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Sulfonic Acid Doped Thermoreversible Polyaniline Gels. 2. Influence of Sulfonic Acid Content on Morphological, Thermodynamical, and Conductivity Properties Tushar Jana and Arun K. Nandi* Polymer Science Unit, Indian Association For The Cultivation of Science, Jadavpur, Calcutta 700032, India Received February 19, 2001. In Final Form: May 25, 2001 Thermoreversible gelation behavior of polyaniline (PANI) in the presence of varying concentrations of sulfonic acids are studied. Four different sulfonic acids are used, e.g., dinonylnaphthalene sulfonic acid (DNNSA), dinonylnaphthalene disulfonic acid (DNNDSA), (()-camphor-10-sulfonic acid (CSA), and n-dodecyloxo sulfonic acid (DOSA). The composition range studied here is WPANI ) 0.05-0.80 (WPANI is the weight fraction of PANI in the gel). It has been found from the SEM study that for all the sulfonic acids in the composition range WPANI ) 0.05-0.40, fibrillar network is present. The TEM study also supports the above viewpoint. The thermodynamic study of the gels has been done by DSC-7, and for all the systems broad peaks consisting of two fused gel melting/gel formation peaks are observed. After proper deconvolution of the two peaks the gel melting/gel formation temperatures are measured. When they are plotted with WSO3H (weight fraction of sulfonic acid), typical phase diagrams consisting of two almost parallel curves are found. They are explained by considering the lamellar model where the bilayer and monolayer portions of the surfactant form different crystalline domains. The higher melting point curve is due to the former and the lower one is due to the later in each phase diagram. The lowering of gel melting/gel formation points with increase in PANI concentration of each curve has been attributed to the dilution effect of PANI in the gel. The corresponding enthalpy values (∆H) also show similar decrease with increasing PANI concentration and are explained also from the dilution effect of PANI. The conductance of these gels varies with weight fraction of PANI (WPANI), showing a maximum, and it has been explained by considering that conductivity in the gel is due to both intrachain and interchain contributions. Attempt is made to discuss the above results by using a molecular model of the PANI-DOSA system with the help of MMX program.
Introduction In recent years the preparation and properties of PANIsulfonic acid systems are highly studied, because of their high conductance value and easy processibility.1-10 Major emphasis has been given to the preparation of doped samples and the study of their conductivity, morphology, and optical behavior and blending with other polymers etc.1 Among the sulfonic acids (()-camphor-10-sulfonic acid (CSA) is much used for this purpose. The high solubility of PANI-CSA in m-cresol has been attributed to the molecular recognition effect.3,4 Kinlen et al. developed an emulsion process for synthesizing sulfonic acid doped organically soluble PANI with dinonylnaphthalenesulfonic acid (DNNSA) and dinonylnaphthalenedisulfonic * To whom correspondence should be addressed; e-mail: psuakn@ mahendra.iacs.res.in. (1) 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. (2) Xia, Y.; Wiesinger, J. M.; MacDiarmid, A. G. Chem. Mater 1995, 7, 443. (3) Ikkala, O. T.; Pietila L.-O.; Ahjopalo, L.; Osterholm. H.; Passiniemi. P. J. J. Chem. Phys. 1995, 103, 9855. (4) Vikki, T.; Pietila, L.-O.; Osterholm, H.; Ahjopalo, K.; Takala, A.; Toivo, A.; Levon, K.; Passiniemi, P.; Ikkala, O. Macromolecules 1996, 29, 2945. (5) Kinlen, P. J.; Liu, J.; Ding, Y.; Graham, C. R.; Remsen, E. E. Macromolecules 1998, 31, 1735. (6) Kababya, S.; Appel, M.; Haba, Y.; Titelman, G. I.; Schmidt, A. Macromolecules 1999, 32, 5357. (7) Olinga, T. E.; Fraysse, J.; Travers, J. P.; Dufresne, A.; Pron, A. Macromolecules 2000, 33, 2107. (8) Paul, R. K.; Pillai, C. K. S. Synth. Met. 2000, 114, 27. (9) Haba, Y.; Segal, E.; Narkis, M.; Titelman, G. I.; Siegmann, A. Synth. Met. 2000, 110, 189. (10) Vikki, T.; Ruokolainen, J.; Ikkala, O, T.; Passiniemi, P.; Isotalo, H.; Torkkeli, M.; Serimaa. R. Macromolecules 1997, 30, 4064.
