This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014 New J. Chem.

Cite this:DOI: 10.1039/c4nj01558d

Tough hyperbranched epoxy/neem-oil-modifiedOMMT thermosetting nanocomposite with anantimicrobial attribute

Bibekananda De,a Kuldeep Gupta,b Manabendra Mandalb and Niranjan Karak*a

In the present study, a high performance, tough, antimicrobial, hyperbranched epoxy nanocomposite is

fabricated by the incorporation of neem oil-immobilized organo-modified montmorillonite nanoclay.

The immobilization of neem oil with organo-modified montmorillonite nanoclay is performed by the

combined effect of mechanical and ultrasonic forces under ambient conditions. This immobilization is

confirmed by FTIR and XRD studies. FTIR, XRD, SEM and TEM analyses also confirm the formation of the

exfoliated nanocomposite. The dose-dependent enhancements of tensile strength (50%), elongation at

break (3-fold), toughness (5.5-fold) and thermal stability (20 1C) of the pristine epoxy thermoset are

observed for the nanocomposite. Antimicrobial studies are performed by growth curve and zone-of-

inhibition analyses against different bacteria and a fungus at different doses of neem oil-immobilized

organo-modified montmorillonite nanoclay. The nanocomposite with 50 wt% neem oil-immobilized

organo-modified montmorillonite nanoclay (2.5 wt%) shows significant activity against biofilm formation

compared to the pristine thermoset. Thus, the studied nanocomposite has strong potential as a high-

performance functional material.

Introduction

Polymer nanocomposites are one of the most active areas ofdevelopment in the domain of nanotechnology and materialsscience. The incorporation of nanoclays into various polymersto enhance properties like mechanical, thermal, physical, barrier,have been extensively reported over the last two decades.1–3 Thehigh surface area, favorable intercalation chemistry and naturalavailability of nanoclays make them the most popular among allthe nanomaterials.4,5 They have a high tendency to absorb orintercalate different bio-molecules like drugs or biocides,6,7

organo-molecules8 or different nanoparticles9,10 between theirlayer structures, and thus recently they have attracted consider-able attention in the biological field. However, high moistureabsorption and incompatibility with hydrophobic polymersrestrain the incorporation of clay particles into the polymermatrix. Thus, organically modified clay is largely used in polymernanocomposites, and this modification can be performed eitherby surface functionalization or by the cation exchange process.11–13

The modification of clay is mainly performed to achieve improved

properties as well as to obtain a new set of desired properties.Among different matrices for the fabrication of polymer–claynanocomposites, epoxy resins are extensively used. Furthermore,the design of hyperbranched architecture in the epoxy structureoffers a golden feather into it. This is because of their highsolubility, low viscosity, and high reactivity, in addition to the factthat thermosets also possess high tensile strength, high modulus,high stiffness, high thermal stability and high chemical resis-tance.14–16 However, such approaches cannot properly address themain drawback of low toughness or the highly brittle nature ofthe epoxy thermoset. Thus, this observation prompted researchersto investigate further modification of the conventional OMMT insuch a way that the abovementioned problem of epoxy thermosetcan be addressed.

On the other hand antimicrobial polymer nanocompositesare promising materials in advanced surface coatings fordestructing microorganism in different fields including marineindustries because of slow release of the active agents. Tradi-tionally, antimicrobial materials contain one or more toxiccompounds like biocides and metal nanoparticles, which maycause human health as well as environmental hazards dueto leaching problem.17–19 Therefore, a natural antibiotic- orbiopesticide-immobilized, clay-reinforced polymer nanocompositeis preferred, although no such report is found, except our earlierreport.6 Moreover, the active agent used earlier, H. aromatica, hadthe problem of volatility and availability. On the other hand,

a Advanced Polymer and Nanomaterial Laboratory, Department of Chemical Sciences,

Tezpur University, Napaam-784028, Assam, India.

