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Photoluminescence of epoxy/clay nanocomposites.

INTRODUCTION

The mosi widely used plastic scintillators are based on polystyrene with added fluorescent dye molecules. However, it has been shown that scintillators based on other polymers exhibit several advantages [I, 2]. In particular, epoxy systems are easy to shrink very little during polymerization.

Recently, transparent polymer nanocomposites films with low coefficient of thermal expansion have been prepared using clays [3, 4]. Both properties are relevant to plastic scintillators. On the other hand, to optimize the energy transfer process between the host and the dopants, the polymer fluorescence spectrum must overlap as well as possible with the dye absoiption spectrum. Albeit the dielectric properties of epoxy/montmorillonite-clay nano composites have proven to be affected by the nanostructu-ration imposed by the interactions among their components, little attention has been paid up to now to the analysis of the photoluminescence (PL) in these nanocom-posites [5].

Montmorillonite (MMT) is a clay mineral often used due to the special interlayer structure of its stacked silicate sheets (thickness ~1 nm, length ~130-180 nm), which chemical formula is [[M.sup.+].sub.y]([Al.sub.2-y][Mg.sub.y])([Si.sub.4])[O.sub.10][(OH).sub.2].[nH.sub.2]O [6]. When nanoscale dispersion of this layered clay is achieved, intercalated and/or exfoliated structures are usually observed. In the intercalated form, polymer chains are introduced between the layers that retain their ordered multilayer morphology. Jn the exfoliated form, the silicate layers are fully delaminated and individually dispersed in the polymer matrix [7, 8]. Ion exchange of the [Na.sup.+] and [Ca.sup.2+] gallery cations in the natural MMT by alkylammo-nium and alkylphosphonium ions may facilitate intercalation of the polymer and exfoliation of the silicate layers by lowering their surface energy and improving their wettability by hydrophilic polymers, or by increasing the gallery spacing. These clays are further called organoclays (OMMT) [9].

Recently, several papers have been published on the optical and electronic properties of conjugated polymer/ MMT-clay nanocomposites [10-12J. Nanoscale architecture has been used to control the physical conformation of the polymer chains and the way the chains pack together, which in turn affect the luminescent efficiency and the energy transfer in conjugated polymer [13, 14J. Specifically, the red-shift or blue-shift of the PL spectra of the poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) loaded with organoclays was attribuled to changes of chain conformation from compact coil to extended coil or vice versa, respectively [15, 16].

Polymer chains of linear polymers and polymer networks in the vicinity of nanofillers tend to be stretched leading to conformational changes [17]. These changes may also affect the PL of non-conjugated polymers. Therefore, the major aim of this research was to study the effect of unmodified and modified layered silicate clays on the PL of epoxy/clay nanocomposites. As well, further insights into the effect of the nanoscale architecture are targeted by using halloysite nanoiubes (HNTs). These are multiwalled kaolinite nanotubes, [Al.sub.2][Si.sub.2][O.sub.5][(OH).sub.4].[nH.sub.2]O, which length varies from 1 to 5 pm and outer diameter from 50 to 70 nm [18].

EXPERIMENTAL

Materials

Natural monlmorillonite (MMT: [Cloisite.sup.[R]] [Na.sup.+]), and montmorillonite modified with methyltallow-bis(2-hydrox-yethyl) quaternary ammonium salt (OMMT: Cloisite25 A) were obtained from Southern Clay Products (Gonzalez, TX). Unmodified halloysite nanotubes HNT ' were obtained from NaturalNano (Rochester, NY). The matrix used was a diglycidyl ether of bisphenol A (BDGE: Flex A, epoxy equivalent weight 187) and the hardener was a 4-nonyl-benzyl amine (FlexB), both marketed by F. Parrilla y Compania SA de CV (Mexico, DF). All materials were used without further purification.

Preparation of EpoxylClay Nanocomposites

The nanocomposites were prepared by the solution casting method, with 0, 2, 5, and 15 wl% clay content. The desired proportion of clay particles was first dispersed in 7 ml of chloroform by sonicalion in an ultrasonic bath for 30 min. The epoxy resin was then added and stirred for another 30 min. After evaporating the chloroform by heating the mixture at 60 [degrees] C for 10 min, the hardener was slowly added and sonically dispersed for 5 min. The ratio of resin to hardener, by volume, was 50/ 50. After mixing, starch suspensions were degassed under vacuum, cast in glass molds, and left at room temperature in an open atmosphere for 5 days to form the polymer.