acid (DNNDSA) and these systems when treated with surfactants, e.g., bezyltrimethylammonium chloride, show a 5-fold increase in conductivity due to the formation of interconnected network structure.5 Kababye et al.6 reported a synthesis of soluble PANI-dodecylbezenesulfonic acid (DBSA) dispersion and showed from solid state NMR that DBSA is molecularly miscible with PANI. Highly conducting and processable PANI by protonation with novel long side chain sulfonic acids is recently reported by several groups.7,8 PANI-DBSA dispersions have been recently used to prepare blends with other commodity plastics.9 Apart from the above precious uses of the various sulfonic acids in the processing and conductivity enhancement of PANI, it is interesting to prepare thermoreversible gels of PANI with the above sulfonic acids.10,11 These conducting gels have a great importance due to a combination of both the elastic property of the gel and the significant enhancement in electrical conductivity due to network structure formation.12 It also provides easy processibilty for the infusible and intractable PANI.11 Vikki et al. prepared thermoreversible gels of PANI by doping with excess of DBSA in formic acid medium.10 In our earlier communication, we reported that PANI produces thermoreversible gels when doped with any of the sulfonic acid dopants, e.g., DNNSA, DNNDSA, CSA, and dodecyloxo sulfonic acid (DOSA) from formic acid medium. The report was made only for the concentration WPANI ) 0.15, where WPANI represents the weight fraction of PANI in the gel.11 In this report attempt is made to (11) Jana, T.; Nandi. A. K. Langmuir 2000, 16, 3141. (12) Fizazi, A.; Moulton, J.; Pakbaz, K.; Rughooputh, S. D. D. V.; Smith, P.; Heeger, A. J. Phys. Rev. Lett. 1990, 64, 2180.
10.1021/la010253l CCC: $20.00 © 2001 American Chemical Society Published on Web 08/18/2001
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understand the gelation behavior of PANI by varying the composition of the blend. We want to focus here on how the morphology, thermodynamics, and conductivity change by varying the sulfonic acid content of the blend. The aim of this study is, therefore, to observe whether the gel behavior changes by varying the surfactant concentration and also to get a better insight to understanding the gelation mechanism in the PANI-sulfonic acid doped systems. For this purpose we have prepared PANIsurfactant systems of varying compositions, WPANI ) 0.050.80; we have mainly focused our attention on the morphology (both by SEM and TEM), thermodynamical behavior, and conductivity. All these properties are tuned up together to illuminate the gelation mechanism of this new type of gel (gel without a liquid). Molecular modeling through the MMX program for a representative system is also used to understand the lamellar model of this type of gel more clearly. Experimental Section Samples. Polyaniline. Polyaniline was prepared in the laboratory by polymerizing distilled aniline in HCl medium with ammonium persulfate as initiator.13It has been converted to the emeraldine base (EB) form by digesting with NH4OH solution. The molecular weight (Mv) of PANI (EB) measured from intrinsic viscosity measurements in sulfuric acid was 15 500.11 Dopants. Four sulfonic acid dopants of different structures were used in this work. The DNNSA and DNNDSA were a gift from Dr. P. J. Kinlen of Monsanto Company, St. Louis, MO.5 They were the products of King Industries and had the commercial names Nacura-1051 and Nacura-155, for DNNSA and DNNDSA, respectively. They were supplied in diluted condition with 50% (by weight) ethylene glycol butyl ether for DNNSA, and DNNDSA contains 45% isobutyl alcohol. During preparation of gels of different PANI-sulfonic acid concentrations the dilutent concentrations were taken into account for the above two systems. CSA was purchased from Fluka and was used as received. DOSA was obtained by passing an aqueous solution of sodium dodecyl sulfate (SDS) through a column containing acidic resin. The eluent was evaporated and finally dried in a vacuum. Preparation of Gels. PANI (EB) and sulfonic acids were mixed at different compositions by taking the weighed amounts of the components in formic acid (B. D. H., A. R.) in a roundbottomed flask and were stirred with a mechanical stirrer by keeping the round-bottomed flask in a thermostat at 65 °C for 24 h [(PANI + dopant) mixture:formic acid ) 2.5:97.5 (by weight)]. The formic acid was removed by pouring the contents of the round-bottomed flask in a Petri dish and drying in a pull of air at 80°C. Finally the samples were dried in a vacuum at 60 °C for a week.10,11 The above drying procedure completely removes the formic acid, ethylene glycol butyl ether, and isobutyl alcohol. This is evident from the absence of FTIR peaks at 3114, 1124, and