- Email: [email protected]
Conductive fibers from enzymatically synthesized polyaniline
Synthetic Metals 107 Ž1999. 117–121 www.elsevier.comrlocatersynmet
Conductive fibers from enzymatically synthesized polyaniline X. Wang a , H. Schreuder-Gibson b, M. Downey a , S. Tripathy a , L. Samuelson a
b,)
Department of Chemistry and Center for AdÕanced Materials, UniÕersity of Massachusetts Lowell, 1 UniÕersity AÕenue, Lowell, MA 01854, USA b Natick Soldier Center, U.S. Army Soldier and Biological Chemical Command, Natick, MA 01760, USA Received 17 July 1999; accepted 26 July 1999
Abstract Conducting polyaniline ŽPANI. fibers have been spun from a water-soluble form of PANI which was enzymatically synthesized. The enzyme, horseradish peroxidase ŽHRP. was used to polymerize aniline in the presence of sulfonated polystyrene ŽSPS. to directly form a water-soluble, conducting, PANIrSPS complex which combines moderate electrical conductivity with appreciable processability. The PANIrSPS complex was spun into fibers from aqueous solution using a dry-spinning technique. Thermal studies which included thermogravimetric analysis ŽTGA., differential scanning calorimetry ŽDSC., and DMA show that the complex has very good thermal stability and a Tg at 1508C. Mechanical properties of the fibers show a tenacity of 0.34 cNrdtex for the as-spun fibers with an increase to ˚ 0.56 cNrdtex after thermal stretch alignment. Wide angle X-Ray diffraction shows the presence of two weak peaks at d values of 4.16 A ˚ for the drawn fibers, while no crystalline reflections were observed for cast films. The drawn fibers also show an order of and 2.95 A magnitude improvement in conductivity. These results show that some degree of fiber orientation and crystallinity may be induced during processing. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Polyaniline; Solution processing; Fibers; Thermal analysis; Mechanical analysis; X-Ray diffraction
1. Introduction Polyaniline ŽPANI. has received considerable attention over recent years as a promising conducting polymer for many electronic applications w1x. PANI, however, is regarded as an essentially intractable polymer from a processing point of view and as a result, practical applications have remained limited. To address this, many attempts have been made to improve the processability of PANI which may be classified into two main strategies. One approach is to form a highly dispersed blend of PANI with another processable polymer and then thermally process the final composite w2–8x. A major advantage of this approach is that thermal processing is preferable because it is easier and cheaper than solution processing. The disadvantage, however, is that dispersion of PANI in a blend can be difficult, and it often results in a complex two-phase system, where the conductivity of the product depends not only on the connectivity and morphology of the dispersed conductive phase, but also on the interfacial structure. The second widely adopted strategy is to increase the solubility )
Corresponding author. Tel.: q1-978-934-3792; fax: q1-978-4589571; e-mail: [email protected]
of PANI for direct solution processing. These studies have included modifying the main chain with solubilizing substituents, utilizing highly concentrated acids to dissolve the polymer, complexing PANI with solubilizing surfactants or polyelectrolytes, or using functionalized protonic acids to dope PANI, to enable processing from solution w9–16x. Although these methods have improved processability, their full potential is not commercially realized due in part to the harsh synthetic conditions used, environmental incompatibility, and cost prohibitive procedures. Recently, we have reported the enzymatic synthesis of a water-soluble, electrically conducting, PANIrsulfonated polystyrene ŽSPS. complex w17x. In this process, aniline is enzymatically polymerized in the presence of a polyelectrolyte that serves as a matrix within which the monomers align and preferentially react to form water-soluble, electrically active PANI. This new synthetic approach offers ease of synthesis, processability, stability Želectrical and chemical., and environmental compatibility. This process is also advantageous in that numerous polyelectrolytes may be judiciously chosen to enhance the mechanical integrity and processability of the final complex. In the present work, we demonstrate the feasibility of processing a PANIrSPS aqueous solution into fibers using a dry-spinning tech-
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 5 0 - 2
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X. Wang et al.r Synthetic Metals 107 (1999) 117–121
nique. From an industrial point of view, it is advantageous to choose dry-spinning for the formation of continuous filament yarns from solution. This well-established process is a relatively flexible one, and many spinning conditions are easily modified depending on the desired results. In the present work, we describe the dry spinning of enzymatically synthesized PANIrSPS and the resulting thermal, mechanical, electrical, and crystalline properties of the spun polymers.
