1132
Anal. Chem. 1992, 64, 1132-1140
Experimental Aspects of Solid-state Voltammetry T. T. Wooster,t M. L. Longmire,$H.Zhang,§ M. Watanabe," a n d Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 This paper describes the propertles of poly( ether) polymer electrolytes as solvent media for solld-state voltammetry. Experlmental requirements for microelectrode voltammetry and results for the dependency of diffusive transport of electroactlve solutes on polymer solvent molecular weight, structure, and temperature (and related phase state) are described for elghl pdy(ether)s: linear poly(ethylene oxides) MW = 400, 1000, 2000, and 600000 (Me,PEG-400, Me,PEG-1000, and Me,PEG-2000, PEO-600 000), linear poly(propy1ene oxide) MW = 4000 (PPO-4000), the comb polymer poly(bls[(methoxyethoxy)ethoxy]phosphazlne) (MEEP), the block copolymer poly(ether)-poly( urethane urea) (PEUU), and the crosslinked poly(ether) network PEO.
As in any active research area, advances in solid-state chemistry depend on the development of the necessary measurement tools. Our laboratory has been interested in devising electrochemical methodologies (e.g., solid-state voltammetry) applicable to polymeric mixed-valent solids' and to solutions of electroactive monomers in polymer matrices.2 In the latter studies, microelectrode^^^^^ have been used to measure the physical diffusivity2c-dfb-jaand electron-transfer dynamics*' of electroactive monomers dissolved in poly(ether) "polymer electrolyte^"^ and how that diffusivity is affected by plasticization by sorbed small molecules.2a~d,g-m Besides defining avenues to the study of interesting chemical and electrochemical processes in poly(ether)s, these experiments offer an electrochemical entree into solid-state materials on a broader front. Substantial work remains to establish the scope of applicability of microelectrode voltammetry to solid and semisolid materials, including polymers. One aspect of the work involves identifying and comparing the properties of polymers that make them useful as solid-state electrochemical solvents. Our previous work on voltammetry'? in poly(ether) phases focused on the microelectrode methodology and theory and on the interesting chemical and electrochemical effects that were encountered. The purpose of this paper is to present a more coherent examination and comparison of poly(ethers) as solid-state electrochemical solvents. Described here is microelectrode voltammetry and the dependency of diffusive transport of electroactive solute on temperature and the related phase-state of the eight poly(ether)s shown in Figure 1: high MW (600 000) linear poly(ethylene oxide) (PEO-600000), low MW (400, 1000, 2000) methyl end-capped linear poly(ethy1eneoxide) (MezPEG-400, MezPEG-1000, and MezPEG-2000), low MW (4000) linear poly(propy1ene oxide) (PP0-4000), the comb polymer5 poly(his[(methoxyethoxy)ethoxy]phosphazine)(MEEP),the block copolymers poly(ether)-poly(urethane urea) (PEUU), and a cross-linked poly(ether) called7 network PEO. All of these poly(ethers) readily dissolve lithium electrolytes to form solid 'Present address: Department of Chemistry, University of Ver-
mont, Burlington, VT 05405.
*Presentaddress: E. I. Du Pont De Nemours & Co. (Inc.),P.O. Box 1089, Orange, TX 77631-0189. Present address: Department of Chemistry, Duke University, Durham, NC 27706. On leave from the Department of Chemistry, Sophia University, Chiyoda-ku, Tokyo 102, Japan.
or semisolid but ionically conductive solutions, the ionic conductivities of which have been investigated4in the context of "polymer electrolytes".
EXPERIMENTAL SECTION Microelectrode Voltammetry. Figure 2 displays an electrochemical cell'?convenient for voltammetry of polymer solutions of electroactive species. The cell consists of three wire tip electrodes (Pt-microdisk, Pt-wire auxiliary, and Ag-wire pseudoreference electrode)exposed in a polished, insulating platform which can be either coated with a polymer film (which can also be bathed in an appropriate gas) or embedded in a bulk sample of polymer. Such details are governed by the particular polymer electrolyte and by the experimentalobjectives.'?When for example polymer films are used for transport rate measurements,the f i thickness should be large enough to yield semiinfiiite diffusion conditions;2b the thickness needed depends on the value of the redox solute diffusion coefficient in the poly(ether) solvent and can be a few tens of microns. Alternatively, to observe the effects of absorption of a bathing gas component,2b,d,gsm such as plasticization-enhancement of diffusion rates of electroactive solutes, relatively thinner films are chosen so as to promote fast partition-equilibration between vapor and polymer phases. Film-coated cells are typically sealed in a glass container, while maintaining a Nz bath, or in a glovebox or otherwise with minimum exposure to the laboratory air, followed by multiple evacuation and Ar r e f i to establish an inert atmosphere. The glass container temperature was controlled with thermal ribbon wrap or an oil bath and was monitored by T or J type thermocouples(Omega)mounted inside. The polymer film could be exposed to diffusion rate-enhancing (plasticizing) vapors of organic solvents by passing a N2stream with the desired partial pressure of the organic compound through the glass container. On occasion, the microelectrode assembly was simply mounted over a pool of liquid solvent. The electrochemical cell and locally constructed, low-current p ~ t e n t i o s t a t ~ ~ were housed in a Faraday cage during current measurements. PEO-600000/LiCF3S0,. Linear poly(ethy1ene oxide) (Aldrich, average MW 600 000, PEO-600000) was purifieds by passing a saturated aqueous solution through two ion-exchange columns: a strongly acidic cation exchanger (Dowex-50W,Aldrich),followed by a strongly basic anion exchanger (Dowex-1,Aldrich). The purified solution was frozen and lyophilized to a pure white polymer solid which stored in a N,-filled glovebox. LiCF3S03 electrolyte was prepared as in the literat~re.