at 3337 cm-1 for formic acid, ethylene glycol butyl ether, and isobutyl alcohol, respectively.11,14 Morphology Study. The morphology study of the samples was done both by scanning electron microscopy (SEM) and by transmission electron microscopy (TEM). For the SEM study the dried samples were gold-coated and their micrographs were taken in a SEM apparatus (Hitachi S-415 A). The TEM studies of the gel were done by dropping a ∼0.2% (w/v) solution of the gel in formic acid (in acetone for PANI - DNNSA system) onto a carbon-coated copper grid and drying under a vacuum at 35 °C for 3 days. The gel was then observed in the TEM (Hitachi, H-600) in the transmission mode.11 Thermodynamical Study. A Perkin-Elmer DSC-7 was used for this purpose. The PANI gels were taken in large volume capsules (LVC), fitted with O-rings and were tightly sealed with the help of a quick press. For PANI-DNNSA and PANIDNNDSA systems the gels were scanned from -20 to 160 °C and (13) Cao, Y.; Andreatta, A.; Heeger, A. J.; Smith, P. Polymer 1989, 30, 2305. (14) Clothup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy; Academic Press: New York, 1964.
Langmuir, Vol. 17, No. 19, 2001 5769 for the other two systems it was scanned from -20 to 200 °C. After the first scan the samples were kept at the end temperatures for 10 min and were then cooled at the rate of 5 deg/min to -20 °C. Here the samples were kept at equilibrium for 10 min and again heated at the rate of 10 deg/min. The peak temperatures and the enthalpy changes were measured from the computer attached to the instrument. The instrument was calibrated with indium before each set of experiments. Conductivity Measurements. The electrical conductivity of the gels was measured by making pellets of the gel in a press. The pellets had a diameter of 1.3 cm and a thickness of ∼0.07 cm. The conductivity was measured at room temperature (27 °C) by the standard spring-loaded pressure contact four-probe method.15A constant current (I) was passed from a constant direct current source electrometer (Keithley, model 617) and through two adjacent leads of the four probes, and the voltage (V) across the other two leads was measured by using a multimeter (Keithley, model 2000). The conductivity (σ) was calculated from the relation
σ)
ln 2 I Πd V
(1)
where d, the thickness of the pellet, was taken as the average of four measurements at different places using a screw gauge.
Results Morphology. Figure 1 presents the SEM pictures of the PANI-CSA system for the different compositions. It is apparent from the figure that the fibrillar network morphology is clearly seen for WPANI ) 0.05-0.40; however, fibrillar network morphology is not seen for WPANI ) 0.60 and 0.80. In the composition WPANI ) 0.60 some fibrils are seen but no network is clearly observed. However, there is an absence of both network and fibrillar morphology in the composition WPANI ) 0.80. According to the defination of a thermoreversible gel, formation of a fibrillar network is the most important condition;16,17consequently, it may be summarized that the PANI-CSA system produces a thermoreversible gel for the compositions WPANI ) 0.050.40.11 Careful observation of the SEM micrographs also indicates that the fibrillar diameter is larger for lower PANI content in the system. The composition WPANI ) 0.60 has very few fibrillar structures, and they are less in number to make the network. However, no fibrill is formed in the composition WPANI ) 0.80 system, probably because of lack of complete doping of PANI chains with CSA. This probably prevents the chain from becoming extended, and therefore it retains its coil structure. In other systems, e. g., PANI-DNNSA, PANI-DNNDSA, and PANI-DOSA, similar observations are also made. In Figure 2, TEM pictures of PANI-DNNDSA systems are shown. From the figure it is also apparent that a fibrillar network is present for compositions WPANI ) 0.050.40, supporting that thermoreversible gelation occurs for the above composition range. Similar TEM pictures are also true in other systems. Thermodynamic Behavior. In Figure 3 the thermograms of the PANI-DNNDSA system for (a) the first heating of the as-prepared sample, (b) cooling at a rate of 5 deg/min from the melt, and (c) the second heating of the cooled samples of process (b) are shown. It is apparent from the figures that for WPANI ) 0.05-0.60 we obtained endothermic peaks for the heating and exothermic peaks (15) Frommer, J. E.; Chance, R. R. In Encyclopedia of Polymer Science and Engineering, 2nd ed.; Mark, H. F., Bikales, N. M., Overberger; C. G., Menges, G., Eds.; John Wiley & Sons: New York, 1986; Vol. 5, p 473. (16) Daniel, C.; Dammer, C.; Guenet, J. M. Polym. Commun. 1994, 35, 4243. (17) Guenet, J. M. Thermoreversible Gelation of Polymers and Biopolymers; Academic Press: London, 1992.
