- Email: [email protected]
solution processable conducting polyaniline with novel sulfonic acid dopants and its thermoplastic blends
Synthetic Metals 114 Ž2000. 27–35 www.elsevier.comrlocatersynmet
Meltrsolution processable conducting polyaniline with novel sulfonic acid dopants and its thermoplastic blends Raji K. Paul, C.K.S. Pillai ) Council of Scientific and Industrial Research, Regional Research Laboratory, ThiruÕananthapuram Industrial Estate P.O., ThiruÕananthapuram, Kerala 695 019, India Received 24 August 1999; received in revised form 18 January 2000; accepted 19 January 2000
Abstract The effects of three sulfonic acid dopants, sulfonic acid of 3-pentadecylphenol ŽSPDP., sulfonic acid of 3-pentadecylanisole ŽSPDA. and sulfonic acid of 3-pentadecylphenoxy acetic acid ŽSPDPAA. Žsynthesized from an inexpensive naturally existing biomonomer, cardanol. on the electrical conductivity and other properties of polyaniline ŽPANI. were studied. All the three dopants were found to act as very good plasticizing cum protonating agents for PANI. Doping was carried out either by the in situ doping emulsion polymerization route or by mechanical mixing of emeraldine base and the dopant. The protonated complexes obtained by the emulsion polymerisation route exhibited exceptionally high degree of crystalline order and orientation. Highly conducting free-standing films of protonated PANI could be prepared both by the solution casting and by the melt processing techniques. On thermal processing, free-standing flexible films with conductivities as high as 65 S cmy1 could be prepared. PANI protonated with the dopants, SPDA and SPDPAA were blended with polyŽvinyl chloride. ŽPVC. and thin films were prepared by melt processing techniques and the properties of the blends were studied. Percolation threshold is occurring at very low weight percentage of PANI and a maximum coductivity value of 2.5 S cmy1 was obtained for 25 wt.% of PANI for PANI–SPDA–PVC system. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Polyaniline; Novel sulfonic acid dopants; Meltrsolution processability; Thermoplastic blends; PVC; Percolation threshold
1. Introduction Since the discovery of conducting polyacetylene films in 1977 w1x, there has been substantial interest in the scientific and engineering communities in understanding the intrinsically conductive polymers ŽICPs. and finding applications for their unique properties w2x. The most common ICPs are polyaniline ŽPANI., polypyrrole, polythiophene, polyacetylene, polyŽ p-phenylene. and polyŽ pphenylene sulfide.. Since the backbone of these polymers contains conjugated double bonds, they exhibit extraordinary electronic properties, such as low ionization potential and high electron affinity, and, as a result, can be easily reduced or oxidized. These polymers undergo ‘‘doping’’ upon exposure to a protonic acid and become electrically conductive w3x. But these polymers are highly intractable and infusible because of their highly aromatic nature, the
) Corresponding author. Tel.: q91-471-490-674; fax: q91-471-490186. E-mail address: [email protected] ŽC.K.S. Pillai..
interchain hydrogen bondings and the charge delocalization effects w4x. So one of the active research areas of both scientific and industrial importance has been to develop electrically conductive polymers, which are fusiblermelt processable or soluble in common solvents. Over the last decade, significant progress has been made in the processing of ICPs into useful oriented articles. PANI is one of the most promising conducting polymers due to its straightforward polymerisation and excellent chemical stability combined with relatively high levels of conductivity w5x. Emeraldine base form of PANI is soluble only in N-methylpyrrolidone ŽNMP., selected amines, concentrated sulphuric acid and other strong acids. Emeraldine salt is even more intractable. Among the methods to improve the melt and solution processability of PANI, covalent substitution, such as ring substitution w6–8x, N-alkylation w9–11x and protonation with functionalized protonic acids, such as dodecylbenzene sulfonic acid w12,13x, camphorsulfonic acid w14x, phosphoric acid esters w15–18x, etc., merit mention. The use of specific functionalized protonic acid dopants render high solubility to PANI in common solvents w12x, allowing preparation of solution
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 0 0 . 0 0 2 0 6 - X
28
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
cast w19x or melt processable blends with a low percolation limit w20,21x. But according to Pron et al. w22x, the selection of appropriate solvent is of crucial importance for electrical conductivity of solution cast PANI films. For example, PANI protonated with camphor sulfonic acid, in selected solvents, such as m-cresol, induce extended coil conformation of PANI chains, which facilitate polaron delocalization and crystallization of the polymer upon casting w22x. Also m-cresol is suspected to be a cancer causing substance. From the industrial point of view, the fabrication of a thermally processable conducting polymer would be preferable because it is easier and much cheaper. Thermally processable PANI has recently been fabricated using aliphatic and aromatic diesters of phosphoric acid as protonating agents w15–18,22x, where the hydrophobic groups introduced to PANI with each dopant ion lead to its plastification. In our previous paper w18x, we have shown that protonation of PANI with a dopant, 3-pentadecylphenylphosphoric acid induces melt and solution processability to PANI. In this communication, we show that sulfonic acid derivatives of 3-pentadecyl phenol ŽPDP., derived from an inexpensive naturally existing biomonomer, cardanol, are excellent dopants for PANI, which induce plastification leading to thermal processability and solution processability to PANI. The dopants synthesized are sulfonic acid of 3-pentadecylphenol ŽSPDP., sulfonic acid of 3-pentadecylanisole ŽSPDA. and sulfonic acid of 3-pentadecylphenoxy acetic acid ŽSPDPAA.. One of the significant features of the structure of the dopants is that it has a flexible n-alkyl ŽC 15 H 31 . substituent in the meta position of the aromatic ring, which makes the doped PANI melt processable and render high solubility for PANI in common solvents. These dopants, thus, render plasticizing ability to PANI so that free standing flexible films could be prepared by the conventional melt processing techniques and by the solution processing techniques. Thus, for the first time, a method has been developed Žprocess filed for patent in India and abroad. to prepare melt processable conducting PANI doped with sulfonic acid dopants having protonating cum plasticizing ability. We also report the preparation of conductive flexible thermoplastics of protonated PANI by blending with PVC. The conducting thermoplastic films were fabricated by the melt processing techniques and are found to exhibit very low percolation threshold and high transparency.
2. Experimental
2.1. Reagents All chemicals used were purchased from S.D. Fine Chem., Mumbai. Aniline was doubly distilled under vacuum and stored in a refrigerator. All the solvents used were of spectroscopic grade. PVC Ž67–311 extrusion grade. purchased from NOCIL, Mumbai is used for the blending studies.
2.2. Synthesis
2.2.1. The preparation of polyemeraldine base PANI was synthesised chemically according to the method of MacDiarmid et al. w23x. The neutral base form of PANI was obtained by dedoping the PANI–salt in 3 wt.% ammonia solution for 3 h followed by washing with acetone and drying in vacuum for 8 h at 608C.
2.2.2. Synthesis of dopants 2.2.2.1. The synthesis of methyl ether of 3-PDP. The methyl ether of PDP was prepared by reaction of PDP with dimethyl sulfate in aqueous alkali w24x. 2.2.2.2. The synthesis of 3-pentadecylphenoxy acetic acid. 3-Pentadecylphenoxy acetic acid was prepared by reaction of PDP with monochloroacetic acid in alcohol and aqueous alkali w25x. 2.2.2.3. Sulfonation. Sulfonation of PDP, methyl ether of PDP and 3-pentadecylphenoxy acetic acid was carried out by reacting with 98% of concentrated sulfuric acid at 110–1208C w25x.
2.2.3. Doping Protonation of PANI by these dopants were carried out either by in situ doping emulsion polymerisation route or by mechanical mixing method. 2.2.3.1. In situ doping emulsion polymerisation. The in situ doping emulsion polymerisation of aniline in the presence of the dopants was carried out according to the procedure of Osterholm et al. w13x. The polymers obtained by the emulsion polymerisation method using the three dopants are designated as PANI–SPDP 1, PANI–SPDA 1 and PANI–SPDPAA 1. These protonated polymers obtained were then converted to the emeraldine base form by treatment with 3 wt.% of aqueous ammonia solution for 3 h.
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
2.2.3.2. Mechanical mixing method. Emeraldine base of inherent viscosity 1.2 dl gy1 Žmeasured at room temperature in concentrated sulfuric acid. was mixed with the dopant in a molar ratio of 0.5 dopant to polymer repeat unit PhN, using an agate mortar and pestle at room temperature to get PANI–SPDP 2, PANI–SPDA 2 and PANI– SPDPAA 2. Different molar ratios of the PANI–dopant combination were prepared in order to find out the plastification threshold. 2.2.4. Preparation of blends Conducting blends of PANI–SPDA and PANI– SPDPAA with PVC were prepared by mechanical mixing at room temperature for an extended period of time to achieve optimum homogeneity. Various plasticized PANIrPVC ratios were used. The mixture containing PANI and PVC were then hot pressed at 1608C for 15 min so that thin films of approximate thickness of 0.2 mm can be prepared. 2.3. Characterisation methods Inherent viscosities of PANI base and protonated PANI were determined in 0.1% wrw solutions in concentrated H 2 SO4 at 258C using an Ubbelohde viscometer. The FTIR spectra were recorded on a NICOLET IMPACT 400 D spectrophotometer. The UV–VIS spectra were taken in a Shimadzu UV-2100 spectrophotometer. Conductivity was measured by four-probe method. XRD pattern was recorded on a Rigaku Geiger-Flex DrMAX series using CuK a radiation. SEM pictures were recorded using JEOL JSM5600LV Scanning Electron Microscope. DSC thermograms were taken on Differential Scanning Calorimeter 2010 attached to Thermal Analyst 2100. Mechanical properties were determined on an Instron Series IX, Automated Materials Testing System 1.09.