E-mail: karakniranjan@yahoo.com; Fax: +91-3712-267006b Department of Molecular Biology and Biotechnology, Tezpur University,

Napaam-784028, Assam, India. E-mail: mandal@tezu.ernet.in

Received (in Montpellier, France)11th September 2014,Accepted 24th October 2014

DOI: 10.1039/c4nj01558d

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among the different natural biocides, neem (Azadirachta indica)seed oil is a very common and popular agent because of itsnontoxic nature to mammals; moreover, it is a very effectiveantiseptic, antifungal, antibacterial and insecticide.20,21 Crudeneem seed oil contains azadirachtin-like biocidal triterpenoidcompounds, as well as long chain fatty acids like linoleic acid,oleic acid, palmitic acid, and stearic acid.22,23

Thus, in the present study, crude neem seed oil-immobilizedorganically modified montmorillonite clay (OMMT) is used forthe fabrication of high performance antimicrobial hyper-branched epoxy nanocomposites. Here, neem oil will act asan antimicrobial agent, and the fatty acids of the oil maytoughen the whole system by the plasticizing effect. Therefore,the use of hyperbranched epoxy is of particular interest becauseof its easy synthetic accessibility, low viscosity, high solubilityand a large number of end functional groups. In this study,OMMT helps in enhancing the performance of the matrix aswell as providing stability to the active agent.

ExperimentalMaterials

The hyperbranched epoxy resin used in this study was preparedfrom bisphenol-A, triethanol amine and epichlorohydrin by apolycondensation reaction as previously reported.14 The epoxyequivalent, degree of branching and viscosity of the preparedhyperbranched epoxy were 358 g per eq., 0.79 and 19 Pa s(at 25 1C), respectively. Octadecyl amine modified montmorillonitenanoclay (OMMT) was purchased from Sigma-Aldrich, Germany.Crude neem seed oil was acquired from Agri Life, India, and usedafter vacuum drying. Poly(amido-amine) hardener (HY840, aminevalue 5–7 eq. per kg) was obtained from Ciba Giegy, India.Tetrahydrofuran (THF, Merck, India) was used after distillation,and all other chemicals used in this study were of reagent grade.

Immobilization of neem oil on OMMT

Crude neem oil (NO) was immobilized on OMMT by using theeffect of mechanical and ultrasonic forces at room temperature.In a typical process, 0.5 g OMMT was dispersed in 25 mL THFby magnetic stirring for 30 min in a 60 mL glass bottle. 20 wt%of crude neem oil (0.1 g) was added into it and stirred con-tinuously for 5 h, followed by ultrasonication for 10 min at roomtemperature. The neem oil-immobilized OMMT was coded asNO–OMMT. In order to study the effect of excess neem oil,OMMT was immobilized with 50 wt% neem oil (with respect toOMMT) and coded as ENO–OMMT.

Preparation of hyperbranched epoxy/NO–OMMTnanocomposites

The hyperbranched epoxy/NO–OMMT nanocomposites were pre-pared by a solution technique.11,24 Three different weight percent-ages (1, 2.5 and 5) of NO–OMMT dispersed in THF were incorpo-rated into the hyperbranched epoxy and stirred magnetically for 5 hat room temperature, followed by sonication for 10 min by anultrasonic processor (UP200S, Hielscher, Germany) with a standard

sonotrode (tip diameter, 3 mm) at 60% amplitude and 0.5 cycleswith an acoustic power density of 460 W cm�2. An amount of50 wt% (with respect to hyperbranched epoxy) poly(amido-amine)was mixed homogeneously with the abovementioned mixtures andcoated on glass and steel plates. The plates were maintained for24 h under vacuum at room temperature to remove THF and othervolatiles. Initially (when the samples were in a low viscous state),high vacuum was applied to remove THF, and the coated plateswere then maintained on a flat platform to obtain even surface ofthe cast film. Then, the samples were placed again under controlledvacuum, and two consecutive weightings after 24 h and 30 h of theabovementioned coated plates were taken to confirm the completeremoval of THF or other volatiles. During this period, just gelationof the samples was started, and no significant crosslinking wasobserved (swelling value, 72%). Finally, the plates were cured insidea furnace at 100 1C for 60 min, followed by post-curing at 130 1C for30 min. The nanocomposites were coded as PNC1, PNC2.5 andPNC5 for 1, 2.5 and 5 wt% neem oil-modified OMMT, respectively.Similarly, a hyperbranched epoxy nanocomposite with 2.5 wt%unmodified OMMT was prepared (coded as PC2.5) for comparison.The pristine hyperbranched epoxy thermoset was coded as P. Inorder to study the effect of excess neem oil, a 2.5 wt% ENO–OMMT-based nanocomposite was prepared and coded as PENC2.5.

