Protonation of polyaniline with lightly sulfonated polystyrene

Transcription

1 Protonation of polyaniline with lightly sulfonated polystyrene Yiwpirlg Fii, K. A. Weiss'3 Polymer Science Program and Department of Chemical Engineering. University of Connecticut. Storrs. CT , U.S.A. (Received: March ) SUMMARY Protonation of polyaniline base with lightly sulfonated polystyrene in polar solvents such its dimethyl sulfoxide and N-methyl-2-py~olidone was investigated by UV-Vis absorption spectra. As the molar ratio of sulfonated polystyrenelpolyaniline increases, the conversion from polyaniline base form to salt form is observed owing 1 incrensed protonation. The isosbestic point clearly shows that quinoid unit and semiquinoid unit are in equilibrium. They are functions of the sulfonic acid concentration and solvent medi ii. Introduction In the past fifteen years, significant progress has been made on organic conducting polymers with novel electrical, optical and electrochemical properties '.-'). Among these conducting polymers, polyaniline (PANI) shows very promising industrial application, because of its good environmental stability and facile synthesis". lo). Polyaniline can be reversibly doped by protonation of the imine nitrogen atoms through acid-base chemistry, and this is accompanied by a ten order of magnitude increase in conductivity. It can be synthesized by the oxidative chemical or electrochemical polymerization of aniline (AN) in aqueous acidic solution ' I.I3'. The resulting PANI salt (conducting form) is generally not soluble in any organic solvent. and is not fusible. However, PANI base (insulating form)? which can be obtained by treating the conductive form with aqueous NkOH. is soluble in N-methyl-2-pyrrolidone (NMP). N,N'-dimethylpropylencurca (DMPU), and slightly soluble in dimethyl su I fox i de (DMS ). Recently. significant progress has been made in the processing of polyaniline. Cao et al. 'I used functionalized protonic acids, such as dodecylbenzenesulfonic acid (DBSA), to dope PANI base and simultaneously render the resulting PANI coniplcx soluble in common organic solvents. The use of surl'actant counter-ions to induce co-solubility and compatibility with bulk polymers enables the synthesis of highly conducting polyblends with a variety of commercial polymers l5i. Tzou et al. I('' also reported a similar approach by using organic acid its dopants. Others used polymeric acids, such as poly(styrene sulfonic acid) 17-'31, and poly(acrylic acid)ll-?71. Polyaniline doped with these polyelectrolyte polymers exhibited water solubility 'x-2i'. We are interested in preparing conductive polymer blends with improvcd processability and better controlled conductivity. Theoretically. the conductivity of polyani Hiithig & Wept' Verlag. Zug ccc /96/52.5