3. Results and discussions 3.1. Part A: studies on protonated PANI 3.1.1. General obserÕations Sulfonic acid derivatives of PDP and the related compounds are well known for their surfactant properties w24,25x. No report could be located on their use as dopants for PANI. Interesting observations were made on doping PANI with these sulfonic acids having a long hydrophobic hydrocarbon-side chain. The PANI protonated by using these dopants becomes soluble in nonpolar or weakly polar organic solvents, such as xylene, chloroform, tetrahydrofuran, etc. Thin films of PANI doped with SPDP, SPDA and SPDPAA could be easily prepared from xylene and chloroform solutions because the counter ions of the dopants act as surfactants for organic solvents. At the same time, protonation of PANI with these dopants results in a heav-
29
ily plasticized mixture, which exhibits rheological parameters characteristic of a Bingham-type liquid with the viscosity decreasing with the increase of dopant content. The composition required to reach plastification threshold for the protonated PANI with these dopants is for a dopantrPANI ratio greater than 0.3, above which the polymer can be thermally processed to give free-standing flexible films. These sulfonic acid dopants were found to act as an emulsifying cum protonating agents for PANI. This enabled the in situ doping emulsion polymerisation of aniline in the presence of the dopant. The polymerised PANI– dopant complexes had the compositions of PANIŽSPDP. 0.3 , PANIŽSPDA. 0.33 and PANIŽSPDPAA. 0.33 as determined from the observed weight differences between the as-polymerised complex and the corresponding reduced emeraldine base indicating that some deprotonation occurred during the washing procedure of the emulsion polymerised complex. At this level of composition, the plastification of the polymer has occurred and films could be prepared by hot-pressing method. But the hot pressed films of PANI– SPDP 2, PANI–SPDA 2 and PANI–SPDPAA 2 exhibited better flexibility than those of the PANI–SPDP 1, PANI– SPDA 1 and PANI–SPDPAA 1. The emeraldine base synthesized from in situ doping emulsion polymerised complexes exhibited inherent viscosities higher than the PANI base commonly polymerised in acidic aqueous solutions. The use of the bulky dopant as the counter ions increases the chain separation between adjacent PANI chains and consequently decreases the interchain interactions. An increased solubility of the growing chains in the emulsion is thus to be expected, leading to observed increase in viscosity w13x. Table 1 gives the inherent viscosity values of the salt and the corresponding base forms of PANI synthesized by the in situ doping emulsion polymerisation route and the conductivity values on pressed pellets of the protonated PANIs by the emulsion polymerisation method in both xylene and chloroform solvents. The higher inherent viscosity values of 0.91 dl gy1 and 1.49 dl gy1 respectively of the PANI–SPDPAA 1 and its corresponding base in comparison to other dopants could be due to the enhanced solvent dopant interaction. The polar dopant interacts with the less polar solvent Table 1 Effect of different dopants and solvents on the conductivity and viscosity of PANI Sl. no.
1 2 3 4 5 6
Solvent
Xylene Xylene Xylene Chloroform Chloroform Chloroform
Dopant
SPDP SPDA SPDPAA SPDP SPDA SPDPAA
Conductivity ŽS cmy1 .
0.6 2.5 6.2 0.13 0.61 0.84
Viscosity, hin Ždl gy1 . Salt
Base
0.84 0.89 0.92 0.85 0.89 0.87
1.22 1.31 1.49 1.26 1.35 1.28
30
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
xylene and it permits the PANI chain to occupy an expanded coil like structure as evidenced from the UV–VIS spectral analysis.
3.1.2. Spectral analysis The FTIR spectra of all these acid protonated PANIs exhibit spectral features characteristic of the protonated state. All the three spectra of the doped PANIs were almost identical. Broad absorbances at about 3440 cmy1 , 1560 cmy1 , 1480 cmy1 , 1128 cmy1 and 800 cmy1 appear in the spectra of all the PANI salts. The band at 3440 cmy1 is due to the N–H stretching band. The bands corresponding to quinoid ŽN5Q5N., 1593 cmy1 and benzenoid ŽN–B–N., 1506 cmy1 ring stretching modes of polyemeraldine base are shifted towards lower frequencies, 1560 cmy1 and 1480 cmy1 respectively. The band at 1128 cmy1 is typical of the protonated state of PANI. The absorption around 800 cmy1 is characteristic of 1,4 substituted phenyl rings. The peaks characteristic of the dopants were also present in the FTIR spectra of all the protonated PANIs. The UV–VIS spectrum of the protonated polymers were measured in various solvents such as xylene, tetrahydrofuran, chloroform and m-cresol. Fig. 1Ža. represents the UV–VIS spectra of PANI–SPDP and PANI–SPDA in xylene, tetrahydrofuran and in chloroform and all these spectra exhibit strong evidences for the protonation of PANI as seen from the bands at ca. 444 and 800 nm. Fig. 1Žb. exhibits the differences of the UV–VIS spectrum of PANI–SPDPPA in both xylene and chloroform, showing a free carrier tail occurring in the NIR region of the spectra of PANI–SPDPAA in xylene. Fig. 1Žc. gives the UV–VIS spectrum of the protonated complexes in m-cresol. The behaviour of PANI–SPDA in m-cresol indicates a change in molecular confirmation from ‘‘compact coil’’ to the ‘‘expanded coil’’ like structure. This conformational change is, however, not complete in the cases of PANI– SPDP and PANI–SPDPAA in m-cresol as evidenced from the localized polaron at about 900 nm. This type of behaviour indicates that the polar solvent m-cresol interacts more strongly with the dopant anion of SPDA than does with that of SPDPAA. In the case of PANI–SPDPAA in xylene, the polar moieties of the dopant interacts more strongly with the less polar xylene. This leads to the change in molecular confirmation from ‘‘compact coil’’ to the ‘‘expanded coil’’ like structure for PANI–SPDPAA in xylene. MacDiarmid and Epstein w26x explained this confirmational change as the secondary doping effect. For PANI–SPDP and PANI–SPDA in solvents such as xylene, tetrahydrofuran, chloroform, etc. it is possible that the polymer-solvent interactions are negligible and this effect tends to favor a compact coil confirmation of the polymer chain. But for PANI–SPDA in m-cresol, the doped polymer chain-solvent interaction will increase and the solvation of the positive andror negative ions associated with
Fig. 1. Ža. UV–VIS spectrum of PANI-SPDA in Ža. THF, Žb. xylene Žc. CHCl 3 and PANI-SPDP in Žd. THF, Že. xylene Žf. CHCl 3 . Žb. UV–VIS spectrum of PANI-SPDPAA in Ža. xylene and Žb. inchloroform. Žc. UV–VIS spectrum of Ža. PANI-SPDA Žb. PANI-SPDPAA and Žc. PANI-SPDP in m-cresol.
the polymer will increase resulting in an expansion of the initial compact coil confirmation of the PANI chain. 3.1.3. ConductiÕity studies Conductivity values were measured on pressed pelletsrfilms by the four-probe method. Table 1 gives the conductivity values obtained for the emulsion polymerised samples. The PANI–SPDPAA 1 exhibited a maximum conductivity value of 6.2 S cmy1 at room temperature compared to the conductivity values of PANI–SPDP 1 Ž0.6
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
S cmy1 . and PANI–SPDA 1 Ž2.5 S cmy1 .. This can be understood as previously explained from the dopant-solvent interaction taking place between SPDPAA and xylene. The conductivity values are comparatively lower in the case of polymers obtained using chloroform. The room temperature conductivity values for the mechanically mixed PANI–SPDP 2, PANI–SPDA 2 and for PANI–SPDPAA 2 are 0.82, 1.84 and 3.31 S cmy1 , respectively. The slightly higher conductivity values for the emulsion polymerised samples compared to the mechanically mixed samples is due to the more homogeneous protonation of PANI achieved by the in situ doping achieved through the emulsion polymerisation route. The protonated PANI complexes become highly soluble in nonpolar or weakly polar organic solvents such as xylene, chloroform, THF, etc., because of the presence of a long aliphatic hydrocarbonside chain of the dopant and so thin films of doped polymer could easily be prepared by the solution casting method from these solvents and from m-cresol. PANI– SPDA in m-cresol exhibited a conductivity value of 12 S cmy1 and PANI–SPDPAA in xylene exhibited a conductivity value of 13 S cmy1 , whereas the conductivity values reported for PANI–CSA–m-cresol system and PANI– DBSA–m-cresol system were above 100 S cmy1 . The lower values of conductivity of the present doped PANI systems might be due to the bulky nature of the dopant, which hinders its easy diffusion into the polymer backbone. The protonated PANI of 1:0.5 molar ratio forms a heavily plasticized mixture so that free-standing flexible films could be prepared by the hot pressing method. Fig. 2 represents the log conductivity against the temperature of pressing for PANI–SPDP 2, PANI–SPDA 2 and for PANI–SPDPAA 2. A maximum conductivity value of 65 S11cmy1 was obtained for the PANI–SPDPAA 2 film pressed at 1408C. This can be compared to the conductivity value of 70 S cmy1 reported in the case of the melt processable PANI obtained by doping with diphenyl phosphate w15–17,22x. So, it appears that the present conductiv-
Fig. 2. Log conductivity vs. pressing temperature for films of Ža. PANISPDPAA 2 Žb. PANI-SPDA 2 and Žc. PANI-SPDP 2.