~ Stock solutions for film-casting,containing electroactivesolute, PEO-600000, and electrolyte, were prepared in 9:l CH3CN/ CH30H. The ether 0xygen:lithium ion concentrationratio is either 161or 8 1 (0:Li);the former gives the maximum ionic conductivity according to previous studies.l0 PEO-600OOO films are cast onto the Figure 2 microcell by evaporating single or multiple droplets of the casting solution. For films of thickness exceeding 50 pm, the polymer film is readily peeled from the microcell assembly for thickness measurement. Thinner polymer films can be employed for fast gas/polymer phase equilibration, at the expense of introducing a cylindrical diffusion geometry.2b Me2PEG-400/LiC104, Me2PEG-1000/LiC104, Me2PEG2000/LiC104. These lower molecular weight, methyl end-capped poly(ethy1ene glycol) dimethyl ethers (PolysciencesInc., nominal MW 400, 1000, 2000; number average MW 334, 898, and 2065, respectively, by our own gel permeation analysis) were dried in vacuo overnight at 50 O C and stored under N2 Me,PEG-400 is a highly viscous liquid (polymer melt) at room temperature whereas Me,PEG-lOOO and Me2PEG-2000are soft waxes melting at T, = 32 and 50 "C, respectively. The polymer solutions were prepared by mixing appropriate amounts of LiC10, electrolyte (Aldrich,to give 0:Li = 16:l) and the desired electroactive solute in Me,PEG-400, Me2PEG-1000,or Me,PEG-2000 (warmingthe
0 1992 American Chemical Society 0003-2700/92/0364-1132$03.00/0
ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992
x
X-O-fCW2-r~-O-);;
-
X.Y
Y
MEEP
: PEO
H
X=CH,,
X-H,
1133
Me2PEG
Y=H:
L(O-CH2-CH2t
0-CH,
PPO
Y=CH,:
PEUU CH,
0
ftp
ON-:
c - N -~ c 2H H
I
I
)-6- N - @ H ~ ~ N - c?- 0 - f ~ ~CH~~ -o 0
-N-&HI C H N~
H
H
-
+
H m
H
pco
A
Network PEO
Nco
C H o+cH~-cH,-o-);;H ~
Flgure 1. Structures of poty(ether) solvents: linear poly(ethylene oxide), pEO-600 0 0 0 linear methyl end-capped poly(ethyleneoxide) Me2pEG400, Me,PEG1000, Me2P€G2000(MW = 400, 1000, 2000, respectively); linear poly(propyiene oxide) PPO-4000 (MW = 4000); the comb polymer po~(bb[(methoxyethoxy)ethoxy]phosphazine) MEEP the block copolymer poty(ettw+po!y(urethane urea) EUU; and a cross-linkedpolymer, network PEO.
,.-------\.,
I
Polymer Film
I
I -
T -A g
Epoxy
_.
WE radius=
' O e 0 lJmREF. WE. AUX. Or
12.5 vm
Figure 2. Schematic diagram of the microelectrode cell, wfth 10- or 25-pm (diameter) Pt-microdisk electrode and 500-pm (diameter) pt auxiliary and Ag reference electrodes.
latter two to ca.60 "C). Dissolutionoccurs readily in Me2PEG-400 at room temperature with stirring and sonication. CH30H (50 ~ 0 1 % )was sometimes added to the Me2PEG-1000 and Me2PEG-2000melts at 60 "C to promote dissolution, after which the methanol was removed by vacuum evaporation at room temperature for 24 h. Visibly thick films of polymer electrolyte solution are easily formed on the Figure 2 cell platform with polymer melt droplets. PPO-4000/LiC104. A 1:l (v/v) mixture of linear poly(propylene oxide) (Polysciences,nominal MW 4000) and CH30Hwas purified with ion-exchange columns as for PE0,8 and a small amount of activated alumina was stirred in the eluted mixture for 24 h. CH30H was removed by vacuum pumping at 80-100 "C, and the purified polymer was stored in a desiccator. Dissolution of LiC104 (0:Li = 25:l) and electroactive solutes was typically aided by adding 2:l v/v CH30H to the viscous liquid polymer and then removing the CH30H under vacuum. A ca. 30-pL droplet of the viscous polymer solution cast on the microcell forms a suitably thick film. MEEP/LiCF3S03. The comb polymer poly(bis[(methoxyethoxy)ethoxy]phosphazine), MEEP,2fwas generously donated by Prof. D. F. Shriver. The vacuum-dried MEEP was stored in a N2glovebox where it was combined with dried LiS03CF3at OLi = 16:l by dissolving in dry THF. The MEEP/LiS03CF3polymer electrolyte, isolated by vacuum evaporation of the solvent, was dissolved in 9:l CH3CN/CH30H in a N2 glovebox; this stock casting solution was thereafter handled on the benchtop in capped vials. Electroactive species were dissolved in portions of the
casting solution, droplets of which were evaporated to make films typically 1-20 pm thick. PEUU/LiC104. The block copolymer poly(ether)-poly(urethane urea), PEUU, was prepared from 4,4'-methylenebis(pheny1 isocyanate),1,2-propylenediamine,and PEO (averageMW = 3000) according to a published procedure.6 NJV-Dimethylformamide (DMF, EM Science),distilled under dry Ar and stored under N2, was used to make a casting solution of the electroactive solute, LiC104 (0:Li = 55:l) and 5% (by weight) PEUU. Films of (dry) thickness >50 pm were formed from droplets cast on the Figure 2 platform by evaporating the DMF at 80 "C for 30 min followed by vacuum evacuation for 24 h at room temperature. Network PEO/LiC104. Network PE07 was prepared by cross-linking triol poly(ethy1eneoxide) (Daichi Kogyo Seiyaku, Polysciences). average M W 3000), with tolyl 2,4diisocyanate (TDI, Triol PEO was dried under vacuum at 80 "C for 48 h; TDI was distilled under vacuum, and methyl ethyl ketone (MEK, Fisher) was distilled at atmospheric pressure. All reagents were stored in a N2box, where appropriate amounts of triol PEO, TDI,LiC104 (0:Li = 50:1), and electroactive species were codissolved in MEK and a droplet of the polymerizing mixing was placed directly on the Figure 2 microcell platform to form, in place,21a film of the cross-linked polymer. The electrochemical cell was sealed in a glass container and the cross-linking reaction completed by transferring the sealed container to a 80 "C oven for 30 min. The resulting f h (thickness>lo0 pm) adheres tightly to the microcell surface. Electrochemical measurements were performed, after drying at room temperature under vacuum for 24 h, without opening the sealed container. The network PEO cross-linking reaction can alternatively be performed7 by casting thin films onto flat glass and thermally treating as above. This procedure yields free standing films, voltammetryof which can be obtained by pressing the film against the electrode platform surface, but which is not as reproducible as that from cross-linked-in-place network PEO which seems to form a more reliable interfacial contact with the microdisk electrode. Forming Polymer Films. The optimal method to prepare a film depends on the particular polymer. The simplest is to cast a solution of polymer, electrolyte, and electroactive species from a volatile solvent; this typically produces smooth pinhole free polymer films whose finished thickness (1-50 pm) depends on the volume and concentration of the casting solution. PEO-