2 488 Y. Fu, R. A. Weiss line can be varied from lo-'" to 1 S/cm as a function of doping level. In reality, the conductivity of polyaniline increases dramatically at low doping level and levels off at ca. 1% doping "). Here, lightly sulfonated polystyrene (HSPS) was chosen as the polymer matrix, because the randomly placed sulfonic acid groups on the polystyrene chains act as a dilute acid in an organic medium. We expect HSPS will protonate the imine nitrogen sites of polyaniline, while possibly retaining the processability of polystyrene. This molecular protonation should promote compatibility between polyaniline and polystyrene within the blends, and simultaneously transform the insulating polyaniline base form to the metallic conducting form, thereby rendering the blends conductive. The use of polymeric dopants may significantly improve the stability of the resulting polymer blends, because small molecule dopants tend to migrate out of the polymer matrix. In this communication, we present direct evidence of doping PANI through protonation by sulfonic acid groups of lightly sulfonated polystyrene in non-aqueous solutions and its dependence on the solvent environment. Experimental part Aniline (99%) was obtained from Fisher and distilled before use. Spectroscopic grade dimethyl sulfoxide (DMSO), and N-methyl-2-pyrrolidone (NMP) were also obtained from Fisher. PANI hydrochloride powder was synthesized by the oxidative polymerization of aniline in 1. M aqueous HCI with ammonium peroxydisulfate (APS) as the oxidant, as described previously''). The powder was converted to PANI base (emeraldine base) by treatment with.5 M aqueous ammonia solution followed by drying under vacuum. Polystyrene (an = 1, aw = 28) was sulfonated in dichloroethane at 5 C with acetyl sulfate following the procedure of Makowski et a1.29) The sulfonated polystyrene was recovered by steam striping, washed with methanol and dried under dynamic vacuum. The sulfonic acid concentration was determined to be 5.3 mol-% by titration of the polymer in toluenehethanol mixed solvent (vol. ratio 9/1) with NaOH in methanol. PANVHSPS blends with different molar ratios were prepared by mixing appropriate volumes of two solutions, 1 mm PANI base (based on the approximate aniline repeating unit -C,H,NH-) in DMSO or NMP and 1 mm HSPS (based on the styrene unit) in the same solvent, to give a clear solution. UV-Vis absorbance spectra were recorded with a Perkin-Elmer Lambda 6 UVNIS spectrophotometer. For solution measurements, the concentration of PANI was kept constant at.6 mm, while the concentration of HSPS was varied to get the desired [-HSO,H]/[AN] ratios. Results and discussion The UV-Vis absorption spectra of different ratios for PANIMSPS in DMSO solution are shown in Fig. 1. The PANI base has two absorption peaks at 335 nm and 64 nm. The absorption peak at 335 nm is due to the n-n* transition of benzenoid rings, while the absorption peak at 64 nm is due to the n-x* transition of quinoid rings on the PANI chain. In the PANI base form, half of the N atoms exist as amine groups and the other half as imine groups. Since only the imine groups can be doped by acid, the maximum doping level of the PANI is.5 equivalent protons. When

3 Protonation of pol yaniline with lightly sulfonated polystyrene 489 Fig. I. UV-Vis absorption spectra of PANI base at different concentration of HSPS in DMSO, concentration of PANI is kept constant at.6 mm, (a)[-so,h]/[ani = ; fb) [-SO,H]/[AN] =.75: (c) J-SO,H]/[AN] = I. 1 c.6 a 8.5 u1 n Q.L " 32 LOO L Wavelength in nrn PAN1 base solution was mixed with HSPS in DMSO solution, a clear solution resulted. This indicates that the HSPS can induce solubility of the polyaniline salt in the same way as an organic acid dopant. As the molar ratio of [HSPSj/[PANI] was increased, the color of the solution gradually changed from blue to green due to increased protonation of the imine sites of PANI. The UV-Vis spectra showed a new absorption peak at 82 nm due to protonation of the imine sites of PANI. The absorption peak originates from a polaron band transition, which is also observed by protonation with organic acid in solution I"). As the molar ratio of [HSPSJ/[PANI] increased, the intensity of the 82 nm also increased and correspondingly, the intensity of the quinoid absorption peak at 64 nm decreased. A clear isosbestic point at 73 nm was observed, which indicates that the sulfonic acid groups directly protonate the imine nitrogen and transform the quinoid units into semiquinoid units as shown below: This conversion between quinoid and semiquinoid units is reversible and quantitative. The amount of' sulfonic acid required to reach the maximum doping level, however, was greater than the theoretical.5 equivalent. This reduction of effective protonation of' quinoid rings of PANI may be due either to conformation hindrance of the polymeric sulfonic acid groups andor to solvent effects. The influence of the