Characterization of the EpoxylClay Nanocomposites

The dispersion of the clays in the epoxy matrix was studied by X-ray diffraction (XRD) and transmission electron microscopy, TEM. X-ray diffraction analyses were carried out on a Siemens D5000 diffractometer, with Cu Ka radiation at 35 kV and 25 mA. The samples were scanned in step mode by 2 [degrees] /min scan rate in the range 2 [theta] (10 [degrees] . All diffractograms were normalized at the same total area under the scattering curve over the measured Bragg angles. [The,.sub.d](00I) spacing in the MMT was calculated according to Bragg\s equation, [lambda] = 2d sin [theta] . TEM micro-graphs were taken from ullralhin sections with a JEOL JEM-1010 TEM, using an acceleration voltage of 60 kV. A JEOL JSPM-5200 atomic force microscope (AFM) was used to investigate the surface morphology of the nano-composites. Thin sections (about 500 nm) were cut from the molded specimens and AFM images were taken in tapping mode.

A Nicolet 510 spectrometer operating at 2 [cm.sup.-1] resolution was used to obtain the Fourier transform infrared spectroscopy (FTIR) spectra in the wavenumber range of 4000-400 [cm.sup.-1]. The electron absorption spectra of the films were recorded on a Shimazdu 240IPC spectrophotometer in diffuse reflection mode by using a ISR Shi-madzu integrating sphere and BaS04 pellets as standards. Luminescence measurements were performed on a Jobin Yvon Horiba FLUORO MAX-P luminescence spectrophotometer.

RESULTS AND DISCUSSION

FTIR Analysis

Figure 1 shows the normalized IR spectra of the cured cpoxy, MMT, OMMT and HNTs clays, and epoxy/clay nanocomposites in the 4000-2500 [cm.sup.-1] region. The IR spectrum of the epoxy sample (Fig. la) presented a broad band at 3423 [cm.sup.-1] attributable to OH groups and intense multiplete bands at the 2960-2920 [cm.sup.-1] region due to the C--H stretching in the methyl and methylene groups. The distinctive band related to the aromatic content of the epoxy at 3032 [cm.sup.-1] was also observed [19, 20].

[FIGURE 1 OMITTED]

Different bands were observed around 3620 and 3695 [cm.sup.-1] in the IR spectrum of the HNT (Fig. la), attributable to the two [Al.sub.2]OH stretching bands. The small shoulder at 3450 [cm.sup.-1] could be ascribed to OH groups H-bonded to interlayer water [21]. The band corresponding to OH stretching of lattice water related to silicates also appears around 3450 [cm.sup.-1] in the IR spectra of the MMT and OMMT (Fig. lb and c), as well as the absorption band due to "free" hydroxyl groups around 3630 [cm.sup.-1] [22]. Also, the IR spectrum of the OMMT showed intense C--H absorption and stretching peaks at 2921 [cm.sup.-1] and 2852 [cm.sup.-1], confirming the presence of methylene links in the tallow of the salt (Fig. lc).

The OH band associated to the cured epoxy increased and shifted to 3395 [cm.sup.-1] in the IR spectra of the epoxy/ HNTs nanocomposites. This behavior was also observed in the IR spectra of the epoxy/MMT (~3401 [cm.sup.-1]) and epoxy/OMMT [(~3411cm.sup.-l]) nanocomposites; in addition, the band associated to free-clay hydroxyl groups in the epoxy/OMMT (3630 [cm.sup.-1]) disappeared, confirming that OH groups present in these clays formed hydrogen bonds with the OH groups of the amine-cured samples (Fig. la-c) [20],

Clay Dispersion

Figure 2 shows that the basal distance [d.sub.001] = 1.15 nm of the MMT (related to silicate interlayer spacing [6]) expanded to 1.37, 1.46, and 1.44 nm for the epoxy/MMT nanocomposites loading 2, 5, and 15 wt% MMT, respectively. While, Fig. 3 shows that [d.sub.001] = 1.87 nm of the OMMT expanded to 3.45, 3.25, and 3.56 nm for the epoxy/OMMT nanocomposites loading 2, 5, and 15 wt%, respectively. This behavior clearly indicates that for either of these clays, the polymer chains were intercalated into the silicate layers. The larger increase of interlayer distance of the nanocomposites with OMMT, compared to nanocomposites with MMT, may be related to the long alkyl chains present in the organic modified montmoril-lonite allowing a major amount of polymer intercalation and to the matching of polarity of the clay surface with that of the polymer [23]. Other authors have previously reported dispersion of layered clays in epoxy matrixes [24-26].

