JAST
2012 September;3(2):193-201.
Published online 2012 May 21.
doi:http://dx.doi.org/10.5355/JAST.2012.193
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| Copyright ⓒ 2010 Journal of Analytical Science & Technology |
| Thermal Degradation Behavior of Polythiophene-modified Montmorillonite Nanocomposite |
| Amir Meguedad1*, Nassira Benharrats1 |
| 1LPPMCA, Department of Chemistry, Faculty of Sciences, University of Sciences and Technology Oran, BP 1505 Al M뭤aouer, Oran 31000, Algeria. |
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Corresponding Author:
amir meguedad ,Tel: 213775331533, Email: supernova_amir@yahoo.fr |
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ABSTRACT |
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| Polythiophene (PTh) nanoparticles was synthesized by cationic surfactant assisted dilute polymerization method using FeCl3 as oxidant. The physical characterizations of the synthesized PTh nanoparticles were studied by FT-IR and XRD, DSC, SEM.
Polythiophene (PTh)/clay nanocomposites were successfully synthesized with montmorillonite modified with Cetyl-Trimethyle - Ammonium Bromide (CTAB) and Didocyl Dimét hyl-Ammonium Bromide (DDAB), 1-bromohexadecane (MSAB).
The thermal degradation behavior of polythiophene (PTh) in PTH/Na+-montmorillonite (Na+-MMT) nanocomposites prepared by in-situ intercalative polymerization of thiophene into Na+-MMT has been investigated by thermogravimetric analysis (TGA) and X-ray diffraction (XRD), SEM.
It was found that the PTh obtained by cationic surfactant assisted dilute polymerization method had better capacitor performances than the same obtained by the conventional chemical and electrochemical polymerization methods.
The nanocomposites suggest that the PTh chains for PTh/Na+-MMT nanocomposites are more thermally stable than those for a pure PTh. This improvement in the thermal stability for the nanocomposites is attributed to the presence of Na+-MMT nanolayers with a high aspect ratio acting as barriers, thus shielding the degradation of PTh in the nanogalleries and also hindering the diffusion of degraded PTh from the nanocomposites. The shielding effect of the nanolayers is found to be significant as the Na+-MMT content in the PTh/Na+-MMT nanocomposites is increased. The XRD patterns of the nanocomposites after TGA measurements indicate that the basal spacing (d001) of the PTh/Na+-MMT nanocomposites is almost intact, implying that the thermal decomposition of the PTh chains is believed to occur mainly outside the silicate layers.
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| Keywords: Nanocomposite, Polythiophene ,Thermal stability,Thermogravimetry |
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Introduction |
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Conducting polymers such as polypyrrole (PPy), polyaniline (PANI) and polythiophene (PTh) have been the subject of considerable recent interest because of their unique properties such as high electrical conductivity, good environmental stability in doped and neutral states [1,2], ease of synthesis [3] and numerous potential applications including rechargeable batteries.
[4], electronic and optical devices [5], and light emitting diodes [6]. Characteristic properties which make the conducting polymers potentially attractive for such a wide range of applications are strongly dependent on synthetic procedures, type of doping, morphology and other variables [4]. However, since the work on polymer layered silicate nanocomposites by Toyota researchers [7, 8], much research has been carried out in this field. With the addition of a very small amount of organically-modified clay, these nanocomposites show a significant increase in many properties, including mechanical properties, thermal stability, flame retardancy and barrier properties. Montmorillonite (MMT) is the common natural clay, which is most often used; since it is hydrophilic, it is necessary to exchange the inorganic Na+ or Ca+2 with organic cations to render the gallery space more organophilic to permit the entry of organic polymers into this space. This is usually accomplished by ion exchange of MMT using quaternary ammonium. [9, 10].
The main objective of this paper is to study the thermal degradation behavior of PTh chains in PTh/ Na+-MMT nanocomposites prepared by the in-situ intercalative polymerization of thiophene in the interlayer space of Na+-MMT. The thermal stability of PTh chains in the nanocomposites is also compared with that for a simple PTh and MMT. |
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Materials and Methods |
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2.1 Materials
Thiophene monomer was distilled under reduced pressure and stored in a refrigerator prior to use. The bentonite used for this study was natural montmorillonite of MAGHNIA in Algeria purchased from E.C.V.O (Company of Ceramics and Glass of the West).