31
Fig. 3. Plot of ln resistance vs. Ty1 r4 of Ža. PANI-SPDPPA 2 film at 1408C and Žb. PANI-SPDA 2 film at 1408C. The normalised resistance vs. temperature plot is shown in the inset.
ity value of 65 S cmy1 is the highest so far reported for a thermally processable PANI doped with sulfonated dopants. The PANI–SPDA 2 film pressed at 1408C gave a conductivity value of 42 S cmy1 . But for PANI–SPDP 2 film the conductivity is comparatively less because of the presence of a hydroxyl group. All these protonated polymers are thermally stable up to 2008C for preparing highly conducting films by the melt processing method. There is no significant decrease for the conductivity value for the melt-processed film at 2008C also. PANI doped with 3pentadecylphenoxy acetic acid itself is giving a conductivity in the order of 10y2 S cmy1 . Fig. 3 represents a plot of ln of normalised resistance vs. Ty1 r4 for films of Ža. PANI–SPDA 2 and Žb. PANI–SPDPAA 2 pressed at 1408C. It shows that ln of normalised resistance is proportional to Ty1 r4 in the temperature range 150–50 K indicating the three-dimensional variable range hopping conduction. Normalised resistance measured in the temperature range 300–50 K is shown in the inset of Fig. 3. The data are analysed for Mott’s variable range hopping conduction in three dimensions in the temperature range 150–50 K. 3.1.4. Morphology and crystallinity Fig. 4 represents the XRD patterns of PANI base, PANI–SPDP 1, PANI–SPDA 1 and PANI–SPDPAA 1. The in situ doping emulsion polymerised compounds exhibit high degree of crystalline order and orientation compared to the PANI base and the mineral acid doped PANI because of the more homogeneous protonation of PANI achieved by the emulsion polymerisation technique. The PANI–SPDPAA 1 is having comparatively higher conductivity and crystalline nature because of the solvent-dopant interaction as discussed earlier where the highly polar dopant interacts strongly with the solvent xylene. In the case of protonated PANI systems the conductivity is directly related to crystallinity Žbecause of the more ordered structure. and this may be the reason why a higher conductivity is achieved for the emulsion polymerised PANI– SPDPAA 1.
32
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
1, PANI–SPDPAA 2 and PANI–SPDPAA 2 film at 1408C.. The figure shows that the grains are loosely packed and distinguishable in nature in the case of PANI base whereas the SEM picture of the emulsion polymerised sample shows better cohesion and higher aggregation indicating higher molecular weight build up. In the case of the plasticized PANI–SPDPAA 2, PANI grains are swollen and continuous in nature, which upon hot pressing gives the free-standing flexible films. Fig. 5Žd. represents the PANI–SPDA 2 film at 1408C, typical of the hot pressed films of the plasticized PANI. 3.2. Part B: studies on thermoplastic blends
Fig. 4. X-ray diffraction patterns of Ža. emeraldine base, Žb. PANI-SPDP 1, Žc. PANI-SPDA 1, Žd. PANI-SPDPAA 1.
Fig. 5 represents the SEM photographs typical of these sulphonic acid-doped PANIs ŽPANI base, PANI–SPDPAA
The preceding section discussed studies carried out on the doping behaviour and meltrsolution processability of PANI doped with three novel sulfonic acid dopants having long flexible side chain. Highly conducting free-standing films of protonated PANI could be prepared both by the solution casting and by the melt processing techniques. Melt processability could further be improved by blending with conventional thermoplastic polymers to incorporate attractive mechanical and other properties w27x. There have been reports on blending of PANI through electrochemical w28x and chemical w29x polymerisation of aniline onto polymer substrates or by blending of soluble or processable conducting polymers with insulating polymers in solution w12,19x or melts w15,16,21x. Several conductive com-
Fig. 5. SEM photographs of Ža. PANI base, Žb. PANI-SPDPAA 1, Žc. plasicized PANI-SPDPAA 2 and Žd. PANI-SPDPAA 2 film at 1408C.
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
Fig. 6. Conductivity vs. PANI content in the blends of Ža. PANIŽSPDA. 0.5 , Žb. PANIŽSPDPAA. 0.5 . Pressing temperature, 1608C; pressing time, 15 min.
posites of PANI with vinyl polymers, such as polystyrene w30x, polyŽalkyl methacrylates. w12,19,31–33x, polyŽvinyl chloride., PVC w15,16,34–37x and polyŽethylene-co-vinyl acetate. w38x were reported. A few conductive blends are already in commercial use. Ikkala et al. w27x prepared conductive polymer blends by blending thermoplastic polymers with Neste complex ŽPANI–DBSA prepared by thermal doping. using conventional melt-processing techniques. Shacklette et al. w39x reported that Versicon ŽAllied Signal., a conductive form of PANI is dispersible in polar thermoplastic matrix polymers such as polycaprolactone and polyŽethylene terephthalate glycol.. We observed that the blends of doped Žwith SPDA and SPDPAA. PANI with PVC gave highly conducting thermoplastic films with low percolation threshold and high transparancy. The conducting films with uniform morphology were fabricated by the melt processing techniques. Fig. 6 represents the log conductivity vs. the content of PANI–SPDA and PANI–SPDPAA in PVC. The conductivity values obtained for PANI–SPDA–PVC polyblend
33
are higher compared to those of PANI–SPDPAA–PVC system. In the case of PANI–SPDA–PVC system, a 2 wt.% fraction of PANI itself is giving a conductivity value of 3.4 = 10y3 S cmy1 with no indication of a sharp percolation threshold. On the other hand, a 2 wt.% of PANI in the case of PANI–SPDPAA–PVC system exhibited a conductivity value of 8.2 = 10y5 S cmy1 . Laaska et al. w15x reported that the percolation threshold is observed for a 25 wt.% of the electroactive component in the case of PANI protonated with DBSA and their blends with PVC. In the present case, a lower percolation threshold is obtained because the plastification of PANI strongly facilitates the mixing of the components of the blend. The dispersion of PANI grains in the PVC matrix was, thus, considerably enhanced by the presence of the plasticizer, which apparently loosened the PANI grain–grain adhesion forces as reported by Pron et al. w40x In the case of blends of conducting polymers, a low content of the conducting phase is desirable for applications because an excessive amount may distort other properties of the matrix material, such as mechanical strength or colour. Thus, the remarkable reduction in the percolation threshold is achieved by the presence of the flexible side chains of the dopants, which plasticizes PANI. Actually, the incorporation of flexible side chains to the rigid-rod backbone remarkably improves the solubility and lowers the melting or softening points for the stiff polymers. In the case of the blends of PANI with SPDPAA and SPDA, the long-side chains of the dopants could even induce molecular miscibility of the two rigid polymers. Fig. 7 represents a plot of ln of normalised resistance vs. Ty1 r4 for a polyblend film of PANI–SPDA–PVC system, having 25 wt.% of PANI. It shows that ln of normalised resistance is proportional to Ty1 r4 in the temperature range 35–300 K, indicating a three-dimensional variable range hopping conduction. Normalised resistance measured in the temperature range of 300–35 K is shown in the inset of Fig. 7. The data are analysed for Mott’s
Fig. 7. Plot of ln resistance vs. Ty1 r4 of PANI-SPDA-PVC blend with 25 wt.% of PANI. The normalised resistance–temperature plot is shown in the inset.