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The dispersion of the HNTs was evaluated from TEM micrographs. The TEM image of the epoxy/HNTs nano-composite loading 2 wt% clay showed several agglomerates of a few HNTs in the epoxy matrix (see Fig. 4). Hydrogen bonds between the HNTs and the amine-cured samples seem not to be enough to achieve individually separated HNTs. By using another curing agent, HNTs could be dispersed very uniformly in an epoxy resin [27].

[FIGURE 4 OMITTED]

Electronic Absorption of EpoxylClay Nanocomposites

The UV-vis absorption spectra of cured epoxy and epoxy/clay nanocomposites are shown in Fig. 5. The cured epoxy presents a broad intense band around 260 nm, whereas all epoxy/clay nanocomposites present the same band and also an intense band around 380 nm. The strong absorption at 250-270 nm has been related to benzene chromophore in the epoxy resin [28]. While the strong absorption near 380 nm could be associated to hydroxyl ions in the clays, according to PL studies in oxides [29]. The position of the band around 260 nm in the absorption spectra of them all does not change with clay type or clay concentration, excluding ground state interactions.

[FIGURE 5 OMITTED]

Luminescence of Epoxy/clay Nanocomposites

The PL emission and excitation spectra of the cured epoxy excited at 250 nm are illustrated in Fig. 6. Two emission bands were found at 328 and 403 nm. The strong emission at 328 nm has been attributed to bisphenol and the weak emission around 403 nm to an amine group [30]. The excitation spectra for the two bands corroborate that the two emissions are due to different species, the excitation maxima are quite different (300 nm for emission at 328 nm and 335 nm for emission at 403 nm).

[FIGURE 6 OMITTED]

Cured epoxy and epoxy/MMT nanocomposites showed almost identical PL behaviors Fig. 7. No band shift was observed as MMT concentration was varied, indicating that the band gaps of the cured epoxy and the epoxy/ MMT nanocomposites were essentially identical.

[FIGURE 7 OMITTED]

On the other hand, the PL spectra of the nanocompo-sites with either OMMT or HNTs displayed significant differences, compared with the spectrum of the cured ep-oxy resin (see Fig. 8). The presence of either of these clays resulted in more highly structured spectra compared with spectra of nanocomposites with MMT, two emission bands were found at 360 and 383 nm and a small shoulder at 403 nm. Also, a red-shift (by around 50 nm, maximum) of the band associated to the bisphenol groups (328 nm) was observed. This indicates that important structural reorganization of the polymer chains occurred during curing of the base resin due to presence of OMMT or HNTs.

[FIGURE 8 OMITTED]

Electrostatic forces between the ammonium ions of the OMMT and the charged clay particles prevent recoiling and molecular motion of the intercalated epoxy molecules adjacent to the clay surface [31]. A similar effect could be produced by the hydrogen bonds formed with OH groups located both at the edges and surface defects of OMMT and HNTs and OH groups in the amine-cured epoxy [5, 27]. FTIR measurements confirmed association of the clays and epoxy resin through hydrogen bonds, as indicated before. Therefore, a planar conformation of the polymer chains is assumed in the excited state not only in the interlayer space of the OMMT but also in the vicinity of the HNTs. As a result, not only the emission spectra is better resolved compared with their absorption spectrum but also it is red-shifted; this behavior has also been reported for p-terphenyl molecules [32].

The PL excitation spectra of the epoxy/5% OMMT and epoxy/5% HNT nanocornposites are shown in Fig. 9. The excitation bands corresponding to the 383 and 403 nm emission bands showed to be parallel, with similar shapes and maxima at 340 and 350 nm for the nanocornposites with OMMT and 350 and 353 nm for the nanocornposites with HNTs. This indicates that these two emissions arise from the same chromophore and could be attributed to amine groups, according to the excitation spectrum of the epoxy resin. The difference in maxima is explained by geometric rearrangements. In the same way, the excitation bands corresponding to the 360 nm emission could be attributed to bisphenol groups. Similar spectra were observed for nanocornposites loading 2 and 15% clay (not presented).

