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Langmuir 2002, 18, 5720-5727
Sulfonic Acid Doped Thermoreversible Polyaniline Gels. 3. Structural Investigations Tushar Jana, Jhunu Chatterjee†, and Arun K. Nandi* Polymer Science Unit, Indian Association For The Cultivation Of Science, Jadavpur, Calcutta-700032, India Received March 12, 2002. In Final Form: April 29, 2002 The surface morphology of the thermoreversible polyaniline (PANI) gels prepared with dinonylnaphthalenesulfonic acid (DNNSA), dinonylnaphthalenedisulfonic acid (DNNDSA), (()-camphor-10-sulfonic acid (CSA), and n-dodecyloxy sulfonic acid (DOSA) are studied using atomic force microscopy (AFM) for 15% PANI concentration (w/w). The AFM study clearly reveals the formation of lamellar morphology in the gel. X-ray scattering experiments of these gels also support the lamellar structure formation, and the lamellar thickness measured from it remains invariant with composition for the PANI-DNNSA and PANI-DNNDSA systems. In PANI-CSA and in PANI-DOSA systems the lamellar thicknesses vary with PANI concentration. Both X-ray diffraction and electron diffraction experiments on the gels of different compositions of PANI-DNNSA and PANI-DNNDSA systems indicate new spacings of lower dhkl values which are invariant with composition. Similar observations are also found in PANI-CSA and PANIDOSA systems. These results indicate the formation of new unit cells in the lamella due to the microcrystallization of elongated surfactant tails formed under the doped condition. The dhkl values characterizing the lamellar thickness, bilayer thickness, and monolayer thickness are discussed in view of molecular mechanics calculations using the MMX program. The composition dependency of the lamellar thickness is discussed from the cohesive force of the surfactant tails within the lamella.
Introduction Polyaniline (PANI) is an important conducting polymer, but it is difficult to process because of its highly aromatic nature, interchain hydrogen bonding, and charge delocalization.1,2 Recently, long-chain sulfonic acids and phosphonic acids have been used both as doping agents and also as processing aids3-14 of this important polymer. Various techniques, e.g. dispersion polymerization,4,9 emulsion polymerization,14 enzymatic polymerization,12,13 mechanical mixing,5 solvent cast methods,8 etc., are used to produce the PANI-sulfonic acid and PANI-phosphonic acid complexes. However, a slightly different approach of doping with sulfonic acid using the swelled PANI lattice * To whom correspondence should be addressed. E-mail: psuakn@ mahendra.iacs.res.in. † Present address: Dept. of Mechanical Engineering, FAMUFSU College of Engineering, Tallahassee, FL 32310. (1) Gettinger, C. L.; Heeger, A. J.; Pine, D. J.; Cao, Y. Synth. Met. 1995, 74, 81. (2) Vikki, T.; Pietila¨, L-O ¨ .; Osterholm, H.; Ahjopalo, L.; Takala, A.; Toivo, A.; Levon, K.; Passiniemi, P.; Ikkala, O. Macromolecules 1996, 29, 2945. (3) Menon, R.; Yoon, C. O.; Moses, D.; Heeger, A. J. In Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker Inc.: New York, 1998; p 27. (4) Kinlen, P. J.; Liu, J.; Ding, Y.; Graham, C. R.; Remsen, E. E. Macromolecules 1998, 31, 1735. (5) Zheng, W.-Y.; Wang, R.-H.; Levon, K.; Rong, Z. Y.; Taka, T.; Pan, W. Macromol. Chem. Phys. 1995, 196, 2443. (6) Vikki, T.; Ruokolainen, J.; Ikkala. O. T.; Passiniemi, P.; Isotalo, H.; Torkkeli, M.; Serimaa, R. Macromolecules 1997, 30, 4064. (7) Vikki, T.; Isotalo, H.; Ruokolainen, J.; Passiniemi, P.; Ikkala, O. Synth. Met. 1999, 101, 742. (8) Olinga, T. E.; Fraysse, J.; Travers, J. P.; Dufresne, A.; Pron, A. Macromolecules 2000, 33, 2107. (9) Kababya, S.; Appel, M.; Haba, Y.; Titelman, G. I.; Schmidt, A. Macromolecules 1999, 32, 5357. (10) Jana, T.; Nandi, A. K. Langmuir 2000, 16, 3141. (11) Pron, A.; Osterholm, J. E.; Smith, P.; Heeger, A. J.; Laska, J.; Zagorska, M. Synth. Met. 1993, 57, 3520. (12) Nagarjan, R.; Tripathy, S.; Kumar, J.; Bruno, F. F.; Samuelson, L. Macromolecules 2000, 33, 9542. (13) Nagarajan, R.; Liu, W.; Kumar, J.; Tripathy, S. K.; Bruno, F. F.; Samuelson, L. A. Macromolecules 2001, 34, 3921. (14) Paul, R.; Pillai, C. K. S. Synth. Met. 2000, 114, 27.