Na+ -montmorillonite clay (Na-MMT) with a cation exchange capacity (CEC) value of about 90 mmol/100g. Ferric chloride (FeCl3), chloroform (CHCl3), methanol (CH3OH, Aldrich). Cetyl-Trimethyle - Ammonium (CTAB), Didocyl Dimét hyl-Ammonium Bromide (DDAB), 1-bromohexadecane (MSAB).
2.2 Preparation of organophilic montmorillonite
Organophilic montmorillonite OMMT was prepared by cationic exchange between Na+ or Ca+2 in MMT galleries and a reactive intercalating CTAB agent in an aqueous solution. 2.5 g of MMT was suspended in 250 ml of distilled water. An aqueous solution of 3mmole CTAB was added gradually. After stirring of 4 h at ambient temperature, the exchanger clay was filtered and washed with distilled water until no chlorite ion was detected with 0.1 AgNO3 solutions. It was then dried in a vacuum oven at room temperature. OMMT was obtained, ground with a mortar, and sieved using a Cugriddle with 280 mesh. This OMMT is denoted as CTAB-MMT. By the same preparation method, the denoted DDAB-MMT, MSAB-MMT organophilic montmorillonite the concentrations for DDAB used were 0.9 CEC (Cation Exchange Capacity), MSAB-MMT can be obtained from the exchange of 12.5 g with 4.15 g MSAB.
2.3 Synthesis of polythiophene
2 ml of thiophene was taken in a titration flask containing 70 mL CHCl3. 9 gr of FeCl3 was added to 180 mL CHCl3. This mixture was stirred and added to the stirring solution of thiophene in CHCl3. Then the whole mixture was kept for magnetic stirring. After 24 h stirring the compound was filtered and the precipitate which was black in color was washed first with CHCl3 and then with CH3OH. During this procedure the color of the precipitate changes from black to brown.
2.4 Preparation of polythiophene nanoparticles
The PTh nanoparticles was synthesized by cationic surfactant assisted dilute polymerization method. The synthesis procedure is as follows; thiophene monomer (1.0 g) and 0.0034 mol of surfactant (CTAB) were dissolved in 30 ml of distilled water by constant stirring for 15 min. The oxidant, such as 0.055 mol of FeCl3 solution was added dropwise to the monomer-surfactant solution under stirred condition [11]. The preliminary polymerization process was identified by the color change (brown) of the reaction mixture. The polymerization process was allowed to constant stirring for 24 h at 30 캜. The dark-brown precipitate of polythiophene was collected by filtration of the reaction mixture using methanol and distilled water until colourless filtrate was obtained. The obtained PTh nanopowder was dried in a vacuum oven at 80캜 for 6h. this PTh nanoparticles is denoted as PTh-CTAB. By the same preparation method, , the denoted PTh-DDAB.
2.5 Synthesis of polythiophene / nanocomposite
Nanocomposite of polythiophene montmorillonite was synthesized with montmorillonite modified with CTAB and DDAB to compare their electric conductivities and their thermal stability.PTH/Na+-MMT nanocomposites were prepared by the in-situ intercalative polymerization of thiophene as previously described. One example of typical synthetic procedures is as follows: 0,012mol of thiophene was mixed with 35 ml of CHCl3 . FeCl3 dissolved in 90 ml of CHCl3 and was added to the reactional mixture and each of the OMMT is 1wt%, 3wt% of the suspensions was added to the mixture. Here, thiophene monomer in suspension was intercalated into the OMMT galleries. These suspensions were added into reactor vessels, bubbled with N2 gas to remove oxygen and then Polymerization is carried out during 24 hours at ambient temperature. After being dried under vacuum for 24 h at ambient temperature, the nanocomposite products were obtained.
2.6 Conductivity properties
The room temperature conductivity of all the PTh/Na-MMT nanocomposites are summarized in Table I. Variation of the electric conductivity of Nanocomposite Polythiophènes-OMMT (CTAB):It can be seen that the electrical conductivity of nanocomposites decreased with PTH content in nanocomposite. The conductivity of PTH-CTABMMT shows the highest value of all studied nanocomposites.