2. Experimental 2.1. Materials Horseradish peroxidase ŽHRP; EC 1.11.1.7. was purchased from Sigma. Aniline Žpurity 99.5%., polyŽsodium 4-styrenesulfonate. Žmolecular weight of 70 kDa., and all other chemicals were purchased from Aldrich Chemicals and used as obtained.
spinneret Ž0.004 in.., at a flow rate of 0.025 mlrmin, and finally into a 70-cm long hot air gap Ž0.1 mrs, 508C.. The resulting monofilament was wound up on a bobbin at a speed of 1–5 mrmin and dried in air. Typical fiber diameter was approximately 100 mm. 2.4. Bulk solid and film preparation 2.4.1. Solid The complex solution was dried under dynamic vacuum at 708C for 12 h to form a dark green solid for further studies. 2.4.2. Film The concentrated solution was poured onto a grooved Teflon surface and dried first at 608C under an IR light for 24 h and then under dynamic vacuum at 608C for 12 h. This procedure gave isotropic films with an average thickness of 0.2 mm. 2.5. Characterization
2.2. Enzymatic synthesis of the polymer complex The polymerization of aniline was carried out at room temperature according to the method previously described w17x. SPS was first dissolved in 200 ml of 0.1 M sodium phosphate buffer, pH 4.3, in concentrations ranging from 0.1 to 0.3 M Žbased on monomer repeat unit., followed by an equal molar addition of aniline with constant stirring. One milliliter of HRP stock solution Ž10 mgrml. was then added to the solution. To initiate the reaction, a stoichiometric amount of 30% hydrogen peroxide diluted in 30 ml water was added incrementally over 1.5 h with constant stirring. The reaction was allowed to continue for a minimum of 3 h with constant stirring at room temperature. The final solution was dark green and dialyzed Žcutoff molecular weight of 3000. for 20 h to remove any unreacted monomer, oligomers, and phosphate salts. 2.3. Fiber spinning A 30–35 wrwt.% PANIrSPS solution was prepared by concentrating the polymer solution by rotary evaporation. The solution was then deaerated for 30 min using a sonicator and transferred to a stainless steel 1 cm3 microsyringe. The spinning apparatus was set up in the vertical configuration. A motor was used to drive the PANIrSPS fluid in the stainless steel syringe through a single-hole
The UV–vis spectra were recorded on a Perkin-Elmer Lambda-9 UVrvisrnear-infrared spectrophotometer. The molecular weight was determined by light-scattering measurements with a Wyatt Technology DAWN F laser photometer. Thermal properties of the polymer films and fibers Ždried in a vacuum oven at 708C for 12 h. were studied using thermogravimetric analysis ŽTGA., differential scanning calorimetry ŽDSC., and DMA ŽTA Instruments Inc... The electrical conductivity of the films and fibers were measured using the four-point and two-point methods, respectively, with a Keithley 619 electrometerr multimeter. Fibers were tested for tenacityrelongation according to procedure ASTM 2101Ž1Y gauge length, 65% RH.. Tenacity is expressed as cNrdtex; 1 dtex is 1 g in weight per 10,000 meters. The X-ray patterns were collected from parallel-mounted fibers across the fiber axes and from isotropic films using CuK a radiation in reflection mode.
3. Results and discussion 3.1. Characterization of the PANI r SPS complex solution Fig. 1 shows the mechanism of the enzymatic polymerization of aniline in the presence of SPS, where the
Fig. 1. Mechanism of the enzymatic polymerization of aniline with the presence of SPS.
X. Wang et al.r Synthetic Metals 107 (1999) 117–121
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Fig. 4. Thermogravimetric scan of the SPSrPani complex.
3.2. Thermal and mechanical characterization of the bulk PANI r SPS complex Fig. 2. UV–vis absorption of the PANIrSPS complex.
SPS matrix serves several critical functions towards the formation of a water-soluble, electrically active PANIrSPS complex. One is to preferentially align the aniline monomers and promote a more ordered para-directed reaction; second, is to provide counterions for doping to the electrically active form; and lastly, to provide the necessary anionic charges on the complex to maintain water solubility for processing. To confirm the presence of the electrically active PANI and formation of the complex, UV-visible absorption and light-scattering measurements were carried out. As shown in Fig. 2, the characteristic 414 nm and 780 nm absorption peaks which are assigned to polaron band transitions are present and confirm the presence of the partially oxidized emeraldine salt or conductive form of PANI w18x. Lightscattering was used to study the weight average molecular weight of the PANIrSPS complex and SPS solution. Since it is difficult to differentiate the molecular weight of just the PANI in the PANIrSPS complex, a comparison of the complex vs. SPS only was used to estimate complex formation and extent of polymerization. It was found that the molecular weight of PANIrSPS complex was significantly higher than that observed for the pure SPS. This indicates that polymerization of aniline is occurring and that the complex behaves as one system.
Fig. 3. Thermogravimetric scan of SPS.