34
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
variable range hopping conduction in three dimensions in this temperature range. Fig. 8 represents the SEM micrographs of surfaces of films obtained by hot pressing of PVC blends of PANI– SPDA–PVC and PANI–SPDPAA–PVC having 20 wt.% of PANI content. The blend morphology was studied to elucidate the structure responsible for the electrical conductivity behaviour. In the case of PANI–SPDA–PVC system, there is evidence for a continuous network formation of the conducting phase. But, in the case of PANI– SPDPAA–PVC, even though the network formation is observed, there is evidence for PANI–SPDPAA agglomerates, which is evenly distributed in the polymer matrix. Roichman et al. w37x have also reported a similar type of
Fig. 9. Tensile strength vs. PANIŽSPDA. 0.5 content in PANI-SPDA-PVC system.
distribution of agglomerates of PANI–DBSA in the polymer matrix for a solution mixing blend of PANI–DBSA– PVC system. The formation of the agglomerates interferes with the conduction phenomenon and eventually lowers conductivity for the PANI–SPDPAA–PVC system. PVC, being a highly polar polymer, enhances the blending and compatabilization with the less polar SPDA-doped PANI rather than with the polar SPDPAA-doped PANI. Therefore, better conducting network is formed in the polyblend of PANI–SPDA–PVC and 2 wt.% of PANI in PVC itself is giving a conductivity value in the order 10y3 S cmy1 . A better uniform distribution of doped PANI was observed at lower wt.% levels by the scanning electron microscopy studies. It is possible that the conductive phase may exhibit special morphologies of self assembled interpenetrating polymer networks as explained by Heeger w41x. The most characteristic feature of PANI containing blends obtained by using functionalised acids is the possibility of preparation of transparent films and the possibility of keeping the mechanical properties of the composites at a level essentially equivalent to the host bulk polymer. Fig. 9 represents the tensile strength of the polyblend PANI– SPDA–PVC vs. the PANI content. The tensile strength is
Fig. 8. SEM photographs of Ža. PVC film, Žb. PANI-SPDA-PVC with 20 wt.% of PANI, Žc. PANI- SPDPAA-PVC with 20 wt.% of PANI.
Fig. 10. DSC thermograms of Ža. PVC itself, Žb. 10 wt.% of Žc. 20 wt.%, Žd. 30 wt.% of PANI in PANI-SPDA-PVC.
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
decreasing rapidly with increase in the content of plasticised PANI and the blend containing 25% of PANI is having a tensile strength of 6 MPa and a conductivity value of 2.5 S cmy1 . Fig. 10 represents the DSC thermograms of PANI– SPDA–PVC blends. The miscibility of polymer blends is commonly ascertained through Tg measurements. The Tg increases with increasing PANI content in the blend and this increase with the PANI content is taken to indicate the miscibility as reported by Ong et al. w42x. The blends of PANI with thermoplastic polymers are expected to have a number of applications, such as electrostatic dissipation ŽESD., static discharge and electromagnetic interference shielding ŽEMI.. The required conductivity levels are approximately 10y5 –10y9 S cmy1 for ESD and greater than 1 S cmy1 for EMI. And because of the high surface-to-volume ratio resulting from a fractual network structure of the conducting phase, it is expected that the blends of doped Žwith SPDA and SPDPAA. PANI with PVC can be suggested for the potential applications.
4. Conclusion Three novel functionalized sulfonic acid dopants, SPDP, SPDA and SPDPAA, were found to impart meltrsolution processability and protonating ability to PANI. The significant aspect is that plasticization of PANI could be simultaneously achieved during the process of protonation and the protonated PANI could be thermally processed and highly conducting free-standing flexible films, which are thermally stable up to 2008C could be prepared. A maximum conductivity value of 65 S cmy1 was obtained for a film of PANI–SPDPPA at 1408C. Also, highly conductive thermoplastic films could be prepared by the blending of protonated PANI with PVC and these films could exhibit low percolation threshold and high transparency. And because of the high surface-to-volume ratio resulting from a fractual network structure of the conducting phase, these blends can be suggested for the potential applications.
References w1x C.K. Chiang, C.R. Fincher, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau, A.G. MacDiarmid, Phys. Rev. Lett. 39 Ž1977. 1098. w2x T.A. Skotheim ŽEd.., Handbook of Conducting Polymers Vols. 1 and 11 Decker, New York, 1989. w3x M.K. Traore, W.T.K. Stevenson, B.J. Mc Cormic, R.C. Dorey et al., Synth. Met. 40 Ž1991. 137. w4x T. Vikki, L.O. Pietila, H. Osterholm, L. Ahjopalo, A. Takala, A. Toivo, K. Levon, P. Passiniemi, O. Ikkala, Macromolecules 29 Ž1996. 2945.
35
w5x J.-C. Chiang, A.G. MacDiarmid, Synth. Met. 13 Ž1986. 193. w6x A.G. MacDiarmid, A.J. Epstein, Faraday Discuss. Chem. Soc. 88 Ž1989. 317. w7x G. D’Aprano, M. Leclerc, G. Zotty, G. Schiavon, Chem. Mater. 7 Ž1995. 33. w8x Y. Wei, W.N. Focke, G.E. Wnek, A. Ray, A.G. MacDiarmid, J. Phys. Chem. 93 Ž1989. 495. w9x W.-Y. Zheng, K. Levon, J. Laakso, J.-E. Osterholm, Macromolecules 27 Ž1994. 7754. w10x O. Oka, Jpn. Kokai Tokkyo Koho JP 95,239,207 w93,239,208x ŽCl.C08G73r00.17 Sept. Ž1993., JP Appl. 91r193, 565, 09 July Ž1991. Ca; 120–135499m. w11x C.H. MaCoy, I.M. Lorkovic, M.S. Writon, J. Am. Chem. Soc. 117 Ž1995. 33. w12x Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 Ž1992. 91. w13x J.E. Osterholm, Y. Cao, F. Klavetter, P. Smith, Synth. Met. 55–57 Ž1993. 1034. w14x Y. Cao, P. Smith, Polymer 34 Ž1993. 3139. w15x J. Laaska, A. Pron, M. Zagorska, S. Lapkowski, S. Lefrant, Synth. Met. 69 Ž1995. 113. w16x A. Pron, J. Laska, J.E. Osterholm, P. Smith, Polymer 34 Ž1993. 4235. w17x J. Laska, A. Pron, S. Lefrant, J. Polym. Sci., Part A: Polym. Chem. 33 Ž1995. 1437. w18x R.K. Paul, V. Vijayanathan, C.K.S. Pillai, Synth. Met. 104 Ž1999. 189. w19x C.Y. Yang, Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 53 Ž1993. 293. w20x Y. Cao, P. Smith, A.J. Heeger, PCT patent Application WO 22r22911, 1992. w21x O.T. Ikkala, J. Laakso, K. Vakiparta, E. Virtanen, H. Ruohonen, H. Jarvinen, T. Taka, P. Passiniemi, J.E. Osterholm, Y. Cao, A. Andreatta, P. Smith, A.J. Heeger, Synth. Met. 69 Ž1995. 97. w22x A. Pron, W. Luzny, J. Laaska, Synth. Met. 80 Ž1996. 191. w23x G.E. Asturias, A.G. MacDiarmid, R.P. McCall, A.J. Epstein, Synth. Met. 29 Ž1989. E157. w24x S. Caplan, US PATENT 2310146, 1943. w25x S.C. Sethi, B.C. Subba Rao, S.B. Kulkarni, S.S. Katti, Ind. J. Tech. 1 Ž1963. 348, and the references cited therein. w26x A.G. MacDiarmid, A.J. Epstein, Synth. Met. 65 Ž1994. 103. w27x O.T. Ikkala, J. Laakso, K. Vakiparta, E. Virtanen, H. Ruohonen, H. Jarvinen, J. Taka, P. Passiniemi, J.E. Osterholm, Synth. Met. 69 Ž1995. 97. w28x S.A. Chen, W.G. Fang, Macromolecules 24 Ž1991. 1242. w29x H. Hsu, Synth. Met. 41 Ž1991. 671. w30x E. Ruckenstein, S. Yang, Synth. Met. 53 Ž1993. 283. w31x C.K. Subramaniam, A.B. Kaiser, P.W. Gilberd, C.J. Liu, B. Weissling, Solid State Commun. 97 Ž1992. 235. w32x L. Terlemezyan, M. Mihailov, B. Ihanov, Polym. J. 27 Ž1995. 867. w33x S. Yang, E. Ruckenstein, Synth. Met. 59 Ž1993. 1. w34x P. Banerjee, B.M. Mandal, Synth. Met. 74 Ž1995. 257. w35x C.K. Subramaniam, A.B. Kaiser, P.W. Gilberd, C.J. Liu, B. Weissling, J. Polym. Sci., Polym. Phys. Ed., 31 Ž1993. 14, 25. w36x C. Conn, N. Booth, J. Unsworth, Adv. Mater. 7 Ž1995. 790. w37x Y. Roichman, M.S. Silverstein, A. Siegmann, M. Narkis, J. Macromol. Sci., Phys. B 38 ŽIe2. Ž1999. 145. w38x W. Zheng, K. Levon, T. Taka, J. Laakso, J.E. Osterholm, J. Polym. Sci., Polym. Phys. Ed. 33 Ž1995. 1289. w39x L.W. Shacklette, C.C. Han, M.H. Luly, Synth. Met. 57 Ž1993. 3532. w40x Pron, Y. Nicolau, F. Genoud, M. Nechtschein, J. Appl. Polym. Sci. 63 Ž1997. 971. w41x A.J. Heeger, Trends Polym. Sci. 3 Ž1995. 39. w42x H. Ong, S.H. Goh, H.S.O. Chan, Polymer 38 Ž1997. 1065.