Characterization

FTIR spectra were recorded on a Nicolet FTIR spectrometer(Impact-410) using a KBr pellet. The wide-angle X-ray diffrac-tion patterns of the nanomaterials and the nanocompositeswere recorded by a Miniflex (Rigaku Corporation, Japan) X-raydiffractometer using CuKa radiation (0.154 nm). The morpho-logy of the hyperbranched epoxy/NO–OMMT nanocompositeswas studied by high-resolution transmission electron micro-scopy (HRTEM, JEOL, JEMCXII, transmission electron micro-scope, Japan, operating at 200 kV). The tensile strength of thepristine thermoset and the nanocomposite films (size: 60 �10 � 0.3 mm3) were measured by universal testing machine(UTM WDW10, China) with a 500 N load cell at a crossheadspeed of 10 mm min�1 using the standard test ASTM D822.A scratch hardness test (ASTM G171) was performed using ascratch hardness tester (Sheen Instrument Inc. Ltd, UK) on thesurface of the glass-coated thermoset films (size: 75 � 25 �0.3 mm3). Impact testing was carried out by an impact tester(S. C. Dey & Co., India) as per the standard falling ball method(ASTM D1709) using the steel plate-coated thermoset films(size: 150 � 50 � 0.3 mm3). The bending test of the thermosetfilms was carried out by the ASTM D522 method using amandrel with a diameter 1–100 mm. All of the tests for themeasurement of mechanical properties were repeated fivetimes and average values were taken. The thermal stability ofthe thermosets was measured by thermogravimetric analysis(PerkinElmer TG4000) with a nitrogen flow rate of 30 mL min�1

and a heating rate of 10 1C min�1 from 30 to 700 1C. Thechemical resistance test was carried out in various chemicalenvironments like aqueous NaOH (5%), aqueous HCl (10%),aqueous NaCl (20%), aqueous ethanol (20%) and tap water toinvestigate the effects of these chemicals on the films of a

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pristine hyperbranched epoxy thermoset and its nanocomposite.The films were cut into small pieces (1 cm2, 0.1–0.15 g) andmaintained in 100 mL amber bottles containing the above-mentioned media at an ambient temperature (25 1C). Thepercentage of weight loss of the films was measured after15 days of testing.

Antimicrobial study of the nanocomposites

Antimicrobial tests were performed by a well diffusion methodas reported in previous studies.25,26 Staphylococcus aureus(MTCC 3160) and Bacillus subtilis (MTCC 121) as gram positivebacterial strains; Klebsiella pneumoniae (MTCC 618) and Pseudo-monas diminuta (MTCC 3361) as gram negative bacterial strainsand Candida albicans (MTCC 3017) as a fungal strain were usedin the antimicrobial assay. 200 mL of a log phase culture of thetest microbes was seeded on the surface of the Muller-Hintonagar (potato dextrose agar for fungal study) on Petri dishes. Thenanocomposites were dispersed in sterilized DMSO, and 100 mLof them were placed into 6 mm diameter wells. In one well,DMSO was taken as a blank, and in another well, Gentamicin(Nystatin for fungal), was used as a positive control. The zone ofinhibition diameters were measured using a transparent rulerafter incubation for 24 h at 37 1C (for bacteria) and for 48 h at28 1C (for fungus). For growth curve analysis of the microbes, thecultures were taken in conical flasks. 200 mL of the samples wereadded to the corresponding conical flasks and incubated for 24 hat 37 1C (for bacteria) and for 48 h at 28 1C (for fungus). Oneconical flask without the sample was taken as the control fortesting each microbe. The growth of the microbes was measuredby checking optical density (OD) at 620 nm after every 2 h, andthe OD was taken up to 20 h.

Biofilm formation study

Biofilm formation on P and PENC2.5 was studied by means of amicrotiter plate biofilm assay.27,28 In the present study, a slightmodification was performed; herein, direct thermoset films(size 1 cm2) were used instead of microtiter plates. The filmswere incubated for 72 h in a potato dextrose broth (PDB)medium. The films were gently washed thrice with phosphatebuffer saline (PBS, pH 7.4) to remove the planktonic bacteria.Then, the films were resuspended and homogenized in PBS byrigorous vortexing for 5 min, and the cells were serially dilutedand plated onto PDB agar. Finally, colony-forming units (CFU)were enumerated after 48 h of incubation at 28 1C.