[FIGURE 9 OMITTED]

Although the conformation of individual chains in the films cannot be determined from AFM images, the matrixes of the nanocornposites with OMMT and HNTs showed distinct phase morphology, compared with nanocornposites with MMT (see Fig. 10). At this scale, the epoxy resin largely penetrated only into the OMMT and HNT agglomerates wetting the nanoparticles of which they are formed and leading, therefore, to nanostructurcs with large populations of extended coiled chains [33].

[FIGURE 10 OMITTED]

In Fig. 11, maximum PL intensity versus the clay content of the nanocomposites is displayed. There was a severe reduction in the PL intensity of epoxy resin when MMT or OMMT were added to elaborate nanocomposites, independently of clay content. It has been proposed that the PL intensity of UV-irradiated epoxy resins is mainly affected by the concentration of OH groups that can solvate electrons providing stable trapping sites [34]. Modification on the OH content of the nanocomposites with all three types of clays can be observed in their respective FTIR spectra (see Fig. 1). MMT and OMMT seem to severely inhibit the formation of these trapping sites. Although HNTs tended, in general, to produce the same effect as MMT and OMM, at 5 wt% HNTs the PL intensity increased. This behavior could be related to the large amount of polymer chains that penetrated into the HNTs agglomerates (as seen in the AFM micrographs) restringing the local motion of the main chain. It has been stated that stabilization of trapped electrons is related to local motions of the polymer chain [35], Photoluminescence quenching in poly(3-hexylthiophene) (P3HT)/ OMMT nanocomposites has also been reported. The PL quenching of the P3HT was ascribed to rapid charge transfer from the photo excited states [36]. On the other hand, it has been reported that intercalation of MEH-PPV chains into the galleries of OMMT led to an increased PL intensity which was attributed to excitations being prevented from finding low-energy trap sites and to increased excitation beam paths [12]. This complex PL intensity behavior of solid polymers with nanoclays merits more extensive future studies.

[FIGURE 11 OMITTED]

CONCLUSIONS

It was observed that incorporation of MMT, OMMT, and HNTs clays affects the PL behavior of the amine-cured epoxy. The changes in the emission bands of the epoxy resin can be interpreted in terms of modifications to the polymer chain conformation (more coplanar) in the excited states. It is notable that the red-shift in the PL emission spectra of the nanocomposites was only observed when a large amount of epoxy resin penetrated into the clay agglomerates (OMMT and HNTs) independently of the presence of intercalated clay structures (MMT and OMMT), as noticed by AFM images. This then allows the tuning of the wavelength of light emission in the 328-375 nm region when OMMT and HNTs are added to amine-cured epoxy resins to overlap with the dye absorption spectrum in plastic scintillators based on this polymer.

FTIR measurements showed that hydrogen bonds between OH groups in the clays and the OH groups in the resin epoxy vary as a function of the concentration and type of clay. Since the PL intensity was greatly decreased at almost any clay concentration, this suggests that the trapping sites formed from hydroxyl groups are inhibited by these interactions. However, the small PL efficiency of the nanocomposite with 5 wt% HNTs indicated more complex underlying mechanisms.

ACKNOWLEDGMENTS

The authors thank M.E. Sanchez (ENCB-IPN), M. Guerrero, and Z. Rivera (ClNVESTAV, Dpto. de Fisica) for technical assistance.

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Margarita Mondragon, (1) Miguel Angel Cortes, (1) Eduardo Arias, (2) Ciro Falcony, (3) Orlando Zelaya-Angel (3)

(1). Instituto Politecnico Nacional, ESIME Azcapotzaico, Av de las Granjas 682, 02250 Mexico, D.F. Mexico

(2). Centro de investigacion en Quimica Aplicada, CIQA, Boulevard Enrique Reyna 140, 25253 Saltillo, Coahuilla, Mexico

(3). Centro de Investigacion y de Estudios Avanzados del IPN, Departamento de Fisica, Apdo. Postal 14-740, Mexico, D.F. Mexico

Correspondence to: Dr. Margarita Mondragon; e-mail: mmondragon@ipn.nix

DOI 10.1002/pen.21970

Published online in Wiley Online Library (wileyonlinelibrary.com).

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Author:Mondragon, Margarita; Cortes, Miguel Angel; Arias, Eduardo; Falcony, Ciro; Zelaya-Angel, Orlando
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1MEX
Date:Sep 1, 2011
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