Conjugated polymers for thin film coating process

Zhexiong Tang, Wenguang Li, Cheryl Nowak, and Sze YangDepartment of Chemistry, University of Rhode Island, Kingston, RI 02879

ABSTRACT

In this paper, we present the syntheses of the conjugated polymers of different“effective molecular weights.” The goal is to be able to optimize the thin film coatingprocess without compromising the electronic, optical and electroactive properties.

Our strategy for controlling the effective molecular weight is to synthesize adouble-strand conducting polymer that is an inter-polymer complex of a polymericdopant and a conjugated polymer. The first strand of the two-strand complex is either apolyaniline or a polypyrrole chain, while the second strand is a poly(acrylic acid). Themolecular weight of poly(acrylic acid) is used as a parameter for adjusting the coatingand film-forming properties.

We found that the inter-polymer complexes are dispersible in water. Thedispersion stability is strongly dependent on the molecular weight of the poly(acrylicacid) strand. The dispersion is more stable for polymers synthesized with highermolecular weight of poly(acrylic acid). We also found that the particle size and particlemorphology are dependent on the effective molecular weight.

INTRODUCTION

Conjugated polymers are used in light emitting diodes, thin film transistors,electrochromic devices, chemical sensors, and anticorrosion coatings. A key element inthese devices is a thin layer of π-conjugated polymer. This thin film is coated on asubstrate and is part of a multi-layer structure in the device. For the device to perform,the thin film should have good mechanical property. The coated thin film needs to beadherent to the substrate, and the polymer needs to be elastic enough to maintain a stableinterface between the contacting layers of the devices.

For traditional organic coatings, the molecular weight (MW) of the polymer is animportant parameter for optimizing both the coating process and the thin film mechanicalproperties. High molecular weight is often needed for adhesion to the substrate, for theelasticity of the polymer film, for optimizing the coating process, and for a good film-formation property.

Molecular weight has not been studied as a parameter to optimize the thin filmcoating process of the π-conjugated polymer because of the difficulty in controlling andmeasuring its molecular weight. This difficulty is understandable given the complexityof the polymerization process of the conjugated polymers. Although the requiredconjugation length for optical and electronic properties can be satisfied at relatively lowmolecular weight, a good film forming property usually requires a much highermolecular weight.

Mat. Res. Soc. Symp. Proc. Vol. 708 © 2002 Materials Research Society

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In this article, we report an approach that controls the “effective molecularweight” for the conjugated polymer for coating process. The idea involves the use of aninter-polymer complex of conducting polymer. This molecular complex consists of twopolymers that form a double-strand molecular complex [1]. The first strand is aconducting polymer, e.g., polyaniline (PAN) or polypyrrole (PPy). The second strand isa polymeric dopant, e.g., poly(acrylic acid), (PAA). The two strands of the componentpolymers are non-covalently bonded in a largely side-by-side arrangement. We use thesymbols PAN:PAA and PPy:PAA to represent the complex. A structural representationfor PPy:PAA is shown in figure 1.

Figure 1. An inter-polymer complex between polypyrrole and poly(acrylic acid).PPy:PAA. Here A is the carboxylate functional group of PAA.

In this double-strand inter-polymer complex, the first strand provides thenecessary opto-electronic property, while the second strand provides the properties forcoating process and thin film formation properties. By controlling the molecular weightof the polymeric dopant (the second strand) the “effective molecular weight” of the inter-polymer complex is adjusted for optimizing the coating process and the film formingproperties.

In this paper, we report the synthesis of the molecular complexes PAN:PAA andPPy:PAA with MW of PAA ranging from 20,000 to 750,000. By using PAA of differentchain lengths we altered the “effective molecular weight” of the inter-polymer complex.

To facilitate the coating process, it is desirable to disperse the π-conjugatedpolymer in water or a solvent. We are interested in the question of whether a highmolecular weight complex is easily dispersible in water to facilitate the coating process.One could speculate that when the molecular weight of the complex is too high, thepolymer might not be dispersible. What we found is contrary to this expectation. Thestability of the dispersion is found to be better for the higher molecular weight complex.We have studied the morphology of these complexes and found a correlation among theeffective molecular weight, the dispersibility and the morphology of the polymericproducts.