in the formic acid medium produces thermoreversible PANI gels.6,10,15 In the doping process, the long-chain sulfonic acids replace the doped formic acid molecules, which become removed during the drying processes. The long-chain sulfonic acids yield thermoreversible PANI gels when they are used in larger concentrations (>50% w/w).7,15 The conducting polymer gels are of great interest for material scientists because of the excellent combination of their elastic property and significant electrical conductivity.16 In the gel the conductivity is a function of both intrachain and interchain contributions16,17 and it is much higher than that of the solvent cast film.18 In the above PANI-sulfonic acid gels the dependency of conductivity with sulfonic acid concentration is interesting and shows a maxima in each case. The conductivity has been found to vary both with the doping level and the cross-linking density.15 The elastic behavior of the conducting polymer gels has been studied by Vikki et al., and they have observed almost a complete recovery for repeated runs when the samples are compressed to 80% of its original thickness.6,7 The PANI gels exhibit reversible first-order phase transition, and a SEM study revealed the presence of a three-dimensional network structure.10,15 Thus, the PANI-sulfonic acid systems fulfill all the characteristics of thermoreversible gel formation.19,20 The structures of these gels are not yet clear and are important to elucidate for designing and fabricating new materials. PANI under doped conditions with sulfonic acid behaves like a comb-shaped polymer.5,6 The comb-shaped (15) Jana, T.; Nandi, A. K. Langmuir 2001, 17, 5768. (16) Fizazi, A.; Moulton, J.; Pakbaz, K.; Rughooputh, S. D. D. V.; Smith, P.; Heeger, A. J. Phys. Rev. Lett. 1990, 64, 2180. (17) Xia, Y.; Wiesinger, J. M.; MacDiarmid, A. G.; Epstein, A. J. Chem. Mater. 1995, 7, 443. (18) Malik, S.; Jana, T.; Nandi, A. K. Macromolecules 2001, 34, 275. (19) Guenet, J. M. Thermoreversible Gelation of Polymers and Biopolymers; Academic Press: New York, 1992. (20) Daniel, C.; Dammer, C.; Guenet, J. M. Polym. Commun. 1994, 35, 4243.
10.1021/la025724y CCC: $22.00 © 2002 American Chemical Society Published on Web 06/22/2002
Doped Thermoreversible Polyaniline Gels
polymers, e.g. poly(alkyl acrylates), are reported to undergo gelation via crystallization of the side chains in the appropriate solvent.21 However, Vikki et al.6 proposed that the PANI-dodecylbenzenesulfonic acid (DBSA) gel is formed due to cross-linking through localized mesomorphic domains. Therefore, there is a lot of ambiguity in the mechanism of gelation in this type of polymer. In the PANI-dinonylnaphthalenesulfonic acid (DNNSA), PANI-dinonylnaphthalenedisulfonic acid (DNNDSA), PANI-camphorsulfonic acid (CSA), and PANI-dodecyloxy sulfonic acid (DOSA) gels, it has been proposed on the basis of thermodynamic study that gelation may occur due to the side chain crystallization of the dopants anchored from the PANI chains.15 The resulting system may, therefore, produce a lamellar structure as in many other gels.22,23 From the phase diagrams and enthalpy of gel fusion/gel formation values it has been proposed that there are both monolayer and bilayer microcrystallizations of the doped surfactant molecules to form the lamella.15 Here we present some direct experimental evidence for the formation of lamellar structure in this PANI-sulfonic acid gel. The structures of these gels studied by atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS), and electron diffraction are presented. All these results indicate the presence of lamellar structure in the gel. The dependency of the lamellar thickness on composition is studied by WAXS experiment, and a probable explanation for the compositional variation of lamellar thickness of the PANI-sulfonic acid gels is presented. Molecular mechanics calculation (MMX program) is used to interpret the molecular cause of lamella formation. Experimental evidence for the formation of microcrystallites of the surfactant tails within the lamella is also presented from both WAXS and electron diffraction. Experimental Section Samples. Polyaniline [emeraldine base (EB) form] was synthesized in the laboratory by polymerizing aniline in HCl medium with ammonium persulfate initiator and digesting it with NH4OH solution.24 The molecular weight (Mv) from intrinsic viscosity in H2SO4 at 25 °C was 15 500.10 The dopants DNNSA and DNNDSA were gifts from Dr. P. J. Kinlen, Monsanto Co., St. Louis, MO.4 CSA was purchased from Fluka, and DOSA was obtained by passing an aqueous solution of sodium dodecyl sulfate (SDS) through a column containing acidic resin followed by drying. The PANI-sulfonic acid gels were prepared from formic acid medium using the procedure described earlier.10,15 PANI (EB) and sulfonic acids, mixed at different compositions (w/w) were taken in formic acid (BDH, AR) and stirred for 24 h at 65 °C. The formic acid was removed by a pull of air at 80 °C on a Petri dish. These gels were