CHAPTER 4 INFRARED SPECTROSCOPY STUDIES

Transcription

1 CHAPTER 4 INFRARED SPECTROSCOPY STUDIES

2 CHAPTER 4 INFRARED SPECTROSCOPY STUDIES 4.1 General Introduction Many polymers crystallize and many others do not. Since crystals have a well ordered periodic structure we would not expect to be able to crystallize those polymers which do not possess a periodic structure. On the other hand it can be anticipated that linear polymer molecules with regular periodic structures might arrange themselves side by side to form crystals. The (CH)x prepared by the method of Shirakawa has a fibrillar morphology [l].the fibers are highly crystalline, 75 to 90% of the scattered X-ray intensity constituting sharp Debye Scherrer reflections. But it was found that all forms of polypyrrole reported so far are extremely poorly crystalline. So the structure of this material is not obtained using X-ray methods L21.A useful and simple method of structure determination is with the help of infrared spectroscopy. The infrared spectra of polyacetylene, the first conducting polymer, was first reported by H. Shirakawa and S. Keda in 1971 [31. They have carried out the analysis of normal vibrational modes of polyacetylene and related polymers and have discussed the results in relation to structure of the polymers. the

3 In the case of polypyrrole, the IR spectra in its oxidized and neutral forms were first reported by Streat et al. in 1982 [ 4] and the characteristic peaks of pyrrole were assigned by Furakawa and et al. in 1984 [5]. Later a number of researchers prepared different types of polypyrroles using different preparation techniques and several types of anions, and used IR spectroscopy to determine the presence of those anions and their effect on the structure and properties of the polypyrrole samples. Mitsuhara Asano et al. [6] prepared polypyrrole film from vapour phase using iodine as the counter anion. They compared the IR spectrum of this iodine polypyrrole with those of other polypyrroles prepared electrochemically using different types of counter anions and found that the structure of Iodine polypyrrole is the same as electrochemically prepared polypyrroles. IR spectroscopy is being increasingly employed for the structural charecterization of conducting polymers and to investigate the effect of different anions on their properties. 4.2 IR spectroscopy of polymers In attempting to identify and characterize polymeric materials almost invariably recourse will be made to vibrational spectroscopy. When used either alone or in conjunction with other physicochemical techniques, vibra-

4 tional spectroscopy is capable of providing detailed information on polymer structure [7]. Infrared radiation of frequencies less than about cm-i is absorbed and converted by an organic molecule into energy of molecular rotation. This absorption is quantized, thus a molecular rotation spectrum consists of discrete lines. Infrared radiation in the range from about cm-l is absorbed and converted by an organic molecule into energy of molecular vibration. This absorption is also quantized, but vibrational spectra consist of band rather than lines because a single vibrational energy change is accompanied by a number of rotational energy changes. It is with these vibrational - rotational bands, particularly those occurring between 4000 cm-i and 600 cm-l, that we shall be concerned. The frequency or wavelength of absorption depends on the relative mass of the atoms, the force constants of the bands, and the geometry of the atoms. When molecular vibrations result in a change in the bond dipole moment, as a consequence of charge in the electron distribution in the band, it is possible to stimulate transitions between energy levels by interaction with electromagnetic radiation of appropriate frequency. In effect, when the vibrating dipole is in phase with electric vector of the incident radiation the vibrations are enhanced and there

5 is transfer of energy from the incident radiation to the molecule. It is the detection of this energy absorption which constitutes IR spectroscopy. polymers are usually examined in the solid state and the nature of the technique imposes some limitations on the form of sample which are studied [el. In general, solids have strong IR absorption which means that the samples have to be relatively thin or the absorbing species only present in low concentration when dispersed in a transparent medium. The simplest physical form is thin film prepared either by solvent casting or by hot press- ing. Film thickness are typically in the range to 0.05 mm [8]. Standard sample preparation techniques used for IR examinations of non-polymeric materials including filled KBr discs are also useful when examining polymeric materials [8J. The method requires that the KBr and the polymer sample are finally ground, dispersed to give an approximately 1% concentration of the polymer and pressed in a die using a combination of pressure and vacuum to produce a thin disc. A problem for most polymers arises due to their toughness which means that it is difficult to grind them to a sufficiently fine particle size. The use of mulls although feasible is not generally recommended for polymers [El.