Thermal studies of the PANIrSPS complex vs. SPS were carried out. The TGA curves of the SPS powder and PANIrSPS solid are given in Figs. 3 and 4, respectively. As shown in Fig. 3, the first significant weight loss of SPS is at approximately 1008C and is due to water evaporation. The first degradation of SPS is observed at 275–3508C with a corresponding 3.8% loss in weight. Further degradation of SPS occurs at 430–6008C with a dramatic weight loss of approximately 23.62%. When PANI is complexed with the SPS, significant improvements in thermal stability are observed. Fig. 4 shows TGA curves for the SPSrPANI complex. Other than moisture loss, there is no significant weight loss observed up to 3408C. The first significant weight loss attributed to PANIrSPS complex degradation occurs from 3408C to 4308C with a corresponding weight loss of only 7.9%. As the scan continues, an 18.2% loss of weight observed between 4308C and 6008C is explained by further degradation of the complex. These differences in the SPS and PANIrSPS thermal decomposition behavior further supports the presence of a complex. In addition, these TGA results show very good thermal stability of the PANI complex, compared with other doped forms of PANI such as hydrochloric acid, hydrosulfuric acid, and camphorsulphonic acid-doped PANI w19,20x. The DSC results of the PANIrSPS solid ŽFig. 5. show a single broad endothermic peak on the first scan with a
Fig. 5. DSC of the complex first scan Ž1., second scan Ž2..
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X. Wang et al.r Synthetic Metals 107 (1999) 117–121
778C for PANIrSPS and 508C for SPS may be attributed to the conformational changes of side chain groups. 3.3. Film and fiber studies
Fig. 6. DSC of SPS first scan Ž1., second scan Ž2..
peak at 968C, which is again most likely due to water loss, as was observed previously in the TGA. If the sample is cooled at 108Crmin and scanned a second time, the endothermic peak is no longer observed. These observations are further evidence that the endothermic peak is due to initial water loss and not a melting of any crystalline regions in the sample. As expected with a rigid backbone polymer, the presence of a glass transition temperature is not observed over the temperature range investigated. For SPS ŽFig. 6. the first scan is similar to that of the complex, while in the second scan, a weak transition temperature at about 1328C is observed. The DMA dynamic loss tangent Žtan d . of PANIrSPS and SPS cast films from dynamic tensile stretching experiments are shown in Fig. 7. For both PANIrSPS and SPS, three transitions are observed in the loss tangent marked a , b , and g . The a transition due to sample softening caused by the glass transition, occurs at about 1508C for the complex and 1408C for SPS. The higher Tg of PANIrSPS than that of SPS suggests that the complex has a more rigid chain conformation. The b transitions at about 1008C for both samples agree with changes in the TGA which are associated with water. Thus, the transition at 1008C could be due to the loss of water which in turn causes a change in hydrogen bonding. The g transition at
Fig. 7. Temperature dependence of loss tangent and storage modulus of the complex Ž1. and SPS Ž2..
Fig. 8 shows an optical microscope image of a PANIrSPS dry-spun fiber. The dry-spun fibers are homogeneous, with a very smooth surface and are free-standing fibers that have an average diameter of 100 mm. Tensile tests show that the fibers formed from this dry-spinning method have a tenacity and elongation of 0.34 cNrdtex and 8.5%, respectively. After thermal stretching of the fibers Žat 1808C., the tenacity increases to 0.56 cNrdtex and the elongation decreases to 6.03%, indicating that the polymer chains are able to align to some degree as was also determined in similar fiber-forming studies w23x. This observed tenacity of PANIrSPS complex as-spun fiber is also comparable to most PANI fibers made by other solution processing methods Ž0.18–0.3 cNrdtex. w21–24x. The conductivity of the films and dry-spun fibers were found to be approximately 10y4 Srcm and 10y3 Srcm, respectively. The higher conductivity of the fibers suggests that the orientation of the polymer chains improves the electrical conductivity of the complex. It is believed that the overall lower conductivity of PANIrSPS complex compared to that of pure chemically synthesized PANI Ž1–10 Srcm. is due to decreased interchain diffusion of the charge carriers caused by separation of the PANI chains from the SPS molecules. Also, the PANI in the complex is not completely in the doped state. Our previous work showed that by either increasing the ratio of PANI in the complex or by exposing the complex to HCl vapor, the conductivity could be increased another one to two orders of magnitude. In this work, we exposed the fibers to HCl vapor for 2 h and the conductivity of the fibers increased to 10y2 Srcm. It is expected that further enhancement of the conductivity of the fibers could be achieved by optimizing the parameters of the polymerization and formation procedures. X-ray diffraction profiles of a dry-spun fiber and a cast film are shown in Fig. 9. Two weak crystalline reflections
Fig. 8. Optical microscope image of the dry-spun fibers.