1134
ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992
600000, PEUU, and MEEP films were made in this manner. While MezPEG-400,MezPEG-lOOO,MezPEG-2000,and PPO-4ooo films can also be cast from solutions, casting of their melts gives dry films more reliably uncontaminated by plasticizing organic monomers. The poly(ether) electrolytes avidly adsorb moisture and polar solvents, enhance the ionic conductivity and electroactive solute diffusivity, and can degrade voltammetric behavior. Network PEO is insoluble,so electrolyte and electroactive species must be added either by soaking a prepared film in a solution of electrolyte and electroactive species or by adding the electrolyte and electroactivespecies in the polymer cross-linking mixture. The latter procedure is preferred for quantitative work but is limited to electroactive species that are unreactive in the cross-linking chemistry. Solubility of Electroactive Species. The characteristics of poly(ether)s favoring their use in metal deposition and energy storage studies4parallel those important for use in electrochemical voltammetry.2 The coordinative reactivity of poly(ether)s promotes electrolyte salt dissolution and ion mobility through (i) a high ligand-dipoleconcentration, (ii) a flexible polymer backbone allowing chain reorganization and multiple coordinationto a single cation, and (iii) a low cohesive energy density (e.g., weak polymel-polymer intera~tions).~J~ For voltammetric application, the important additional requirement is solubilization of interesting electroactive species at levels greater than ca. 1m M the typical signal/noise ratio in voltammetric currents would generally make work in poly(ethers)sdifficult at lower concentrations. Although we have not systematically evaluated solubilities,it appears that relatively low-dipole species like CpzFe and [M(bpy),l2+(M = Ru, Co, Os, Fe, etc.) exhibit lower solubilitiesthan do alkali-metal salts. Likewise,solubilities of metalloporphyrins like [Fe(tetraphenylporphyrin)Cl],with difficultly displaced axial ligands, are typically low but are higher when weakly axially coordinated,Le., [Fe(TPP)(PFB)].Similarly,the lithium salt of the radical anion of tetracyanoquinodimethane (TCNQ-) is solublez1in network PEO to ca. 0.1 M6and [Fe(tetrakis(o-aminophenyl)porphyrin)Cl] is soluble in MezPEG containing a zinc salt as electrolyte.I5 A PEO appendage is a powerful solubilizer; ferrocene with a covalently attached PEO tailznis miscible in any proportion with a PEO solvent of similar MW. Thus, while a satisfactory level of electroactive species solubility in poly(ether)s cannot as a practical matter be taken for granted, important tactics are available to enhance dissolution.
RESULTS AND DISCUSSION Voltammetric Measurements of Diffusion of Electroactive Solutes. While voltammetric principles are the same whether in fluid or solid-state electrolyte solution, there are important differences in the execution of solid-state experiments. Briefly, the following points summarize some previous observations. (a) Ionic conductivities of polymer electrolytes are typically meager (a ,good" poly(ether) conductivity is 10klO" S/cm2), so to minimize iR, effects, we use microelectrode^^*^ with associated small currents and microarrays%J3for their associated small current paths. (b) Molecular solute diffusivities in polymer electrolyte solvents generally parallel electrolyte ion m~bilities'~ as reflected in ionic condu~tivities;~ both are small and quite variable, changing over orders of magnitude with temperature,za*d,esf,h phase-state of the polymer solvent (partly crystalline vs amorphous),2a,d,e,fsh the electroactive solute, the concentration of electroactive solute and electrolyte,zhand sorption of vapors of plasticizing organic monomers.2e,g,m These diffusivity variations are chemically informative but lead to variable2' diffusion geometries in the depletion layer around the microelectrode: radial, linear, or mixed linearradial. An appropriate repertoire of voltammetric data analysis is consequently required to properly evaluate electroactive solute diffusion coefficients.2i (c) The small microdisk electrode currents coupled with slow diffusion make trace concentration measurements unrealistic in solid polymers with this electrode geometry. Useable
concentrations typically exceed 1-5 mM, and in one casezn where the diffusion coefficient dipped to cmz/s it proved convenient that the electroactive species concentration was 0.43 M. The development of microband, band array, and microhole array experiments may ameliorate this problem. (d) The physical rigidity and nonvolatility of the polymer electrolytes facilitate unusual cell designs; e.g., the polymer solution can be a thin film with a polymer/gas contact with consequent opportunities to rapidly add or remove volatile reagents and to design new kinds of gas sensors.2g" (e) The slow physical diffusivities of electroactive solutes enhance, in a relative sense, the role of electron self-exchange between the oxidized and reduced members of the electroactive solute in the electrode diffusion layer. This electron self-exchange chemistry can enhance the apparent diffusivityZdJof the redox solute by amounts related to the value of the