Results and discussionFormation and characterization of neem oil-immobilizedOMMT

Neem oil-immobilized OMMT was formed by combining theeffect of mechanical shearing and ultrasonication forces. Theseforces help in the dispersion of OMMT and the interaction ofits layers by neem oil due to the presence of polar groups inboth. The immobilization of neem oil into OMMT was firstcharacterized by a FTIR study (Fig. 1). In FTIR spectra, the

following bands were found: for neem oil nmax (cm�1): 3472(–OH), 2928 (–CH), 1750 (CQO, ester) and 1458 (C–O, ester); forOMMT: 3640 (–NH), 3414 (–OH), 2928 (–CH), 1037 (Si–O), 527(Al–O–Si) and 462 (Si–O–Si);6,10 and in case of NO–OMMT nmax

(cm�1): 3627 (–NH), 3427 (–OH), 2928 (–CH), 1736 (CQO, ester),1627 (CQO, amide), 1471 (C–O, ester), 1037 (Si–O), 527 (Al–O–Si)and 462 (Si–O–Si). The shifting of the ester linkages (CQO and C–O) of neem oil from 1750 and 1458 cm�1 to 1736 and 1471 cm�1

in NO–OMMT reveals the presence of different interactions likehydrogen bonding and different polar–polar interactions, of theneem oil with the OMMT clay. The presence of amide linkage inNO–OMMT along with ester linkage confirms that few estergroups of neem oil chemically interact with the amine groups(octadecyl amine) of OMMT. The interaction of neem oil with theclay galleries was also confirmed by XRD study (Fig. 2). In the XRDpatterns, the basal peak (d001) of OMMT was shifted from 2y = 4.21to 2.71 after the immobilization of neem oil. Thus, layer spacingincreased by 1.17 nm after immobilization, as can be calculatedfrom XRD data using Bragg’s equation. This shifting confirmedthat neem oil is immobilized into the clay galleries, and thefatty ester chains of neem oil intercalated into the layers byinteracting with them.

Fig. 1 FTIR spectra of OMMT, neem-oil (NO), NO–OMMT and nano-composite (PNC2.5).

Fig. 2 XRD patterns of OMMT, NO–OMMT and PNC2.5.

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Formation and characterization of hyperbranchedepoxy/NO–OMMT nanocomposites

The hyperbranched epoxy/NO–OMMT nanocomposites wereprepared by a solution technique using the combined effectof mechanical shearing force and ultrasonication. The compo-sitions of the components in the nanocomposites are given inTable 1. The prepared nanocomposites were characterized byFTIR, XRD, SEM and TEM analyses. In the FTIR spectrum ofPNC2.5, the bands were found at nmax (cm�1): 3400 (–OH), 2928(–CH), 1640 (CQO, amide), 1458 (C–O), 1037 (Si–O), 565 (Al–O–Si)and 462 (Si–O–Si). Herein, the shifting of –OH and Al–O–Si bandsof NO–OMMT from 3427 to 3400 and from 527 to 565 cm�1 afterformation of the nanocomposite is due to the presence ofdifferent interactions such as hydrogen bonding, polar–polar,etc., of NO–OMMT with the hyperbranched epoxy and thepoly(amido-amine) hardener.11 In the nanocomposite, the esterlinkage of NO–OMMT at 1736 cm�1 completely vanished, whereasonly amide linkage was found at 1640 cm�1. This confirmed thatall of the ester linkages of NO–OMMT are chemically reacted withthe hardener during the curing time and converted into amidelinkages. In the XRD pattern, d001 diffraction peak of NO–OMMTat 2y = 2.71 completely vanished after the formation of thenanocomposite (PNC2.5), as shown in Fig. 2. This may be dueto the strong interactions of the polymer chains with clay galleries,which cause complete delamination of the clay layers. It may alsohappen due to the masking effect of the clay with the polymer asthe amount of clay was very less (2.5 wt%). The TEM images(Fig. 3) disclose the actual picture of the structure of the nano-composites. The images reveal the disordered exfoliated structureof the NO–OMMT layers in the hyperbranched epoxy matrix. Thisdisordered exfoliated structure is due to the strong physico-chemical interactions of the chains of hyperbranched epoxywith NO–OMMT galleries, which give uniform dispersion of theplatelets in the matrix. The exfoliated interlayer spacing of the

