Conductive Polymers and Plastics in Industrial Applications Part 6 ppt

The electrochemical synthesis of conducting polymer colloids is based upon removing the polymer as it is formed above the electrode surface and stabilizing into colloidal particles with

solubility.13,14The major drawback of such systems is that organic solvents such as dichloro-methane, m-cresol and N-methylpyrrolidone must be used, which are industrially

undesirable

Two potential product areas can be explored for large scale electrochemical production

of conducting polymers, these being aqueous colloidal dispersions or water soluble polymers

A brief description of these polymer technologies is discussed below

CONDUCTING POLYMER COLLOID PROCESSING

To overcome the water solubility limitations of conducting polymers, aqueous dispersions that are sterically stabilized15-21have been prepared More recent approaches involve the use

of a core/shell approach where an inner inert colloidal core, such as SiO2or TiO2, is coated with the conducting polymer, in the presence of a steric stabilizer, in order to yield uniform colloid morphology and particle size distributions.22-25All of these approaches involve the production of the conducting polymer componentvia a chemical oxidative route.26,27

This technique involves oxidizing the monomer using a chemical agent and has limitations as dis-cussed above

The electrochemical synthesis of conducting polymer colloids is based upon removing the polymer as it is formed above the electrode surface and stabilizing into colloidal particles with a polymeric surfactant (i.e., a steric stabilizer) In the early stages of electropolymerization, the process of oxidation and oligomerization is said to occur within the diffusion layer above the electrode surface.28-31 As polymerization continues, the oligomer solubility (in the electrolyte solution) is exceeded and subsequently precipitates onto the electrode surface If the electrolyte flows across the electrode surface fast enough, it

is possible for the polymer to be swept away from the electrode before deposition occurs.32-34 This process is further facilitated by the presence of the steric stabilizer.35The final product is

a colloidal conducting polymer that is doped with the anion of the supporting electrolyte used during synthesis

ELECTROCHEMICAL REACTOR DESIGNS

Two electrochemical cells designs were developed “in-house” at IPRI utilizing porous reticu-lated vitreous carbon (RVC – ERG Aerospace) foam electrodes RVC is a three dimensional electrode material with a surface area to volume of 65.6 cm2/cm3at a porosity of 100 pores per inch (PPI) RVC was chosen as a replacement for the traditional plate electrode in order to maximize the electrode surface area and minimize the cell volume The two designs consid-ered in this cell development study were either a facing anode/cathode design (Figure 1) or an sandwiched anode between two opposing cathodes configuration (Figure 2) Each cell com-partment is separated by an anion exchange membrane (NeoseptaTM) and protected by

ing filter paper between the membrane and electrode to minimize fouling Electrolyte was flowed from separate catholyte and anolyte reservoirs

EVALUATION OF ELECTROCHEMICAL REACTORS

In order to establish that the electrochemical cells were operating at optimal efficiency a se-ries of standardized tests have been developed so that different cells can be compared to one another The theoretical and experimental details are discussed below

The cells were modelled as a Plug Flow Reactor type reactor Equations exist for the de-sign and evaluation of each type of reactor.36Parameters valid for continuous flow reactors are based upon the assumption of mass transport limited conditions The essential equations for this evaluation process are:

Mass Transfer Coefficient (km)

iL limiting current

z number of electrons of reaction

F Faraday constant

Figure 1 Parallel anode/cathode cell Figure 2 Sandwiched anode configuration.

C concentration of reactant

In some cases it is preferable to use eq [1] as:

In general, and particularly for cells with plate electrodes, it is possible to use:

A S = A/V R

VR volume of reactor (containing electrode)

For cells with three-dimensional electrodes:

Ve electrode volume

Fractional Conversion (XA)

X A = (C (in) - C (out) )/C (in)

or

C(in) concentration of reactant IN

C(out) concentration of reactant OUT

For a PFR operated in single pass mode, and assuming first order mass controlled kinet-ics, the following relationship will hold:

C (out) = C (in) exp[-(k m A e /Q v )] [6]

Qv volumetric flow rate

X A = 1-exp[-{k m A e /V R }τ]

or

τ residence time (VR/QVor Ve/QVfor three dimensional porous electrode) For a PFR operated inrecirculation mode:

X A t PFR

, = 1-exp[-(t X A PFR

T

τT residence time in holding tank

FIGURES OF MERIT

Space-time Yield (ρst)

m mass of product

In terms of km, at 100% current efficiency,φ

M molecular weight of reactant

Current Efficiency (φ)

Qp charge required to produce product

QT total charge used

EXPERIMENTAL

CHEMICALS Potassium hexacyanoferrate (II) and sodium nitrate were used as supplied from Ajax (Analar) Milli-Q grade water was used in all experiments

A PAR 363 potentiostat/galvanostat was used in all efficiency tests All potentials were mea-sured versus a Ag/AgCl reference electrode A Shimadzu UV-1601 UV-Visible spectrophotometer was used for all concentration determinations Data acquisition was made using a Maclab™ (AD Instruments) interface using Chart V3.3.5 software

PROCEDURES Each Cell design was tested by preparing a 1 L anolyte solution consisting of 2 mM