self-exchangerate constant k,, analogous to effects known for polymer film coatings on electrodes contacted by fluid e l e c t r ~ l y t e s . ~Transport ~J~ measurements in situations where only physical diffusion is thought to be important give physical diffusion coefficients, Dphys,whereas diffusion coefficients measured when there is a potential (verifed or not) for electron self-exchange to enhance the transport are termed apparent values, DaFp. (0 Specific changes in electrochemical reactivity may be provoked by slow polymer dipole relaxations to which heterogeneous electron-transfer rates respond%in ways analogous to those known in fluid media solvent dynamics,17by altered ligand association-dissociation rates and by ion association processes. Voltammetry and diffusivity measurements presented below involve a range of relative diffusion layer thicknesses and microdisk radii r, T = 4D,,,t/r2. For potential steps and diffusion-controlled electrode reaction rate, radial,3 linear,ls and mixedlg radial-linear diffusion geometries prevail when 7 > 50, T < 0.02, and 50 > 7 > 0.02, respectively, requiring use of the equations ilim = 4nrFDC (1)
i = nFAl/iSC/fi
(2)
i = 4nrFDC(0.7854 + 0 . 8 8 6 2 ~ - + ~ /0.2146e-'".78237~"2 ~ I (3) For a microdisk with r = 5 pm, the steady currents of eq 1 will be observed a t long times or with slow potential scans when Dspp> cmz/s. At this electrode, when D,, < 10-lo cm2/s,eq 2 describes the chronoamperometric response, and potential sweep cyclic voltammograms have the familiar peaked current shape. Intermediate Dappvalues require use of eq 3. Temperature Dependence of Diffusion in Polymer Solvents. Using the appropriate diffusion theory as indicated above, example results of diffusivity measurements in the various poly(ether) solvents are summarized in Table I and in Figure 3. The values cited are apparent values (De,,,) and may contain some enhancement effects of electron self-exchange. However for reasons discussed below, electron-exchange enhancement of transport rates is not expected to be a numerically large factor. We will first discuss the general features of these data; the individual poly(ether)s are discussed in later sections. The temperature dependencies of electroactive solute diffusivitiea fall into two groups: poly(ether) solvents that display (Figure 3B) smooth but curved Arrhenius plots and those which display (Figure 3A) curved Arrhenius lines at higher temperatures and a sharp discontinuity leading to much slower diffusion below a certain temperature. In Figure 3A the discontinuities in Dappvalues occur a t temperatures that increase with the molecular weight of the poly(ether) solvent and which are approximately the same
ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992
1135
Table 1. Diffusion Coefficients of Electroactive Solutes in Acetonitrile and Eight Poly(ether) Solvents
redox probe
acetonitrile
PEO600000 2 x lo"
CpFeCpCHZN(CH,)+,
(RTP
CpFeCp CpFeCp-CO,H
8.1 X (68 "Oh
2.6 x
(RVh
1.4 X
(RTlh CpFeCp-C02-NaS
Me2PEG- Me2PEG- ME2PEG4006
l000b
1.4 x 10-7
1.7 x 10-7 (67 "C)'
(RVh 2.8 x 10-7
2000b
PPOc
MEEPd
PEUUe
network PEOf 1.1 x 10-7 (60 "C)'
(42 "Cy 1.1 x 104
3 x 10-7 (RT)t,' (RTlh 1.6 x 10-7 2.2 x 10-9 7.3 x 10-11 4.7 x 10-8 2.9 X 10" (68 "C) (RVh (RT)' (RT)' (RT) 6.9 x 10-7 2.9 x 10-7 2.1 x 10-7 1.5 x 10-7 (60"CY (65 "Qh (62 "CIh (63 "C)
3 x 10-9 6.3 X lo4 (65 "C)' (65 "C)' 8.5 X 10-l' 1.4 x 10-7 (65 "C)' (RTIh 3.9 x 10-9 (65 "C)' 3.8 X lo4
(RIYh
6.4 x 10-7
CplzFe [Os(phen),] 2+/3t
1.5 x 10-7 (65 oC)h 2.2 x 10-8
5.3 x
lo"
(RVh
3 x 10-7
(RT)gsh
(RVh
5.2
1.0 x 10-6
(RTlh
1.6 X
(RVh
1.7 X
2.6 X lo4 (35Ih
1.8 x 10-7
2 x 10-7
1.2 x 1o-g 4.2
(65 "C)'
(RT)s
(RVh
1.4 x 10-7 (65 oC)h 1.1x 10-6 5.9 x 10-7 2.4 x 10-7 (RT)S* (66 "C)' (RTIh 2.9 X
(65 "C)'
4 x 10-7
X
(65 oC)h
(RT)' 9.7 x 10-9 (61 "C)'
2.3 3.7 x 10-10 1.5 x 10-7 (60 "C)' (RT)' 1.1 x 104 1.6 X lo-' (60 "C)' (RT)'
X
10"
(RT)g* X
10"
(RT)g*
4.0 X 10" (65 "C)h 9.0 x 10-8
(RTY
3.0
X
lo4
(RVh
(RVh a PEO-600 000 is poly(ethy1ene oxide) (average MW = 600 000). * ME2PEG-400,ME2PEG-1000,ME2PEG-2000are poly(ethy1ene glycol) dimethyl ester (average MW = 400, 1000, 2000, respectively). cPPO is poly(propy1ene oxide) (average MW = 4000). dMEEP is poly(bis[(methoxyethoxy)ethoxy]phosphazine.2f e PEUU is segmented copolymer, poly(ether)-poly(urethane urea). 'Network PEO is a network cross-linked poly(ethy1ene oxide). #Bathed in nearly saturated acetonitrile vapor at room temperature; ca. 15% vol acetonitrile uptake." hMeasured by steady-state cyclic voltammetry using eq 2. 'Measured by fitting chronoamperometric data to eq 4. temperatures (1000,2000, and 600000 MW poly(ether), 32, 50, and 65 OC, respectively) at which endothermic features can be observed in differential scanning calorimetry for samples of these poly(ether)s containing the same quantities of lithium electrolyte. The discontinuity coincides with the formation or dissolution of ordered (e.g., crystalline) regions4 in the poly(ether), which impede molecular diffusion and ionic mobility. Diffusion and ionic transport are thought to occur principally in the amorphous polymer regi~n.~~'@' Little is known about the details of how the partial crystallinity impedes diffusion, and transport' phenomena are consequently more difficult to interpret in the partly crystalline phase. The Figure 3B data show no D,, discontinuities since none of these polymers exhibit crystallinity in the temperature intervals shown. That is, these are profiles of diffusion of the electroadiveprobe through entirely amorphous polymer solids. These Arrhenius plots are nonlinear, as are those in Figure 3A above the melt transition temperature T,. Nonlinear Arrhenius behavior is characteristic21of polymeric media, in which the temperature scale must be referenced against the glass transition temperature Tgof the polymer. Curved Arrhenius plots like those in Figure 3 can be analyzed with the semiempirical Williams-Landel-Ferry (WLF) equation21a which describes the temperature dependence of dielectric and mechanical relaxation in the context of free volume theory.21b This equation contains two constants ("universal parameters") that depend only on the chain fluctuations providing solute mobility and not on the chemical nature of the polymer, so similar non-Arrhenius behavior of different poly(ether)s is expected. The data in Table I and Figure 3 show that D,, values also vary with the molecular weight of the h e a r chain poly(ether) by large or small amounts, depending on whether comparison