TEMPLATE GUIDED SYNTHESIS

The method for synthesizing the molecular complexes PAN:PAA and PPy:PAA isrelated to a previously reported template-guided synthetic strategy [2,3]. This strategyinvolves two steps: First the monomers of aniline or pyrrole are adsorbed onto the

A- A- A- A- A- A- A- A- A- A-A-

+ +NN N N NNN N

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polyacrylic acid backbone (the template polymer). Then the adsorbed monomers arepolymerized to form a π-conjugated polymer. The conjugated polymer becomes a partof the inter-polymer complex [4].

As an example, we illustrate this synthetic strategy for forming PAN:PAA. Whenaniline (AN) is mixed with an aqueous solution of PAA, the AN monomers are adsorbedonto the PAA polymer backbone due to hydrophobic, electrostatic and hydrogen bondinginteractions. There is also a contribution due to an electrostatic interaction known as“counter-ion condensation” [5]. The “counter-ion condensation” comes from anelectrostatic interaction that is particularly strong between the anilinium ions and thepartially ionized polyelectrolyte PAA.

After the aniline monomers are adsorbed onto PAA to form (AN)x:PAA, theaniline monomers are polymerized by an oxidant (Na2S2O8) to form PAN. The resultingpolymer PAN is topologically entangled with PAA to form the molecular complexPAN:PAA. The molecular complex is dispersible in water because the PAA strand ofthe inter-polymer complex is water-soluble [6]. The molecular complex is stable underboth acidic and basic conditions [7]. It is stable with respect to heat and resists dedopingin neutral distilled water.

PAN:PAA AND PPY:PAA INTER-POLYMER COMPLEXES WITH DIFFERENTMW OF PAA

In order to compare the properties of the inter-polymer complexes, we performeda series of syntheses that resulted in products with a fixed mass ratio between PAN andPAA but with different molecular weight of PAA. The mole ratio between the acrylicacid monomer and aniline was kept at the ratio of (AN):(AA) = 1:2. A set of reactionswith different molecular weight of PAA was carried out with the same concentration ofreactants and under the same reaction conditions. Poly(acrylic acid) with molecularweights of 20,000, 50,000, 90,000, 250,000, 450,000, and 750,000 were used tosynthesize PAN:PAA(MW) with different molecular weights of PAA. Since thepolymerization reactions for PAN were performed under the same conditions, sameprocedures, same concentration of oxidants, and the same acid and monomerconcentrations, we assume that the average chain length of the resulting PAN is the samefor all the complexes. For a PAA linear chain of MW=750,00, there could be more thanone PAN chain attached to the PAA strand in a PAN:PAA(750,000) complex. While forPAN:PAA (MW=20,000), there could be more than one PAA polymer chain attached toa PAN chain.

Ppy:PAA inter-polymer complexes were synthesized with the same procedure.

EXPERIMENTAL

Materials

Poly(acrylic acid) were purchased from two suppliers, Aldrich Chemicals andPolysciences. Polyacrylic acid 25% solution in water, molecular weight 20,000, 50,000,and 90,000 were obtained from Polysciences. Poly(acrylic acid) powder (<0.1% cross-linked), MW: 2,000, 250,000, 450,000 were obtained from Aldrich Chemicals. Also,

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Poly(acrylic acid) 35% solution in water with molecular weight of 100,000 (weightaverage), 250,000 (viscosity average) were obtained from Aldrich Chemicals. Anilineand pyrrole from Aldrich was purified by distillation under vacuum. Sodium persulfateand ammonium persulfate were purchased from Aldrich.

Synthesis

The syntheses were performed as replicas of identical reactions for the purpose ofcomparing the properties of the products. Each reaction in a group of replicated reactionswas performed using exactly the same procedure, with exactly the same monomer andreagent concentrations, except that the molecular weight of the PAA template is varied.The concentration of all the reagents including that of PAA was the same for a set ofreactions. As an illustrative example, we describe the synthesis procedure for one inter-polymer complex PPy:PAA(MW=250,000) which is part of a group of polymerssynthesized at the same time with the same procedure.

Dissolve 0.875 grams Poly(acrylic acid)(250,000 MW, Polysciences) in 25 gramsdistilled water. Stir for 2 hours to make the PAA dissolve completely. Add 0.62 g anilineinto the PAA aqueous solution to form a highly viscous solution of the (AN)x:PAAadduct. Stir for 4 hours to allow equilibrium adsorption. The monomer mole ratiobetween the acrylic acid monomer and aniline is (AA) : (AN) = 2:1. To the solutions prepared in the last step, add 4 ml 3 M nitric acid to each of thesolutions. Stir for two hours. Dissolve 0.006 mol ammonium persulfate in 10 ml distilledwater. Add the ammonium persulfate solution to the solution of (AN)x:PAA drop bydrop. The solution first turns to blue color and then turns to dark green. Continue to stirfor 24 hours. The solutions are a green homogeneous dispersion of the PAN:PAAproduct.