2.7 Characterization polythiophene / nanocomposites
The X-ray diffraction (XRD) data were recorded for thin film samples using a siemens D5000 Diffractometer (University ORAN Es-Senia, Algeria) instrument with CuK? radiation (?= 1.5405 Å).
Differential scanning calorimetry was carried out on a Perkin-Elmer Pyris 1 DSC at heating rate of 10캜/min under N2 flow from 0 to 600캜.
The scanning electron microscopy (SEM) image of the residue experiment was taken using a Hitachi S- 4700, FE-SEM. Thermogravimetric analyses (TG/DTA) were performed on a Shimadzu DT 40 instrument. |
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Results and Discussion |
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Fig.1 shows the FTIR spectra of nanoparticules of polythiophene (b : CTAB, a : DDAB). The characteristic vibration peaks of polythiophene and polythiophene with sulfactent CTAB and DDAB are at 1600cm-1 (C=C), 3420 cm-1( =C-H bending ), 2970 and 1550cm-1(C-H aliphatic), 1100 et 1218 cm-1 (C-H bending), 673 cm-1 (C-S-C).
IR spectra of polythiophene - DDAB and polythiophene- CTAB show in addition to the peaks of the Polythiophene the presence of the peaks relating to the surface-active used [12].
We notice a reduction in the absorption peak of C=C and an increase the intensity of C-S- C for the both surfactant used. Small peaks ranging from 2800 to 3000 cm-1, which are assigned to the aliphatic C-H stretching modes, associated with the long alkyl.
We also note a displacement of the peaks of vibrations of = C-H and deformations of C-H towards the high frequencies.
The wide angle X ray diffraction patterns of polythiophene and polythiophene-DDAB are presented in (Fig.2). The broad diffraction peak of polythiophene is at about 18.56�. For polythiophene- DDAB there is a weak peak at 55.45�.
X-ray diffraction study was used to find out the crystalline nature of PTh nanoparticles with surfactant CTAB (Figure 3). There was a partially crystalline broad peak centered at near 2Ɵ value of 22.2�. The strong diffraction peak associated with the chain?to-chain stacking distance of about 2Ɵ= 22.2� is due to the amorphously packed polythiophene main chain [13, 14].
The photograph obtained by FE-SEM (Figure. 4a and b) show the morphology of the structure of pure polythiophene and the polythiophéne-CTAB.
Fig. 4 (a) shows a fibrous profile for Polythiophene while on the figure 4(b) we notice variable aggregates of sizes.
The presence of surfactant in the reactional medium favours the formation of nanoparticules which is incorporated between them. A stronger enlargement would have enabled to us to see the nanoparticules.
(Fig.5) (a) shows the DSC traces of the pure polythiophene and polythiophene-CTAB, polythiophene-DDAB. The pure polythiophene has an endotherm at 170캜 corresponding to its glass transition temperature, followed by a second endotherm at 262캜 corresponding to the reticulation of the chains polymeric. The third endotherm at 330캜 corresponding to the decomposition of polymer.
The polythiophene-CTAB has an endotherm at 180캜 corresponding to the departure of the surfactant, followed by a second at 220캜 corresponding to the deprotonation of the chains of polythiophene. Degradation has starts at 280 캜 for polythiophene and at 400 캜 for the polythiophene -CTAB.
The polythiophene-DDAB has an endotherm at 180캜 corresponding to the departure of the surfactant,. Degradation has starts at 270 캜 for polythiophene and at 360 캜 for the polythiophene -DDA.
Fig. 6 shows the FTIR spectra of OMMT and Na-MMT. The characteristic vibration peaks of OMMT and Na-MMT are at 3600 cm-1 (O-H stretching), 1050 cm-1 (Si-O stretching), 630 cm-1(Al-O stretching) and 520 cm-1 (Si-O bending), but for MSAB-MMT there is an obvious new absorbing peaks at 1720cm-1(C=O stretching). Moreover, for CTAB-MMT and DDAB-MMT there is a more intense absorption peak of C-H stretching at 2920 cm-1, 1480cm-1 (CH3 bending). The wide-angle X-ray diffraction patterns of OMMT and Na-MMT are presented in.