is made a t temperatures below or above the melt value T,. Comparing Me2PEG-400,MezPEG-lOOO,and MezPEG-2000 above T, (see CpFeCpCOzH at 60 "C, Table I) reveals a variation in D, of ca. 4-fold; D,, decreases further in PPO-4OOO which may be a combined effect of structure and MW. The differences in D, are large at T < T, in Me2PEG-400,Me2PEG-1000,and MqPEG-2000, but this is an indirect effect on transport that occurs through changes in poly(ether) crystallinity with MW. That is, variations in molecular diffusivity, which reflect the dynamics of poly(ether) chain segmental mobility: are mildly dependent on MW and strongly dependent on temperature and poly(ether) phaseState. Among the other poly(ether)s, the data show that diffusion rates in network PEO are comparable to those in the linear Me2PEGs and substantially faster than those in MEEP and P E W . Even though physically rigid and cross-linked, network PEO is a favorable solid-state poly(ether) medium by combining advantages of an amorphous state with relatively long chain intervals between cross-link sites. Table I also gives examples of D,, values measured when small volatile organic molecules have been sorbed into the polymer phase (see PEO-600 OOO), causing diffusion-plasticization.2a,dvea,mThe diffusivity changes can be substantial and are rapidly reversible, disappearing when the volatile species desorbs, so they probably involve changes in the amorphous phase as opposed to changes in crystallinity. Plasticizationz2can be used to manipulate solute diffusivity and as a gas detectorzggmfor the volatile species, but its molecular details remain unclear.2" The disadvantage of the plasticization effect is that small molecule impurities (notably water) can severely affect quantitative measurements aimed at diffusivity intrinsic to the poly(ether1 unless proper at-
1136
ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992
[Co(bpy),)"
CpFeCpC0,H Me,PEG-lOOO
PEO-600,000
/
-6.0 1
E; U
i
/
-7.0 -6*5 -7.5
0 -8.0 0 -8.5
t
,
CpFeCpC0,H Me,PEG- 2 0 0 0
voltammetric oxidation revealed effects of TCNQ-/O self-exchange of only ca. 5-fold increase of Dapprelative to DPh, even though D,,, was made to vary (by temperature and electrolyte concentration in the network PEO solvent) by >l@-fold. The interpretation is that, when Dph, is made to become smaller and smaller, electron transfer occurs at distances (6) increasingly larger than collision-contact values (A). Then, 6 > A and according to the reaction rate vs distance relation k6 = k Aexp[-/3(6 - A)], the time scale of delivering electrons at long distance, even though slower than at the contact rate (kA),remains competitive with the time s d e of diffusion over the distance 6 - A. This was demonstratedz3by comparing the constant of the electron transfer
-10.5'".'
2.25
2.50
2.75
3.00
1000/T
3.25
3.50
to the summation of the time constants for diffusion of electron donor and acceptor together and for reaction at collision-contact
(K)
CpFeCpC0,H PPO-4000,
-6.0
\
CpFeCpC0,Network- , P E O
The TCNQ-IO data indicate that variation in the reaction distance 6 as D,, varies causes the time scales of eqs 5 and 6 to maintain roughly equal values irrespective of the value of Dphys.The principal point to be made about diffusivity measurements in polymer solvents, then, is that while electron 0 -as self-exchange may affect the measured Dapp,the resulting errors are in the range of factors of 2-3 difference between D,, and the true DphWConsidering the wide variability of Dapp,the electron self-exchange effect is thus of modest pro- 10.0 portion. - 10.5 In response to an interesting reviewer question, eq 4 applies 2.25 2.50 2.75 3.00 3.25 3 . 5 0 in the absence of electrical gradienta as might be present from large uncompensated resistance effects. These were absent 1000/T ( K ) in the present study. Electrical gradients would enhance the Figure 3. Arrhenlus plot of D, p. Panel A: ( 0 )ferrocenecarboxylic rate of electron transfers. acid in Me,PEGlOOO, 25 pm (dkmeter);(0)ferrocenecarboxylic acid in Me,PEG2000, 25 pm (diameter);(0)[Co(bpy),](PF,), in E O Characteristics of t h e Eight Poly(ether) Polymer 600000, 10 pm (dlameter). Panel B: (0) ferrocenecarboxylic acid Solvents. Voltammetry in Linear High Molecular Weight in PPO-4000, 25 pm (diameter), )(. sodium ferrocenecarboxylate in PEO. PEO-600000 was used in our earliest voltammetric network PEO, 10 pm (diameter),(A)[C~(bpy)~l(PF,), in MEEP, r = 5 experimentsza3 but it has proven to be a complex solvent pm; (A)ferrocenecarboxyllc acid In PEUU, 25 pm (diameter). All because of its multiphase nature. There are three phases in measurements were under inert gas. PEO-6000004JoJ"b at room temperature; (i) crystalline PEO, tention is given to maintaining anhydrous conditions. (ii)stoichiometric crystalline PEO/electrolyte complex, and Electron-Hopping Enhancement of Diffusion. The (iii)amorphous PEO. From DSC, NMR, and ac conductivity diffusion constants in Table I and Figure 3 are as noted above measurements the former two phases melt at =65 and >135 uncorrected for any enhancement effects arising from electron OC, respectively. Thus even above the 65 "C T,, the PEOself-exchangereactions in the mixed-valent diffusion layer 600 000/LiCF3S03system may not be completely amorphous; around the e l e c t r ~ d e . ~ ~ JThe ~ ~actual J ~ J ~D,,~ ~ ~values for the remaining crystalline polymer/electrolytecomplex