Conformations of polyaniline in polymer blends

Conformations of polyaniline in polymer blends

Journal of Molecular Structure 701 (2004) 13–18 www.elsevier.com/locate/molstruc Conformations of polyaniline in polymer blends Jadwiga Laska* Facult...

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Journal of Molecular Structure 701 (2004) 13–18 www.elsevier.com/locate/molstruc

Conformations of polyaniline in polymer blends Jadwiga Laska* Faculty of Materials Science and Ceramics, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Krako´w, Poland Received 30 March 2004; revised 19 May 2004; accepted 19 May 2004

Abstract Conformational studies of polyaniline (PANi) in its doped, i.e. conducting form, have been performed. The main goal of this study was to determine how the conformations depend on a dopant, solvent and a polymer matrix in polyaniline blends with classic polymers such as poly(methyl methacrylate), polystyrene, cellulose derivatives, polyamides, etc. The obtained results shown that even slight changes in polymer conformations can be easily checked by means of UV – vis –NIR or NIR only spectroscopy. On the basis of the described results, prediction of macroscopic properties of PANi samples, for example, conductivity, at the stage of preparation is possible. q 2004 Elsevier B.V. All rights reserved. Keywords: Polyaniline; UV –vis –NIR spectroscopy; Polymer blends; Conformations

1. Introduction In the group of conducting polymers, polyaniline is the one most often studied because of its most promising applicability. Polyaniline molecule consists of two segments, a flat structure of two imine groups and a quinoid ring, and tetrahedral segments of two amine groups separating three benzenoid rings (Fig. 1). Because of the presence of aromatic rings, the polyaniline chain should be rigid and mostly turn into an expanded coil conformation. However, separation of each two aromatic rings with a nitrogen atom enables for the coil conformation (Fig. 2). The situation in the PANi chain changes upon protonation. This process is accompanied by creation of positive charges on the nitrogen atoms (see Fig. 1). Their repulsion should cause straightening of the chain, which is extremely beneficial for delocalization of the electrons (charges) along the chain, and creation energetically most favourable polaronic structure. The problem, however, is not so simple. The polymer conformation can be strongly influenced by a dopant, as well as a solvent used during the sample preparation. For instance, if the anions of the dopant for the reason of their bulkiness cannot quickly diffuse in between the chains * Tel.: þ48-12-617-2331; fax: þ 48-12-633-7161. E-mail address: [email protected] (J. Laska). 0022-2860/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2004.05.021

following the delocalizing positive charges, one can expect that the chain expansion will be hindered. Xia et al. [1] have recently shown that the conformations of polyaniline chain can be determined on the basis of its UV – vis –NIR spectra. Two kinds of spectra of polyaniline salts have been recorded (Fig. 3) [2 –6]. One of them shows three bands, at 360 nm (3.42 eV), 440 nm (2.80 eV) and 780 (1.58 eV). In the other, only two bands appear, namely at 440 nm (2.80 eV) and a broad band starting at around 800 nm and extending far to near infrared region. The band at 360 nm is assigned to the p – p* electronic transition, while bands, both at 440 and . 780 nm, are connected with creation of polarons [7]. The band extending from 800 nm toward the near infrared region is called a free-carrier tail and is characteristic for highly conductive substances, such as metals, polyacetylene doped with iodine, or poly( p-phenylenevinylene) doped with AsF5. For the comparison, UV – vis spectrum of non-conducting emeraldine base is also shown in Fig. 3. It consists of two bands, at 340 nm (3.8 eV) and 660 nm (2 eV). The band at 340 nm comes from the p – p* electronic transition, and the band at 660 nm is assigned to electronic transformation of benzenoid ring into the quinoid one B ! Q [8,9]. High conductivity means delocalization of electrons, and in organic compounds it is strictly conditioned by flattening the structure of molecule in the region of delocalization. In polymers, a flat structure is the most likely when the chain is straightened. In consequence the spectra, at the same

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2. Experimental

Fig. 1. Chemical structure of polyaniline (a) emeraldine base, (b) protonated PANi in polaronic form.

time, gives us a lot of information both about macroscopic and molecular properties of the compound. Because UV – vis – NIR spectroscopy brings clear information on the conformations of the molecules both in the solution and solid state, it can be a simple and effective method of determining of the quality (usefulness) of samples from the point of view of conductivity as soon as at the stage of preparation. The method can be the most valuable in investigating conducting blends, especially for determining the influence of a dopant, a solvent, and also a polymer matrix on the conformation of the polymer.

Blends of doped polyaniline with classic polymers such as poly(methyl methacrylate) (PMMA), polystyrene (PS), nylon 6, ABS, and cellulose derivatives were obtained in the following way: powder of polyaniline was mixed with liquid DiOHP in molar ratio (y) of phosphate:PANi ranging from 0.5 to 1.0. The samples are ascribed in the paper as PANi(DiOHP)y. The mixture becomes a black paste of doped polyaniline with a green tint. A solution of a classic polymer in m-cresol or chloroform was prepared, and the doped PANi was added in amounts of 2– 45 wt%. A mixture was stirred overnight, then films were casted by evaporating the solvent. They are ascribed as blends I if m-cresol was used as a solvent or blends III if the solvent was chloroform. Some films prepared in m-cresol were casted after earlier centrifugation of the mixture and separation of insoluble parts (these films are ascribed as blends II). Blends containing camphorsulfonic acid (CSA) instead of diisooctyl phosphate (DiOHP) were prepared similarly in spite that all three components were put together into the solvent and then stirred. That means that the protonation of polyaniline occurred together with blending the polymers. UV – vis spectra were recorded on Hewlett-Packard spectrometer.