In Fig.7, the broad diffraction peak of Na-MMT is at about 6.59� (d= 1.34mm). For MSAB-MMT there is weak peak at 6.02� (d= 1.43mm), which is shifted from that of Na-MMT. The basal spacing between the sheets of the clay is not effectivety increased by MSAB. Strong diffraction peaks at 2?= 3.5� and 2?= 3.6� are the diffraction peaks of the crystal surface (001) of CTAB-MMT and DDAB-MMT respectively. The basal spacings between the sheets of CTAB-MMT and DDAB-MMT are much wider than that of MSAB-MMT since the alkyl chains of CTAB and DDAB are much longer than that of MSAB. The basal spacings of CTABMMT and DDABMMT are proximally equals. This indicates that the basal spacings of the modified clay are greatly influenced by the length of the alkyl chain and by volume of the substituent of the intercalating agent.
The third endotherm between 450 at 500캜 corresponding to the departure of structural OH.
Fig. 9. shows the wide-angle X-ray diffraction patterns of PTh/ clay nanocomposites containing different modified clay.
For the PTh/ CTAB-MMT nanocomposites, there is no obvious peak in the testing range of 2?, implying the clay layers have become disordered and thus partly exfoliated PTh/ clay nanocomposites have been formed. The diffraction peaks of the PTh /MSAB-MMT nanocomposites completely disappear, indicating that the structures of these nanocomposites are exfoliated.
However, the strong diffraction peak of PTh/DDAB-MMT nanocomposites is obviously different from that of PTh/clay anocomposites made with the reactive modified agents, indicating that the structure of PTh/ clay nanocomposites is intercalated and the clay layers are still well-ordered.
Scanning electron microscopic analyses Atypical scanning electron micrograph for the pure Na-MMT and PTh/DDAB–MMT nanocomposite is shown in (Fig.1)0.
The SEM images of pure Na-MMT, PTh/Na-MMT nanocomposites are shown in (Fig.1)0 a, b. Figure 10 a shows sheet-like plates of the clay. In (Fig.1)0 b, it can be seen that the clay layers are dispersed uniformly and homogeneously in the polymer matrix and the interlayer spacing of Na-MMT is expanded, which is evidence for the intercalated morphology. These SEM results were in good agreement with the results of the XRD patterns. The thermal characterizations of the nanocomposites include thermogravimetric analysis (TGA) and differential thermal analysis (DTA). Weight losses (%) versus temperature (캜) curves for pure Na-MMT, pure PTh, and PTh/ DDAB-MMT nanocomposite are shown in (Fig.1)1. The TGA curve of pure Na-MMT shows two stages of weight loss ((Fig.1)1a). The first weight loss in Na-MMT below 100 캜 (≈5.47%) is a result of the release of free water.
The second weight loss around 600 캜 is associated with the dehydroxylation of silicate structure [15, 16]. The total weight loss is only 13.94% up to 800 캜. As can be expected, Na-MMT shows a high thermal stability. As shown in (Fig.1)1b, a small weight loss (≈3.03%) just below 100 캜 is attributed to the loss of absorbed water of pure PTh. The second weight loss is nearly 63.00% at 800 캜, which was attributed to thermal decomposition of the polythiophene chains. In contrast to PTh, the weight loss of the PTh /DDAB-MMT nanocomposite decreased and exhibited a weight loss of 15.50% at 800 캜. Incorporation of polythiophene with Na-MMT would be expected to enhance the thermal stability of PTh /DDAB-MMT nanocomposite relative to that of PTh. This enhanced thermal stability of the PTh /DDAB-MMT nanocomposite is due to the restricted thermal motion of the PTh in the clay galleries.
The DTA curves of pure Na-MMT, pure PTh, and PTh /DDAB-MMT nanocomposite were also shown in (Fig.1)1. In the DTA curve of the pure Na-MMT, the endothermic peaks which result from dehydration at 75 캜 and dehydroxylation at 635 캜 are observed. Pure PTh shows the endothermic peaks, which corresponds to dehydration (80 캜) and the structure starts to decompose at 280 캜. Figure 11c indicates that, in the nanocomposite, the peaks occurring at 95 and 625 캜 results from the dehydration of nanocomposite and the decomposition of PTh, respectively. The thermal decomposition of PTh /DDAB-MMT shifted the higher temperature than pure PTh, which implies that the composite system has the enhanced thermal stability due to the intercalation of PTh between clay layers.