phase the electroactive solutes in Figure 3 may be somewhat smaller depends on the (thought to have a 4 1 OLi than the Dappvalues cited, particularly when the solute's electrolyte concentration and can range%up to ca. 30%. The homogeneous electron self-exchange rate constant, k,,, is large presence of the salt-rich polymer/electrolyte complex also and ita physical diffusion rate (Dphys) is small. D- is enhanced lowers the electrolyte concentration in the amorphous regions by electron self-exchange according to the equation15 where ion transport and molecular diffusion occur. It is also likely that electroactive solutes are excluded from the crystalline regions, thus increasing their concentrations in the (4) amorphous ones. Background currents in PEO-600 000/LiCF3S03 are low where k6 is the exchange rate constant between electroactive between +1.0 and -1.2 V (va Ag wire, Figure 4E). The sharply donor and acceptor at reaction center-to-center distance 6 and rising cathodic background may represent reduction of acidic (total) concentration C. While eq 4 suggests that Dappmight, (hydroxy or water) impurities and/or oxygen; the background when k66'C is large, become orders-of-magnitudelarger than typically vary widely with polymer thermal history, Dph,, the literature relating to this s u b j e ~ t ~ indicates ~ ~ ~ ~ J ~ J currents ~ humidity, and residue of casting solution.24 effects of 10-fold and less and generally factors of 2 or 3. The [ C ~ ( b p y ) , ] ~ +voltammetry /~+ in PEO-600 000 shown A recent studyz3has suggested that electron transfer at a in Figure 5 (right side) illustrates the effects of temperature longer distance than the collision-contact distance (A) genin a poly(ether) that at room temperature is 70-90% cryserally assumed15J6may function as a self-limiting mechanism tallineZ0vaand offers little amorphous space for electroactive to prevent large differences in Dap, and Dph, from occurring. solute and electrolyte ion transport. The room-temperature Studies23 of TCNQ- diffusivity enhancement during its
-6.5 -7.0 -7.5 0 -8.0
A
U
--
ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992
A
1137
25OC
B 75°C
4 10.2 nA ----
-*,,
PI-
C 2.0
0.0
1.0
-2.0
E (Vvs.A;)' Flgure 4. Background cyclic voltammograms of (A) Me,FTG400/ LiCIO, (0:Li = 16:1), 25 OC, dry N, 10 mV/s, 25 p m (diameter): (B) Me2PEG1000/LiCi0, (0:Li = 16:1), 25 OC, dry N, 10 mV/s, 25 p m (diameter): (C) Me2PEG2000/LiCI0, (0:U = 161), 25 OC, dry N, 10 mV/s, 25 p m (diameter); (D) PPO-4OOO/LlCIO, (0:Li = 25:1), 25 OC, room air, 20 mV/s, 10 p m (diameter): (E) PE0-600,000/ucF3S0, (0:U = 15:1), 25 OC, exposed to acetonitrile-saturated N, vapor, 20 mV/s, 25 p m (diameter); (F) network PEO/LiCIO, (0:Li = 50:1), 25 OC, dry N, 50 mV/s, 10 p m (diameter): (G) MEEP/LiCF,SO, (0:Li = 41), 60 O C , dry N, 100 mV/s, 10 p m (diameter): (H) PEUU/LICIO, (0:Li = 55:1), 25 OC, dry N, 10 mV/s, 25 p m (diameter). E (VVS. Ag)
0
12.5 PA
4
30.6OC
43.1°C I
0.57
51.5OC
I12.5pA
57.0°C
0.80
7O.O0C 1.0 (xs E (V vs.Ag)
0
8O.O0C
10.05nA Figure 5. (Left) cyclic voltammograms for 49 mM [Os(phen),](PF,), in PEG600 OOO/UCF,SO, exposed to increasing partial pressures of acetonitrile vapor. (Right) cyclic voltammetry of 20 mM [Co(bpy),](PF,), in PE0-600000/UCF3S0, In dry N, 20 mV/s, 25 p m (diameter), T = 30-80 OC (0:Li = 16:l).
voltammetry correspondinglydisplays small currents and large resistance effects; voltammetry is generally difficult. The diffusion rate increases with temperature (Figure 5), and above the 65 "C melt temperature the voltammetry is better behaved. The left-hand side of Figure 5 illustrates diffusion-plasticization by sorbed acetonitrile of [ O ~ ( p h e n ) ~diffusivity ]~+
145°C
Figure 6. Photomicrographs of a PEO-600 OOO/LiCF3S03film on the surface of a microelectrode cell: region 1, PEO/electrotyte complex; region 2, microcrystalline E O ; WE, 25 p m (diameter) working electrode: REF, 0.35 mM Ag quasi-reference electrode; (A) 25 OC; (B) 75 OC; (C) 145 OC.
in PEO-600 000/LiCF3S03. These data are excerpted from earlier2"results and are presented here simply for comparison. The transport-enhancing effect of diffusion-plasticization can be comparable to that of temperature, as shown in Table I by comparing Dappin room-temperature acetonitrile-plasticized polymer with that in the PEO-600000 melt. The waveshape changes in Figure 5 (left) are simply a result of changes in electrode diffusion geometry caused by changes in the D,, values. We have used these enhancement effects for gas detection.2g*mThe rapidity and reversibility of the plasticization effect implies that this phenomenon involves solely the amorphous region since polymer recrystallization is typically a rather slow process. Another complexity of PEO-600 000/LiCF3S03, revealed (Figure 6) by microphotography of a film on a Figure 2 cell, is that its crystalline regions can have dimensions larger than that of the microelectrode, leading to uncontrolled variation in the polymer phase that contacts the electrode (the small shiny dot on the right side of the microphotograph). At room temperature (Figure 6A) there are clearly two distinct regions in the polymer. With increasing temperature, one are4 region 2, has melted by =70 OC (Figure 6B); the large spherulites in the other, region 1,began melting a t =135 "C which is shown in Figure 6C at 145 "C. By reference to the literature melting transitions,loJ*b region 1is the PEO/electrolyte complex and region 2 the PEO crystalline region. The microdisk electrode is obviously partly blocked by a polymer/complex spherulite at temperatures below 135 "C, which would yield an artifically
1198
ANALYTICAL CHEMISTRY, VOL. 64, NO, 10, MAY 15, 1992
A
A "
3
10.1 n A
I
A
PAI
25
+B 0.5 n A I
rc
1.0
0.5
0.5
0
Flgwe 7. Cyclic voltammetry of 5 mM ferrocenecarboxylic acid in (A) Me,PEG2000/LiCI04, (6) Me,PEGl OOO/LiC104, and (C) Me,PEG 4OO/LICiO,. Condltlons (0:LI = 16:l): at 25 O C in dry N,, 10 mV/s, 25 pm (diameter).