Fig. 2. Expanded coil (a) and coil (b) conformation of polyaniline.

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Fig. 3. UV–vis spectra of: (a) and (b) protonated polyaniline, (c) emeraldine base.

NIR spectra were recorded on the Biorad FTS-6000 spectrometer.

3. Results and discussion Fig. 4 shows UV – vis and NIR spectra of blends of PANi(DiOHP)0.5 with PMMA. The blends were prepared by mixing plasticized PANi(DiOHP)0.5 with poly(methyl methacrylate) PMMA in m-cresol (blends I and II) or chloroform (blend III), followed by evaporation of the solvent. The blend I differs from the blend II by the way of preparation, i.e. the blend II was casted from the solution after centrifuging insoluble particles. The method of preparation has visible influence on the absorption in the near infrared region, i.e. the shape of the free-carrier band has different shapes. It must be added, though, that the solvent influence is undoubtedly. Spectra of all films contain three characteristic bands, at 360, 440 and 780 nm. Significant differences in the spectra of the given samples are visible in the near infrared region. In the case of films casted from chloroform reasonably narrow band with maximum at 780 nm is observed and it is the only band in that region. The absence of any band above 1000 nm means that no full delocalization of electrons occurs. Consequently one can conclude that the molecules are in the coil conformation. When the films were casted from m-cresol beside the band at 780 nm also band extending above 1000 nm appears. In the UV – vis region also two bands are observed. Coexistence of all four bands at 360, 440, 780 and . 1000 nm means coexistence of both conformations. If the films were casted from the solution obtained by centrifuging off the insoluble part, the band at . 1000 nm shows distinct maximum at 1440 nm, and

Fig. 4. (a) UV–vis and (b) NIR spectra of conducting blends of polyaniline with PMMA. Blends I and II were prepared by stirring 5 wt% of PANi(DiOHP)0.5 and 95 wt% of PMMA in m-cresol, blends III—in chloroform. Blends II were additionally centrifuged, and insoluble parts were separated before film casting.

completely disappears above 2100 nm. If the films were casted from a suspension (without centrifugation) the intensity of the band is evenly high in the whole NIR range. The appearance of the maximum at 1440 nm means that the delocalization of polarons is in some degree hindered. It can be affected by a creation of NHþ 2 groups, which lacking the pair of electrons at the nitrogen atoms efficiently isolate polaronic segments. Such situation is possible only if the excess of the dopant is present, i.e. the content of the dopant molecules is higher than half of the number of nitrogen atoms (y . 0:5). Elemental analysis of the investigated films proves that this situation exist in case of the blends II. In agreement with this conclusion is dependence of UV – vis – NIR spectra of blends II on the dopant

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Fig. 5. NIR spectra of PMMA blends I containing different molar ratios ðyÞ of DiOHP to PANi.

content shown in Fig. 5. For blends containing DiOHP/ PANi with the molar ratio 0.5 the NIR spectrum is analogical as for blends I (also y ¼ 0:5). For the blends representing y . 0:5 the intensity of the band decreases above 1700 nm, and at 1440 a maximum become visible. Spectra of blends containing polystyrene, which were casted from m-cresol without centrifugation, have shape characteristic of expanded coil conformation (Fig. 6). They consist mainly of two bands at 440 nm and the free-carrier tail. The maximum at 780 is practically not visible in UV –vis spectra, and only slightly marked in NIR spectra, which proves that polyaniline takes mainly the conformation of the expanded coil. The centrifuged samples give spectra characteristic of coiled conformation with a maximum at 780 nm. It must be added that films casted from chloroform show spectra exactly the same as blends III of PMMA, i.e. characteristic of coiled chain. Comparing the spectra of blends of PMMA with these of PS we can assume that the PANi chain conformation is also influenced by the polymer matrix and not only by a solvent. Because of the presence of carbonyl groups in the chain, PMMA can create hydrogen bonds with amine groups of polyaniline. As the PMMA itself shows tendency to take a coil conformation it become a pattern for interacting polyaniline molecules. As m-cresol has an acidic nature and can reversibly protonate PANi, some molecules not interacting with PMMA became expanded chains and give the free-carrier tail in NIR spectra. On the other hand, polystyrene is known as showing a strong tendency for separation from other polymers in blends and copolymers (e.g. HIPS, ABS, SBS). In the mixture with polyaniline it also creates separate phase, and the conformation is influenced only by the solvent. This hypothesis is confirmed by spectra of blend containing other polymer matrixes, such as cellulose derivatives, ABS or nylon (Fig. 7). The molecules of cellulose and its

Fig. 6. (a) UV– vis and (b) NIR spectra of conducting blends of polyaniline with PS. Blends were prepared by stirring 5 wt% of PANi(DiOHP)0.5 and 95 wt% of PS in m-cresol. Films of blends I were obtained by evaporation of m-cresol, blends II were additionally centrifuged, and m-cresol insoluble parts were separated before film cast. Conductivity of the blends in brackets.