X. Wang et al.r Synthetic Metals 107 (1999) 117–121
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equipment and advice, Mr. F. Zhou and Dr. L. Li for conductivity measurements, Drs. S. Jena, R. Tirasirichai, and Mr. N. Ramaswamy for their assistance in the use of light-scattering and TA facilities.
References
Fig. 9. WAXS patterns of the fibers Ž1. and isotropic film Ž2..
˚ Ž2 u s are observed for the fiber at d values of 4.16 A ˚ . Ž . 21.38 and 2.95 A 2 u s 36.80 plus a strong silicon reflection from the substrate. No crystalline reflections were detected for the cast film and indicates that fiber processing induces a degree of crystallization of the PANIrSPS complex. 4. Conclusion We have used a dry-spinning technique to process a new, enzymatically synthesized form of PANI. These results show that the polymer complex has appreciable processability and thermal stability. The fibers show improved tensile strength, conductivity, and crystallinity over cast films suggesting that chain alignment is induced during processing. This spinning approach in conjunction with the enzymatic polymerization is particularly attractive in that it is simple Žno need for solvent recovery., environmentally safe, and versatile towards optimizing the final fiber properties.
Acknowledgements We thank Dr. W. Liu and Prof. Jayant Kumar for helpful discussions, Betty Ann Welsh for the dry-spinning
w1x A.G. MacDiarmid, J.-C. Chiang, M. Halpern, W.-S. Huang, S.-L. Mu, N.L. Somasiri, W. Wu, S.I. Yaniger, Mol. Cryst. 121 Ž1985. 173. w2x P. Passiniemi, J. Laakso, H. Osterholm, M. Pohl, Synth. Met. 84 Ž1997. 775–776. w3x S.W. Byun, S.S. Im, Synth. Met. 69 Ž1995. 219–220. w4x M. Ahlskog, H. Isotalo, O. Ikkala, J. Laakso, H. Stubb, J.-E. ¨ Osterholm, Synth. Met. 69 Ž1995. 213–214. w5x O.T. Ikkala, J. Laakso, K. Vakiparta, E. Virtanen, H. Ruohonen, H. ¨ Jarvinen, T. Taka, P. Passiniemi, Synth. Met. 69 Ž1995. 97–100. ¨ w6x S.J. Davies, T.G. Ryan, C.J. Wilde, G. Beyer, Synth. Met. 69 Ž1995. 209–210. w7x A. Pron, W. Luzny, J. Laska, Synth. Met. 80 Ž1996. 191–193. w8x B. Wessling, Synth. Met. 93 Ž1998. 143–154. w9x G.W. Heffner, D.S. Pearson, C.L. Gettinger, Polym. Eng. Sci. 35 Ž1995. 860–867. w10x G.W. Heffner, W.E. Rochefort, D.S. Pearson, Polym. Eng. Sci. 35 Ž1995. 868–875. w11x Y. Cao, P. Smith, A. Heeger, Synth. Met. 32 Ž1989. 263. w12x Y. Cao, P. Smith, A. Heeger, Synth. Met. 48 Ž1992. 91. w13x C.L. Gettinger, A.J. Heeger, D.J. Pine, Y. Cao, Synth. Met. 74 Ž1995. 81–88. ¨ w14x Y.-H. Liao, K. Levon, J. Laakso, J.-E. Osterholm, Macromol. Rapid Commun. 16 Ž1995. 393–397. w15x L.F. Sun, H.B. Liu, R. Clark, S.C. Yang, Synth. Met. 84 Ž1997. 67–68. w16x Y. Cao, G.M. Treacy, P. Smith, A.J. Heeger, Appl. Phys. Lett. 60 Ž1992. 2711. w17x W. Liu, J. Kumar, S.K. Tripathy, K. Senecal, L. Samuelson, J. Am. Chem. Soc. 121 Ž1. Ž1999. 71–78. w18x J.M. Ginder, A. Epstein, J. Phys. Rev. B. 42 Ž1990. 10674. w19x L. Abell, S.J. Pomfret, P.N. Adams, A.P. Monkman, Synth. Met. 84 Ž1997. 127–128. w20x A. Boyle, J.F. Penneau, E. Genies, ` C. Rikel, J. Polym. Sci., Part B: Polym. Phys. 30 Ž1992. 265–274. w21x P.W. Gibson, H.L. Schreuder-Gibson, D. Rivin, AIChE J. Ž1998., in review. w22x B.R. Mattes, H.L. Wang, D. Yang, Y.T. Zhu, W.R. Blumenthal, M.F. Hundley, Synth. Met. 84 Ž1997. 45–49. w23x C.-H. Hsu, A.J. Epstein, Synth. Met. 84 Ž1997. 51–54. w24x Y.Z. Wang, J. Joo, C.-H. Hsu, A.J. Epstein, Synth. Met. 68 Ž1995. 207–211.