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Figure 1. SEM pictures of the PANI-CSA system for different compositions. WPANI: a, 0.05; b, 0.25; c, 0.40; d, 0.60; e, 0.80.
for the cooling processes. On the other hand, PANI (EB) does not show any phase transition under the above conditions.11 Apart from the fibrillar network structure, another important criteria for the thermoreversible gels is the presence of reversible first-order phase transition.16,17 Therefore, the presence of reversible first-order phase transition together with the fibrillar network structure confirms that the PANI-DNNDSA system (WPANI ) 0.05-0.40) forms thermoreversible gels. However, for the WPANI ) 0.60 system, lack of fibrillar network structure indicates that it does not produce any thermoreversible gel. In this system some fibrillar crystallites are also formed showing reversible first-order phase transition, but they are insufficient in concentration to produce the network. Similar observations are found in other systems. It must be pointed out here that the thermograms are broad in nature and careful observation indicates that there are two peaks fused together during both the heating and cooling processes. The occurrence of the two peaks in the thermograms of polymer-solvent type thermoreversible gels is due to the polymer-solvent compound formation.18-22 However, in the surfactant-mediated PANI gels, such formation of polymer-solvent compounds is (18) Guenet, J. M.; Mckenna, G. B. Macromolecules 1988, 21, 1752.
not possible, since in the gel only PANI and surfactant are present, as evidenced from the FTIR spectra.11 In Figure 4 the phase diagrams of the PANI-sulfonic acid gels are presented. The phase diagrams are drawn by proper deconvolution of the two peaks and measurement of the individual peak temperature. The presence of two almost parallel curves (except for PANI-DOSA system) for both melting and cooling processes is interesting. Drawing attention to the lamellar model of the polymersurfactant system,11 it has been considered that the sulfonic acid head is anchored from the nitrogen atoms of PANI and the nonpolar hydrocarbon tails become extended forming a monolayer and a bilayer (where overlapping of hydrocarbon tails occurs). Both the monolayer and the bilayer are capable of crystallization.The crystalline domains formed from the monolayer portion have lower melting points than those formed from the bilayer because of the higher density of the crystalline elements in the bilayer. During cooling two exothermic peaks are observed; the higher temperature peak is due to the crystallization of the bilayer and the lower one is due to the crystallization (19) Saini, A.; Spevacek, J.; Guenet, J. M. Macromolecules 1998, 31, 703. (20) Guenet, J. M. Thermochim. Acta 1996, 284, 67. (21) Mal, S.; Nandi, A. K. Langmuir 1998, 14, 2238. (22) Dikshit, A. K.; Nandi, A. K. Macromolecules 2000, 33, 2616.