small D,, value. Since in another instance the microelectrode might be remote from a spherulite, producing larger Dapp values, Dapptemperature profiles for PEO-600 OOO like that in Figure 3A have a generally reproducible shape but are poorly reproducible in absolute values of Dapv Cooling the polymer in Figure 6C to room temperature gave no optical sign of recrystallization even after more than 24 h. Slow recrystallization is well-known in the polymer literature, and in chromatography experiments, PEO-600 000 electrolyte solutions can remain homogeneous for days after heating above the second melting transition.2g Diffusivity measurements in PEO-600 OOO are more reproducible when preceded by a thermal cycling, in the metastable amorphous state. Voltammetry in Linear Low Molecular Weight PEO: MepEG-400, Me8EG-1000, Me8EG-2000. Lower molecular weight linear poly(ether)sare lesa crystalline and lower-melting than PEO-600000/LiCF3S03and are simpler to use. Methyl end-capped poly(ethy1eneglycols) are available in a variety of molecular weights; used here are MW = 400 (a highly viscous liquid) and 1000 and 2000 (soft waxes melting at T , = 32 and 50 OC, respectively, to highly viscous fluids). These polymers exhibit reasonably small background currents (Figure 4A-C) and respectable potential limits, between +1.5 and ca-2.0 V (vs Ag). At higher temperatures, the background currents are larger and the potential window is smaller. D,, in these three poly(ether)s spans a range of 2000 (Table I) in the case of ferrocenecarboxylic acid. As illustrated in Figure 7,diffusion in Me2PEG-400is fast enough to establish radial diffusion-controlled steady-state voltammetry whereas that in Me2PEG-2000and Me2PEG1000is so slow that linear diffusion prevails. In the melt states, on the other hand, diffusion coefficients of CpFeCpC02H vary only by 3.5-fold from Me2PEG-400to Me2PEG2000,still in order of M W but with much less substantial differences. These diffusivity features are more reproducible than those in PEG800 OOO, so these materials are attractive as polymer solvents. Voltammetry in Low Molecular Weight Poly(propy1ene oxide): PPO-4OOO. Commercially available PPO-4OOO (Figure 1)is a racemic mixture, and the associated steric factors inhibit chain folding and crystallinity; this polymer is therefore a completely amorphous viscous liquid a t room temperature. The pendant methyl group also sterically hinders cation ~olvation,4.'~ which depresses ionic conductivity. PPO-400 becomes more viscous with added electrolyte26and/or electroactive solute. The attractive amorphous characteristic of PPO-4000 is unfortunately tempered by a relatively narrow potential window, ca. +1.0 to -0.75 V vs Ag for PPO/LiClO, (0:Li =
0
-0.5
E (VvsAg)
I
I
Flgure 8. Cyclic voltammetry of 10 mM ferrocenecarboxylicacid in ppO/LICI04 (0:Li = 25:l) at (A) 25 O C In dry N,, (6) 25 O C plasticized In acetonttrilesaturated N, bathing vapor, (C) 60 O C In dry N,. Conditions: 50 mV/s, 25 pm (diameter). E(VvsAg)
1io
?
Oi 5
1.0 1
E (V vs. Ag) 0.5 1
9
U
Flgure 8. Cyclic voltammetry in MEEP/LCF,SO, (OLi = 4:l) of (A) 100 mM [OS(phen),](PF&at 35 O C and (6) 65 mM [Co(bpy),](PF6), at 100 O C , In dry N,, 50 mV/s, 10 pm (diameter).
25) (Figure 4D). The source of the background currents is unknown, and a variety of purification attempts did not improve the situation. Voltammetry within the limits of Figure 3D is however well-behaved. Room-temperaturevoltammetry of ferrocenecarboxylicacid in PPO-4000 (Figure 8A) gives a diffusion coefficient (Table I) intermediate between MepEG-400, and Me2PEG-1000and MePEG-2000, but at 63 "C, above T,,, for the latter materials, the D,, for CpFeCpC02H increases smoothly with MW in the series Me2PEG-400> Me2PEG-1000> Me2PEG-2000> PPO-4OOO, reflecting the combination of both the changing molecular weight and the varying degree of crystallinity of the various polymers. We have little data on diffusion-plasticization effects in entirely amorphous polymer melts, but experiments in PP0-4000 show it does occur. Currents in room-temperature voltammetry of CpFeCpC02H in acetonitrile vapor-bathed PPO-4OOO are enhanced (Figure 8B) compared to dry PP0-4OoO (Figure 8A). The diffusivity enhancement is however less than that observed in the substantially crystalline PEO600000 (Figure 5A). Voltammetry in the Comb Polymer MEEP. Another structural tactic that breaks up polymer crystallinity is pendant poly(ether) side chains on another polymer backbone. This method has been utilized6 in poly(bis[(methoxyethoxy)ethoxy]phosphazine) where short-chain methyl end-capped poly(ether)s are attached to the poly(ph0sphazine) backbone (MEEP, Figure 1). MEEP derives its physical rigidity from entanglement of the poly(phosphazene) backbone, while the pendant ethoxymethoxy chains can form ionically conductive6polymemalt electrolyte complexes. The short pendant poly(ether) chains inhibit crystallization, so this polymer is also entirely amorphous. Examples of solid-state voltammetry in MEEP are shown in Figure 9 for [Os-
ANALYTICAL CHEMISTRY, VOL. 64, NO. 10, MAY 15, 1992 .A. O.88nAI
'O
E ( V05 vsAg)
0
6
h&== I
I
1.0
0
E ( V v s Ag) Flgure 10. Cyclic voltammetry of (A) 21 mM [Os(vpy),(bpy),](PF,), in PEW/ucIo, (OU = 55:l) at 25 "C exposed to s a m t e d acetonbile bathing vapor, 20 mVls, r = 12.5 pm, (B) 5 mM CpF&pCH,N(CH,),+ dissolved in network PEO/LiCIO, (0:Li 35 "C. 5 mV/s.
