Partial Sulfonation of Polyaniline NanofibersDerek Perry and David M. Sarno

Chemistry Department, Queensborough Community College of CUNY, Bayside, NY 11364

INTRODUCTION & BACKGROUND:Polyaniline (PANI) is among the most highly studied conducting polymers because it is easily prepared, its properties are readily varied by common synthetic reactions, and it exhibits good environmental stability. In its emeraldine oxidation state, it can be reversibly switched between conductive (emeraldine salt) and insulating (emeraldine base) forms by protonic doping/dedoping (exposure to acid or base, respectively). Applications of PANI include batteries, sensors, and other electronic devices.

Emeraldine base (PANI-EB)Emeraldine salt (PANI-ES)

N N N NH H H H

x+ +A- A-

N N N N

H H

x

– H+

+ H+

Composites of PANI nanofibers with gold nanoparticles (Au-NPs) can lead to new materials with novel optical and electronic properties, especially for sensors and catalysis (Guo, Macromol. 2010, 43, 10636). Because there is no specific interaction between PANI and Au-NPs, loading of Au-NPs onto the fibers is not very reproducible, thus the applications of their composites are limited. In an earlier study, we found that using sulfonated polyaniline (SPANI), which has negatively charged substituents, significantly improved the deposition of Au-NPs with positively charged ligands. In contrast, Au-NPs with negatively charged ligands did not deposit well on SPANI, presumably due to electrostatic repulsion.

The images above show almost no nanostructure because sulfonation increases the water solubility of PANI, which degrades the fibers. By developing a reliable method for partial sulfonation of PANI fibers, it may be possible to limit the water solubility, and also to provide sufficient charge to attract Au-NPs. To this end, we have modified a method published by Barbero and co-workers (Acta Polym. 1999, 50, 40) used to sulfonate bulk PANI via nucleophilic addition of sulfite ions onto the polymer. A similar method was used to sulfonate PANI fibers anchored to a surface (Epstein, Nat. Nanotechnol. 2007, 2, 354), but sulfonation of free-standing nanofibers has not been previously reported. Our goal is to modify the fiber surface while leaving the fiber core insoluble.

SPANI / (−) Au-NPs SPANI / (+) Au-NPs

SEM images showing deposition of Au-NPs on SPANI. NPs are seen as bright spots.

EXPERIMENTAL METHOD: PANI-ES nanofibers were prepared by rapid mixing of equal volumes of aniline (0.8M) and ammonium persulfate (0.1M), each prepared in 1M HCl. It was left undisturbed for 24 hr at room temperature and then split into portions that were dialyzed in 1M NH4OH to obtain deprotonated PANI-EB nanofibers. After excess base was removed by centrifugation, the samples were dialyzed in 0.1M Na2S2O5 at room temperature for the desired times. The products were dialyzed in deionized water. Droplets of the dispersion were dried directly on sample stubs for SEM. FTIR samples were filtered and dried in air.

N N N NH H H H

x+ +

SO3- SO3

- SO3- SO3

-

SPANI: sulfonated polyaniline in its emeraldine salt form

PANI-ESnanofibers

PANI-EBnanofibers

Partially sulfonatednanofibers

1M NH4OH 0.1M Na2S2O5

FTIR after exposure of nanofibers to 0.1M Na2S2O5

ACKNOWLEDGMENTS: Silvia Salamone, Yiran Chen, Jaspreet Kaur, Engred Vanegas, Bhawanie Persaud and Dr. Paris Svoronos are thanked for their support and contributions to this work. Financial support from the QCC Chemistry Department, CUNY Research Scholars Program, MSEIP, PSC-CUNY Research Award (65401-0043, 67311-0045), NSF-MRI (DMR 0722607), and NSF-STEP (DUE 0652963) is gratefully acknowledged.