nanocomposite is shown in Fig. 3(b), in which we can see thatinterlayer distance between the two clay layers increases up to8 nm. The strong dispersion of the NO–OMMT clay in thehyperbranched epoxy matrix was found from the SEM image ofthe fracture surface of the nanocomposite. From the SEMimages, it can be seen that the surface of the nanocomposite,PNC2.5 (Fig. 4b), is rougher than the pristine hyperbranchedepoxy, P (Fig. 4a). This indicates that the crack propagation willbe less in rougher surface from the planner manner as thecrack tip is distorted by clay platelets, and thus crack propaga-tion will be more difficult.29

Curing study

The curing study of the hyperbranched epoxy resin and all of itsnanocomposites was performed with 50 wt% poly(amido-amine) hardener at 100 1C for 1 h, followed by post curing at130 1C for 30 min. The swelling value of pristine thermoset, aswell as its nanocomposites, was checked in THF at room tempera-ture (25 1C). From Table 2, it can be found that the swelling valueof P decreased after the formation of nanocomposites both withunmodified OMMT (PC2.5) and NO–OMMT at lower amounts(PNC1). However, the swelling values of the nanocompositesincreased at higher amounts of NO–OMMT (PNC5) and immobi-lized neem oil (PENC2.5). This may be due to the fact that athigher amounts of NO–OMMT or immobilized neem oil, the fattyester of neem oil provides a plasticizing effect and thus large freevolume to the system, which may help to penetrate the solventmolecules.

Mechanical properties of the nanocomposites

The mechanical properties of the pristine hyperbranched epoxythermoset and its nanocomposites with unmodified and neemoil modified OMMT are given in Table 2. Herein, more than50% increment of tensile strength and more than 3-fold incre-ment of elongation at break were found for the pristinehyperbranched epoxy thermoset after the formation of thenanocomposite with 2.5 wt% of NO–OMMT. However, a 40%increment of tensile strength and no change in elongation atbreak were observed after the formation of the nanocompositewith 2.5 wt% of unmodified OMMT. Thus, the increment oftoughness, i.e. the area under stress–strain curves (Fig. 5) ofhyperbranched epoxy thermoset, was much higher in PNC2.5(2994 MPa) than PC2.5 (834 MPa). This is because of theplasticizing effect of fatty ester chains of neem oil inside the

Table 1 Composition of the components in nanocomposites

Composition (parts) P PC2.5 PNC1 PNC2.5 PNC5 PENC2.5

Epoxy 100 100 100 100 100 100Poly(amido-amine) 50 50 50 50 50 50OMMT 0 2.5 0.83 2.08 4.17 1.67Neem oil 0 0 0.17 0.42 0.83 0.83

Fig. 3 TEM images of PNC2.5: (a) 200 nm and (b) 50 nm magnification.

Fig. 4 SEM images of the fracture surface of (a) pristine hyperbranchedepoxy thermoset (P) and (b) nanocomposite (PNC2.5).

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clay platelets, which increases the flexibility and toughness ofthe material by increasing the free volume between the mole-cules. Thus, it provides more ways of energy dissipation by themobility of exfoliated clay platelets11 and by the segmentalmotion of the aliphatic ester moieties in their molecularchains.30,31 It has already been shown through XRD study thatneem oil increased the spacing (1.17 nm) between the claylayers, which helps to penetrate the hyperbranched epoxychains into the clay galleries. This helps in the formation ofstrong interactions between polymer chains and clay layers,which in turn enhances the tensile strength significantly.Elongation at break increases with increase in the amount ofNO–OMMT or immobilized neem oil (in the case of PENC2.5).This is due to the increase of plasticization effect of neem oil.However, tensile strength decreases at a higher loading ofNO–OMMT (PNC5) and a high amount of immobilized neemoil (PENC2.5). This is because of a decrease in interactionsbetween hyperbranched epoxy and clay platelets at higheramounts of clay loading by the aggregation of some clayplatelets, whereas in the case of high amounts of immobilizedneem oil, the amount of aliphatic fatty ester chains areincreased in the entire system, which provides a more plasticiz-ing effect to the system. The nanocomposites exhibited highscratch hardness, impact resistance and flexibility as the

toughness of the hyperbranched epoxy was increased drama-tically after the formation of the nanocomposites. However,these differences could not be measured as the values for thenanocomposites reached the highest limit of the instrumentsfor scratch hardness (10 kg) and impact resistance (100 cm);and the lowest limit of the instruments for flexibility evalua-tion (1 mm bending diameter of mandrel). Nanocompositesabsorbed the highest limit of impact energy, which is attrib-uted to the presence of aliphatic fatty esters of neem oil andether linkages of hyperbranched epoxy, which dissipate theimpact energy by the segmental motion in their molecularchains.30,31