6 There are no rigid rules for interpreting an infrared spectrum. Certain requirements, however, must be noted before an attempt is made to interpret a spectrum [9]: 1. the spectrum must be adequately resolved and of ade adequate intensity. 2. the spectrum should be that of a reasonably pure compound. 3. the spectrometer should be calibrated so that the bonds are observed at their proper frequencies or wave lengths. Proper calibration can be made with reliable standards, such as polystyrene film. 4. the methods of sample handling must be specified. If a solvent is employed, the solvent, concentration, and cell thickness should be indicated. The two important areas for a preliminary examinations of a spectrum are the region cm-i and the cm-i region. The high frequency portion of the IR spectrum is called the functional group region. The characteristic stretching frequencies of important functional groups such as OH, YH, and C=O occur in this portion of the spectrum [9].

7 The lack of strong absorption bands in the cm-l region generally indicates a non aromatic structure. Aromatic and heteroaromatic compounds display strong outof-plane C-H bending and ring bending absorption bands in this region that. can frequently be correlated with the substitution pattern. The intermediate portion of the spectrum, cm-l, is usually referred to as the "finger print" region. The absorption pattern in this region is frequently complex, with the bands originating in interacting vibrational modes Hetero aromatic compounds The spectra of heteroaromatic compounds result primarily from the same vibrational modes as observed for the aromatics. The aromatic type compounds give rise to a large number of very sharp characteristic bands, so that their identific!ation by infrared method is usually straightforward. Furthermore the changes in certain regions which result from substitution are largely independent of the nature of the substituents so that it is usually possible to determine, also, the degree and type of substitution present [9].

8 4.3.2 C-H stretching vibrations Hetero aromatics, such as pyridines, pyrazines, pyrroles, furans, and thiophenes, show C-H stretching bands in the 3077-,3003 cm-i region [9] N-H stretching frequencies Hetero aromatics containing N-H group show N-H stretching absorption in the region of cm-l. The position of absorption within this general region depends upon the degree of hydrogen bonding, and hence upon the physical state of the sample or the polarity of the sol- vent. Pyrrole and indole in dilute solution in nonpolar solvents show a sharp band near 3495 cm - 1, concentrated solution shows a widened band near 3400 cm-l. ~oth bands also may be seen at intermediate concentrations [9] Ring stretching vibrations (skeletal bands) Ring stretching vibrations occur in the region between cm-l. The absorption involves stretching and contraction of all of the bonds in the ring and interaction between these stretching modes. The band pattern and the relative intensities depend on the substitution pattern and the nature of the substituents. Furans,

9 pyrroles and thiophenes display 2 to 4 bands in this region [ C-H out-of-plane bending The C-H out-of-plane bending ( d CH) absorption pattern of the hetero aromatics is determined by the number of adjacent hydrogen atoms bending in phase. The absorption data for the out-of-phase C-H bending ( C H) and ring bending ( $ ring) modes of pyrrole is given below [ 9 ]. Pyrrole - position of substitution-2-acyl, Phase - solid, )( CH or $ ring modes are at cm-i and 755 cm-i. In order tso detect the anion bands in conducting polypyrroles, it is necessary to select an anion which absorbs in a window in the polypyrrole spectra, for - 1 example, below 800 cm, because the region from 10 to 400 cm -1 contains the pyrrole ring vibrations and the anion vibrations. That is the anion vibration bands are masked by the pyrrole ring vibrations [9]. In the case of polyacetylene the spectrum of neutral polymer is charac- terized by a series of sharp peaks, whereas that of the