CONCLUSIONS & FUTURE WORK:• PANI nanofibers can be modified via nucleophilic addition of sulfite ions

• FTIR shows clear evidence of sulfonation with Na2S2O5, consistent with the literature (Barbero, Acta Polym. 1999, 50, 40)

• SEM shows increasing fiber degradation with increasing exposure time to salt, consistent with increasing sulfonation, but results are inconsistent

• Shorter deprotonation times appear to limit sulfonation, which maintains fiber quality

• Extent of sulfonation is unknown and will be determined via elemental analysis

• Effects of reaction temperature and salt concentration will be studied

Evidence of sulfonation is seen in the FTIR spectra of the products

SUMMARY OF FTIR DATA:• PANI-EB spectra are identical for all exposure times in NH4OH

• Spectra are similar for all exposure times in Na2S2O5 (48 & 72hr exposures not shown)

• New peaks near 1065 cm-1 and 1020 cm-1 indicate sulfonation

• New peaks near 1125 cm-1 and 880 cm-1 indicate 1,2,4-trisubstitution, consistent with addition of sulfonate groups to the disubstituted PANI backbone

• Upwards sloping baseline is evidence of protonation after exposure to Na2S2O5,consistent with the addition mechanism

• Salt solution is acidic: Na2S2O5 produces HSO3− in water, which protonates PANI

• Spectra do not indicate the extent of sulfonation

Partial sulfonation of PANI-EB nanofibers by dialysis in 0.1M Na2S2O5

The PANI-ES nanofibers were deprotonated in 1M NH4OH because PANI-EB is more readily sulfonated than PANI-ES. Reducing the exposure time in the base, may limit the extent of deprotonation, thereby making fewer sites available for sulfonation and reducing fiber degradation.

PANI deprotonated in NH4OH for 1hr

% T

rans

mitt

ance

Wavenumbers (cm-1)

PANI-EB

1 hr salt exposure

5 hr salt exposure

24 hr salt exposure

C=NC-C C-H

2000 1800 1600 1400 1200 1000 800 600

PANI deprotonated in NH4OH for 24hr

Wavenumbers (cm-1)

% T

rans

mitt

ance

PANI-EB

1 hr salt exposure

5 hr salt exposure

24 hr salt exposureC=N

C-C C-H

2000 1800 1600 1400 1200 1000 800 600

Nanofibers sulfonated after 24hr deprotonation in NH4OH1hr exposure to Na2S2O5

20 μm

5hr exposure to Na2S2O5

20 μm

24hr exposure to Na2S2O5

20 μm

PANI-EB after 24hr NH4OH

10 μm

48hr exposure to Na2S2O5

20 μm

72hr exposure to Na2S2O5

20 μm

Nanofibers sulfonated after 1hr deprotonation in NH4OHThe length of time the nanofibers were in 0.1M Na2S2O5 at room temperature was increased and fiber quality was examined.

5hr exposure to Na2S2O5

20 μm20 μm

1hr exposure to Na2S2O5

24hr exposure to Na2S2O5

20 μm

PANI-EB after 1hr NH4OH

10 μm

20 μm

72hr exposure to Na2S2O5

20 μm

48hr exposure to Na2S2O5

Na2S2O5

PANI-EB nanofiber

Partially sulfonated PANI-ES surfacewith PANI-EB core

Nucleophilic addition of sulfite ions to PANI-EB

N

N

2 H+ + SO3-2

N

N

+ B-

H

H

SO3-

H

N

N

SO3-

H

H

+ BH

+

PANI-EB PANI-ES

The “conventional method” of PANI synthesis (Stejskal, Pure Appl. Chem. 2002, 74, 857) results in irregular and granular particles. However, high surface area PANI nanofibers are easily synthesized with high yields by rapid mixing of reagents at room temperature (Kaner, Angew. Chem. Int. Ed. 2004, 43, 5817).

SUMMARY OF SEM DATA:• Exposure to the base does not change the morphology of PANI nanofibers.

• PANI-ES nanofibers deprotonated in 1M NH4OH for only 1hr show little evidence of fusing or degradation following sulfonation.

• Degradation is more pronounced following 24hr deprotonations, suggesting greater solubility due to a greater degree of sulfonation.

• Representative images are shown above, but the appearance of the sulfonated nanofibers varies from batch to batch and within a single batch.

PANI-ES nanofibers

10 μm