finally dried at 60 °C in a vacuum for 1 week and were stored in a vacuum desiccator. Microscopy. Atomic force microscopy (AFM) was used to determine the microstructure of the gel. A D-3000 Nanoscope (Digital Instruments, Santa Barbara, CA) AFM instrument was used in the tapping mode at a resonance frequency of the tip end equal to ∼300 kHz. A small amount of the gel was placed on a glass slide, and a thin smear was drawn with that using a glass coverslip. Similar objects found by primary surface focusing step were further scanned, and the pictures were taken in the amplitude mode. Some of these pictures were further observed using 3-D software. The electron diffraction studies of the samples were done using a TEM apparatus (Philips CM 200) at the National Metallurgical Laboratory, Jamshedpur, India, operating at 200 kV. The samples (21) Plate, N. A.; Shilaev, V. P. Comb shaped Polymers and liquid Crystals; Plenum Press: New York, 1987. (22) Flory, P. J. Faraday Discuss. 1971, 57, 7. (23) Nijenhuis, K. t. Adv. Polym. Sci. 1997, 130, 1. (24) Cao, Y.; Andreatta, A.; Hegger, A. J.; Smith, P. Polymer 1989, 30, 2305.
Langmuir, Vol. 18, No. 15, 2002 5721 were prepared by dropping an ∼0.2% (w/v) solution of the gel in formic acid (in acetone for the PANI-DNNSA system) onto a carbon-coated copper grid and were dried in a vacuum at 35 °C for 3 days. The electron diffraction experiments were done on the network structure of the sample in the instrument, and the photographs were taken at a camera length equal to 1 m. X-ray Diffraction Experiment. The X-ray diffraction experiments were performed by a Rigaku DMAX 2500 diffractometer at the National Chemical Laboratory, Pune, India. The instrument had a rotating anode generator with a copper target, and it was operated at 40 kV and 150 mA. All the experiments were performed in the reflection mode. The samples were operated in the continuous mode at a scan speed of 2°/min with RINT 2000 wide-angle goniometer and a scintilation counter detector. Both the microscopy and the X-ray diffraction experiments were performed under extremely dry conditions. Molecular Modeling. The molecular modelings of these gels were performed using a molecular mechanics (MMX) program.25 For simplicity two polymer chains, each with four monomeric units, remaining side by side were considered to constitute the polymer layer. The surfactants were incorporated into the polymer layers in such a way that the aminic nitrogen of PANI was connected through H-bonding from the sulfonyl oxygen and the iminic nitrogen was connected through H-bonding from the hydroxyl group of sulfonic acid.6,10,15 The surfactant tails were drawn in the extended chain conformation, and the lamellar distances were fixed at those obtained from X-ray analysis. After energy minimization through the MMX program, we inquired the various distances and these are shown in the respective figures.
Results Atomic Force Microscopy (AFM). In Figure 1 the AFM pictures of the gels (WPANI ) 0.15) are presented together with that of pure PANI (EB). It is apparent from the figures that the morphologies of the PANI gels are different from that of pure PANI (EB). In the former systems bright stripes are seen and they are absent in the PANI (EB). In analogy with the lamellar morphology of block-copolymers studied by AFM,26 the bright stripes may be attributed to the lamella of PANI-surfactant systems and the valleys may be attributed to the excess of nonlamellar sulfonic acid/PANI chains present in the system. This is in contrast to the surface morphology studied by SEM where fibrillar networks are clearly seen. Though both instruments characterize the morphology of the surfaces, the resolution power of SEM is 500-600 times lower than that of AFM. Consequently, AFM resolves the surface structure very well and it shows that each fibril (primary object) has a lamellar organization. A representative three-dimensional AFM picture of a PANI-sulfonic acid doped gel is shown in Figure 1f. This figure also clearly shows the lamellar structure of the PANI-DNNSA gels. Literature reports of AFM studies of polymer gels usually indicate the fibrillar network or globular morphology;27-29 however, in some cases polymer strands are also observed.29 Our results clearly show the presence of lamellar structure in sulfonic acid doped PANI gels. In PANI-HCl doped and in PANI-DBSA doped systems, previous workers observed granular morphology from STM studies and they attributed it to nonuniform doping (25) Gajewski, K. E.; Gilberr, M. H. In Advances in Molecular Modeling; Liotta, D., Ed.; JAI Press: Greenwich, CT, 1990; Vol. 2. (26) Magonov, S. N.; Whangbo, M.-H. Surface Analysis with STM and AFM-Experimental and Theoretical Aspects of Image Analysis; VCH: Weinheim, Germany, 1996; p 303. (27) Suzuki, H.; Suzuki, A. Colloids Surf., A 1999, 153, 487. (28) Curran, M. D.; Stiegman, A. E. J. Non-Cryst. Solids 1999, 249, 62. (29) Power, D.; Larson, I.; Hartley, P.; Dunstan, D.; Boger, D. V. Macromolecules 1998, 31, 8744.