10 oxidized polymer consists of a few, more intense, very broad peaks. The situation is completely different in the case of polypyrrole, where both the neutral and oxidized forms of the polymer are characterized by similar relatively sharp specftra. Furakawa et al. [S] assigned that the peaks around 1300, 1400, 1450 and 1550 cm-' were due to the skeletal vibrations of the pyrrole ring on the basis of frequency shift on C-deuteration. The peaks at about 3400 cm-i and cm-i are the characteristic peaks of pyrrole. The peak which appears as a shoulder at about 2900 cm-i is due to the C-H stretching. The peak around 780 cm-i shows that the polypyrrole is polymerized through the reaction at oc position [lo]. 4.4 Work included in this chapter Infrared spectroscopy is being increasingly employed for the structural determination of conducting polymers and to investigate the effect of different anions on their properties. In this chapter the author reported the detailed infrared spectroscopy studies of different types of poly- pyrroles prepared by chemical and electrochemical methods

11 2-3- using (Moo4) and (PO4) as counter anions. The effects of preparational temperatures on the structural changes of polypyrrole samples are also investigated. 4.5 Experimental details The details of experimental methods of preparation of 2- polypyrrole Samples using (Moo4) and (PO4) 3- counter anions by electrochemical and chemical methods of polymer- ization are described in sections A, B, C, D, and E of chapter 3. The spectra of the prepared samples were recorded using a Shimadzu model IR 470 spectro- photometer and F.T.I.R. were recorded using a Bruker model IF S66V spectrophotometer by KBr Pellet method, in the 400 to 4000 cm-' range. The principle and the specimen preparation for IR spectra are described in chapter 2, section

12 4.6 Results and discussion Infrared spectra of [PPy(Mo04) ] films prepared electrochemic!ally using aqueous solution of ammonium molybdate and trace of H2S04 as electrolyte 2 - Figure 4.1 shows the infrared spectrum of PPy(MoO4) sample prepared using aqueous solution of ammonium molybdate and H2S04 as electrolyte. The spectral characteristics are given in table 4.1. The peaks at about 3400 cm-' and cm-i are the characteristic peaks 2- of pyrrole. They suggest that PPy(Mo0 ) film maintains 4 the structure of pyrrole without deformation [61. The peaks around cm-lr cm-' are assigned to the skeletal vibrations of the pyrrole ring which is in agreement with the interpretation of Furakawa et a1 [S]. 2 - Also PPy(Mo04) has a shoulder due to C-H stretches at 2900 cm-l. The peak at 940 cm-' is the characteristic peak of molybdate ion. The peaks near 780 cm-' suggest that the P P (~00~)~- ~ film is polymerized through the reaction at q position [lo]. The peak at 1120 cm -1 is the 2- characteristic peak of (SOq) ion [ll]. 2 - Figure 4.2 gives the infrared spectra of PPy(Mo04) film prepared electrochemically at a different temperatures +s"c. From the table 4.2 it is found that

13 Wave number (ern-') 4 Figure 4.1. IR spectrum of PP~(MOO 12- prepared using aqueous solution of am8nium molybdate and ~ $ 0 as ~ electrolyte.

14 Table 4.1 Assignment of IR spectra Sample Peak position Assignment (cm ) 3400 N-H stretches 2900 C-H stretches 1400 C-C, C-N ring stretching - PPy(Mo0 ) 2 film prepared 1290 C-H and N-H using aimoniurn golybdate deformation and H2S04 at 30 c 1120 S-0 stretches CH deformation 940 Characteristic peaks of Mo 780 Polymerization at =G position

15 PI u c m Wave number (cm-') -t Figure 4.2. IR spectra of PPy(Mo8 12- film prepared electrochemically at 5 8.

16 Table 4.2 Assignment of IR spectra Sample Peak posftion Assignment (cm N-H stretches 2900 C-H stretches C-C, C-N ring 1540 stretching PPy(Mo0 ) film prepared using a&oniumomolybdate 1290 C-H and N-H and H2S04 at 5 C deformation 1120 S-0 stretches CH deformation 940 Characteristic peaks of Mo 780 Polymerization at aposition