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Figure 2. TEM picture of the PANI-DNNDSA system for different compositions. WPANI: a, 0.05; b, 0.40; c, 0.60. Table 1. Exothermic and Endothermic Peak Temperatures and Their Enthalpy Changes (after Deconvolution) for Cooling from the Melt (at 5 deg/min) to -20 °C and Subsequent (Second) Heating at 10 deg/min for PANI-Sulfonic Acid Gels PANI-DNNSA cooling
PANI-DNNDSA
2nd heating
cooling
PANI-CSA
2nd heating
cooling
PANI-DOSA
2nd heating
cooling
2nd heating
temp, enthalpy, temp, enthalpy, temp, enthalpy, temp, enthalpy, temp, enthalpy, temp, enthalpy, temp, enthalpy, temp, enthalpy, °C J/g °C J/g °C J/g °C J/g °C J/g °C J/g °C J/g °C J/g 0.05 40.7 58.5 0.15 41.5 59.0
2.2 1.6 0.9 1.6
56.8 78.8 54.2 77.5
1.8 1.9 1.2 2.8
35.0 63.7 35.5 59.0
1.7 7.8 1.3 7.5
57.6 83.4 52.1 77.1
7.7 16.9 3.9 2.3
53.3 80.3 51.7 94.2
5.1 6.7 2.3 2.6
80.9 122.5 78.2 106.9
0.7 28.5 5.6 6.2
0.25 35.5 52.7 0.40 37.3 52.9 0.60 36.5 52.0
1.2 2.1 0.8 2.2 0.9 1.4
52.5 72.8 51.7 74.8
2.0 3.3 0.8 4.8
37.1 52.3 33.8 51.7 33.3 52.8
1.0 1.3 1.0 1.5 1.2 1.5
49.2 74.1 45.9 73.8 43.9 73.5
1.9 4.1 1.4 2.5 1.2 2.2
92.9 97.2 74.9 98.1
0.7 0.3 3.3 2.2
70.0 101.2 63.8 135.9
7.5 10.0 3.0 4.6
103. 9 54.9 112. 1
13.5 1.8 1.3
41.9 55.1
3.7 2.2
54. 2 134 .8 51. 9 76. 5 116 .1 52. 2 105 .8 88. 3 116 .1
10.4 11.0 8.2 1.6 1.3 0.6 3.9 2.9 3.8
of the monolayer. Due to the denser molecular arrangement of the bilayer, it crystallizes faster than the monolayer. It is apparent from the phase diagrams (Figure 4) and also from Table 1 that the melting points decreases with decrease in WSO3H for almost all the systems. A probable reason is that PANI itself is acting as a diluent for the crystallites formed during gelation. As the PANI concentration increases, the concentration of the crystallites decreases, causing a depression of the melting point.23 However, some anomaly in the gel formation and melting point data (second heating) in Table 1 have been observed for higher PANI content of PANI-CSA and PANI-DSA systems. No definite reason for this anomaly is known to us. Nonetheless, it may be surmised that the phase diagrams are satisfactorily explained from the lamellar model proposed for thermoreversible PANIsurfactant gels.11 In Figure 5 the enthalpy values for the gel melting process are also plotted with WSO3H of the as prepared gels, and it is apparent from the figures that the ∆H values
decrease with decrease in WSO3H for both the peaks. The decrease of ∆H values with increase in WPANI is also due to the dilution effect of PANI on the surfactant crystallites (for both monolayer and bilayer) formed during gelation.23 The ∆H values in Table 1 also satisfactorily obey the above conclusion. Thus the thermodynamic data support the lamellar model proposed for the PANI-surfactant gel11 with the presence of two types of crystallites in the lamella. Conductivity. In Figure 6 the conductivity of the gels is plotted with weight fraction of PANI (WPANI) in the gel. It is apparent from the figure that there is a maximum in conductivity with composition. The maximum conductivity observed in these gels is ∼2 s/cm for the PANIDOSA system. Of the four systems, the PANI-DNNSA system shows the lowest conductivity. PANI in its emeraldine (EB) base form is nonconducting; however, by doping with protons it becomes conducting due to the formation of its metalic salt form.24 The initial growth of conductivity with increase of WPANI may arise from the actual decrease of dopant concentration, which has no
(23) Mandelkern, L. Crystallization of Polymers; McGraw-Hill: New York, 1964.
(24) Gettinger, C. L.; Heeger, A. J.; Pine, D. J., Cao, Y. Synth. Met. 1995, 74, 81.
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Figure 3. (a) Melting endotherms of the PANI-DNNDSA system of the as-prepared gels for indicated compositions (WPANI) (HR ) 10 deg/min). (b) Cooling exotherms from 160 °C at the rate of 5 deg/min of PANI-DNNDSA systems at indicated compositions (WPANI). (c) Melting endotherms of the PANIDNNDSA system prepared from Figure 3b for the indicated compositions (WPANI).