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Figure 1. AFM pictures of PANI-surfactant gels taken in amplitude mode for (a) PANI-DNNSA, (b) PANI-DNNDSA, (c) PANI-CSA, (d) PANI-DOSA, (e) PANI, and (f) PANI-DNNSA (three-dimensional).
of PANI.30,31 However, Zheng et al. also observed some stripelike morphology and attributed it to the lamellar structure in the PANI-DBSA system.5 The difference between the morphologies of the acid-doped PANI samples of previous workers5,30,31 is probably due to the different preparation procedure. In the formic acid medium the doping of sulfonic acids is uniform giving a regular lamellar morphology. However, for hydrochloric acid doped samples lamellar morphology may not be observed since the molecular force for the lamella formation is inadequate (see Discussion). X-ray Diffraction. In Figure 2 the X-ray diffraction patterns of the PANI-DNNSA system are shown from (30) Bonnell, D.; Angelopoulos, M. Synth. Met. 1989, 33, 301. (31) Jeon, D.; Kim, J.; Gallagher, M. C.; Willis, R. F. Science 1992, 256, 1662.
the 2θ value of 2°. It is apparent from the figure that there are peaks in the smaller diffraction angles (2θ < 5°) for all the compositions; however, in the higher diffraction angle region (2θ > 5°), the crystalline peaks are not prominent except for the diffractrogram of WPANI ) 0.40. A careful observation of the diffractrograms of WPANI ) 0.15-0.25 indicates the presence of very small peaks which are also identified by the computer attached to the instrument. This sample also exhibits a good electron diffraction pattern (presented later). A comparison of the X-ray diffractrogram of PANI WPANI ) 0.40 with that of WPANI ) 1.0 (i.e. PANI EB) reveals that the peaks are different. This indicates that some new crystallites are formed in this gel and at this composition their concentration is higher than those of the compositions WPANI ) 0.05-0.25. For WPANI ) 0.60 apart from the peaks in higher
Doped Thermoreversible Polyaniline Gels
Langmuir, Vol. 18, No. 15, 2002 5723
Figure 4. X-ray diffraction pattern of WPANI ) 0.40 for (A) PANI-DNNDSA and (B) PANI-CSA. Inset: CSA diffraction pattern.
Figure 2. X-ray diffraction patterns of PANI-DNNSA gels for the indicated weight fraction of PANI.
Figure 3. X-ray diffraction patterns of PANI-DOSA gels for the indicated weight fraction of PANI.
angle region there are peaks also in the lower angles (d ) 32.7, 16.1, and 9.5 Å). The reason for the lower angle peaks in this composition is probably the same as in the other compositions. The absence of network structure at this composition may be due to the lack of sufficient concentration of fibrils required for its formation. A comparison of the AFM picture and lower angle X-ray diffraction data clearly shows that the d value of ∼30 Å corresponds to the lamellar thickness of the gel samples of this system. However, we are unable to measure the long distance because of the limitation of the instrument. So, lamellar structure is present for all the compositions (i.e. WPANI ) 0.05-0.60) studied here. In Figure 3 the X-ray diffractrograms of the PANIDOSA system for the different compositions are shown with the diffractrogram of pure DOSA. It is apparent from the figure that there are intense diffraction peaks at
smaller angles (2θ < 5°) and these are absent in either the DOSA or in PANI (EB) diffractrograms (Figure 2). So like the former system, here also the smaller angle peaks are probably due to lamellar structure formation. In the region 2θ > 5°, DOSA itself has intense peaks, but in the gel some of these peaks are absent (e.g. 2θ ) 17.3, 17.5, 26.2) and some new peaks are observed. So to identify these new peaks the DOSA peaks should be subtracted from the gel diffractrograms to avoid any confusion that may arise from the unreacted DOSA in the system. The other two systems, i.e. the PANI-DNNDSA and PANI-CSA systems, also have similar X-ray diffraction