Meltrsolution processable conducting polyaniline with novel sulfonic acid dopants and its thermoplastic blends Raji K. Paul, C.K.S. Pillai ) Council of Scientific and Industrial Research, Regional Research Laboratory, ThiruÕananthapuram Industrial Estate P.O., ThiruÕananthapuram, Kerala 695 019, India Received 24 August 1999; received in revised form 18 January 2000; accepted 19 January 2000
Abstract The effects of three sulfonic acid dopants, sulfonic acid of 3-pentadecylphenol ŽSPDP., sulfonic acid of 3-pentadecylanisole ŽSPDA. and sulfonic acid of 3-pentadecylphenoxy acetic acid ŽSPDPAA. Žsynthesized from an inexpensive naturally existing biomonomer, cardanol. on the electrical conductivity and other properties of polyaniline ŽPANI. were studied. All the three dopants were found to act as very good plasticizing cum protonating agents for PANI. Doping was carried out either by the in situ doping emulsion polymerization route or by mechanical mixing of emeraldine base and the dopant. The protonated complexes obtained by the emulsion polymerisation route exhibited exceptionally high degree of crystalline order and orientation. Highly conducting free-standing films of protonated PANI could be prepared both by the solution casting and by the melt processing techniques. On thermal processing, free-standing flexible films with conductivities as high as 65 S cmy1 could be prepared. PANI protonated with the dopants, SPDA and SPDPAA were blended with polyŽvinyl chloride. ŽPVC. and thin films were prepared by melt processing techniques and the properties of the blends were studied. Percolation threshold is occurring at very low weight percentage of PANI and a maximum coductivity value of 2.5 S cmy1 was obtained for 25 wt.% of PANI for PANI–SPDA–PVC system. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Polyaniline; Novel sulfonic acid dopants; Meltrsolution processability; Thermoplastic blends; PVC; Percolation threshold
1. Introduction Since the discovery of conducting polyacetylene films in 1977 w1x, there has been substantial interest in the scientific and engineering communities in understanding the intrinsically conductive polymers ŽICPs. and finding applications for their unique properties w2x. The most common ICPs are polyaniline ŽPANI., polypyrrole, polythiophene, polyacetylene, polyŽ p-phenylene. and polyŽ pphenylene sulfide.. Since the backbone of these polymers contains conjugated double bonds, they exhibit extraordinary electronic properties, such as low ionization potential and high electron affinity, and, as a result, can be easily reduced or oxidized. These polymers undergo ‘‘doping’’ upon exposure to a protonic acid and become electrically conductive w3x. But these polymers are highly intractable and infusible because of their highly aromatic nature, the
) Corresponding author. Tel.: q91-471-490-674; fax: q91-471-490186. E-mail address: [email protected] ŽC.K.S. Pillai..
interchain hydrogen bondings and the charge delocalization effects w4x. So one of the active research areas of both scientific and industrial importance has been to develop electrically conductive polymers, which are fusiblermelt processable or soluble in common solvents. Over the last decade, significant progress has been made in the processing of ICPs into useful oriented articles. PANI is one of the most promising conducting polymers due to its straightforward polymerisation and excellent chemical stability combined with relatively high levels of conductivity w5x. Emeraldine base form of PANI is soluble only in N-methylpyrrolidone ŽNMP., selected amines, concentrated sulphuric acid and other strong acids. Emeraldine salt is even more intractable. Among the methods to improve the melt and solution processability of PANI, covalent substitution, such as ring substitution w6–8x, N-alkylation w9–11x and protonation with functionalized protonic acids, such as dodecylbenzene sulfonic acid w12,13x, camphorsulfonic acid w14x, phosphoric acid esters w15–18x, etc., merit mention. The use of specific functionalized protonic acid dopants render high solubility to PANI in common solvents w12x, allowing preparation of solution
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 0 0 . 0 0 2 0 6 - X
28
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
cast w19x or melt processable blends with a low percolation limit w20,21x. But according to Pron et al. w22x, the selection of appropriate solvent is of crucial importance for electrical conductivity of solution cast PANI films. For example, PANI protonated with camphor sulfonic acid, in selected solvents, such as m-cresol, induce extended coil conformation of PANI chains, which facilitate polaron delocalization and crystallization of the polymer upon casting w22x. Also m-cresol is suspected to be a cancer causing substance. From the industrial point of view, the fabrication of a thermally processable conducting polymer would be preferable because it is easier and much cheaper. Thermally processable PANI has recently been fabricated using aliphatic and aromatic diesters of phosphoric acid as protonating agents w15–18,22x, where the hydrophobic groups introduced to PANI with each dopant ion lead to its plastification. In our previous paper w18x, we have shown that protonation of PANI with a dopant, 3-pentadecylphenylphosphoric acid induces melt and solution processability to PANI. In this communication, we show that sulfonic acid derivatives of 3-pentadecyl phenol ŽPDP., derived from an inexpensive naturally existing biomonomer, cardanol, are excellent dopants for PANI, which induce plastification leading to thermal processability and solution processability to PANI. The dopants synthesized are sulfonic acid of 3-pentadecylphenol ŽSPDP., sulfonic acid of 3-pentadecylanisole ŽSPDA. and sulfonic acid of 3-pentadecylphenoxy acetic acid ŽSPDPAA.. One of the significant features of the structure of the dopants is that it has a flexible n-alkyl ŽC 15 H 31 . substituent in the meta position of the aromatic ring, which makes the doped PANI melt processable and render high solubility for PANI in common solvents. These dopants, thus, render plasticizing ability to PANI so that free standing flexible films could be prepared by the conventional melt processing techniques and by the solution processing techniques. Thus, for the first time, a method has been developed Žprocess filed for patent in India and abroad. to prepare melt processable conducting PANI doped with sulfonic acid dopants having protonating cum plasticizing ability. We also report the preparation of conductive flexible thermoplastics of protonated PANI by blending with PVC. The conducting thermoplastic films were fabricated by the melt processing techniques and are found to exhibit very low percolation threshold and high transparency.
2. Experimental
2.1. Reagents All chemicals used were purchased from S.D. Fine Chem., Mumbai. Aniline was doubly distilled under vacuum and stored in a refrigerator. All the solvents used were of spectroscopic grade. PVC Ž67–311 extrusion grade. purchased from NOCIL, Mumbai is used for the blending studies.
2.2. Synthesis
2.2.1. The preparation of polyemeraldine base PANI was synthesised chemically according to the method of MacDiarmid et al. w23x. The neutral base form of PANI was obtained by dedoping the PANI–salt in 3 wt.% ammonia solution for 3 h followed by washing with acetone and drying in vacuum for 8 h at 608C.
2.2.2. Synthesis of dopants 2.2.2.1. The synthesis of methyl ether of 3-PDP. The methyl ether of PDP was prepared by reaction of PDP with dimethyl sulfate in aqueous alkali w24x. 2.2.2.2. The synthesis of 3-pentadecylphenoxy acetic acid. 3-Pentadecylphenoxy acetic acid was prepared by reaction of PDP with monochloroacetic acid in alcohol and aqueous alkali w25x. 2.2.2.3. Sulfonation. Sulfonation of PDP, methyl ether of PDP and 3-pentadecylphenoxy acetic acid was carried out by reacting with 98% of concentrated sulfuric acid at 110–1208C w25x.
2.2.3. Doping Protonation of PANI by these dopants were carried out either by in situ doping emulsion polymerisation route or by mechanical mixing method. 2.2.3.1. In situ doping emulsion polymerisation. The in situ doping emulsion polymerisation of aniline in the presence of the dopants was carried out according to the procedure of Osterholm et al. w13x. The polymers obtained by the emulsion polymerisation method using the three dopants are designated as PANI–SPDP 1, PANI–SPDA 1 and PANI–SPDPAA 1. These protonated polymers obtained were then converted to the emeraldine base form by treatment with 3 wt.% of aqueous ammonia solution for 3 h.
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
2.2.3.2. Mechanical mixing method. Emeraldine base of inherent viscosity 1.2 dl gy1 Žmeasured at room temperature in concentrated sulfuric acid. was mixed with the dopant in a molar ratio of 0.5 dopant to polymer repeat unit PhN, using an agate mortar and pestle at room temperature to get PANI–SPDP 2, PANI–SPDA 2 and PANI– SPDPAA 2. Different molar ratios of the PANI–dopant combination were prepared in order to find out the plastification threshold. 2.2.4. Preparation of blends Conducting blends of PANI–SPDA and PANI– SPDPAA with PVC were prepared by mechanical mixing at room temperature for an extended period of time to achieve optimum homogeneity. Various plasticized PANIrPVC ratios were used. The mixture containing PANI and PVC were then hot pressed at 1608C for 15 min so that thin films of approximate thickness of 0.2 mm can be prepared. 2.3. Characterisation methods Inherent viscosities of PANI base and protonated PANI were determined in 0.1% wrw solutions in concentrated H 2 SO4 at 258C using an Ubbelohde viscometer. The FTIR spectra were recorded on a NICOLET IMPACT 400 D spectrophotometer. The UV–VIS spectra were taken in a Shimadzu UV-2100 spectrophotometer. Conductivity was measured by four-probe method. XRD pattern was recorded on a Rigaku Geiger-Flex DrMAX series using CuK a radiation. SEM pictures were recorded using JEOL JSM5600LV Scanning Electron Microscope. DSC thermograms were taken on Differential Scanning Calorimeter 2010 attached to Thermal Analyst 2100. Mechanical properties were determined on an Instron Series IX, Automated Materials Testing System 1.09.