Purification

An aqueous dispersion of pure PAN:PAA or PPy:PAA were obtained by dialysis.The low molecular weight reaction products, the sulfate ions and the nitrate ions wereremoved by dialysis through a semi-permeable membrane with molecular weight cut offof 3,500. Repeated dialysis first in acidic solutions and distilled water leads to completeremoval of low molecular weight impurities.

Reaction yield

The purified reaction product was dried and the mass of the polymer wasdetermined gravimetrically. The reaction yields for all samples were found to be between92% and 95%. Previous studies had shown that samples purified by the dialysistreatment contain exclusively the inter-polymer complex and the elemental compositionof the complex is consistent with that for a PAN to PAA mole ratio of 1:2. Thus thetotal amount of the reaction product is the same for all syntheses.

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Dispersion stability

The dialyzed reaction products are initially dispersible in water. The particle sizeis, however, not the same for the complexes synthesized with different MW of PAA.The particles of PAN:PAA(MW 250,000) could penetrate the pores (about 0.5 to 1micron) of filter papers while the particles of PAN:PAA(MW 20,000) are completelyblocked in a filtration process. The effect can be visualized by placing a small drop ofeach solution on a sheet of filter paper in a manner similar to the introduction of samplefor paper chromatography. The solution from PAN:PAA(MW 250,000) spreads with thewater to form a circular area with green colored stain on the filter paper. The particlesfrom a suspension of PAN:PAA (MW 20,000) are trapped at the center of the circularwhile the water in the solution spreads. Figure 2 shows the particle size does not seem tochange for the first 4 days after the reaction.

Figure 2. Spreading of the water dispersion of the reaction product on filter paper. ThePAN:PAA (MW 250,000 and 450,000) diffuse with the water through the porous filterpaper, while the complexes synthesized with PAA MW < 90,000 do not spread with thewater.

The UV-Vis spectra of the purified molecular suspension of PAN:PAA are shownin figure 3. The absorption spectra have the characteristic absorption bands forpolyaniline and polypyrrole. While the qualitative features for the dispersed complexesare the same, the quantitative aspect of the absorption intensity depends on the molecularweight of the complex. All samples for figure 3 contain the same amount (mass) of thedispersed inter-polymer complexes, yet the absorption band intensities are different fordifferent molecular weights of PAA.

The explanation for this observation is that these samples contain dispersedparticles of different sizes. When an aggregated particle is large enough to be opticallyopaque, it does not contribute to the spectra. The absorption band intensity will be mostintense when the particles are small enough to allow the green light (λ 560 nm) to be

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partially transmitted through the particle. In principle, the optically opaque particlesshould contribute to a background “black” absorption, but the contribution is negligibledue to their small physical cross section compared with the total cross sectional area ofthe light beam of the spectrometer. Thus for PAN:PAA and PPy:PAA dispersions, thosesynthesized with PAA MW < 90,000 have large aggregated particles, while thosesynthesized with PAA MW of 250,000 and 450,000 are much less aggregated.

Figure 3. UV-Vis spectra of the water dispersed PAN:PAA complex with the same massof the dispersed material in each solution. Molecular weight of PAA, from bottom totop, 20,000, 50,000, 90,000, 250,000, and 450,000.

The particle size in the solution influences the long-term stability of the waterdispersion. Figure 4 shows the pictures of 4 samples synthesized simultaneously usingthe exactly the same concentrations and the same procedure, except that the PAAtemplate molecular weight used was different (PAA MW = 20,000, 50,000, 90,000, and250,000). Although all solutions were dispersed initially at the time of synthesis, thecomplexes with low PAA molecular weights eventually precipitated out. The pictures inFig. 4 show that PAN:PAA complexes with MW = 20,000 and 50,000 were precipitatedfrom the dispersion within 5 weeks. The solution with PAN:PAA(MW 250,000)remained a stable dispersion for more than a year.