Figure 4. Phase diagrams of the PANI-sulfonic acid systems after proper deconvolution of the peaks for the first heating process. The open symbols are for the higher melting peak, and the closed symbols are for the lower melting peaks: (a) PANIDNNSA, (b) PANI-DNNDSA, (c) PANI-CSA, and (d) PANIDOSA, systems.
electronic conduction, followed by an increase of the concentration of conducting salt of PANI. Similar increase in conductivity at low PANI concentration was also observed by Vikki et al. for PANI-DBSA gels.10 In all the gels, fibrillar networks are observed, and with increasing PANI concentration the network density becomes increased (cf. Figure 1) up to WPANI ) 0.4. In the gel the
conductance may be considered as consisting of two contributions,2,12 e.g., σgel ) σ (intrachain, interchain), and when no network is present the interchain contribution is absent. Consequently, for WPANI > 0.4 conductivity should decrease due to the absence of the network structure. This explanation is found to be obeyed satisfactorily for PANI-CSA and PANI-DNNDSA systems,
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Figure 6. Plot of log conductivity vs WPANI in the gel for different sulfonic acid dopant gels: 4, PANI-DNNSA system; 2, PANIDNNDSA system; b, PANI-CSA system; O, PANI-DOSA system.
Figure 5. Plot of enthalpy changes with weight fraction of sulfonic acid (WSO3H) for PANI-sulfonic acid systems during first heating of the as-prepared gel. The open symbols are for the higher melting peak, and the closed symbols are for the lower melting peak: (a) PANI-DNNSA, (b) PANI-DNNDSA, (c) PANI-CSA, and (d) PANI-DOSA systems.
where maxima occurs at WPANI ) ∼0.4. On the other hand, in PANI-DOSA and PANI-DNNSA systems the maximum is at WPANI ) 0.25 and 0.22, respectively. This indicates that the above relation is not simply obeyed in all these gels. Here an attempt is also made to explain the conductivity from the doping level and in Figure 7 the conductivity is plotted with doping level (atomic ratio of S/N). The doping level is calculated from the weights of PANI and sulfonic acid taken in the experiments to prepare the gel, because in the gel preparation procedure there is no loss of sulfonic acid or PANI. In the figure, with increasing the sulfonic acid concentration (i.e., S/N ratio) the conductance increases due to increasing concentration of doped PANI. At S/N ) 1, PANI is fully doped and conductance should show a maximum. However, in the plots it is clear that the maxima in all the cases are at S/N values lower than unity. This indicates that not only the intramolecular part but also the intermolecular part of σ is operative at these regions, and due to the larger network density the maximum conductivity shifts to the S/N value lower than unity. In Table 2 the WPANI for S/N )1 and those for maximum conductivity of Figures 6 and 7 are shown. It is apparent from the table that the maximum occurs at higher WPANI than for S/N )1, and this may be due to the increasing cross-linking density with increase in PANI
Figure 7. Plot of log conductivity (S/cm) vs doping level (S/N ratio) for different sulfonic acid dopant gel: 4, PANI-DNNSA system; 0, PANI-DNNDSA system; b, PANI-CSA system; O, PANI-DOSA system. Table 2. Comparison of WPANI Values for Maxima in Figures 6 and 7 and for S/N Ratio ) 1 WPANI for samples
S/N ) 1
maxima in plot 6
maxima in plot 7
DOSA CSA DNNSA DNNDSA
0.25 0.28 0.16 0.25
0.25 0.40 0.22 0.40
0.28 0.44 0.19 0.38
concentration in the gel. So it may be surmised from these results that conductivity of the PANI gel is a function of both doping level and cross-linking density. Discussion Here we discuss our thermodynamic results through a molecular model. Molecular modeling of the lamellar structure of these gels has been done with the help of the MMX program.25 For simplicity we have considered four (25) Gajewski, K. E.; Gilberr, M. H. In Advances in Molecular Modeling; Liotta, D., Ed.; JAI Press: Greenerick, CT, 1990; Vol. 2.
