Polyaniline synthesis: influence of powder morphology on conductivity of solution cast blends with polystyrene

Synthetic Metals 98 Ž1999. 201–209

Polyaniline synthesis: influence of powder morphology on conductivity of solution cast blends with polystyrene Y. Roichman a , G.I. Titelman a , M.S. Silverstein a b

a,)

, A. Siegmann a , M. Narkis

b

Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel Department of Chemical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel Received 17 August 1998; revised 2 October 1998; accepted 2 October 1998

Abstract Synthesis of polyaniline ŽPANI. was performed under different conditions followed by dedoping, redoping with dodecyl benzene sulfonic acid ŽDBSA. and then blending with PS. The morphologies of the as-polymerized, doped and blended PANI were studied. The main polymerization stages seem to include: PANI oligomers assembling into nuclei, nuclei growing into primary particles Ž10 nm., primary particles assembling into aggregates Žf 0.5 mm. and aggregates assembling into agglomerates Žf 10 mm.. The morphology of the as-polymerized PANI was found to be strongly related to the rate of oxidant addition, synthesis duration and synthesis temperature. This morphology dominates the effects of DBSA doping and dispersing the resulting PANI–DBSA in the matrix polymer. A fine PANI–DBSA powder with weakly bound aggregates is likely to disperse well in a solvent and hence promote the formation of the desired fine-network morphology and yield a low percolation threshold and high conductivity. Synthesis at a high oxidant addition rate, an excess of oxidant, a relatively high polymerization temperature and a short synthesis duration should diminish the tendency to form dense complex structures. These dense structures prevent efficient DBSA doping, deaggregation and the desired fine-network dispersion of PANI–DBSA in the blends. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Polyaniline; Polystyrene; Electrical conductivity; Polymer; Blend

1. Introduction Blending of intrinsically conductive polymers ŽICP. with thermoplastic polymers through solution processing has recently been reported to be quite efficient we.g., Refs. w1,2xx. As the ICP dispersion is mixed with a polymer dissolved in a common solvent, the type of dispersion achieved is determined by the compatibility between the ICP and the solvent, and between the ICP and the matrix polymer. To compatibilize an ICP with a solvent and a thermoplastic matrix Cao et al. w1x have suggested a counter ion doping method, namely, doping the ICP with an appropriate functionalized protonic acid, which interacts with the matrix in a cast film. The morphology of solution-cast ICPrpolymer blends is often characterized by aggregation of the ICP within the insulating matrix. The morphology that evolves depends

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on the ability of the solvent to finely disperse the ICP and on the flocculation of the dispersed ICP when blended with the polymer solution. It has been suggested w3x that the dispersion process may involve swelling the ICP aggregates in a solvent to promote deaggregation into primary particles. The size, structure and nature of the dispersion determine the blend’s electrical characteristics, namely, the percolation threshold and the conductivity above the threshold. It stands to reason that aggregate density, size and inter-particle adhesion forces significantly affect the solvent’s ability to swell and disintegrate the ICP. Among the recently reported w4x effects of synthesis conditions on the structure of as-polymerized ICP powders is that of the polymerization rate determining the powder classification from fine to coarse. In this study, the ICP investigated was polyaniline ŽPANI. synthesized by chemical oxidation polymerization. This synthesis procedure involves the addition of an oxidant to aniline dissolved in an aqueous sulfuric acid solution resulting in a powder, PANI salt. The PANI–H 2 SO4 powder is first dedoped and then redoped with a suitable

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 8 . 0 0 1 9 0 - 8

Y. Roichman et al.r Synthetic Metals 98 (1999) 201–209

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Table 1 Synthesis parameters of PANI-H 2 SO4 polymerizations Code