3. Results and discussions 3.1. Part A: studies on protonated PANI 3.1.1. General obserÕations Sulfonic acid derivatives of PDP and the related compounds are well known for their surfactant properties w24,25x. No report could be located on their use as dopants for PANI. Interesting observations were made on doping PANI with these sulfonic acids having a long hydrophobic hydrocarbon-side chain. The PANI protonated by using these dopants becomes soluble in nonpolar or weakly polar organic solvents, such as xylene, chloroform, tetrahydrofuran, etc. Thin films of PANI doped with SPDP, SPDA and SPDPAA could be easily prepared from xylene and chloroform solutions because the counter ions of the dopants act as surfactants for organic solvents. At the same time, protonation of PANI with these dopants results in a heav-
29
ily plasticized mixture, which exhibits rheological parameters characteristic of a Bingham-type liquid with the viscosity decreasing with the increase of dopant content. The composition required to reach plastification threshold for the protonated PANI with these dopants is for a dopantrPANI ratio greater than 0.3, above which the polymer can be thermally processed to give free-standing flexible films. These sulfonic acid dopants were found to act as an emulsifying cum protonating agents for PANI. This enabled the in situ doping emulsion polymerisation of aniline in the presence of the dopant. The polymerised PANI– dopant complexes had the compositions of PANIŽSPDP. 0.3 , PANIŽSPDA. 0.33 and PANIŽSPDPAA. 0.33 as determined from the observed weight differences between the as-polymerised complex and the corresponding reduced emeraldine base indicating that some deprotonation occurred during the washing procedure of the emulsion polymerised complex. At this level of composition, the plastification of the polymer has occurred and films could be prepared by hot-pressing method. But the hot pressed films of PANI– SPDP 2, PANI–SPDA 2 and PANI–SPDPAA 2 exhibited better flexibility than those of the PANI–SPDP 1, PANI– SPDA 1 and PANI–SPDPAA 1. The emeraldine base synthesized from in situ doping emulsion polymerised complexes exhibited inherent viscosities higher than the PANI base commonly polymerised in acidic aqueous solutions. The use of the bulky dopant as the counter ions increases the chain separation between adjacent PANI chains and consequently decreases the interchain interactions. An increased solubility of the growing chains in the emulsion is thus to be expected, leading to observed increase in viscosity w13x. Table 1 gives the inherent viscosity values of the salt and the corresponding base forms of PANI synthesized by the in situ doping emulsion polymerisation route and the conductivity values on pressed pellets of the protonated PANIs by the emulsion polymerisation method in both xylene and chloroform solvents. The higher inherent viscosity values of 0.91 dl gy1 and 1.49 dl gy1 respectively of the PANI–SPDPAA 1 and its corresponding base in comparison to other dopants could be due to the enhanced solvent dopant interaction. The polar dopant interacts with the less polar solvent Table 1 Effect of different dopants and solvents on the conductivity and viscosity of PANI Sl. no.
1 2 3 4 5 6
Solvent
Xylene Xylene Xylene Chloroform Chloroform Chloroform
Dopant
SPDP SPDA SPDPAA SPDP SPDA SPDPAA
Conductivity ŽS cmy1 .
0.6 2.5 6.2 0.13 0.61 0.84
Viscosity, hin Ždl gy1 . Salt
Base
0.84 0.89 0.92 0.85 0.89 0.87
1.22 1.31 1.49 1.26 1.35 1.28
30
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
xylene and it permits the PANI chain to occupy an expanded coil like structure as evidenced from the UV–VIS spectral analysis.
3.1.2. Spectral analysis The FTIR spectra of all these acid protonated PANIs exhibit spectral features characteristic of the protonated state. All the three spectra of the doped PANIs were almost identical. Broad absorbances at about 3440 cmy1 , 1560 cmy1 , 1480 cmy1 , 1128 cmy1 and 800 cmy1 appear in the spectra of all the PANI salts. The band at 3440 cmy1 is due to the N–H stretching band. The bands corresponding to quinoid ŽN5Q5N., 1593 cmy1 and benzenoid ŽN–B–N., 1506 cmy1 ring stretching modes of polyemeraldine base are shifted towards lower frequencies, 1560 cmy1 and 1480 cmy1 respectively. The band at 1128 cmy1 is typical of the protonated state of PANI. The absorption around 800 cmy1 is characteristic of 1,4 substituted phenyl rings. The peaks characteristic of the dopants were also present in the FTIR spectra of all the protonated PANIs. The UV–VIS spectrum of the protonated polymers were measured in various solvents such as xylene, tetrahydrofuran, chloroform and m-cresol. Fig. 1Ža. represents the UV–VIS spectra of PANI–SPDP and PANI–SPDA in xylene, tetrahydrofuran and in chloroform and all these spectra exhibit strong evidences for the protonation of PANI as seen from the bands at ca. 444 and 800 nm. Fig. 1Žb. exhibits the differences of the UV–VIS spectrum of PANI–SPDPPA in both xylene and chloroform, showing a free carrier tail occurring in the NIR region of the spectra of PANI–SPDPAA in xylene. Fig. 1Žc. gives the UV–VIS spectrum of the protonated complexes in m-cresol. The behaviour of PANI–SPDA in m-cresol indicates a change in molecular confirmation from ‘‘compact coil’’ to the ‘‘expanded coil’’ like structure. This conformational change is, however, not complete in the cases of PANI– SPDP and PANI–SPDPAA in m-cresol as evidenced from the localized polaron at about 900 nm. This type of behaviour indicates that the polar solvent m-cresol interacts more strongly with the dopant anion of SPDA than does with that of SPDPAA. In the case of PANI–SPDPAA in xylene, the polar moieties of the dopant interacts more strongly with the less polar xylene. This leads to the change in molecular confirmation from ‘‘compact coil’’ to the ‘‘expanded coil’’ like structure for PANI–SPDPAA in xylene. MacDiarmid and Epstein w26x explained this confirmational change as the secondary doping effect. For PANI–SPDP and PANI–SPDA in solvents such as xylene, tetrahydrofuran, chloroform, etc. it is possible that the polymer-solvent interactions are negligible and this effect tends to favor a compact coil confirmation of the polymer chain. But for PANI–SPDA in m-cresol, the doped polymer chain-solvent interaction will increase and the solvation of the positive andror negative ions associated with
Fig. 1. Ža. UV–VIS spectrum of PANI-SPDA in Ža. THF, Žb. xylene Žc. CHCl 3 and PANI-SPDP in Žd. THF, Že. xylene Žf. CHCl 3 . Žb. UV–VIS spectrum of PANI-SPDPAA in Ža. xylene and Žb. inchloroform. Žc. UV–VIS spectrum of Ža. PANI-SPDA Žb. PANI-SPDPAA and Žc. PANI-SPDP in m-cresol.
the polymer will increase resulting in an expansion of the initial compact coil confirmation of the PANI chain. 3.1.3. ConductiÕity studies Conductivity values were measured on pressed pelletsrfilms by the four-probe method. Table 1 gives the conductivity values obtained for the emulsion polymerised samples. The PANI–SPDPAA 1 exhibited a maximum conductivity value of 6.2 S cmy1 at room temperature compared to the conductivity values of PANI–SPDP 1 Ž0.6
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
S cmy1 . and PANI–SPDA 1 Ž2.5 S cmy1 .. This can be understood as previously explained from the dopant-solvent interaction taking place between SPDPAA and xylene. The conductivity values are comparatively lower in the case of polymers obtained using chloroform. The room temperature conductivity values for the mechanically mixed PANI–SPDP 2, PANI–SPDA 2 and for PANI–SPDPAA 2 are 0.82, 1.84 and 3.31 S cmy1 , respectively. The slightly higher conductivity values for the emulsion polymerised samples compared to the mechanically mixed samples is due to the more homogeneous protonation of PANI achieved by the in situ doping achieved through the emulsion polymerisation route. The protonated PANI complexes become highly soluble in nonpolar or weakly polar organic solvents such as xylene, chloroform, THF, etc., because of the presence of a long aliphatic hydrocarbonside chain of the dopant and so thin films of doped polymer could easily be prepared by the solution casting method from these solvents and from m-cresol. PANI– SPDA in m-cresol exhibited a conductivity value of 12 S cmy1 and PANI–SPDPAA in xylene exhibited a conductivity value of 13 S cmy1 , whereas the conductivity values reported for PANI–CSA–m-cresol system and PANI– DBSA–m-cresol system were above 100 S cmy1 . The lower values of conductivity of the present doped PANI systems might be due to the bulky nature of the dopant, which hinders its easy diffusion into the polymer backbone. The protonated PANI of 1:0.5 molar ratio forms a heavily plasticized mixture so that free-standing flexible films could be prepared by the hot pressing method. Fig. 2 represents the log conductivity against the temperature of pressing for PANI–SPDP 2, PANI–SPDA 2 and for PANI–SPDPAA 2. A maximum conductivity value of 65 S11cmy1 was obtained for the PANI–SPDPAA 2 film pressed at 1408C. This can be compared to the conductivity value of 70 S cmy1 reported in the case of the melt processable PANI obtained by doping with diphenyl phosphate w15–17,22x. So, it appears that the present conductiv-
Fig. 2. Log conductivity vs. pressing temperature for films of Ža. PANISPDPAA 2 Žb. PANI-SPDA 2 and Žc. PANI-SPDP 2.