17 0 the PP~(MOO~)'- film prepared at +5 C has some additional peaks at about 1740 cm-l, 1640 cm-l, 1660 cm-i and cm. Also the peaks of PPy(Mo04) prepared +30 C were broader than those of the film prepared at +5Oc. These results suggest that the molecular structure of polypyrrole film prepared at lower temperature is more ordered than those prepared at higher temperatures [12] Infrared spectra of [PPy(MoO )'-I films prepared 4 electrochemically using aqueous solution of molybdic acid as electrolyte Figure 4.3 shows the infrared spectrum of PPy(Mo sample prepared using aqueous solution of molybdic acid as the electrolyte. The spectral characteristics are given in table 4.3. The peaks at about 3400 cm-i and cm-i are the characteristic peaks of pyrrole. They suggest that P P (~00~ ~ 1 '- film maintains the structure of pyrrole - without deformation [6]. The peaks around cm cm-l are assigned to the skeletal vibrations of the pyrrole ring which are in aggrement with the interpretation of Furakawa et a1 [ 51. Also ppy MOO^)^- has a shoulder due to C-H stretches at 2900 cm". The peak at 940 cm-i is the characteristic peak of molybdate ion. The peaks near 780 cm-i suggest that the P P (~00~ ~ )*- film is polymerized through the reaction at oc position [lo].

18 2 - Figure 4.3. IR spectra of PPy(Mo04) sample prepared using aqueous solution of molybdate acid as the electrolyte.

19 Table 4.3 Assignment of IR spectra Sample Peak posftion Assignment (cm ) N-H stretches C-H stretches C-C, C-N ring stretching PPy (MOO ) 2- film prepared 1290 using m%lybdic acid C-H and N-H deformation -CH deformation Characteristic peaks of Mo polymerization at oc position

20 4.6.3 Infrared spectra of PP~ (~0~) 3- film prepared electrochemically using aqueous solution of orthophosphoric acid as electrolyte Figure 4.4 shows the infrared spectrum of PPy(Po4) 3 - film prepared at room temperature. The peaks at about 3420 cm cm-1 are the characteristic peaks of pyrrole and they suggest that PPy(Po4) 3 - maintains the structure of pyrrole [ 6 1. The peaks around cm-' and cm-' are assigned to the skeletal vibrations 3 of the pyrrole ring. Also PPy(Po4) - has a shoulder due -1 to C-H stretches at 2900 cm-l. The peak at 960 cm is the characteristic peak of PO4 ions. The peak near 780 cm suggests that the PPy(P04) film is polymerized through the reaction at oc position [lo]. The assignment of these peaks are given on the table F.T.I.R. spectra of PPy(P04) films prepared chemically using aqueous solution of orthophosphoric acid in acetonitrile as oxident Figure 4.5 shows the infrared spectrum of PPy(P04) film prepared chemically at room temperature using H3P04 in acetonitrile as oxident. The spectrum shows a number of absorption peaks. The spectral characteristics are given in table 4.6. The peaks at about, 3400 cm-1 and

21 % Wave number (cm ) 3 Figure 4.4. IR spectra of PP~(PO~)'- film prepared at room temperature.

22 Table 4.4 Assignment of IR spectra Sample Peak pos tion Assignment - f (cm ) N-H stretches C-H stretches PPy(P0 ) 3- film prepared 1680 electr8chemically using 1580 aqueous solution of 1420 orthophosphoric acid as electrolyte C-C, C-N ring stretching C-H deformation Characteristic peaks of PO4 Polymerization at position

23 I 1 I LOO Wave number (cm-l Figure 4.5. FTIR spectra of PPy(P0 ) film prepared chemically using H3PO4 ia acetonitrile as oxident. I

24 Table 4.5 Assignment of FTIR spectra Sample Peak pogftion Assignment (cm N-H stretches 2922 C-H stretches PP~(PO ) film prepared 1630 C-C, C-N ring chemicilly using 1400 stretching orthophosphoric acid in acetonitrile as 1296 C-H and N-H electrolyte deformation 1020 C-H deformation 990 Characteristic peaks of PO4 780 Polymerization at sc position