Conductive fibers from enzymatically synthesized polyaniline X. Wang a , H. Schreuder-Gibson b, M. Downey a , S. Tripathy a , L. Samuelson a
b,)
Department of Chemistry and Center for AdÕanced Materials, UniÕersity of Massachusetts Lowell, 1 UniÕersity AÕenue, Lowell, MA 01854, USA b Natick Soldier Center, U.S. Army Soldier and Biological Chemical Command, Natick, MA 01760, USA Received 17 July 1999; accepted 26 July 1999
Abstract Conducting polyaniline ŽPANI. fibers have been spun from a water-soluble form of PANI which was enzymatically synthesized. The enzyme, horseradish peroxidase ŽHRP. was used to polymerize aniline in the presence of sulfonated polystyrene ŽSPS. to directly form a water-soluble, conducting, PANIrSPS complex which combines moderate electrical conductivity with appreciable processability. The PANIrSPS complex was spun into fibers from aqueous solution using a dry-spinning technique. Thermal studies which included thermogravimetric analysis ŽTGA., differential scanning calorimetry ŽDSC., and DMA show that the complex has very good thermal stability and a Tg at 1508C. Mechanical properties of the fibers show a tenacity of 0.34 cNrdtex for the as-spun fibers with an increase to ˚ 0.56 cNrdtex after thermal stretch alignment. Wide angle X-Ray diffraction shows the presence of two weak peaks at d values of 4.16 A ˚ for the drawn fibers, while no crystalline reflections were observed for cast films. The drawn fibers also show an order of and 2.95 A magnitude improvement in conductivity. These results show that some degree of fiber orientation and crystallinity may be induced during processing. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Polyaniline; Solution processing; Fibers; Thermal analysis; Mechanical analysis; X-Ray diffraction
1. Introduction Polyaniline ŽPANI. has received considerable attention over recent years as a promising conducting polymer for many electronic applications w1x. PANI, however, is regarded as an essentially intractable polymer from a processing point of view and as a result, practical applications have remained limited. To address this, many attempts have been made to improve the processability of PANI which may be classified into two main strategies. One approach is to form a highly dispersed blend of PANI with another processable polymer and then thermally process the final composite w2–8x. A major advantage of this approach is that thermal processing is preferable because it is easier and cheaper than solution processing. The disadvantage, however, is that dispersion of PANI in a blend can be difficult, and it often results in a complex two-phase system, where the conductivity of the product depends not only on the connectivity and morphology of the dispersed conductive phase, but also on the interfacial structure. The second widely adopted strategy is to increase the solubility )
Corresponding author. Tel.: q1-978-934-3792; fax: q1-978-4589571; e-mail: [email protected]
of PANI for direct solution processing. These studies have included modifying the main chain with solubilizing substituents, utilizing highly concentrated acids to dissolve the polymer, complexing PANI with solubilizing surfactants or polyelectrolytes, or using functionalized protonic acids to dope PANI, to enable processing from solution w9–16x. Although these methods have improved processability, their full potential is not commercially realized due in part to the harsh synthetic conditions used, environmental incompatibility, and cost prohibitive procedures. Recently, we have reported the enzymatic synthesis of a water-soluble, electrically conducting, PANIrsulfonated polystyrene ŽSPS. complex w17x. In this process, aniline is enzymatically polymerized in the presence of a polyelectrolyte that serves as a matrix within which the monomers align and preferentially react to form water-soluble, electrically active PANI. This new synthetic approach offers ease of synthesis, processability, stability Želectrical and chemical., and environmental compatibility. This process is also advantageous in that numerous polyelectrolytes may be judiciously chosen to enhance the mechanical integrity and processability of the final complex. In the present work, we demonstrate the feasibility of processing a PANIrSPS aqueous solution into fibers using a dry-spinning tech-
0379-6779r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 Ž 9 9 . 0 0 1 5 0 - 2
118
X. Wang et al.r Synthetic Metals 107 (1999) 117–121
nique. From an industrial point of view, it is advantageous to choose dry-spinning for the formation of continuous filament yarns from solution. This well-established process is a relatively flexible one, and many spinning conditions are easily modified depending on the desired results. In the present work, we describe the dry spinning of enzymatically synthesized PANIrSPS and the resulting thermal, mechanical, electrical, and crystalline properties of the spun polymers.