K4Fe(CN)6.3H20(aq)and 0.5 M NaN03(aq) A 1 L catholyte solution of 0.5 M NaNO3(aq) The anolyte and catholyte solutions were then flowed through their respective compartments at

20, 40 and 60 mL/min by peristaltic pumps Efficiency tests were carried out at a constant po-tential of +0 8 V in single pass mode

A chronoamperogram at the applied potential was recorded for both the duration of cell flushing and sample collection Initially, 1 to 3 cell volumes were passed through the cell at +0.8 V, at the flow rate being investigated, and discarded to waste With the potential still be-ing applied, a minimum of 1 cell volume was then collected The collection time (sec) and actual volume collected (mL) were also determined The charge passed was then determined

by integrating the area under the curve from the end of the data acquisition and back for the to-tal collection time (t sec) The concentration of Fe(CN)63- produced from Fe(CH)64- test solution was determined by UV-Vis Spectroscopy

RESULTS AND DISCUSSION

Analysis of the current efficiency parameters for both cell designs in-dicated that at + 0.8 V both cells de-signs had efficiencies of 99 % or better

The dependence of the mass transfer coefficient is shown in Fig-ure 3 At large residence times, or low flow rates, the mass transfer process in the sandwich configura-tion is slightly higher At shorter residence times, higher flow rate, both cell designs have similar mass transfer characteristics due to greater turbulence induced with in

Figure 3 Effect of sandwich and parallel configuration on the mass

transfer coefficient (KmAe/VR).

the RVC electrodes at higher flow rates The effect of this turbulence is characterized by the rapid increase in mass transport at lower residence times (or higher flow rates)

The relationship of space time yield,ρst, to residence time, Figure 4, shows that the sandwich configura-tion is most efficient at all flow rates investigated The sandwich configu-ration has higher ρst values due to minimizing the effect of electrode shielding and thereby utilizing more

of the available anode surface In the sandwich configuration,ρstdecreases

as the electrolyte residence time with

in the cell is also reduced This is in-dicative of insufficient electrolyte contact with the electrode at shorter residence times, but also shows that the cell was operating near its optimal

at the lowest flow rate Increasing the flow rate from 20 to 60 mL/min re-duces the space time yield by 15% In the case of the parallel configuration, the space time yield was at a maxi-mum at shorter residence times This result gives further evidence that this cell design was far less efficient than the sandwich configuration

The fractional conversion, Fig-ure 5, of Fe(CH)64- to Fe(CN)63- was better than 99% for both cell designs The sandwich configuration was again a more efficient cell design Interestingly, fractional conversion in-creased with decreasing residence time, or increasing flow rate, due to enhanced electrolyte turbulence

Figure 5 Effect of sandwich and parallel configuration on Fractional

Conversion.

Figure 4 Effect of sandwich and parallel configuration on the space time

yield, ρst

EXAMPLES OF POLYMERS SYNTHESIZED USING THESE CELL DESIGNS

A number of conducting polymer colloids have been syn-thesized using these cell designs Colloids such as polypyrrole-nitrate,2-4 polypyrrole-lactoferrin,37 polyaniline-polystyrene sulfonate/camphorsulfonic acid38have been successfully synthesized A TEM, Fig-ure 6, of a typical polypyrrole nitrate colloid shows typi-cal colloid morphologies achieved when using these cells A major feature of this synthesis technique is that the colloids formed have a controllable and uniform size distributions

CONCLUSIONS

In this paper we have presented a method of characterizing the efficiency and performance of electro-chemical flow cells utilizing three-dimensional reticu-lated vitreous carbon foam electrodes Cell design characterization is critical for the successful implementa-tion and scale up of electrochemical cell, especially with respect to the scale up from laboratory to prototype and commercial conducting polymer syn-thesis

REFERENCES

1 J.N Barisci, P.C Innis, L.A.P Kane-Maguire, I.D Norris and G.G Wallace.,Synth Met., 1997, 84, 181-182.

2 J.N Barisci, C.Y Kim, D.Y Kim, J.Y Kim, J Mansouri, G.M Spinks and G.G Wallace.,Colloids and Surfaces A., 1997,

126, 129-135.

3 J Barisci, J Mansouri, G Spinks, G Wallace, D.Y Kim and C.Y Kim,Synth Met., 1997, 84, 361-362.

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12 E.E Havinga, W ten Hoeve, E.W Meijer, and H Wynberg.,Chem Mater., 1989, 1, 650-659.

13 Y Cao, P Smith and J Heeger.,Synth Met., 1992, 48, 91.

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17 B Vincent.,Polym Adv Tech., 1995, 6, 356-361.

Figure 6 TEM of Polypyrrole-nitrate colloids.

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19 H Eisazadeh, G.Spinks and G.G Wallace.,Mater Forum, 1993, 17, 241-245.

20 S.Y Luk, W Lineton, M Keane, C DeArmitt and S.P Armes., J Chem Soc., Faraday Trans., 1995, 91, 905-910.

21 S.P Armes and B Vincent.,J Chem Soc., Faraday Trans., 1987, 228-290.

22 S.P Armes, S Gottesfeld, J.G Beery, F Garzon,Agnew., Polym., 1991, 32, 2325-2330.

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24 S Maeda and S.P Armes.,Synth Met.,1995, 69, 499-500.