25 cm-' are the characteristic peaks of pyrrole. They suggest that PPy(P0 ) film prepared by chemical 4 oxidative polymerization of pyrrole using orhophosphoric acid in acetonitrile is maintaining the structure of pyrrole without deformation. The peaks around cm and cm may be assigned to the skeletal vibrations of the pyrrole ring. Also PPy(P04) has a shoulder due to C-H stretches at 2900 cm-l. The peak about 990 cm-i is the characteristic peak of PO F.T.1.R spectra of PPy(P04) powder using orthophosphoric acid in acetone as oxidant In the previous section we discussed the IR spectrum of chemically synthesized polypyrrole film using H3P04 in acetonitrile as the oxidant was discussed. But in this case instead of acetonitrile acetone was used as the reacting medium. Polypyrrole in powder form was obtained by this method and figure 4.6 shows the infrared spectra of PPy(P0 ) by using acetone and orthophosphoric acid. The 4 number of absorption peaks obtained in this case is better than in the previous case (4.6.4). The spectral characteristics are given in the table 4.6.

26 Figure 4.6. FTIR spectra of PP~(PO powder, using orthophosphoric acid in aaetone as oxident.

27 Table 4.6 Assignment of FTIR spectra Sample Peak posftion Assignment (cm ) N-H stretches C-H stretches Shoulder due to C-H stretches PPy(P0 ) powder 1690 prepar$d chemically using orthophosphoric acid in acetone as electrolyte 1360 C-C, C-N ring stretching C-H deformation Characteristic peaks of PO 4 Polymerization at o~ position

28 The peaks obtained at about 3400 cm-l, cm-i are the characteristic peaks of pyrrole. They suggest that the PPy(Mo04) obtained by this method is maintaining the structure of pyrrole without deformation. The peaks around cm-l, cm-' are the skeletal vibrations of the pyrrole ring on the basis of frequency shift on C- deuteration. Also this PPy(P0 ) has a shoulder due to CH 4 stretches at 2400 cm-l. The peak at 960 cm - 1 is the characteristic peak of (PO ) ion. 4 Conclusion The IR spectra of polypyrrole samples prepared by electrochemical and chemical methods had been recorded and analysed. The observed peaks were in good agreement with results of earlier works.

29 References 1. J.P. Pouget, "Electronic properties of polym'ers and related compounds" Proceedings of an International Winter School, Kirchberg, Tirol, Feb. 23, March 1, 1985, H. Kuzmanes, M. Mehring and S. Roth (eds.), Springer Verlas, New York, p G.B. Street, "Hand book of conducting polymers" ed. by T.A. Skotheium, Marcel Dekker, New York, Vol.1, (1986), p H. shirakawa, S. Ikeda, Polym. J., 2, (19711, G.B. Street, T.C. Clarke, K. Krounbi, K.K. Kanazawa, V. Lee, P. Pfluser, J.C. Scott and G. Weister, Mol. Cryst. Liq. Cryst., 83, (1982), Y. Fujii, Y. Furukawa, H. ~akeuchi, and I. Harada, Ann. Meet. Chem. Soc. Jpn. Tokyo, Apr. 1984, Mitsuharu Asano, Masui Inoue, Yoshiaki Takai, Teruyoshi Mizutani and Masayuki Ieda, Jap. J. Appl. Phys. 28, (1989), H.W. Siesler and K. Holland Moritz, Infrared and Raman Spectroscopy of Polymers, Marcel Dekker, New York and Basel, (1980). 8. Polymer characterization, Physical techniques, D. Campbell and J.R. White, Chapman and Hall, New York, 1989, p "Spectromet.ric Identification of Organic Compounds", Robert M. Silverstein, G. Clayton Basster, Terence C. Morrill, John Wiley & Sons, New York, 1981, p C.J. Poucheri, The Aldrich Library of F.T. IR spectra Aldrich Chemical Company INC, Milwaukee, "Infrared spectra of complex molecules", L.J. Bellamy, Melhuen & Co. Ltd., London, John Wiley and Sons, INC, New York, (1962). 12. M. Ogawawara, K. Funashashi, T. Demura, T. Hasiwara and K. Iwata, Synth. Met., 14, (19861,