The PTh obtained by cationic surfactant assisted dilute polymerization method had better capacitor performances than the same obtained by the conventional chemical and electrochemical polymerization methods.
A series of Polythiophene (PTh)/clay nanocomposites were successfully synthesized with montmorillonite modified with Cetyl-Trimethyle - Ammonium Bromide (CTAB) and Didocyl Dimét hyl-Ammonium Bromide (DDAB), 1-bromohexadecane (MSAB).
The thermal properties of nanocomposites were investigated by TGA and DTA. Intercalated nanocomposites are found to be more thermally stable than pure PTh and Na-MMT clay induced the thermal stability. The overall thermal stability trends as follows: pure Na-MMT> PTh/Na-MMT> pure PTh was suggested, respectively. X-ray diffraction results confirmed the insertion of PTh between the interlayers of Na-DDABMMT and the increase in basal spacing (d001) of the nanocomposites.
A modest increase of up to 0.53 nm in layer spacing was observed in the PTh intercalated nanocomposites.
FTIR spectra are also confirming the formation of intercalated PTh in Na-DDABMMT layers. The scanning electron microscopy (SEM) images showed the intercalation of PTh between the clay layers in nanoscale. We also measured the room temperature conductivity of the composites.
With increasing content of PTh, conductivity of composites increased. |
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Conclusions |
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The PTh obtained by cationic surfactant assisted dilute polymerization method had better capacitor performances than the same obtained by the conventional chemical and electrochemical polymerization methods.
A series of Polythiophene (PTh)/clay nanocomposites were successfully synthesized with montmorillonite modified with Cetyl-Trimethyle - Ammonium Bromide (CTAB) and Didocyl Dimét hyl-Ammonium Bromide (DDAB), 1-bromohexadecane (MSAB).
The thermal properties of nanocomposites were investigated by TGA and DTA. Intercalated nanocomposites are found to be more thermally stable than pure PTh and Na-MMT clay induced the thermal stability. The overall thermal stability trends as follows: pure Na-MMT> PTh/Na-MMT> pure PTh was suggested, respectively. X-ray diffraction results confirmed the insertion of PTh between the interlayers of Na-DDABMMT and the increase in basal spacing (d001) of the nanocomposites.
A modest increase of up to 0.53 nm in layer spacing was observed in the PTh intercalated nanocomposites.
FTIR spectra are also confirming the formation of intercalated PTh in Na-DDABMMT layers. The scanning electron microscopy (SEM) images showed the intercalation of PTh between the clay layers in nanoscale. We also measured the room temperature conductivity of the composites.
With increasing content of PTh, conductivity of composites increased. |
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Acknowledgement |
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| We are thankful to Oran University Research Fund and Center National of Research for their financial support to this work. |
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FIGURES |
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Fig.1
FTIR spectra of polythiophene, polythiophene-DDAB, polythiophen e-CTAB. |
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Fig.2
XRD patterns of polythiophene-DDAB. |
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Fig.3
XDR patterns of polythiophene and polythiophene-CTAB |
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Fig.4
FE-SEM micrographs of (a) polythiophene and (b) polythiophene –CTAB. |
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Fig.5
DSC patterns of polythiophene and polythiophene-CTAB, polythiophene-DDAB. |
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Fig.6
FTIR spectra of the organophilic Na-MMT; (a) Na-MMT, (b) MSAB-MMT, (c) Na-MMT-DDAB, (d) Na-MMT-CTAB. |
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Fig.7
XDR patterns of Na-MMT, MSAB-MMT, DDAB-MMT, and CTAB-MMT. |
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Fig.8
DSC patterns of organophilic montmorillonite a) CTAB-MMT, b) DDAB-MMT. |
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Fig.9
XDR patterns of PTh/ clay nanocomposites containing different modified. |
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Fig.10
SEM images of a) pure Na-MMT, b) PTh / DDAB–MMT noncomposites. |
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Fig.11
TGA and DTA curves of a pure Na-MMT, b pure PTh, c PTh/ DDAB-MMT nanocomposite obtained in nitrogen atmosphere at heating rate of 10 캜/min. |
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TABLES |
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Table.1
Variation of the conductivity of Nanocomposite Polythiophéne/OMMT. |
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