Water

H 2 SO4

Aniline

Ammonium persulfate

Synthesis temperature

Oxidant addition rate a wgrŽh l.x

Oxidant addition time

1 2 3 4 5 6 7

1000 g 1000 g 1000 g 1100 g 1000 g 1100 g 70 l

98 g 98 g 98 g 49 g 98 g 98 g 1l

28 g 28 g 28 g 14 g 28 g 28 g 1.2 l

68.5 g q 200 g H 2 O 68.5 g q 200 g H 2 O 68.5 g q 200 g H 2 O 34.3 g q 100 g H 2 O 68.5 g q 200 g H 2 O 22.8 g q 100 g H 2 O 3 kg q 10 l H 2 O

08C 08C 08C 08C 258C 08C 08C

161 268 89 161 161 161 184

1h 10 min 3h 30 min 1h 20 min 15 h

a

Gram oxidant per hour per liter water.

functionalized protonic acid, dodecyl benzene sulfonic acid ŽDBSA., in order to induce processability in xylene and compatibility with PS. The effects of synthesis conditions, namely, oxidant addition rate, temperature, oxidantrmonomer ratio and concentration of reactive-components on the morphology of the as-polymerized PANI powders and on PSrPANI– DBSA blends was investigated.

was slightly slower than instantaneously adding the complete quantity. The low oxidant addition rate used was based on literature reports of the longest duration of effective synthesis w4x. Dedoping PANI–H 2 SO4 was performed by mixing it with a 1% wt. aqueous ammonium hydroxide solution for 3 h, filtering the resulting PANI base, rinsing first in water, then in ethanol and drying in vacuum at 608C for 24 h. Redoping with DBSA was performed by mixing the dry PANI base with a 1 M solution of DBSA in ethanol for 24 h, filtering the resulting PANI–DBSA powder and drying in vacuum at 608C for 24 h. The PS used was Carmel Olefins ŽIsrael. general purpose grade HH 102 E ŽMFI s 4 gr10 min..

2. Experimental 2.1. Materials PANI was synthesized by chemical oxidation polymerization in an aqueous solution of sulfuric acid at 08C. The oxidant Žammonium peroxide sulfate. was slowly added to an aqueous solution of aniline in sulfuric acid. The resulting green powder ŽPANI salt doped with H 2 SO4 ., which precipitated after 15 min, was rinsed first in ethanol, then in water and each powder was dried in a Buchner funnel after each rinse for 60 min. The powder was weighed, dried in vacuum for 24 h at 608C and weighed again. The water content of the powder was calculated from the mass difference before and after drying for this synthesis procedure. The synthesis duration is defined as the oxidant addition time plus an additional 15 min, used to ensure the termination of polymerization. The seven different synthesis conditions ŽTable 1. included changes in oxidant addition rate, oxidant content, synthesis temperature, reactive components concentration and anilineroxidant molar ratio. The highest oxidant addition rate used

2.2. Blends A 10% wt. solution of PS in xylene was prepared by dissolving the polymer in xylene at 1008C for 10 min. A 10% wt. dispersion of PANI–DBSA in xylene was prepared by mixing the powder in xylene at 1008C for 30 min. Polymer blends were prepared by mixing different ratios of the PS solution and the PANI–DBSA dispersion at 1008C for 10 min. The mixtures were cast and left to dry at room temperature for 24 h and then in vacuum at 608C for 24 h. The films resulting from this process will be referred to as ‘solution cast’ blends. 2.3. Morphological characterization 2.3.1. Optical microscopy Films for optical microscopy ŽOM. were prepared by dipping a microscope slide in the blend solution. The film