2. Experimental 2.1. Materials Horseradish peroxidase ŽHRP; EC 1.11.1.7. was purchased from Sigma. Aniline Žpurity 99.5%., polyŽsodium 4-styrenesulfonate. Žmolecular weight of 70 kDa., and all other chemicals were purchased from Aldrich Chemicals and used as obtained.
spinneret Ž0.004 in.., at a flow rate of 0.025 mlrmin, and finally into a 70-cm long hot air gap Ž0.1 mrs, 508C.. The resulting monofilament was wound up on a bobbin at a speed of 1–5 mrmin and dried in air. Typical fiber diameter was approximately 100 mm. 2.4. Bulk solid and film preparation 2.4.1. Solid The complex solution was dried under dynamic vacuum at 708C for 12 h to form a dark green solid for further studies. 2.4.2. Film The concentrated solution was poured onto a grooved Teflon surface and dried first at 608C under an IR light for 24 h and then under dynamic vacuum at 608C for 12 h. This procedure gave isotropic films with an average thickness of 0.2 mm. 2.5. Characterization
2.2. Enzymatic synthesis of the polymer complex The polymerization of aniline was carried out at room temperature according to the method previously described w17x. SPS was first dissolved in 200 ml of 0.1 M sodium phosphate buffer, pH 4.3, in concentrations ranging from 0.1 to 0.3 M Žbased on monomer repeat unit., followed by an equal molar addition of aniline with constant stirring. One milliliter of HRP stock solution Ž10 mgrml. was then added to the solution. To initiate the reaction, a stoichiometric amount of 30% hydrogen peroxide diluted in 30 ml water was added incrementally over 1.5 h with constant stirring. The reaction was allowed to continue for a minimum of 3 h with constant stirring at room temperature. The final solution was dark green and dialyzed Žcutoff molecular weight of 3000. for 20 h to remove any unreacted monomer, oligomers, and phosphate salts. 2.3. Fiber spinning A 30–35 wrwt.% PANIrSPS solution was prepared by concentrating the polymer solution by rotary evaporation. The solution was then deaerated for 30 min using a sonicator and transferred to a stainless steel 1 cm3 microsyringe. The spinning apparatus was set up in the vertical configuration. A motor was used to drive the PANIrSPS fluid in the stainless steel syringe through a single-hole
The UV–vis spectra were recorded on a Perkin-Elmer Lambda-9 UVrvisrnear-infrared spectrophotometer. The molecular weight was determined by light-scattering measurements with a Wyatt Technology DAWN F laser photometer. Thermal properties of the polymer films and fibers Ždried in a vacuum oven at 708C for 12 h. were studied using thermogravimetric analysis ŽTGA., differential scanning calorimetry ŽDSC., and DMA ŽTA Instruments Inc... The electrical conductivity of the films and fibers were measured using the four-point and two-point methods, respectively, with a Keithley 619 electrometerr multimeter. Fibers were tested for tenacityrelongation according to procedure ASTM 2101Ž1Y gauge length, 65% RH.. Tenacity is expressed as cNrdtex; 1 dtex is 1 g in weight per 10,000 meters. The X-ray patterns were collected from parallel-mounted fibers across the fiber axes and from isotropic films using CuK a radiation in reflection mode.
3. Results and discussion 3.1. Characterization of the PANI r SPS complex solution Fig. 1 shows the mechanism of the enzymatic polymerization of aniline in the presence of SPS, where the
Fig. 1. Mechanism of the enzymatic polymerization of aniline with the presence of SPS.
X. Wang et al.r Synthetic Metals 107 (1999) 117–121
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Fig. 4. Thermogravimetric scan of the SPSrPani complex.
3.2. Thermal and mechanical characterization of the bulk PANI r SPS complex Fig. 2. UV–vis absorption of the PANIrSPS complex.
SPS matrix serves several critical functions towards the formation of a water-soluble, electrically active PANIrSPS complex. One is to preferentially align the aniline monomers and promote a more ordered para-directed reaction; second, is to provide counterions for doping to the electrically active form; and lastly, to provide the necessary anionic charges on the complex to maintain water solubility for processing. To confirm the presence of the electrically active PANI and formation of the complex, UV-visible absorption and light-scattering measurements were carried out. As shown in Fig. 2, the characteristic 414 nm and 780 nm absorption peaks which are assigned to polaron band transitions are present and confirm the presence of the partially oxidized emeraldine salt or conductive form of PANI w18x. Lightscattering was used to study the weight average molecular weight of the PANIrSPS complex and SPS solution. Since it is difficult to differentiate the molecular weight of just the PANI in the PANIrSPS complex, a comparison of the complex vs. SPS only was used to estimate complex formation and extent of polymerization. It was found that the molecular weight of PANIrSPS complex was significantly higher than that observed for the pure SPS. This indicates that polymerization of aniline is occurring and that the complex behaves as one system.
Fig. 3. Thermogravimetric scan of SPS.
