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Materials Letters 61 (20

On the use of TEM in the characterization of nanocomposites

Orietta Monticelli a,⁎, Zenfira Musina a, Saverio Russo a, Sara Bals b

a Dipartimento di Chimica e Chimica Industriale, Università di Genova and INSTM NIPLAB, Genova research unit, via Dodecaneso, 31, 16146 Genova, Italyb EMAT (Electron Microscopy for Materials Science), University of Antwerp, Groenenborgerlaan 171, B-2050 Antwerp, Belgium

Received 27 February 2007; accepted 15 November 2006Available online 5 December 2006

Abstract

Both an organically modified commercial clay of montmorillonite type (MMT) and its nanocomposites, based either on polyamide 6 (PA6) oran epoxy resin, as matrix polymer, have been characterized by transmission electron microscopy (TEM). Sample micrographs, taken at increasingexposure times (te), have shown the gradual disappearance of clay layers, because of an amorphisation of the MMT crystalline structures caused byprolonged sample exposure to electron beam. Indeed, the above phenomenon, which is mostly evident in the case of intercalated nanocomposites,makes the detection of the layered silicate dispersion in the polymer matrix rather difficult and compels to perform TEM measurements using veryshort exposure times. Moreover, the microscopy accelerating voltage has turned out to affect sample stability; namely, when decreasing the aboveparameter, the disappearance of clay structure occurs at lower exposure times.© 2006 Elsevier B.V. All rights reserved.

Keywords: Layered silicate; Nanocomposites; TEM; Structure modification

1. Introduction

Among the inorganic materials used as additives orreinforcements to improve the properties of polymers, layeredsilicates, such as MMT, have recently received a great dealof attention [1,2]. To achieve a better interaction with organicpolymers, the cations (typically sodium), present on the surfaceof MMT to balance the net negative charge of the aluminium/magnesium silicate layer, are exchanged with organic moleculescontaining a cationic group, e.g. alkyl ammonium ions, in orderto produce an organoclay. Several research groups have de-scribed organoclay nanocomposites based on a variety of poly-mers including polystyrene [3], epoxy resins [4], polyamides[5], polyurethanes [6], etc. Generally, the structure of nano-composites has been studied using WAXD analysis and TEMobservations. Due to its easiness and availability, WAXD is themost common technique used to probe the nanocompositestructure. By monitoring the position, shape, and intensity ofthe basal reflections from the distributed silicate layers, the

⁎ Corresponding author. Tel.: +39 010 3536196; fax: +39 010 3536198.E-mail address: orietta@chimica.unige.it (O. Monticelli).

0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.matlet.2006.11.086

nanocomposite structure (intercalated or exfoliated) may beidentified. Indeed, in an exfoliated nanocomposite, theextensive layer separation associated with the delamination ofthe original silicate layers in the polymer matrix eventuallyresults in the disappearance of any coherent X-ray diffrac-tion from the distributed silicate layers. On the other hand,for intercalated nanocomposites the finite layer expansionassociated with the polymer intercalation results in the ap-pearance of a new basal reflection corresponding to a largergallery height.

Although WAXD offers a convenient and simple method todetermine the average interlayer spacing of the silicate layers inthe pristine layered silicates and in the intercalated nanocom-posites, the above technique cannot determine the spatialdistribution of the silicate layers as well as any structural non-homogeneity in the observed nanocomposites, namely thesimultaneous occurrence of both intercalated and exfoliatedstructures. Thus, nanocomposite structures cannot be studiedsolely by WAXD patterns. Conversely, TEM allows a qualita-tive evaluation of the internal structure and spatial distributionof the various phases through direct visualization. The abovetechnique can also prove, besides the occurrence of intercalatedand exfoliated nanocomposites, the formation of flocculated

Fig. 1. TEM micrographs of neat Cloisite 30B® and histograms of the relative population as a function of clay interlayer distance: (a) te=0 s, (b) te=30 s, (c) te=60 sand (d) te=120 s.

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structures [7–9]. Although TEM represents a powerful tool forthe characterization of nanocomposites, problems concerningtedious sample preparation and modification of the latter haveto be considered. In this respect, even though the possiblestructural changes of clay minerals, caused by TEM analysis,

Fig. 2. ED patterns of neat Cloisite 30B®: (a) te=0

are well documented in mineralogy [10–12], as far as nano-composite study is concerned the above drawback has not beenanalyzed in detail. On this basis, the present work dealswith the influence of TEM electron beam exposure on themorphological studies of both a widely used commercial

s, (b) te=30 s, (c) te=60 s and (d) te=120 s.

Table 1d-Spacing of Cloisite 30B® at various exposure times by TEM analysis

Exposure time (s) d1 (Å) d2 (Å) d3 (Å) d4 (Å) d5 (Å) d6 (Å) d7 (Å)

0 4.41 2.52 2.20 1.72 1.48 1.28 1.2330 4.25 2.50 1.23 – – – –60 4.23 2.47 – – – – –120 – – – – – – –

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organoclay and its polymer-based nanocomposites, eitherexfoliated or intercalated.

2. Experimental

An organically modified MMT (OMMT) (commercial name:Cloisite 30B®) was supplied by Southern Clay (USA). It containsmethyl, tallow, bis-2-hydroxyethyl, quaternary ammonium salt asorganic compatibilizer. The above clay is characterized by aninterlayer distance (d001) of 1.8 nm.

Nanocomposites based on either PA6 or an epoxy resin,synthesized from bisphenol A diglycidyl ether (DGEBA) andtris-(2-aminoethyl)amine (TREN), and containing 5 wt.% ofCloisite 30B®, were prepared according to the procedure de-scribed respectively in [13] and [14]. WAXD characterizationevidenced the occurrence of a fully exfoliated nanocomposite inthe case of PA6/Cloisite 30B® samples, while the epoxy resinturned out to be intercalated in the clay structure.