Table 2 Characterization of PANI–H 2 SO4 powders Code

Water content wgrgx

Bulk density wgrccx

Doping level w%x

Conductivity before drying wSrcmx

Conductivity after drying wSrcmx

Yield w%x

1 2 3 4 5 6

4.5 7 4.2 5.4 5.9 7.6

0.127 0.113 0.144 0.091 0.089 0.233

32.6 26.5 26.2 36.3 21.4 31.8

0.6 0.5 0.7 2.2 0.8 0.7

0.1 0.1 0.1 0.1 0.04 0.1

69.3 71.1 69.3 72.2 74.7 23.9

Y. Roichman et al.r Synthetic Metals 98 (1999) 201–209

structure was studied using a Zeiss Axiophot OM in the transmission mode. 2.3.2. Scanning electron microscopy The film’s fracture surface morphology was characterized using a JEOL, JSM-840 scanning electron microscope ŽSEM. in the SE mode at 10 kV. The samples were freeze fractured in liquid nitrogen and subsequently gold sputtered. The PANI powders were analyzed using a ZEISS FEO-982, High Resolution Scanning Electron Microscope ŽHRSEM. at 1 kV. No special sample preparation was required for the HRSEM. 2.4. Electrical conductiÕity The electrical conductivity was measured using the 4-point probe technique, applying a voltage between 1 and

203

40 V. PANI samples for conductivity measurements were prepared by compression molding, while the blends’ conductivity was measured on cast films. Silver paint was used to reduce contact resistivity.

3. Results 3.1. PANI–H2 SO4 The as-polymerized PANI–H 2 SO4 powders were first characterized for polymerization yield, water content Žbefore drying., bulk density of the loose powder after drying, doping level Žby weight. and conductivity before and after

Fig. 1. HRSEM micrographs of PANI-base from different syntheses. Ža. Synthesis a1—reference synthesis; Žb. Synthesis a2—higher oxidant addition rate; Žc. Synthesis a3—lower oxidant addition rate; Žd. Synthesis a4—lower reactive–component concentration; Že. Synthesis a5—higher synthesis temperature; Žf. Synthesis a6—lower oxidant content.

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Fig. 2. An HRSEM micrograph of Synthesis a7, PANI-base powder at different magnifications.

drying ŽTable 2.. It can be seen that while the conductivity after drying and the doping level of the PANI–H 2 SO4 are relatively independent of synthesis conditions, it is apparent that as the oxidant addition rate decreases ŽSyntheses a2, a1, then a3. the powder structure becomes denser Žbulk density increases and water content decreases.. It is also apparent that a reduction in the amount of oxidant added ŽSynthesis a1 vs. a6. reduces the yield and changes the structure of the powder Žboth bulk density and water content increase.. 3.2. PANI-base The morphology of the PANI-base powders was investigated using HRSEM. Fig. 1 demonstrates the significance of the synthesis conditions in determining the structure of the PANI powder. A comparison of Fig. 1a, b and c illustrates the effect of the oxidant addition rate. An intermediate oxidant addition rate ŽFig. 1a. results in a hierarchy of agglomerates and aggregates where the aggregates are elongated and of intermediate size Ž250 = 100 nm.. A substantial increase in the oxidant addition rate ŽFig. 1b. yields a reduction in the aggregate size to a level indistinguishable in the HRSEM images, while a decrease in the oxidant addition rate ŽFig. 1c. yields an increase in the aggregate size. The synthesis with lower concentration of reactive components ŽSynthesis a4. yielded an inhomogeneous powder. Some zones contain aggregates ŽFig. 1d. while others are similar in structure to the powder resulting from the

synthesis with a fast oxidant addition rate ŽFig. 1b.. It is suggested that the lower concentration caused significant inhomogeneity in the synthesis reactor and hence inhomogeneity in the structure of the resulting powder. The effects of polymerization rate on powder morphology were investigated by comparing the syntheses at a high oxidant addition rate at 08C ŽFig. 1b. and an intermediate oxidant addition rate at 258C ŽFig. 1e.. Both syntheses have a higher polymerization rate than Synthesis a1 and their structure is similar suggesting a relationship between polymerization rate and powder morphology. The oxidantraniline ratio influences both the yield and the morphology. A low oxidantraniline ratio yielded small aggregates with a high bulk density ŽFig. 1f.. The effect of a stochiometric oxidantraniline ratio ŽSynthesis a7. is demonstrated in Fig. 2. The PANI agglomerates are elongated with a substantially higher aspect ratio than those formed when a larger oxidantr aniline ratio was used ŽFig. 1.. It should be noted that the stochiometric synthesis lasted 24 h, 15 h oxidant addition plus 9 h stirring, while the other synthesis durations were from 1 to 3 h. Moreover, Synthesis a7 was performed at the pilot scale which influences the hydrodynamics of the polymerization system. A few conclusions regarding the polymerization process can be presently drawn. 1. The polymerization kinetics influences the morphology of the resulting PANI powder. A fast polymerization process, induced by either a high oxidant addition rate or by a high temperature, results in a small aggregate morphology.