behavior; the former behaves like the PANI-DNNSA system whereas the latter behaves like the PANI-DOSA system. CSA itself has strong diffraction peaks, but in its gel some peaks are absent and also some new peaks are formed as in the PANI-DOSA system. Representative diffractograms of WPANI ) 0.40 are presented in Figure 4 for the PANIDNNDSA and PANI-CSA systems. In the inset of the figure the CSA diffraction pattern is shown, and it is clear that in the PANI-CSA gel (WPANI ) 0.40) its major diffraction peaks are almost lost. In Tables 1-4 the d spacings and the intensity ratio I/I0 of the various peaks of the X-ray diffraction patterns are presented. For the PANI-CSA and PANI-DOSA systems the respective CSA and DOSA peaks are subtracted from their gel spectra during the compilation. Therefore, in Tables 1-4, the d spacings are of completely new origin except some peaks of PANI (EB) particularly at higher PANI (EB) concentration (denoted by asterisks). These data may reflect the spacings of the crystallites formed from the elongated surfactant tails, in the doped condition. Besides, the lamellar thickness, the bilayer and monolayer thicknesses of surfactant tails may also be obtained from the data of Tables 1-4. In the discussion attempt is made to correlate the X-ray data with the approximate molecular models of these gels. Electron Diffraction. To confirm that there are crystalline entities in the PANI-DNNSA and in the PANI-DNNDSA gels (as the X-ray diffraction intensities are poor), the electron diffraction patterns of both the systems are performed for different gel compositions. The representative electron diffraction patterns for the compositions WPANI ) 0.05, 0.25, and 0.40 are presented in Figure 5 for the PANI-DNNSA system. All these diffractrograms show the presence of concentric rings indicating that the samples contain polycrystalline enti-
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Table 1. dhkl Values and I/I0 for PANI-DNNSA Systems WPANI ) 0.05
WPANI ) 0.15
WPANI ) 0.25
WPANI ) 0.40
WPANI ) 0.60
dhkl (Å)
I/I0
dhkl (Å)
I/I0
dhkl (Å)
I/I0
dhkl (Å)
I/I0
dhkl (Å)
I/I0
31.0
100
29.4 20.1 8.75
100 46 9
4.50
14
31.0 23.5 9.25 5.84a 4.64
100 85 10 8 16
100 30 38 43 19
11
3.54
100 10 11 7 10 10 8
32.7 16.0 9.5 5.8 4.51
3.53
31.53 17.14 8.93 5.63 4.94 4.61 3.61
a
8
3.67a 3.37a 3.06
49 89 8
dhkl values for PANI (EB). Table 2. dhkl Values and I/I0 for PANI-DNNDSA Systems WPANI ) 0.05
WPANI ) 0.15
WPANI ) 0.25
WPANI ) 0.40
WPANI ) 0.60
dhkl (Å)
I/I0
dhkl (Å)
I/I0
dhkl (Å)
I/I0
dhkl (Å)
I/I0
dhkl (Å)
I/I0
26.4 21.1 15.91 11.80
4 100 22 18
27.05 18.78 12.35 9.50 8.70
100 25 8 6 5
25.96 17.14 13.29 9.02
41 100 70 52
26.35
100
25.22
62
13.08
48
12.62 8.97
58 70
8.45 6.18
22 9
5.69
50
5.28 4.48 4.28 3.17 2.22 2.03
11 23 25 9 12 7
57 43 43 19 28
4.51
29
67
3.58 3.39a
100 54
a
4.58
6
4.61
35
8.79 6.32 5.82 5.20 4.48
3.18
1
3.60
17
3.56
dhkl values for PANI (EB). Table 3. dhkl Values and I/I0 for PANI-CSA Systems (Subtracted from CSA Peaks) WPANI ) 0.05
WPANI ) 0.15
WPANI ) 0.25
WPANI ) 0.40
WPANI ) 0.60
dhkl (Å)
I/I0
dhkl (Å)
I/I0
dhkl (Å)
I/I0
dhkl (Å)
I/I0
43.0
100
36.8 23.2 16.1 14.7 13.5 11.2
3 1 38 21 14 9
27.59
21
25.96
36
15.91
17
15.09
24
11.69 9.76 9.06
19 22 25
11.32
2
9.15
9
3.76 a
6
9.11
26
3.57
17
3.55
50
5.98 4.43 3.54 3.31a
100 49 71 48
dhkl (Å)
I/I0
25.2
24
12.9
26
9.71
23
6.00
100
3.52 3.29a
74 42
dhkl values for PANI (EB). Table 4. dhkl Values and I/I0 for PANI-DOSA Systems (Subtracted from DOSA Peaks) WPANI ) 0.05
WPANI ) 0.25
WPANI ) 0.40
WPANI ) 0.60
I/I0
dhkl (Å)
I/I0
dhkl (Å)
I/I0
dhkl (Å)
I/I0
dhkl (Å)
34.61
3
36.03 32.69
66 100
40.12
100
22.64 17.14 11.32 7.52 6.85
4 1 1 1 1
16.81 11.32
14 10
19.84 13.08
27 25
42.03 30.44 20.53 13.91
100 52 13 10
10.71
8.26
27
4.84 4.67
46 52
3.02 2.81
27 82
4.57 4.48 3.01 2.80 a
WPANI ) 0.15
dhkl (Å)
2 2 1 3
5.73
1
4.29
2
I/I0
34 5.94*
13
4.49
21
2.82
18
5.88a 4.80 4.51
70 74 82
2.83 96
dhkl values for PANI (EB).
