112–121.

[8] Y.A. lvarez-Gallego, S. Pereira Nunes, A.E. Lozano, J.G. de la Campa, J. de Abajo,Macromol. Rapid Commun. 28 (2007) 616–622.

[9] R. Dubey, N.S.H.N. Moorthyito, Chem. Pharm. Bull. 55 (2007) 115–117.[10] S. Karatas, N. Kayaman-Apohan, H. Demirer, A. Gungor, Polym. Adv. Technol. 18

(2007) 490–496.[11] Y.Y. Yu, W.C. Chien, T.W. Tsai, Polym. Test. 29 (2010) 33–40.[12] S. Vural, S. Koytepe, T. Seckin, I. Adıguzel, Mater. Res. Bull. 46 (2011) 1679–1685.[13] Y. Yu, S. Qi, J. Zhan, Z. Wu, X. Yang, D. Wu, Mater. Res. Bull. 46 (2011) 1593–1599.[14] B. Lina, H. Liua, S. Zhanga, C. Yuan, J. Solid State Chem. 177 (2004) 3849–3852.[15] J. Xu, R.K.Y. Li, Y.Z. Meng, Y.W. Mai, Mater. Res. Bull. 41 (2006) 244–252.[16] Z.M. Liang, J. Yin, H.J. Xu, Polymer 44 (2003) 1391–1399.[17] M.O. Abdallaa, D. Deana, S. Campbell, Polymer 43 (2002) 5887–5893.[18] J.S. Park, J.H. Chang, Polym. Eng. Sci. 49 (2009) 1357–1365.[19] V. Bugatti, U. Costantino, G. Gorrasi, M. Nocchetti, L. Tammaro, V. Vittoria, Eur.

Polym. J. 46 (2010) 417–427.[20] T. Kashiwagi, R.H. Harris Jr., X. Zhang, R.M. Briber, B.H. Cipriano, S.R. Raghavan,

W.H. Awad, J.R. Shields, Polymer 45 (2004) 881–889.

[21] S. Mallakpour, M. Dinari, Polymer 52 (2011) 2514–2523.[22] C.J.T. Landry, B.K. Coltrain, D.M. Teegarden, T.E. Long, Macromolecules 29 (1996)

4712–4721.[23] H.L. Tyan, K.H. Wei, T.E. Hsieh, J. Polym. Sci. B: Polym. Phys. 38 (2000) 2873–2878.[24] S.H. Anastasiadis, K. Chrissopoulou, B. Frickc, Mater. Sci. Eng. B 152 (2008) 33–39.[25] D.M. Delozier, R.A. Orwolla, J.F. Cahoona, J.S. Ladislawa, J.G. Smith Jr., J.W. Connell,

Polymer 44 (2003) 2231–2241.[26] M. Yin, C. Li, G. Guan, X. Yuan, D. Zhang, Y. Xiao, Polym. Eng. Sci. 49 (2009) 1562–

1572.[27] T.Y. Chao, H.L. Chang, W.C. Su, J.Y. Wu, R.J. Jeng, Dyes Pigments 77 (2008) 515–

524.[28] C.K. Min, T.B. Wu, W.T. Yang, C.L. Chen, Compos. Sci. Technol. 68 (2008) 1570–

1578.[29] H.L. Tyan, C.M. Leu, K.H. Wei, Chem. Mater. 13 (2001) 222–226.[30] R.H. Vora, M. Vora, Mater. Sci. Eng. B 132 (2006) 90–102.[31] S. Mallakpour, M. Dinari, Appl. Clay Sci. 51 (2011) 353–359.[32] Y. Alvarez-Gallego, B. Ruffmann, V. Silva, H. Silva, A.E. Lozano, J.G. de la Campa, S.

Pereira Nunes, J. de Abajo, Polymer 49 (2008) 3875–3883.[33] P.E. Eaton, G.R. Carlson, J.T. Lee, J. Org. Chem. 38 (1973) 4071–4073.[34] L. Zhou, H. Chena, X. Jiang, F. Lu, Y. Zhou, W. Yin, X. Ji, J. Colloid Interface Sci. 332

(2009) 16–21.[35] D.W. Van Krevelen, P.J. Hoftyzer, Properties of Polymers, 3rd ed., Elsevier

Scientific Publishing, New York, 1976.