Table 3 Excess DBSA and conductivity of four syntheses Synthesis number

Oxidant addition rate

Oxidantraniline ratio

Excess DBSAa w%x

Conductivity wSrcmx

1 2 3 7

Intermediate High Low Intermediate

Excess oxidant Excess oxidant Excess oxidant Stochiometric

79 87 99 33

0.7 0.8 0.4 0.3

a

Determined by weighing before and after rinsing with methanol.

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Fig. 3. HRSEM micrographs of PANI–DBSA based on: Ža. Synthesis a1—reference synthesis; Žb. Synthesis a2—higher oxidant addition rate; Žc. Synthesis a3—lower oxidant addition rate; Žd. Synthesis a7—lower oxidant content Žstochiometric oxidantraniline ratio..

2. The synthesis duration influences the morphology of the PANI powder. The longer the synthesis duration the larger the aggregates and agglomerates. 3. Shortening the synthesis duration by reducing the amount of oxidant yields small but dense aggregates. This dense morphology reflects the effect of shortening the duration without modifying the polymerization kinetics. When the oxidant addition rate was increased, yielding a similar synthesis duration with a higher oxidant content, a less dense morphology resulted. 4. The rate of oxidant addition determines the number of new nuclei Žgrowth sites. created at any given time during polymerization. The higher the oxidant addition

rate, the greater the number of nuclei formed and the smaller the aggregates. 3.3. PANI–DBSA The redoping process of PANI with DBSA yields a powder containing PANI–DBSA complex, excess DBSA physically bonded to the PANI–DBSA aggregates and, possibly, a fraction of an undoped PANI ŽPANI-base.. The degree of doping and the content of excess Žphysically bound. DBSA have a significant effect on the dispersion level of PANI in the polymer solution, and the PSrPANI–DBSA blend’s structure and conductivity. The

Fig. 4. HRSEM micrographs of Synthesis a3, PANI–DBSA powder. Ža. Without and Žb. with ultrasonic treatment.

206 Y. Roichman et al.r Synthetic Metals 98 (1999) 201–209 Fig. 5. SEM micrographs of ultrasonic treated PSrPANI–DBSA blends using PANI from different syntheses: Ža. Synthesis a1—reference synthesis; Žb. Synthesis a2—higher oxidant addition rate; Žc. Synthesis a3—lower oxidant addition rate.

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amounts of excess DBSA, as determined by the weight loss on rinsing PANI–DBSA powders with methanol, are presented in Table 3. The large excess DBSA contents, especially in the case of low oxidant addition rate ŽSynthesis a3., is striking. The conductivity of the PANI–DBSA powders seems to scarcely depend on the excess DBSA content. This could imply that the doping process results in a layer of fully doped PANI encompassing a core of undoped PANI aggregates, rather than a homogeneous, low doping level w5x. The morphologies of several PANI–DBSA powders are presented in Fig. 3. Doping does not seem to affect the size and shape of the PANI-base aggregates and agglomerates, as has been noted previously w4x, but rather it encapsulates them with a layer of chemically bound DBSA surrounded by excess DBSA. The thickness of the DBSA layer formed seems to depend on the structure of the PANI-base powder. The denser aggregates ŽFig. 3c and d. seem to have a thin DBSA coating while the less dense aggregates have a thicker DBSA coating ŽFig. 3a and b.. The dark droplets in Fig. 3b are probably excess DBSA which has precipitated during casting. The strength of aggregate bonding in the agglomerates can be estimated by the effects of ultrasonic treatment on the PANI–DBSA dispersions. Although the ultrasonic treatment resulted in deagglomeration in most powders, as demonstrated in Fig. 4, the PANI synthesized with an intermediate oxidant addition rate Že.g., Synthesis a1. was not affected. The strong adhesion between aggregates in this PANI will be discussed below.