ties.32 However, a simultaneous occurrence of the spots is also observed in these systems. The spots may not be due to any single-crystal formation because the spots are (32) Andrews, K. W.; Dyson, D. J.; Keown, S. R. Interpretation of Electron Diffraction Patterns; Plenum Press: New York, 1967.
situated on the rings and also in the gelation condition it is difficult for the single crystals to be produced. The upper limit of the crystal size showing ring patterns by electron diffraction is 102-103 Å, and above it the electron diffraction pattern gives spots.10,32 The polycrystalline materials have a large number of individual randomly oriented crystal-
Doped Thermoreversible Polyaniline Gels
Langmuir, Vol. 18, No. 15, 2002 5725 Table 5. dhkl Values (Å) Calculated from Electron Diffraction Patterns for PANI-DNNSA and PANI-DNNDSA Systemsa PANI-DNNSA
PANI-DNNDSA
WPANI ) WPANI ) WPANI ) 0.05 0.25 0.40 2.44 1.41 1.22 0.86
2.44
3.54
1.35 1.22
2.74 2.33 (2.22) 2.38 1.96 (2.03)
WPANI ) 0.40
3.78 (3.60) 5.22 (5.20)
1.50 a
WPANI ) 0.25
2.44 2.38
1.44 1.42 1.22
WPANI ) 0.05
1.41
3.05 1.99 1.77 1.50
Values in parentheses indicate those found in X-ray results.
rings may be explained from the crystallite size. Probably it is smaller in these samples, and consequently, X-ray diffraction intensities are not strong enough to show any diffraction peak. The electron beam in the transmission electron microscopy is highly intense. and therefore, the cooperative scattering intensity of the crystallites is strong enough to affect the photographic plate. However, in the X-ray diffractrogram of the PANI-DNNSA (WPANI ) 0.40) system, the diffraction peaks are distinctly seen. Though no definite reason is known, probably the higher crosslinking density15 may increase the scattering intensity showing the distinct X-ray diffraction peaks for this composition. In Table 5 the d spacings calculated from the electron diffraction patterns are presented for both the PANIDNNSA and PANI-DNNDSA systems. The dhkl values from electron diffraction are in some cases found to tally with the X-ray dhkl values (given in parentheses) for the PANI-DNNDSA system. Where the X-ray diffraction intensity is poor, electron diffraction experiments may yield the new dhkl values. In the PANI-DNNSA system it is apparent from Figure 5 and Table 5 that the dhkl values are almost invariant with composition, which indicates that the crystallites of surfactant tails formed are of same nature for all the compositions studied here. A detailed analysis of this aspect will be presented in the following section for all the surfactant systems. Discussion
Figure 5. Electron diffraction pattern for PANI-DNNSA gel (camera length ) 1 m) for WPANI: (a) ) 0.05; (b) ) 0.25; (c) ) 0.40.
lites, and each such crystallite gives spots for a particular Millar plane satisfying Bragg’s condition of diffraction by electron beams. These spots lie on a cone of semiverticle angle 2θ (θ ) diffraction angle), and it intersect the plane of the photographic plate giving a ring. When the size of polycrystalline material is large, the number of such individuals is low. Consequently, the diffraction spots will be lower in number to fulfill the ring.32,33 The absence of distinct X-ray diffraction peaks in PANIDNNSA system but the presence of electron diffraction (33) Thomson, G. P.; Cochrane, W. Theory and Practice of Electron Diffraction; Macmillan and Co. Ltd.: London, 1939; p 126.