TEM measurements were performed using a high-resolutiontransmission electron microscope (JEOL 2010). Most of themeasurements were carried out using an accelerating voltage of200 kV, while the exposure times were varied from 0 to 120 s.The neat clay powders were suspended in iso-propanol and adrop of the resultant mixture was deposited on a carbon grid,

Fig. 3. TEM micrographs of the system epoxy resin/Cloisite 3

while nanocomposite ultrathin sections of about 100 nm werecut by a Power TOME X microtome equipped with a diamondknife and placed on a 200-mesh copper grid.

3. Results and discussions

The influence of TEM analysis parameters on the evaluation ofnanostructured system morphology has been studied analyzingpreliminarily a commercial organoclay, namely Cloisite 30B®, andsubsequently the corresponding nanocomposites.

Fig. 1 shows TEM micrographs of the above layered silicate. Theset of photos (from a to d), which deals with the same sample zone, hasbeen taken by increasing the exposure times, i.e. 0, 30, 60 and 120 s.The micrograph, shown in Fig. 1 (a), taken at te of 0 s, reveals thetypical clay structure, characterized by the presence of regular stackingarrangements, made by parallel layers. Interlayer clay distances turnout to vary from 1.01 nm to 1.74 nm in agreement with the averagevalue found by WAXD measurements (1.8 nm). Analyzing themicrographs of Fig. 1(b), (c) and (d), respectively, it comes out thatwhen increasing te, some clay layers seem to disappear while thedistance between some others tends to decrease. As shown in thehistograms of the relative population as a function of the interlayerdistance, also reported in Fig. 1, the above phenomenon causes amodification of the pristine distribution. At last, the overall disap-pearance of any clay structure, at the exposure time of 120 s, com-pletely inhibits the evaluation of the interlayer distance distribution ofthe latter.

In order to better assess the modification of any clay structureelectron diffraction patterns (ED) of the sample at the above expositiontimes have been analyzed (Fig. 2).

As confirmed by the data given in Table 1, the interaction ofelectron beams with the OMMT sample causes a structural modifica-tion of the organoclay with a decrease of d-spacings, up to a completeamorphisation at te of 120 s.

0B®: (a) te=0 s, (b) te=30 s, (c) te=60 s and (d) te=120 s.

Fig. 4. TEM micrographs of the system PA6/Cloisite 30B®: (a) te=0 s, (b) te=30 s, (c) te=60 s and (d) te=120 s.

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As mentioned in the Introduction, clay structure modification byTEM analysis has been widely described [10–12]. Indeed, the abovephenomenon, probably caused by dehydration and collapse of thelayered silicate structure under electron beams, makes the differenti-ation between illite- and smectite-like clay minerals difficult to attainby TEM characterization.

The potential modification of the clay also when dispersed in apolymer matrix has been evaluated in the present study by analyzingtwo nanostructured systems based either on an epoxy resin or PA6,characterized, as already mentioned, by intercalated and completelyexfoliated structures respectively. Fig. 3 shows micrographs of theepoxy resin/Cloisite 30B® system, at different exposure times.

In the micrographs obtained at low exposure times (0 and 30 s), it ispossible to identify a regular stacking arrangement of the clay layers.Indeed, Cloisite 30B® layers are still organized in a parallel way,although their basal spacing turns out to be much higher in thecomposite in comparison to the neat sample, as the layers are irregularlyseparated by≈2.3–4.7 nm of polymer. This finding, in agreement withWAXD results, supports the formation of an intercalated nanocompo-site. As already found for the neat clay, it is evident that also for thissystem by increasing te, the clay structure shows relevant modification.As an artefact, at high exposure times, when some clay layers disappear,the intercalated structure of the epoxy resin/Cloisite 30B® nanocompo-site seems to evolve into an exfoliated one, characterized by clay layerswhich show individual dispersion of delaminated sheets. By using alower accelerating voltage, i.e. 120 kV, clay decomposition seems evenmore evident, as it starts at lower exposure time, i.e. 30 s. The abovefinding underlines that the interaction between the sample and electronbeam, as well as the progressive clay damage is promoted whenelectrons, hitting the sample, hold lower energy.

TEM micrographs of the sample based on PA6, obtained at thepreviously studied exposure times, are shown in Fig. 4.

The presence of single clay layers homogenously dispersed inthe polymer matrix, indicates the formation of a fully exfoliatednanocomposite. Once again, when increasing te, clay structure undergoes

a modification, which is evidenced by the disappearance of some layersand the toning down of others. Nevertheless, also at high exposure timesthe exfoliated structure is still visible. On these grounds, the modificationand decomposition of the layered silicate by TEM analysis turn out to beless challengingwhen exfoliated nanocomposites are studiedwith respectto the characterization of intercalated nanostructure systems.

4. Conclusions

The present study clearly showed some problems involved inthe use of TEM in the characterization of nanocompositesystems based on layered silicates. Indeed, only properexperimental conditions, namely low exposure times and highaccelerating voltage, have to be applied in order to avoid claystructure decomposition, which prevents to obtain, mainly forintercalated nanocomposites, correct information about fillerdispersion in the polymer matrix.

Acknowledgments

The present study was supported by MIUR funds (FIRB2001-Project MAPIONANO). The precious help of Mr.Claudio Uliana in TEM measurements and in many helpfuldiscussions is gratefully acknowledged. In addition, we wouldlike to thank Drs. Alberto Mariani and Simone Bidali (SassariUniversity) for the preparation of the nanocomposites basedon epoxy resin.

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