Thermal studies of the PANIrSPS complex vs. SPS were carried out. The TGA curves of the SPS powder and PANIrSPS solid are given in Figs. 3 and 4, respectively. As shown in Fig. 3, the first significant weight loss of SPS is at approximately 1008C and is due to water evaporation. The first degradation of SPS is observed at 275–3508C with a corresponding 3.8% loss in weight. Further degradation of SPS occurs at 430–6008C with a dramatic weight loss of approximately 23.62%. When PANI is complexed with the SPS, significant improvements in thermal stability are observed. Fig. 4 shows TGA curves for the SPSrPANI complex. Other than moisture loss, there is no significant weight loss observed up to 3408C. The first significant weight loss attributed to PANIrSPS complex degradation occurs from 3408C to 4308C with a corresponding weight loss of only 7.9%. As the scan continues, an 18.2% loss of weight observed between 4308C and 6008C is explained by further degradation of the complex. These differences in the SPS and PANIrSPS thermal decomposition behavior further supports the presence of a complex. In addition, these TGA results show very good thermal stability of the PANI complex, compared with other doped forms of PANI such as hydrochloric acid, hydrosulfuric acid, and camphorsulphonic acid-doped PANI w19,20x. The DSC results of the PANIrSPS solid ŽFig. 5. show a single broad endothermic peak on the first scan with a
Fig. 5. DSC of the complex first scan Ž1., second scan Ž2..
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X. Wang et al.r Synthetic Metals 107 (1999) 117–121
778C for PANIrSPS and 508C for SPS may be attributed to the conformational changes of side chain groups. 3.3. Film and fiber studies
Fig. 6. DSC of SPS first scan Ž1., second scan Ž2..
peak at 968C, which is again most likely due to water loss, as was observed previously in the TGA. If the sample is cooled at 108Crmin and scanned a second time, the endothermic peak is no longer observed. These observations are further evidence that the endothermic peak is due to initial water loss and not a melting of any crystalline regions in the sample. As expected with a rigid backbone polymer, the presence of a glass transition temperature is not observed over the temperature range investigated. For SPS ŽFig. 6. the first scan is similar to that of the complex, while in the second scan, a weak transition temperature at about 1328C is observed. The DMA dynamic loss tangent Žtan d . of PANIrSPS and SPS cast films from dynamic tensile stretching experiments are shown in Fig. 7. For both PANIrSPS and SPS, three transitions are observed in the loss tangent marked a , b , and g . The a transition due to sample softening caused by the glass transition, occurs at about 1508C for the complex and 1408C for SPS. The higher Tg of PANIrSPS than that of SPS suggests that the complex has a more rigid chain conformation. The b transitions at about 1008C for both samples agree with changes in the TGA which are associated with water. Thus, the transition at 1008C could be due to the loss of water which in turn causes a change in hydrogen bonding. The g transition at
Fig. 7. Temperature dependence of loss tangent and storage modulus of the complex Ž1. and SPS Ž2..
Fig. 8 shows an optical microscope image of a PANIrSPS dry-spun fiber. The dry-spun fibers are homogeneous, with a very smooth surface and are free-standing fibers that have an average diameter of 100 mm. Tensile tests show that the fibers formed from this dry-spinning method have a tenacity and elongation of 0.34 cNrdtex and 8.5%, respectively. After thermal stretching of the fibers Žat 1808C., the tenacity increases to 0.56 cNrdtex and the elongation decreases to 6.03%, indicating that the polymer chains are able to align to some degree as was also determined in similar fiber-forming studies w23x. This observed tenacity of PANIrSPS complex as-spun fiber is also comparable to most PANI fibers made by other solution processing methods Ž0.18–0.3 cNrdtex. w21–24x. The conductivity of the films and dry-spun fibers were found to be approximately 10y4 Srcm and 10y3 Srcm, respectively. The higher conductivity of the fibers suggests that the orientation of the polymer chains improves the electrical conductivity of the complex. It is believed that the overall lower conductivity of PANIrSPS complex compared to that of pure chemically synthesized PANI Ž1–10 Srcm. is due to decreased interchain diffusion of the charge carriers caused by separation of the PANI chains from the SPS molecules. Also, the PANI in the complex is not completely in the doped state. Our previous work showed that by either increasing the ratio of PANI in the complex or by exposing the complex to HCl vapor, the conductivity could be increased another one to two orders of magnitude. In this work, we exposed the fibers to HCl vapor for 2 h and the conductivity of the fibers increased to 10y2 Srcm. It is expected that further enhancement of the conductivity of the fibers could be achieved by optimizing the parameters of the polymerization and formation procedures. X-ray diffraction profiles of a dry-spun fiber and a cast film are shown in Fig. 9. Two weak crystalline reflections
Fig. 8. Optical microscope image of the dry-spun fibers.