The observed trend for dispersibility is somewhat counter-intuitive. A naïveexpectation would be that the higher molecular weight complex would be harder to bedispersed. For example, the solubility of polystyrene in xylene decreases as themolecular weight is increased. The observed trend for the PAN:PAA, and PPy:PAAcomplex is opposite to this expectation: The complex with PAA MW = 250,000 is a

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more stable dispersion than that for PAA MW = 50,000. Clearly, the usual argument forthe molecular weight dependent free energy of solution is not a good explanation for thetrend reported in this paper.

In this paper, we presented the inter-polymer complexes synthesized withpersulfate as the oxidant. The complexes can be synthesized with other oxidants. Forexample, hydrogen peroxide could be used as the oxidant with ferric chloride as thecatalyst. Although a similar “effective molecular weight” dependence of dispersibilitywere observed in an ongoing study, the critical molecular weight for precipitation isdifferent from those reported here. It appears that the size of aggregation is dependent onthe oxidant used for synthesis. The results will be presented elsewhere.

Figure 4. Photographs of the water dispersion of the PAN:PAA complexes. Each samplevial contains the same amount of the PAN:PAA complexes. The complexes weresynthesized with different molecular weight of PAA template. PAA molecular weights(from left to right) are 20000, 50000, 90000, and 250000. The pictures show thedispersions one-day after the synthesis (left) and five weeks after the synthesis (right).

TEM IMAGE OF REACTION PRODUCTSThe particle size and the particle morphology of the suspended PAN:PAA and

PPy:PAA complexes were examined with transmission electron microscopy. Therepresentative TEM images for PAN:PAA with different PAA molecular weights areshown in figure 5. The PAN:PAA complexes synthesized from low molecular weightPAA with MW = 20,000, 50,000, and 90,000 show particles that were aggregated from anetwork of fibers. The PAN:PAA complexes synthesized from high molecular weightPAA templates with MW = 250,000, 450,000 and 750,000 were aggregated into smallglobular particles with diameters ranging from 20 to 200 nm.

The particle size of the fibrous network is much larger, with dimensions in therange of 1 to 3 µm. The difference in the particle size correlates with the stability of theaqueous dispersion.

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Figure 5. TEM pictures of the dispersed particles of the PAN:PAA with differentmolecular weight of PAA. From top left, PAA MW = 20000, 50000, 90000, 250000,450000, and 750000.

CONCLUSION

The water dispersible molecular complexes of polyaniline:poly(acrylic acid) andpoly(pyrrole):poly(acrylic acid) were synthesized by a template guided self-assemblymethod. The complexes synthesized may be useful for testing the feasibility of using themolecular weight of the polymeric dopant PAA (or the second strand of the double-strandinter-polymer complex) to control the “effective molecular weight” for optimizing thecoating process and for examining the differences in the film-forming properties.

We found that the dispersibility of the molecular complexes is dependent on themolecular weight of the polymeric complex. The observed trend for the effect ofmolecular weight on polymer dispersibility (or solubility) is unusual. In contrast to thecommonly observed trend of lower solubility for higher molecular weight, the polymericcomplexes are better dispersed for higher molecular weight of PAA. For example, thePAN:PAA complexes with PAA MW > 90,000 are stable dispersions while thosecomplexes with PAA MW<50,000 are easily aggregated and precipitated out of the

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aqueous solution. A similar trend was observed for the polypyrrole:poly(acrylic acid)complexes.

The observed correlation between molecular weight and dispersibility is veryuseful from a practical point of view. A polymeric dispersion with high molecularweight (MW~106) has the advantage of providing good film forming properties. Highermolecular weight coatings are expected to have better adhesion and better mechanicalproperties.

We have also observed an interesting correlation among the MW, thedispersibility, and the morphology of the particles in the dispersion. We found that thelow MW complexes are aggregated into large particles, while the high MW complexesare less aggregated. The low MW complexes aggregated into particles with fibermorphology, while the high MW complexes are small globular particles.

The results of this paper represent the first step in using PAA molecular weightfor endowing a controllable “effective molecular weight” to the PAN:PAA and PPy:PAAcomplexes. The correlation between the molecular weight of the template and thedispersion properties are not yet understood, but a hypothesis is being tested. The π-conjugated polymers with a controllable “effective molecular weight” may be useful foroptimizing the coating process and the thin film properties.

ACKNOWLEDGMENT

This research was partially funded by the URI Transportation Research Center,URI Foundation, and the US Army Research Laboratories. We thank Paul Johnson forhis help in taking the TEM images.

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