31
Fig. 3. Plot of ln resistance vs. Ty1 r4 of Ža. PANI-SPDPPA 2 film at 1408C and Žb. PANI-SPDA 2 film at 1408C. The normalised resistance vs. temperature plot is shown in the inset.
ity value of 65 S cmy1 is the highest so far reported for a thermally processable PANI doped with sulfonated dopants. The PANI–SPDA 2 film pressed at 1408C gave a conductivity value of 42 S cmy1 . But for PANI–SPDP 2 film the conductivity is comparatively less because of the presence of a hydroxyl group. All these protonated polymers are thermally stable up to 2008C for preparing highly conducting films by the melt processing method. There is no significant decrease for the conductivity value for the melt-processed film at 2008C also. PANI doped with 3pentadecylphenoxy acetic acid itself is giving a conductivity in the order of 10y2 S cmy1 . Fig. 3 represents a plot of ln of normalised resistance vs. Ty1 r4 for films of Ža. PANI–SPDA 2 and Žb. PANI–SPDPAA 2 pressed at 1408C. It shows that ln of normalised resistance is proportional to Ty1 r4 in the temperature range 150–50 K indicating the three-dimensional variable range hopping conduction. Normalised resistance measured in the temperature range 300–50 K is shown in the inset of Fig. 3. The data are analysed for Mott’s variable range hopping conduction in three dimensions in the temperature range 150–50 K. 3.1.4. Morphology and crystallinity Fig. 4 represents the XRD patterns of PANI base, PANI–SPDP 1, PANI–SPDA 1 and PANI–SPDPAA 1. The in situ doping emulsion polymerised compounds exhibit high degree of crystalline order and orientation compared to the PANI base and the mineral acid doped PANI because of the more homogeneous protonation of PANI achieved by the emulsion polymerisation technique. The PANI–SPDPAA 1 is having comparatively higher conductivity and crystalline nature because of the solvent-dopant interaction as discussed earlier where the highly polar dopant interacts strongly with the solvent xylene. In the case of protonated PANI systems the conductivity is directly related to crystallinity Žbecause of the more ordered structure. and this may be the reason why a higher conductivity is achieved for the emulsion polymerised PANI– SPDPAA 1.
32
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
1, PANI–SPDPAA 2 and PANI–SPDPAA 2 film at 1408C.. The figure shows that the grains are loosely packed and distinguishable in nature in the case of PANI base whereas the SEM picture of the emulsion polymerised sample shows better cohesion and higher aggregation indicating higher molecular weight build up. In the case of the plasticized PANI–SPDPAA 2, PANI grains are swollen and continuous in nature, which upon hot pressing gives the free-standing flexible films. Fig. 5Žd. represents the PANI–SPDA 2 film at 1408C, typical of the hot pressed films of the plasticized PANI. 3.2. Part B: studies on thermoplastic blends
Fig. 4. X-ray diffraction patterns of Ža. emeraldine base, Žb. PANI-SPDP 1, Žc. PANI-SPDA 1, Žd. PANI-SPDPAA 1.
Fig. 5 represents the SEM photographs typical of these sulphonic acid-doped PANIs ŽPANI base, PANI–SPDPAA
The preceding section discussed studies carried out on the doping behaviour and meltrsolution processability of PANI doped with three novel sulfonic acid dopants having long flexible side chain. Highly conducting free-standing films of protonated PANI could be prepared both by the solution casting and by the melt processing techniques. Melt processability could further be improved by blending with conventional thermoplastic polymers to incorporate attractive mechanical and other properties w27x. There have been reports on blending of PANI through electrochemical w28x and chemical w29x polymerisation of aniline onto polymer substrates or by blending of soluble or processable conducting polymers with insulating polymers in solution w12,19x or melts w15,16,21x. Several conductive com-
Fig. 5. SEM photographs of Ža. PANI base, Žb. PANI-SPDPAA 1, Žc. plasicized PANI-SPDPAA 2 and Žd. PANI-SPDPAA 2 film at 1408C.
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
Fig. 6. Conductivity vs. PANI content in the blends of Ža. PANIŽSPDA. 0.5 , Žb. PANIŽSPDPAA. 0.5 . Pressing temperature, 1608C; pressing time, 15 min.
posites of PANI with vinyl polymers, such as polystyrene w30x, polyŽalkyl methacrylates. w12,19,31–33x, polyŽvinyl chloride., PVC w15,16,34–37x and polyŽethylene-co-vinyl acetate. w38x were reported. A few conductive blends are already in commercial use. Ikkala et al. w27x prepared conductive polymer blends by blending thermoplastic polymers with Neste complex ŽPANI–DBSA prepared by thermal doping. using conventional melt-processing techniques. Shacklette et al. w39x reported that Versicon ŽAllied Signal., a conductive form of PANI is dispersible in polar thermoplastic matrix polymers such as polycaprolactone and polyŽethylene terephthalate glycol.. We observed that the blends of doped Žwith SPDA and SPDPAA. PANI with PVC gave highly conducting thermoplastic films with low percolation threshold and high transparancy. The conducting films with uniform morphology were fabricated by the melt processing techniques. Fig. 6 represents the log conductivity vs. the content of PANI–SPDA and PANI–SPDPAA in PVC. The conductivity values obtained for PANI–SPDA–PVC polyblend
33
are higher compared to those of PANI–SPDPAA–PVC system. In the case of PANI–SPDA–PVC system, a 2 wt.% fraction of PANI itself is giving a conductivity value of 3.4 = 10y3 S cmy1 with no indication of a sharp percolation threshold. On the other hand, a 2 wt.% of PANI in the case of PANI–SPDPAA–PVC system exhibited a conductivity value of 8.2 = 10y5 S cmy1 . Laaska et al. w15x reported that the percolation threshold is observed for a 25 wt.% of the electroactive component in the case of PANI protonated with DBSA and their blends with PVC. In the present case, a lower percolation threshold is obtained because the plastification of PANI strongly facilitates the mixing of the components of the blend. The dispersion of PANI grains in the PVC matrix was, thus, considerably enhanced by the presence of the plasticizer, which apparently loosened the PANI grain–grain adhesion forces as reported by Pron et al. w40x In the case of blends of conducting polymers, a low content of the conducting phase is desirable for applications because an excessive amount may distort other properties of the matrix material, such as mechanical strength or colour. Thus, the remarkable reduction in the percolation threshold is achieved by the presence of the flexible side chains of the dopants, which plasticizes PANI. Actually, the incorporation of flexible side chains to the rigid-rod backbone remarkably improves the solubility and lowers the melting or softening points for the stiff polymers. In the case of the blends of PANI with SPDPAA and SPDA, the long-side chains of the dopants could even induce molecular miscibility of the two rigid polymers. Fig. 7 represents a plot of ln of normalised resistance vs. Ty1 r4 for a polyblend film of PANI–SPDA–PVC system, having 25 wt.% of PANI. It shows that ln of normalised resistance is proportional to Ty1 r4 in the temperature range 35–300 K, indicating a three-dimensional variable range hopping conduction. Normalised resistance measured in the temperature range of 300–35 K is shown in the inset of Fig. 7. The data are analysed for Mott’s
Fig. 7. Plot of ln resistance vs. Ty1 r4 of PANI-SPDA-PVC blend with 25 wt.% of PANI. The normalised resistance–temperature plot is shown in the inset.