207

Fig. 6. Conductivity vs. PANI–DBSA content for PSrPANI–DBSA blends: Ž`. Synthesis a1—reference synthesis; ŽI.. Synthesis a2— higher oxidant addition rate; Že. Synthesis a3—lower oxidant addition rate.

6. The percolation thresholds are in the range 2–6% wt. PANI and the conductivity above percolation is as high as 1 wSrcmx. The sharp increase in conductivity with PANI– DBSA content in the blends containing large PANI–DBSA agglomerates Žintermediate and low oxidant addition rates., is not continuous but occurs in two stages with a plateau between them, implying the existence of two conduction pathways. At low PANI–DBSA contents the conductivity could be limited by the hopping of charge carriers between agglomerates, whereas at higher PANI–DBSA contents such long range hopping would not be necessary and hence the conductivity increases.

3.4. PS r PANI–DBSA blends 4. Discussion The morphologies of PSrPANI–DBSA blends cast from xylene containing PANIs synthesized at different oxidant addition rates are presented in Fig. 5. Two phases are clearly seen: neat PS, characterized by its typical fracture surface, and PANI–DBSA, characterized by its hierarchical structure of aggregates and agglomerates. Between these two phases a layer of DBSA is visible. Wherever this DBSA layer is relatively thick the PS seems to wet the PANI aggregates, otherwise a gap exists between the two phases. The aggregates remain intact upon blending with the polymer solution, i.e., their structure and size were not altered in all the blends studied, in spite of the obvious differences in blend quality. The amount of the DBSA attached to the PANI–DBSA powder and the inter-aggregate adhesion seem to affect the integration of the two polymers. The low bulk density PANI ŽSynthesis a2, high oxidant addition rate., which incorporates a substantial amount of DBSA and has fairly weak inter-aggregate adhesion, is relatively well blended with the PS when compared to the other blends in Fig. 5. The conductivity of different PSrPANI–DBSA blends, as a function of PANI–DBSA content is presented in Fig.

The different stages in preparation of PSrPANI–DBSA blends, namely, PANI–H 2 SO4 synthesis, dedoping, redoping the PANI-base with DBSA and solution blending, all influence the final blend properties. However, the aniline polymerization conditions seem to have the major role, affecting all the processes that follow, and to have the strongest influence on blend morphology and properties. To achieve a high doping level, a fine PANI–DBSA dispersion and a PSrPANI–DBSA blend consisting of fine conductive pathways, it is imperative that the as-polymerized PANI–H 2 SO4 powder contains small, weakly bonded aggregates of low bulk density. The DBSA can penetrate such loose structures relatively easily and this enhances their dispersion in the blending solvent. The polymerization process of aniline is rather complex, with the final product dependent on such synthesis conditions as oxidant content and addition rate, polymerization time, monomerroxidant ratio w4x, as well as on the hydrodynamics of the polymerization system Žreactor size and stirring velocity. w5x. Polyaniline exhibits a complex combination of polymer, metal and salt properties,