Antimicrobial activity of the nanocomposites

Neem oil has been used as a biopesticide for a long time.The antibacterial activity of PNC1, PNC2.5 and PNC5 isshown in Fig. 6. From the figure, it can be seen that thenanocomposite exhibited significant antimicrobial activitytowards gram positive as well as gram negative bacterial strains.The bacterial zones of inhibition for the nanocomposites

Table 2 Performance of pristine hyperbranched epoxy and its nanocomposites

Parameter P PC2.5 PNC1 PNC2.5 PNC5 PENC2.5

Swelling value (%) 24 � 1.0 21 � 0.4 22 � 0.8 24 � 0.2 29 � 0.6 34 � 1.1Tensile strength (MPa) 40 � 1 57 � 1 48 � 4 62 � 3 56 � 2 46.5 � 2Elongation at break (%) 18.5 � 0.5 19 � 1.5 43 � 2 56 � 5 69 � 4 88 � 4Toughnessa (MPa) 540 834 1637 2994 2680 3323Scratch hardnessb (kg) 9.0 � 0.5 10.0 410.0 410.0 410.0 10.0Impact resistancec (cm) 4100 4100 4100 4100 4100 4100Bending diameterd (mm) o1 o1 o1 o1 o1 o1Initial degradation temperature (1C) 267 282 278 285 288 279

a Calculated by integrating the area under stress–strain curves. b Instrument limit of the scratch hardness was 10.0 kg (highest). c Instrument limitof the impact strength was 100 cm (highest). d Instrument limit of the mandrel diameter was 1 mm (lowest).

Fig. 5 Stress–strain profiles of the pristine hyperbranched epoxy and thenanocomposites. Fig. 6 Antibacterial activity of nanocomposites.

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were found to be in the range of 16–18 mm in Fig. 7a.However, from Fig. 7b, it can be found that PNC1, PNC2.5and PNC5 cannot show significant antifungal activity. A slightantifungal activity was observed after the addition of 10 wt%NO–OMMT to the system as shown in Fig. 7b, whereasPENC2.5 exhibited significant antifungal activity towardsCandida albicans fungal strains as shown in Fig. 7c. The fungalgrowth curves for the nanocomposites PNC1, PNC2.5, PNC5and PENC2.5 are shown in Fig. 7d, in which it can be seenthat the nanocomposites inhibit more fungal growth com-pared to the control. The antifungal activity increases withan increase in the amount of NO–OMMT and immobilizedneem oil (PENC2.5). The bacterial growth curves for PNC1,PNC2.5, PNC5 and PENC2.5 are shown in Fig. 8 with grampositive (Bacillus subtilis and Staphylococcus aureus) as wellas gram negative (Klebsiella pneumoniae and Pseudomonasaeruginosa) bacterial strains. In the figure, the bacterial growth

decreases with an increase in the amount of NO–OMMT andimmobilized neem oil (PENC2.5). From this study, it wasfound that PENC2.5 exhibited the highest antimicrobialactivity.

Biofilm formation study

A biofilm formation study was performed only for P andPENC2.5 against Candida albicans fungal strain. Fig. 9 showedadherence of more number of fungus on P film compared toPENC2.5. Thus, it is clearly visible that PENC2.5 inhibitedmore fungal adherence from the surface compared to P, whichis due to the presence of neem oil on the OMMT surface of thenanocomposite. The presence of aliphatic fatty ester chainsof neem oil in the system also increases the hydrophobicityof the thermoset, which inhibits fungal adherence on thesurface.

Fig. 7 (a) Zones of inhibition of the nanocomposites against different bacteria and Candida albicans fungus, (b) antifungal activity of the nanocomposites,(c) antifungal activity of PENC2.5 and (d) fungal growth curves of the nanocomposites.