Fig. 7. NIR spectra of blends containing (a) 5% PANi(DiOHP)0.5 in cellulose propionate, (b) 2% PANi(DiOHP)0.5 in nylon 12 and (c) 2% PANi(DiOHP)0.5 w ABS. All films were casted from m-cresol after centrifugation.

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derivatives are stiff and mostly become expanded chains. As they possess many hydroxyl groups and oxygen atoms they can strongly interact with polyaniline amine groups forcing its molecules to straighten along. In consequence all films of PANi/cellulose derivatives show an intensive band elongating in the whole NIR region, although in several cases strong band at 780 nm is also present. Similarly behave blends of nylon, which as polyamide can create hydrogen bonds with amine through carbonyl group. Nylons are known to create fibres, however, they can coil quite easily. co-Acrylonitrile-co-styrene-co-butadiene (ABS) not surprisingly shows behaviour similar to polystyrene. All discussed blends contained polyaniline doped with diisooctyl phosphate (DiOHP). This dopant, however, influences the conformation of PANi by itself, favouring its coiled conformation. The UV – vis – NIR spectra of PANi(DiOHP)y always consists of three bands at 360, 440 and 780 nm [10]. The change of the dopant immediately is expressed in the change of the shape of the spectra. For instance, applying of camphorsulfonic acid instead of DiOHP in cellulose blend causes disappearance of bands at 360 and 780 and appearance free-carrier tail and increase of a band at 440 nm (Fig. 8). From the presented research we can summarize that chloroform supports coil conformation of polyaniline, and m-cresol expanded coil. This is in agreement with the recently published results that proved solvents chemically neutral to polyaniline, such as chloroform, N-methylpirrolidone, dimethylformamide, or benzyl alcohol favours the coil conformation. Solvents possessing acidic properties with the ability to protonate polyaniline, such as m-cresol, 2-chlorophenol, 1,1,1,3,3,3-hexafluoro-2-propanol, facilitate the chain straightening [2,4,11]. However, if polyaniline is doped with a dopant favouring coil conformation, e.g. DiOHP, both forms will be present even in m-cresol. When doped polyaniline is blended with another polymer it also plays significant role in taking a specific conformations by PANi chain. In general, we can divide polymer matrixes in two groups. One contains polymers possessing hydrogen-bonding groups, like carbonyl group, and the second contains polymers not interacting with PANi—those are mainly polyolefins and polydiens. If polyaniline molecules can hydrogen bond with the polymer matrix their conformation simply follows the matrix pattern, in the other cases conformation depends only on a dopant and a solvent. It is worthy to add that the UV – vis –NIR spectroscopy can be successfully applied for diluted solutions and transparent thin films, but in case of nontransparent samples it is not useful. As PANi has a very strong color both in the doped and undoped form, it is difficult to obtain highly transparent films and solutions. This inconvenience can be reduced by applying only NIR

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Fig. 8. (a) UV –vis spectra of PANi(CSA)0.5, (b) NIR spectra of blend containing 4% PANi(CSA)0.5 in cellulose acetate.

spectroscopy. Near infrared radiation easily pass through optically non-transparent films of PANi blends with the thickness of up to 0.1 mm. Detailed studies of the UV – vis –NIR spectra of doped polyaniline, those found in the literature and the author’s own results, leads to the clear conclusions: the presence of a narrow band at 780 nm proves a coil conformation of the polymer chain, while band extended toward near infrared region—an expanded conformation. Favouring one of the conformations can be connected to conductivity of the sample. Expanding coil makes the possibility for the molecule to become coplanar which make the p-electrons delocalize easy. The delocalization is responsible for the creation of polaron structure of polyaniline and significant increase of conductivity. Consequently, the films casted from chloroform are always less conductive than those casted from m-cresol. Conductivity of some blends together with their spectra is given in Figs. 4 and 6. Also, conductivity of polyaniline doped with DiOHP is lower than that of PANi doped with CSA (5 and 400 S/cm, respectively).

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4. Conclusion

References

Chain conformations of doped polyaniline are influenced by several factors, such as solvents, chemical nature of a dopant, molar ratio of the dopant to PANi, and the polymer matrix in a blend. The conformation, on the other hand, strongly influences the distance of the delocalization of charge-carriers and in consequence conductivity of the sample. The conformation of PANi can be easily determined with NIR spectroscopy that makes it very useful tool for quick and easy estimation of quality of the sample from the point of view of its final conductivity. NIR spectroscopy can be applied during sample preparation before the material is finished. It allows for changing some preparation parameters in the manner to obtain expected properties. Moreover, NIR spectroscopy can be applied for non-transparent solutions and films, which is extremely useful in case of polyaniline samples.

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