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between N---H groups. So it may be surmised from the model that hydrogen bonding may take place between the surfactant and the PANI chains.10,11 Now, if we observe various distances, we see that bilayer thickness would be ∼10 Å, monolayer distance would be ∼9.6 Å, and the distance of PANI chains to surfactant tail would be ∼18 and ∼20 Å. Thus the lamellar model shows that we should get reflection for 2θ corresponding to 20, 18, and ∼10 Å, and the experimental values26 also match these values. Thus the molecular modeling supports the existence of monolayer and bilayer crystals. The packing distance for the bilayer crystals (∼3.7 Å) is much shorter than that of monolayer crystals (∼6.8 Å), indicating that crystalline density of the monolayer is less than that of the bilayer. So the higher melting point of bilayer compared to that of monolayer is successfully explained from the model. Thus molecular modeling using the MMX program also supports the gelation scheme presented earlier11 for the thermoreversible behavior of the PANI-surfactant gel. Conclusion
Figure 8. Molecular modeling of lamellar structure of the PANI-DOSA system after energy minimization through the MMX program (see text). The numbers in the figure indicate the distance (Å) between the points in the energy-minimized structure.
monomeric units and two polymer chains of PANI remaining side by side, to form the polymer layer. DOSA has been chosen as the surfactant due to its simple structure. The aminic nitrogen of PANI has been considered to be connected through H-bonding from the sulfonyl oxygen, and the iminic nitrogen has been considered to form hydrogen bonds through the hydroxyl group of sulfonic acid.10,11 The carbon chain of DOSA has been considered as an extended chain conformation, and also the lamellar distance between the two polymer layers has been fixed at 31 Å.26 After energy minimization of the structure through the MMX program we obtain the molecular model shown in Figure 8. In the model two hydrogen bonds are clearly seen by dashed lines, whereas three other distances (2.7, 2.6, and 2.7 Å) are shorter than the weak hydrogen bonding distance between nitrogen and hydrogen atoms.28 The other three N-H distances are slightly longer than the hydrogen bonding distance (26) Ramesh, C., National Chemical laboratory, Pune, India, personal communication: In the PANI-DOSA system (WPANI ) 0.15) the lowangle WAXS study shows peaks at 2θ ) 2.8, 4.1, 8.0, and 9.55˚ corresponding to d spacings at 31.5, 21.5, 18.2, and 9.3 Å. The d spacing of 31.5 Å corresponds to the lamellar thickness of the PANI-DOSA system (analogous to the PANI-DBSA system27). (27) Zheng, W. Y.; Wang, R. H.; Levon, K.; Rong, Z. Y.; Taka, T.; Pan, W. Macromol. Chem. Phys. 1995, 196, 2443. (28) In ammonium azide the H---N hydrogen bonds have the length 2.98 Å.29 (29) Pauling, L. The Nature of Chemical Bond and the Structure of Molecules and Crystals, 3rd ed.; Cornell University Press: Ithaca, NY, 1960; p 463.
The composition dependence study of PANI-surfactant gel concludes that for WPANI ) 0.05-0.40 thermoreversible gels are observed for PANI-DNNSA, PANI-DNNDSA, PANI-CSA and for the PANI-DOSA systems. The morphology study by SEM and TEM experiments indicate formation of fibrillar network structure in this region. The thermodynamic study indicates a reversible firstorder phase transition during both heating and cooling processes. The thermograms consist of two peaks fused together for both the processes. The phase diagrams contain two almost parallel curves for all the systems during both heating and cooling processes. A probable explanation from the monolayer and bilayer crystallization of the surfactants anchored from the PANI layer is offered. The gel melting and gel formation temperatures decrease with increase in PANI concentration. The ∆H values also exhibit a similar decrease. A probable explanation has been offered by considering PANI as a diluent for the surfactant crystallites. The conductivity varies with the concentration of sulfonic acid in the gel showing a maxima. A probable explanation of the conductivity from the doping level and cross-linking density has been offered. A lamellar model has been developed by the MMX program for the PANI-DOSA system where lamellar dimensions tally with the bilayer and monolayer crystallites supporting our explanation. Acknowledgment. We are grateful to Dr. P. J. Kinlen of Monsanto Company, St. Louis, MO, for his gift of DNNSA and DNNDSA samples. We also gratefully acknowledge the Council of Scientific and Industrial Research, New Delhi [Grant No. 01 (1449)/97 EMR-11] for financial support of the work. We also acknowledge Mr. N. Chowdhury for his help in the SEM and TEM experiments. LA010253L