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Thermal stability

Thermal stability of the pristine hyperbranched epoxywas increased up to 15–20 1C after the formation of nano-composites with NO–OMMT as shown in Fig. 10. The initial(5% weight loss) thermal degradation temperature of the

pristine hyperbranched epoxy thermoset and its nanocompositesare given in Table 2. The thermal stability of the nanocompositesincreases with an increase in the amount of NO–OMMT; however,

Fig. 8 Bacterial growth curves for the nanocomposites.

Fig. 9 Number of Candida albicans adherence on the surfaces of P andPENC2.5.

Fig. 10 TGA thermograms of pristine thermoset and the nano-composites.

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it decreases with an increase in the amount of immobilized neemoil. When the amount of neem oil increases, the amount ofaliphatic fatty ester also increased in the system, which isthermally less stable and thus thermal stability decreases.However, this thermal stability is much higher than the pristinethermoset. The reason for the increase in thermal stability ofthe pristine thermoset after formation of the nanocompositesis due to the intercalation of clay galleries with hyperbranchedepoxy and poly(amido-amine) chains, which restricted thesegmental motion of the polymer chains by different physico-chemical interactions.32 In addition, the improvement in thethermostability of the nanocomposite by the incorporation ofOMMT is due to the fact that clay is a heat insulator and acts asmass transport barrier to the volatile products generated duringdecomposition by providing longer paths for them to travel.28

However, the amount of weight residue at 700 1C is found to behaphazard (Fig. 10) and may be due to the volatilization of themodified nanomaterial, which contains thermolabile aliphaticchains.

Chemical resistance

The chemical resistance of the pristine thermoset and itsnanocomposites was tested in 5% aq. NaOH, 10% aq. HCl,20% aq. NaCl, 20% aq. EtOH and tap water for 15 days, and thepercentage of weight loss of the samples was measured asshown in Table 3. The chemical resistance mainly dependson the cross-linking density and number of chemically stablelinkages present in the system. In the case of polymer nano-composites, it also depends on the interactions between thepolymer and the nanomaterials. A small amount of weight losswas observed both in the pristine hyperbranched epoxy ther-moset as well as in the nanocomposite films. The percentage ofweight loss of nanocomposites in different chemical environ-ments increased at higher amounts of NO–OMMT (PNC5) andimmobilized neem oil (PENC2.5). This may be due to theplasticizing effect of the fatty ester of neem oil. This effectcreates high free volume to the system, which may help topenetrate the chemicals. These penetrated chemicals degradethe uncured parts of the thermosets, and thus weight loss tookplace. PNC1, PNC2.5 and PC2.5 showed superior chemicalresistance to the pristine system due to the presence of strongerphysicochemical interactions like polar–polar, hydrogen bond-ing, intermolecular, and mutual crosslinking between thematrix and the clay layers by the strong dispersion of clayplatelets into the matrix.28

Conclusion

The present study demonstrated a facile approach to obtainthermosetting nanocomposites with significant antimicrobialactivity, high mechanical properties, good thermal stability andhigh chemical resistance through the incorporation of neemoil-immobilized organo-modified montmorillonite in hyper-branched epoxy matrix. The investigation also paves a methodfor addressing the main drawbacks of brittleness and lowtoughness characteristics of the epoxy thermoset. The intrinsicnature of neem oil as a biopesticide provides antimicrobialactivity against different bacterial (both gram positive and gramnegative) and fungal strains. Thus, the study opens up a newavenue for polymer nanocomposites to achieve antimicrobialfunctional material by incorporating neem oil-modified nano-material for its potential industrial applications.

Acknowledgements

The authors express their gratitude to the NRB for financialassistance through grant no. DNRD/05/4003/NRB/251, dated29.02.12. SAIF of NEHU, Shillong, is gratefully acknowledgedfor TEM imaging.

Notes and references

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Table 3 Percentage of weight loss in different chemical environmentsafter 15 days

Chemical environment P PC2.5 PNC1 PNC2.5 PNC5 PENC2.5

Aq. NaOH (5%) 1.50 1.24 1.43 1.52 1.69 2.32Aq. HCl (10%) 0.92 0.87 0.82 0.90 1.02 1.92Aq. NaCl (20%) 0.32 0.22 0.24 0.33 0.45 0.97Aq. EtOH (20%) 0 0 0 0 0 0Water 0 0 0 0 0 0

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