4 49 Y. Fu, R. A. Weiss polymeric nature of acid was evaluated by comparing its effectiveness at protonating PANI base with that of a low molecular weight organic acid, dodecylbenzenesulfonic acid, in DMSO solution. Little improvement in protonation was found for the latter acid, especially for [-SO,H]/[AN] ratios of less than.5, which indicates that conformation hindrance of the polymeric sulfonic acid had little effect on protonation of PAN1 for these dilute concentrations. The protonation of PANI base with HSPS was also studied in NMP solution to assess the solvent effect on doping behavior. Fig. 2 shows the UV-Vis absorption spectra for different molar ratios of [HSPS]/[PANI] in NMP. All the absorption peaks for PANI base were blue-shifted in NMP compared to those in DMSO. The absorption peak due to the R-X* transition of benzenoid rings on PANI chain shifted from 335 nm to 325 nm, while the absorption peak due to the R-R* transition of qui- o,.5 u Wavelength in nrn Fig. 2. UV-Vis absorption spectra of PANI base at different concentration of HSPS in NMP, concentration of PANI is kept constant at.6 mm, (a) [-SO,H]/[AN] = ; (b) [-SO,H]/[AN] = 1.1; (c) [-SO,H]/[AN] = 2.1 ; (d) [-SO,H]/[AN] = 3.2 noid rings shifted from 64 nm to 585 nm. As the [HSPS]/[PANI] ratio increased, a new absorption peak at 83 nm was observed due to the transition from semiquinoid rings. Fig. 2 clearly shows that doping of PANI by HSPS in NMP solution does occur, but the protonation is less effective than in DMSO, and the acid concentration has to be more than doubled to reach the same amount of doping level. Chen et al. studied the doping of PANI by poly(acry1ic acid) (one equivalent) in NMP. They did not observe the semiquinoid peak in solution, though, they did observe the semiquinoid peak for a thin film cast from the same solution. They attributed this phenomenon to strong hydrogen bonding between the carbonyl group of NMP and the carboxylic acid. Our results indicate that a similar competition of the NMP solvent for hydrogen bonds with HSPS also decreased the PANI-protonation efficiency of the polymeric acid, though in our case, we still observe protonation in solution. That suggests that the semiquinoid peak might also be observed for poly(acry1ic acid)/ PANI complexes in NMP solution if the concentration of poly(acry1ic acid) were increased to 2-3 equivalents.

5 Protonation of polyaniline with lightly sulfonated polystyrene 49 I The intensity of the UV-Vis serniquinoid absorption is proportional to the concentration of the protonated state of PANI, and the intensity of the quinoid absorption is proportional to the concentration of the neutral state of PANI. The ratios of absorption intensity of the protonated states and the neutral states (A~L',lliql,i,lOid/Aql,l~l~ill ) are plotted against the ratio of the sulfonic acid group and the aniline repeating unit concentrations, [-SO,H]/[AN], in DMSO and NMP solutions in Fig. 3. When [--SO,H]/[ AN] is less than.5, A,e,l~iyu,n~,id/Aqu,l,~,id is essentially zero in both solvents. which indicates that no significant amount of the protonated state of PANI exists at low [--SO,H]/[AN] ratios. Once the [--SO,H]/IAN] ratio is above.5, however, the A,c,,i,~l,llll~,id/A~,l,inoid ratio in DMSO increases dramatically, while the A,,,,,i c,,,,, lo,d/aql,inr,id Fig. 3. Protonation of PANI base with HSPS as function of [-SO,H[/[AN] ratios in (m) DMSO and (a) NMP I-SO,Hl/ [AN] ratio in NMP increases at a much slower rate. The competing hydrogen bonding between the carbonyl group of NMP and the sulfonic acid may suppress the protonation of PANI base, since it effectively reduces the concentration of available free acid groups that may interact with the PANI. The transition from an insulating form to a metalic conductive form of PANI is evidenced by a steep increase in the concentration of protonated material. A similar observation for the effect of HCI concentration on the protonation of PANI film was also reported by Wan '('I. In conclusion, protonation of PANI base with lightly sulfonated polystyrene was demonstrated for DMSO and NMP solutions. A relatively low concentration of sulfonic acid groups on the polymer, 5.3 mol-72, was sufficient for doping PAN1 and promoting solubility of the resulting macromolecular complexes. The observation of an isosbestic point clearly shows that the quinoid unit and the semiquinoid unit were in equilibrium, and it was a function of the sulfonic acid group concentration. The protonation of polyaniline is retarded in NMP compared to DMSO due to the competition of hydrogen bonding of the NMP solvent to the sulfonic acid group. Ackrro~c.lc.clgernent: We gratefully acknowledge financial support for this work by Conwcticut Imovufio~?.~, hc.

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