25 S Maeda and S.P Armes.,Chem Mater., 1995, 7, 171-178.

26 German Patent P 37 29 566.7 Zipperling Kessler & Co.

27 US Patent Application 823416, 823511 and 823512 Allied-Signal, Zipperling Kessler & Co and Americhem Inc.

28 R John and G.G Wallace.,J Electroanal Chem., 1991, 306, 157.

29 B.R Scharifker and D.J Fermin.,J Electroanal Chem., 1994, 365, 35.

30 B.R Scharifker, E Garcia-Pastoriza and W Marino.,J Electroanal Chem., 1991, 335, 85.

31 D.E Raymond and D.J Harrison.,J Electroanal Chem., 1993, 355, 115.

32 A.F Diaz and B.J Bargon in Handbook of Conducting Polymers, T.A Skotheim Ed., Vol 1,Marcel Dekker, New York,

1986, 81.

33 C.K Baker and J.R Reynolds.,J Electroanal.Chem., 1988, 251, 307.

34 C Lee, J Kwak and A.J Bard.,J Electrochem Soc., 1989, 136, 3720.

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Hydroxyethyl Substituted Polyanilines: Chemistry and Applications as Resists

Maggie A.Z Hupcey

Dept of Materials Science and Engineering, Cornell University, Ithaca, NY

Marie Angelopoulos, and Jeffrey D Gelorme

IBM T.J Watson Research Center, Yorktown Heights, NY

Christopher K Ober

Dept of Materials Science and Engineering, Cornell University, Ithaca, NY

INTRODUCTION

In the field of conducting polymers, solubility of the polymer is a highly desirable quality One method that has been widely used to enhance the solubility of conducting polymers is the incorporation of ring substituents on the polymer backbone.1-3 However ring substituents have generally resulted in a decrease in the conductivity of the polymer Steric constraints im-posed by the substituents disrupt the coplanarity of the polymer chains as well as increase the interchain distance Both factors reduce the mobility of the carriers and as a result lower con-ductivity is exhibited In this work, we wish to report a new series of polyaniline copolymers possessing a hydroxyethyl substituent that improves the solubility of the conducting polymer and yet retains a high conductivity

In the field of microlithography, conducting polymers have potentially many uses.4-6For example, it is thought that damage to the gate oxide and to the resist sidewalls during plasma etching is due to the charging of the insulating resist layer In SEM metrology of devices and masks using a conventional insulating resist, charging of the resist yields errors in the mea-surement, a problem conducting resists have been shown to alleviate.7Conducting polymers have also been proven useful in electron beam (e-beam) lithography, where the exposing radi-ation is a beam of electrons Excess electron charge builds up at the surface of an insulating resist causing severe pattern distortion; with a conducting resist the excess charge is dissi-pated.8With these new hydroxyethyl substituted polyanilines, a simple derivitization to make

a crosslinkable conducting resist is possible

The polyaniline copolymers were synthesized following a well-known procedure.9 Copoly-mer compositions were determinedvia integration on a 300 MHz Varian1

H NMR Molecular weights were measuredvia GPC eluted with NMP with 0.5% LiCl added to each sample For

UV-Visible spectra and conductivity measurements, films were spun coat from 5% (wt/wt) NMP solutions filtered through 5µm filters onto polished quartz discs, then baking the sam-ples in an 85oC oven for 15min Conductivities were measuredvia four point probe Doping

with camphorsulfonic acid (CSA) and acrylamidomethyl-propanesulfonic acid (Aampsa) was done by adding the dopant in the ratio of 2 mole acid per mole repeat unit to a 5% (wt/wt) NMP solution of the base polymer, and processed as for the undoped For HCl doping, the undoped spun coat films were immersed in 1M HCl overnight and dried under light vacuum The methacrylate functionalization was achieved by dissolving the poly(o-hydroxyethyl)aniline homopolymer in cyclohexanone at 3 wt%, adding isocyanoethylmethacrylate (IEM) and stirring for 48 hours until complete disappearance of the isocyanate peak in the IR spectrum

RESULTS AND DISCUSSION

COPOLYMERIZATION Seven copolymers were synthe-sized by varying the molar amount

of o-hydroxyethylaniline mono-mer in the polymono-merization (Figure 1) It was found that the incorpo-rated functionality always ex-ceeded the feed (Table 1) The GPC results show that the molecu-lar weight decreased from a high of

Mn=22K and leveled off at around

Mn=12K for feeds larger than 30% ELECTRONIC PROPERTIES

In the undoped or base form, the hydroxyethyl substituted polymers exhibit a red shift in the exciton absorption in the UV-Visible spectrum as compared to the unsubstituted emeraldine base Aλmaxof 609 nm is observed for the thin film of the unsubstituted polyaniline base whereas aλmaxof 629 nm is observed for the fully substituted poly(o-hydroxyethyl)aniline The fully substituted

Figure 1 Poly(o-hydroxyethyl)aniline copolymer synthesis.