It is apparent from the above results that PANI-sulfonic acid gels have a lamellar structure for all the dopants studied here. The average lamellar thicknesses from AFM study (∼30 A°) tally satisfactorily with the lamellar thicknesses determined from the X-ray diffraction results for the PANI-DNNSA system. It is important to note that in the PANI-DNNSA systems the X-ray results indicate that the lamellar thickness (29.4-32.7 Å) is almost independent of gel composition. This is probably due to the tight nature of the PANI-DNNSA lamella formed from the crystallization of the surfactant tails of doped DNNSA molecules through the interdigitation of the nonyl tails. A molecular model of the PANI-DNNSA lamella is shown in Figure 6, which is the energyminimized structure obtained from the molecular mechanics calculations (MMX program25,34). The tight nature of the lamella may arise due to the stacking of the naphthyl (34) In the molecular models the numbers in the figure indicate the distance (Å) between the points when inquired from the energyminimized structure. The dotted lines at this region indicate H-bonding that occurred during energy minimization. Where H-bonding does not occur, their distances are shown, and these are close to the weak (N-H) H-bonding distance (2.98 Å) found in ammonium azide.35
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Figure 6. Molecular modeling of the lamellar structure of the PANI-DNNSA system after energy minimization through the MMX program. The numbers in the figure indicate the distance (Å) between the points in the energy-minimized structure.
groups together with the nonyl tails in the doped condition of DNNSA. Apart from the van der Waals interaction of dinonyl tails, the interchain attractions through the aromatic naphthyl groups are strong enough to produce the tight lamella. Further evidence of the tight nature of the lamella comes from the bilayer distance (8.5 Å in Figure 6), which remains almost invariant with composition (8.79.5 Å) (Table 1). In the PANI-DNNDSA systems also the lamellar thickness remains invariant with composition (25.2-27.0 Å) (Table 2). From the model (similar to Figure 6) the bilayer thickness is 8.4 Å (cf. Supporting Information), and from the Table 2, it is apparent that the bilayer thickness is also invariant with composition (8.5-9.0 Å). From the molecular model the surfactant chain length is 18.6 Å, and this almost tallies with the experimental values. The dhkl value of 3.58 Å as obtained in many compositions may also be equated to the distances between the two nonyl chains for both PANI-DNNSA and PANIDNNDSA systems. Thus, the dhkl values of the PANIDNNDSA system may be explained. An interesting situation occurs in the PANI-CSA system where the lamella thickness decreases with increasing PANI concentration. CSA is a smaller molecule than any of the sulfonic acids used here. Consequently, chain interdigitation is not possible with this molecule like the others. A probable explanation for the lamellar structure may be due to the fact that CSA itself is capable of crystallization with the other CSA molecules forming the lamella (Figure 7). So it dopes the PANI chains, and in the middle of the lamella the CSA molecules crystallize under conditions somewhat different from those in the pure system.36 So these crystals may be somewhat different from the ordinary CSA crystals, since they crystallize under strained condition within the lamella. With decreasing CSA concentration, the intralamellar CSA molecules decrease thereby decreasing the lamellar thickness, and this may be true for the compositions WPANI ) 0.05 (35) Pauling, L. The Nature of the Chemical Bond and the Structure of Molecules and Crystals, 3rd ed.; Cornell University Press: Ithaca, NY, 1960; p 463.
Figure 7. Molecular modeling of the lamellar structure of the PANI-CSA system after energy minimization through the MMX program. The numbers in the figure indicate the distance (Å) between the points in the energy-minimized structure.
f 0.15 f 0.25, where a decrease in lamellar thickness by 7-9 Å is observed in each step. This may be clarified from the molecular modeling of the PANI-CSA system using the MMX program (cf. Supporting Information). Within the lamella a CSA molecule has dimension of ∼9 Å. The lamellar dimension decreases roughly in this order for varying the composition of the gel as WPANI ) 0.05 f 0.15 f 0.25. So it may be argued from the measured lamellar dimensions that with decreasing CSA concentration from WPANI ) 0.05 f 0.15 f 0.25 in each step one CSA molecule is lost from the lamella. However, for gel compositions of WPANI ) 0.25-0.60 the lamellar dimensions are almost constant because there may be only two CSA molecules attached side by side to the PANI layers in the lamella and this is the minimum requirement for the lamella formation, fixing the lamella thickness at a value of ∼26 Å. In this system one may inquire about the monolayer and bilayer crystallization.10,15 Such a phenomenon is not possible due to the absence of a long-chain structure; however, a careful observation of the model clearly shows that the middle portion of the lamella is highly dense whereas the two sides are less dense. It may, therefore, be appropriate to consider the middle layer to be the bilayer with a thickness of 14.6 Å and the less dense layer (thickness ∼10.6 Å) as the monolayer. Table 3 approximately correlates all these distances with the models. (36) After minimization of an isolated CSA molecule using the MMX program, the maximum value of molecular thickness inquired is about 7.3 Å (Supporting Information Figure S4). The same distance inquired from the CSA molecule within the lamella is 8.9 and 9.0 Å from the Supporting Information Figures S2 and S3. So in the lamella the CSA molecule becomes extended by ∼20%.