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resulting from the charge distributions both on and between the polymeric chains w6x. This uniquely complex molecular identity also lends to the complexity of the polymerization process. The oxidantraniline ratio has the expected effect on the yield of the polymerization; an increase of the ratio increases the yield. The rate of oxidant addition is of great importance, affecting the number and size of the aggregates. The higher the addition rate, the larger the number of nucleation sites formed and the smaller the aggregate size. The oxidant addition rate is also used to determine the duration of the synthesis process. Since aggregates tend to agglomerate during stirring, long stirring times yield a more complex structure. As increasing the oxidation rate both increases aggregate number and decreases stirring time, an intermediate rate is expected to yield the strongest inter-aggregate adhesion. In short-duration syntheses no noticeable agglomerate structure was developed and the resulting aggregates were not dense, while in long-duration syntheses an elongated, highly dense agglomerated structure resulted. In the latter case, process duration is predominant and all other synthesis parameters are of secondary importance. However, for a synthesis of intermediate duration the structure of the agglomerates and aggregates is highly sensitive to the other synthesis conditions. Here, increasing the aggregate concentration in the reactor, through increasing synthesis duration, seems to increase the tendency to agglomerate. These results confirm the expected effects of synthesis duration on aggregate structure. The present observations help describe the synthesis process. Upon the addition of the oxidant the dissolved aniline in the aqueous sulfuric acid solution starts to polymerize and the oxidation state of the chains changes from pernigraniline Žblue dispersion. to emeraldine Ždark green dispersion. w7x. The initially formed oligomers function as growth sites Žnuclei. for polymeric chains. Since the growing polymer chains contain a charge distribution they tend to attract each other and form spherical Žca. 20 nm in diameter., partially crystalline primary PANI particles w8x. Electrical dipoles on the particles’ surface yield both additional chain growth on the surface and short-range particle-particle attraction, leading to the formation of aggregates. The aggregates grow during polymerization, accumulating more primary particles, and consequently forming agglomerates. The aggregate–agglomerate growth largely depends on stirring time Žsynthesis duration.. Dedoping and then redoping with DBSA does not observably alter the structure of the powders. The thickness of the enveloping DBSA layer, however, strongly depends on the powder’s bulk density. A dense structure limits the DBSA penetration into the aggregates and hence reduces the thickness of the doping DBSA layer. The thickness of the DBSA layer and the inter-aggregate adhesion affect the quality of blending with PS. As the powders’ structure does not alter during the post-synthesis processes, the

synthesis has a predominant role in determining the quality and nature of these solution-cast polymer blends. The importance of the dispersion type is manifest in blend conductivity. Higher conductivities are exhibited by blends made with better PANI–DBSA dispersions. The dispersion type is evidently the determining factor in the formation of conductive pathway structures.

5. Conclusions PANI–H 2 SO4 powders were synthesized under different conditions and were then dedoped, redoped with DBSA and blended with PS. The morphologies of the as-polymerized and treated powders and the blends were characterized and correlations between morphology and synthesis conditions were found. Ø The properties of the PANI powder depend on polymerization temperature, oxidant addition rate and the overall concentration of reactive components in water. Ø The main polymerization stages are as follows: PANI oligomers assemble into nuclei, nuclei grow into primary particles Ž10 nm., primary particles into aggregates Žf 0.5 mm. and aggregates into agglomerates Žf 10 mm.. Ø The morphology is strongly related to the rate of oxidant addition. Aggregate size increases and aggregate density decreases with increasing oxidant addition rate. The agglomerate size increases with aggregate number and synthesis duration, thus is particularly large at intermediate oxidant addition rates. Ø The PANI powder is doped inhomogeneously, leaving an appreciable fraction of undoped PANI. The doping occurs mainly at the aggregate surface, with the thinner doped layer on the denser PANI aggregates resulting in a lower conductivity. Ø The quality of the dispersion and blend integration was enhanced through compatibilization with excess DBSA. The best blend integration was observed for the highest amount of excess DBSA, namely, for the least dense PANI with the smallest aggregates. Ø The conductivity of the blends increased with the quality of the PANI–DBSA dispersion. Ø It would be advantageous to synthesize PANI–DBSA under such conditions so as to prevent the formation of a dense complex structure, namely, high oxidant addition rate, an excess of oxidant, a relatively high polymerization temperature and a short synthesis duration.

Acknowledgements The partial support of the Israeli Ministry of Science is kindly acknowledged.

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