34
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
variable range hopping conduction in three dimensions in this temperature range. Fig. 8 represents the SEM micrographs of surfaces of films obtained by hot pressing of PVC blends of PANI– SPDA–PVC and PANI–SPDPAA–PVC having 20 wt.% of PANI content. The blend morphology was studied to elucidate the structure responsible for the electrical conductivity behaviour. In the case of PANI–SPDA–PVC system, there is evidence for a continuous network formation of the conducting phase. But, in the case of PANI– SPDPAA–PVC, even though the network formation is observed, there is evidence for PANI–SPDPAA agglomerates, which is evenly distributed in the polymer matrix. Roichman et al. w37x have also reported a similar type of
Fig. 9. Tensile strength vs. PANIŽSPDA. 0.5 content in PANI-SPDA-PVC system.
distribution of agglomerates of PANI–DBSA in the polymer matrix for a solution mixing blend of PANI–DBSA– PVC system. The formation of the agglomerates interferes with the conduction phenomenon and eventually lowers conductivity for the PANI–SPDPAA–PVC system. PVC, being a highly polar polymer, enhances the blending and compatabilization with the less polar SPDA-doped PANI rather than with the polar SPDPAA-doped PANI. Therefore, better conducting network is formed in the polyblend of PANI–SPDA–PVC and 2 wt.% of PANI in PVC itself is giving a conductivity value in the order 10y3 S cmy1 . A better uniform distribution of doped PANI was observed at lower wt.% levels by the scanning electron microscopy studies. It is possible that the conductive phase may exhibit special morphologies of self assembled interpenetrating polymer networks as explained by Heeger w41x. The most characteristic feature of PANI containing blends obtained by using functionalised acids is the possibility of preparation of transparent films and the possibility of keeping the mechanical properties of the composites at a level essentially equivalent to the host bulk polymer. Fig. 9 represents the tensile strength of the polyblend PANI– SPDA–PVC vs. the PANI content. The tensile strength is
Fig. 8. SEM photographs of Ža. PVC film, Žb. PANI-SPDA-PVC with 20 wt.% of PANI, Žc. PANI- SPDPAA-PVC with 20 wt.% of PANI.
Fig. 10. DSC thermograms of Ža. PVC itself, Žb. 10 wt.% of Žc. 20 wt.%, Žd. 30 wt.% of PANI in PANI-SPDA-PVC.
R.K. Paul, C.K.S. Pillair Synthetic Metals 114 (2000) 27–35
decreasing rapidly with increase in the content of plasticised PANI and the blend containing 25% of PANI is having a tensile strength of 6 MPa and a conductivity value of 2.5 S cmy1 . Fig. 10 represents the DSC thermograms of PANI– SPDA–PVC blends. The miscibility of polymer blends is commonly ascertained through Tg measurements. The Tg increases with increasing PANI content in the blend and this increase with the PANI content is taken to indicate the miscibility as reported by Ong et al. w42x. The blends of PANI with thermoplastic polymers are expected to have a number of applications, such as electrostatic dissipation ŽESD., static discharge and electromagnetic interference shielding ŽEMI.. The required conductivity levels are approximately 10y5 –10y9 S cmy1 for ESD and greater than 1 S cmy1 for EMI. And because of the high surface-to-volume ratio resulting from a fractual network structure of the conducting phase, it is expected that the blends of doped Žwith SPDA and SPDPAA. PANI with PVC can be suggested for the potential applications.
4. Conclusion Three novel functionalized sulfonic acid dopants, SPDP, SPDA and SPDPAA, were found to impart meltrsolution processability and protonating ability to PANI. The significant aspect is that plasticization of PANI could be simultaneously achieved during the process of protonation and the protonated PANI could be thermally processed and highly conducting free-standing flexible films, which are thermally stable up to 2008C could be prepared. A maximum conductivity value of 65 S cmy1 was obtained for a film of PANI–SPDPPA at 1408C. Also, highly conductive thermoplastic films could be prepared by the blending of protonated PANI with PVC and these films could exhibit low percolation threshold and high transparency. And because of the high surface-to-volume ratio resulting from a fractual network structure of the conducting phase, these blends can be suggested for the potential applications.
References w1x C.K. Chiang, C.R. Fincher, Y.W. Park, A.J. Heeger, H. Shirakawa, E.J. Louis, S.C. Gau, A.G. MacDiarmid, Phys. Rev. Lett. 39 Ž1977. 1098. w2x T.A. Skotheim ŽEd.., Handbook of Conducting Polymers Vols. 1 and 11 Decker, New York, 1989. w3x M.K. Traore, W.T.K. Stevenson, B.J. Mc Cormic, R.C. Dorey et al., Synth. Met. 40 Ž1991. 137. w4x T. Vikki, L.O. Pietila, H. Osterholm, L. Ahjopalo, A. Takala, A. Toivo, K. Levon, P. Passiniemi, O. Ikkala, Macromolecules 29 Ž1996. 2945.
35
w5x J.-C. Chiang, A.G. MacDiarmid, Synth. Met. 13 Ž1986. 193. w6x A.G. MacDiarmid, A.J. Epstein, Faraday Discuss. Chem. Soc. 88 Ž1989. 317. w7x G. D’Aprano, M. Leclerc, G. Zotty, G. Schiavon, Chem. Mater. 7 Ž1995. 33. w8x Y. Wei, W.N. Focke, G.E. Wnek, A. Ray, A.G. MacDiarmid, J. Phys. Chem. 93 Ž1989. 495. w9x W.-Y. Zheng, K. Levon, J. Laakso, J.-E. Osterholm, Macromolecules 27 Ž1994. 7754. w10x O. Oka, Jpn. Kokai Tokkyo Koho JP 95,239,207 w93,239,208x ŽCl.C08G73r00.17 Sept. Ž1993., JP Appl. 91r193, 565, 09 July Ž1991. Ca; 120–135499m. w11x C.H. MaCoy, I.M. Lorkovic, M.S. Writon, J. Am. Chem. Soc. 117 Ž1995. 33. w12x Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 48 Ž1992. 91. w13x J.E. Osterholm, Y. Cao, F. Klavetter, P. Smith, Synth. Met. 55–57 Ž1993. 1034. w14x Y. Cao, P. Smith, Polymer 34 Ž1993. 3139. w15x J. Laaska, A. Pron, M. Zagorska, S. Lapkowski, S. Lefrant, Synth. Met. 69 Ž1995. 113. w16x A. Pron, J. Laska, J.E. Osterholm, P. Smith, Polymer 34 Ž1993. 4235. w17x J. Laska, A. Pron, S. Lefrant, J. Polym. Sci., Part A: Polym. Chem. 33 Ž1995. 1437. w18x R.K. Paul, V. Vijayanathan, C.K.S. Pillai, Synth. Met. 104 Ž1999. 189. w19x C.Y. Yang, Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 53 Ž1993. 293. w20x Y. Cao, P. Smith, A.J. Heeger, PCT patent Application WO 22r22911, 1992. w21x O.T. Ikkala, J. Laakso, K. Vakiparta, E. Virtanen, H. Ruohonen, H. Jarvinen, T. Taka, P. Passiniemi, J.E. Osterholm, Y. Cao, A. Andreatta, P. Smith, A.J. Heeger, Synth. Met. 69 Ž1995. 97. w22x A. Pron, W. Luzny, J. Laaska, Synth. Met. 80 Ž1996. 191. w23x G.E. Asturias, A.G. MacDiarmid, R.P. McCall, A.J. Epstein, Synth. Met. 29 Ž1989. E157. w24x S. Caplan, US PATENT 2310146, 1943. w25x S.C. Sethi, B.C. Subba Rao, S.B. Kulkarni, S.S. Katti, Ind. J. Tech. 1 Ž1963. 348, and the references cited therein. w26x A.G. MacDiarmid, A.J. Epstein, Synth. Met. 65 Ž1994. 103. w27x O.T. Ikkala, J. Laakso, K. Vakiparta, E. Virtanen, H. Ruohonen, H. Jarvinen, J. Taka, P. Passiniemi, J.E. Osterholm, Synth. Met. 69 Ž1995. 97. w28x S.A. Chen, W.G. Fang, Macromolecules 24 Ž1991. 1242. w29x H. Hsu, Synth. Met. 41 Ž1991. 671. w30x E. Ruckenstein, S. Yang, Synth. Met. 53 Ž1993. 283. w31x C.K. Subramaniam, A.B. Kaiser, P.W. Gilberd, C.J. Liu, B. Weissling, Solid State Commun. 97 Ž1992. 235. w32x L. Terlemezyan, M. Mihailov, B. Ihanov, Polym. J. 27 Ž1995. 867. w33x S. Yang, E. Ruckenstein, Synth. Met. 59 Ž1993. 1. w34x P. Banerjee, B.M. Mandal, Synth. Met. 74 Ž1995. 257. w35x C.K. Subramaniam, A.B. Kaiser, P.W. Gilberd, C.J. Liu, B. Weissling, J. Polym. Sci., Polym. Phys. Ed., 31 Ž1993. 14, 25. w36x C. Conn, N. Booth, J. Unsworth, Adv. Mater. 7 Ž1995. 790. w37x Y. Roichman, M.S. Silverstein, A. Siegmann, M. Narkis, J. Macromol. Sci., Phys. B 38 ŽIe2. Ž1999. 145. w38x W. Zheng, K. Levon, T. Taka, J. Laakso, J.E. Osterholm, J. Polym. Sci., Polym. Phys. Ed. 33 Ž1995. 1289. w39x L.W. Shacklette, C.C. Han, M.H. Luly, Synth. Met. 57 Ž1993. 3532. w40x Pron, Y. Nicolau, F. Genoud, M. Nechtschein, J. Appl. Polym. Sci. 63 Ž1997. 971. w41x A.J. Heeger, Trends Polym. Sci. 3 Ž1995. 39. w42x H. Ong, S.H. Goh, H.S.O. Chan, Polymer 38 Ž1997. 1065.