Doped Thermoreversible Polyaniline Gels
In sharp contrast to the PANI-CSA system there is an increase in lamella thickness with increasing PANI concentration in the PANI-DOSA gels, and at WPANI ) 0.60 the lamella structure vanishes (Table 4). This may be qualitatively explained from the crystallization forces within the lamella. At higher DOSA concentration all the sites of the PANI chains are fully doped and the interdigitation of the surfactant tails is complete. So there occurs a strong cohesive force between the elongated tails of DOSA molecules. This causes a lowest lamellar thickness value. At lower DOSA concentration, not all the PANI sites are fully doped; rather there may be some vacancy. This may lower the crystallization force causing the lamella to expand, and it is probably the lamellar behavior with increasing the PANI concentration. When the PANI concentration reaches WPANI ) 0.60, probably the vacant positions are large enough to lose the crystallization force and the lamella breaks. The difference in variation of lamellar thickness with composition compared to those of the PANI-DNNSA and PANI-DNNDSA systems is probably due to the absence of aromatic ring structure which has a stronger cohesive force than the aliphatic chains. Now discussion will be made about the dhkl values characterizing the new unit cells produced due to the crystallization of elongated surfactant tails within the lamella. A glance at Tables 1-4 indicates that there are some new (low valued) dhkl data which are absent either in the surfactant or in the PANI crystallites. As discussed earlier, this new dhkl data may arise from the crystallites formed from the overlap of the elongated surfactant tails. The electron diffraction experiments in PANI-DNNSA systems clearly indicate the existence of these crystallites, and the d spacings are independent of compositions as the diffraction rings are equally spaced (Figure 5). In Table 5 the dhkl values calculated from electron diffraction rings are presented and some of these dhkl values tally with the X-ray results (indicated by the values within the parentheses). Now if one wants to compare the lower valued dhkl data with composition, interesting results are obtained. With increasing PANI, dhkl values almost do not change as is apparent from Figure 8. These results clearly indicate that the unit cell parameters are constant for the crystallites produced from the surfactant tails. This indicates that the unit cell nature is independent of PANI concentration in the gel for all the systems. This may offer additional support to the crystallization of the elongated surfactant tails for the lamella development in the PANI sulfonic acid doped thermoreversible gels. Now a question may arise about the number of PANI chains present in the PANI layers of the lamella. A definite answer is not known; however, it may be predicted that the number of PANI chains in the PANI layer is much less than required for its crystallization, which may impart its infusibility.1,2 An approximate idea of it may come from the modeling. The PANI chain has a lateral dimension of 4.5 Å,37 and so to approximately fit the models it may require two PANI chains in each side of the lamella where they may remain as twisted/straight. The exact nature of the PANI layer in the lamella is yet to be understood. (37) Porter, T. L. Atomic Force Microscopy/Scanning Tunneling Microscopy; Cohen, S. H., et al., Eds.; Plenum Press: New York, 1994; p 229.
Langmuir, Vol. 18, No. 15, 2002 5727
Figure 8. dhkl (Å) vs WPANI plots for lower dhkl values (see text) for PANI-DNNSA (O, b), PANI-DNNDSA (4, 2), PANI-CSA (0), and PANI-DOSA (3, 1).
Conclusion This study clearly reveals that the structure of PANIsulfonic acid doped gels is lamellar and the lamella may be formed due to the interdigitation of elongated surfactant tails under doped condition, followed by their microcrystallization. Atomic force microscopy directly shows the existence of lamella, X-ray studies characterize the lamella, and finally the molecular mechanics calculation attempts to offer a molecular view for the formation of the lamella. With increasing PANI concentration in the gels, the lamellar thicknesses vary for the different sulfonic acids characterizing the molecular basis of lamella formation. However, the unit cell parameters of the newly formed surfactant chain crystallites in the lamella remain invariant with composition in all the systems. This indicates that the unit cells formed from the hydrocarbon tails of the surfactants are almost the same for different compositions of the gel. This study clearly indicates how the surfactant tails of doped sulfonic acid molecules may produce a lamella structure with thermoreversible characteristics. Acknowledgment. We gratefully acknowledge Dr. P. J. Kinlen of Monsanto Co., St. Louis, MO, for his valuable gift of DNNSA and DNNDSA samples. We are grateful to the Council of Scientific and Industrial Research, New Delhi [Grant No. 01 (1449)/97 EMR-II], for financial support of the work. We are also grateful to Dr. S. Sivaram and Dr. C. Ramesh, National Chemical Laboratory, Pune, India, for their help in X-ray measurements. Supporting Information Available: Energy-minimized molecular models using a molecular mechanics (MMX) program for PANI-DNNDSA, PANI-CSA of different lamella thicknesses, the CSA molecule, and PANI-DOSA systems. This material is available free of charge via the Internet at http://pubs.acs.org. LA025724Y