X. Wang et al.r Synthetic Metals 107 (1999) 117–121
121
equipment and advice, Mr. F. Zhou and Dr. L. Li for conductivity measurements, Drs. S. Jena, R. Tirasirichai, and Mr. N. Ramaswamy for their assistance in the use of light-scattering and TA facilities.
References
Fig. 9. WAXS patterns of the fibers Ž1. and isotropic film Ž2..
˚ Ž2 u s are observed for the fiber at d values of 4.16 A ˚ . Ž . 21.38 and 2.95 A 2 u s 36.80 plus a strong silicon reflection from the substrate. No crystalline reflections were detected for the cast film and indicates that fiber processing induces a degree of crystallization of the PANIrSPS complex. 4. Conclusion We have used a dry-spinning technique to process a new, enzymatically synthesized form of PANI. These results show that the polymer complex has appreciable processability and thermal stability. The fibers show improved tensile strength, conductivity, and crystallinity over cast films suggesting that chain alignment is induced during processing. This spinning approach in conjunction with the enzymatic polymerization is particularly attractive in that it is simple Žno need for solvent recovery., environmentally safe, and versatile towards optimizing the final fiber properties.
Acknowledgements We thank Dr. W. Liu and Prof. Jayant Kumar for helpful discussions, Betty Ann Welsh for the dry-spinning
w1x A.G. MacDiarmid, J.-C. Chiang, M. Halpern, W.-S. Huang, S.-L. Mu, N.L. Somasiri, W. Wu, S.I. Yaniger, Mol. Cryst. 121 Ž1985. 173. w2x P. Passiniemi, J. Laakso, H. Osterholm, M. Pohl, Synth. Met. 84 Ž1997. 775–776. w3x S.W. Byun, S.S. Im, Synth. Met. 69 Ž1995. 219–220. w4x M. Ahlskog, H. Isotalo, O. Ikkala, J. Laakso, H. Stubb, J.-E. ¨ Osterholm, Synth. Met. 69 Ž1995. 213–214. w5x O.T. Ikkala, J. Laakso, K. Vakiparta, E. Virtanen, H. Ruohonen, H. ¨ Jarvinen, T. Taka, P. Passiniemi, Synth. Met. 69 Ž1995. 97–100. ¨ w6x S.J. Davies, T.G. Ryan, C.J. Wilde, G. Beyer, Synth. Met. 69 Ž1995. 209–210. w7x A. Pron, W. Luzny, J. Laska, Synth. Met. 80 Ž1996. 191–193. w8x B. Wessling, Synth. Met. 93 Ž1998. 143–154. w9x G.W. Heffner, D.S. Pearson, C.L. Gettinger, Polym. Eng. Sci. 35 Ž1995. 860–867. w10x G.W. Heffner, W.E. Rochefort, D.S. Pearson, Polym. Eng. Sci. 35 Ž1995. 868–875. w11x Y. Cao, P. Smith, A. Heeger, Synth. Met. 32 Ž1989. 263. w12x Y. Cao, P. Smith, A. Heeger, Synth. Met. 48 Ž1992. 91. w13x C.L. Gettinger, A.J. Heeger, D.J. Pine, Y. Cao, Synth. Met. 74 Ž1995. 81–88. ¨ w14x Y.-H. Liao, K. Levon, J. Laakso, J.-E. Osterholm, Macromol. Rapid Commun. 16 Ž1995. 393–397. w15x L.F. Sun, H.B. Liu, R. Clark, S.C. Yang, Synth. Met. 84 Ž1997. 67–68. w16x Y. Cao, G.M. Treacy, P. Smith, A.J. Heeger, Appl. Phys. Lett. 60 Ž1992. 2711. w17x W. Liu, J. Kumar, S.K. Tripathy, K. Senecal, L. Samuelson, J. Am. Chem. Soc. 121 Ž1. Ž1999. 71–78. w18x J.M. Ginder, A. Epstein, J. Phys. Rev. B. 42 Ž1990. 10674. w19x L. Abell, S.J. Pomfret, P.N. Adams, A.P. Monkman, Synth. Met. 84 Ž1997. 127–128. w20x A. Boyle, J.F. Penneau, E. Genies, ` C. Rikel, J. Polym. Sci., Part B: Polym. Phys. 30 Ž1992. 265–274. w21x P.W. Gibson, H.L. Schreuder-Gibson, D. Rivin, AIChE J. Ž1998., in review. w22x B.R. Mattes, H.L. Wang, D. Yang, Y.T. Zhu, W.R. Blumenthal, M.F. Hundley, Synth. Met. 84 Ž1997. 45–49. w23x C.-H. Hsu, A.J. Epstein, Synth. Met. 84 Ž1997. 51–54. w24x Y.Z. Wang, J. Joo, C.-H. Hsu, A.J. Epstein, Synth. Met. 68 Ž1995. 207–211.