= 50:1), 25 p m (diameter) at
hen)^]^+/^+ at 35 "C and [Co(l~py)~]~+/~+ at 100 "C, and some diffusion data from a more detailed study" are given in Figure 3B and Table I. The effect of temperature on the electrode diffusion geometry is again evident (Figure 9A vs B) in the voltammetry. The polymer offers a generous potential window, ca. +2.0 to -2.0 V vs Ag (Figure 4G), and well-behaved voltammetry. Diffusion rates of electroactive solutes prove to be rather slow (Figure 3B) compared to the other poly(ether)s. Voltammetry in the Block Copolymer PEUU. The block or segmented copolymer, poly(ethy1ene oxide)-poly(urethane urea), PEUU (Figure l ) , is a thermoplastic elastomer composed of two distinct phases, with distinct boundaries! The general copolymer structure is (A-B), where A is the poly(urethane urea) (hard segment) and B is a poly(ether) (soft segment). At room temperature, the soft segment is above its Tg, giving an elastomeric phase, whereas the hard segment is significantly below its Tg, giving a glassy or semicrystalline p h a ~ e The . ~ ~hard ~ ~ block domains reinforce the copolymer. In the materials used here, the hard segment is 24% (by weight), and the soft segment, poly(ethy1ene oxide, average MW = 3000) is 76%, a compition providing physical rigidity and what is thought6sZ7to be a relatively continuous, amorphous PEO phase. It has been shown6Va that electrolytes like LiC104partition into and form complexes with the soft segmenta and support ionic conductivity there. The background voltammetry (Figure 4H) of dry PEUU/LiC104 (0:Li = 551) reveals appreciable currents but a reasonable potential window of +1.4 to -1.1 V vs Ag. This polymer is a potential model for studying transport in microphase segregated polymers, which is an important technological topic. Currents for electroactive solutes in PEUU at room temperature are quite small and diffusion in this polymer solvent in the dry state is slow (Table I). Diffusion rates in PEUU are modestly enhanced by sorption of acetonitrile vapor, resulting in readily measurable currents, as shown in Figure 10A for [Os(vpy),(bpy)z]z+oxidation. Voltammetry in the Cross-Linked Polymer Network PEO. Covalently cross-linked "network" P E 0 7 is, at room temperature, less than 30% crystalline in the pure state and becomes totally amorphous with the addition of e l e ~ t r o l y t e .Physi~~ d y , network PEO is a rubbery elastomer that, while insoluble in almost all common solvents, is strongly swollen by many. Like other poly(ether)s, network PEO is hygroscopic and readily sorbs small amounts of water. The positive potential limit (Figure 4F) reaches +2.0 V vs Ag, with stringent efforts to exclude water. The negative potential limit lies beyond -2.0 V vs Ag. Like the MezPEG's and MEEP, network PEO has very attractive background current characteristics.
1190
Figure 10B illustrates cyclic voltammetry (of CpFeCpCHzN(CH3)3+)in dry network PEO at 35 "C; the slightly peak shaped voltammogram indicates mixed linear radial diffusion. The Dappdiffusion rates are strongly dependent on temperature and electrolyte concentration, and network PEO has proved to be a powerful medium for exerting major changes in Dappof electroactive solutes without involvement of phase changes or partial crystallinity. We have for example effectedz3 a >lo5-fold variation in D,, of the radical-anion TCNQ- (Dpb is determined from ita TCNQ-/-z voltammetry). The absence of a phase change made possibleB a detailed analysis of how long-distance electron exchange affecta the Dappobserved for TCNQ- in ita TCNQ-IO reaction, for which the second term in eq 4 proves to be significant, as discussed above.
ACKNOWLEDGMEN" This research was supported in part by grants from the National Science Foundation and the Department of Energy (DE-FG05-87ER13675). M.W. acknowledges a sabbatical leave from Sophia University.
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RECEIVED for review December 2,1991. Accepted February 21, 1992.
Quantitation of Acridinium Esters Using Electrogenerated Chemiluminescence and Flow Injection Janet S. Littigt and Timothy A. Nieman* Department of Chemistry, University of Illinois, 1209 West California Street, Urbana, Illinois 61801
Acrklkrlun esters are used as chem#unlnescence (CL) labels
In hmunoasmy. Acrklkrkwn ester CL Is tradtthally triggered by addltlon of a solutlon of H202. Thls paper Is concerned wlth the genoratlon of the reaction-lnltlatlng specles In Sttu (electrochemically) to ellmlnate problems associated wlth clolutlon addttlon. Phenyl acrldlnlum9-carboxylate shows no electrochemldry over the range -1.0 to 4-1.0 V. I n the presence of dlswlved oxygen, as the applled potentlal Is stepped negatlvely, electrogenerated chemllumlnescence (ECL) emklon lntenrlty Increases to a plateau reglon correspodng to the peroxkk plateau of eM-1 oxygen reducth. ECL emlssh lntenrlty Increases as pH Increases from 9 to 12, but decreases at hlgher pH. The rate of form a t h of n o n c ~ l h l n e s c e npeeudobase i between sample h)ectbnand CL observation was 05Udled; at pH 12, that dday tlme should be llmlted to under 0.5 8. The worklng curve dynamlc range covers 4 decades In concentratlon. The detectlon llmlt for acrldlnlum ester-labeled lydne Is 10 fmol.
INTRODUCTION Recent interest in acridinium ester chemiluminescence (CL) has focused on ita application to imm~noassay.'-~Use of
* To whom correspondence should be addressed.
'Current address: T h e Procter & Gamble Co., Miami Valley Laboratories, P.O. Box 398707,Cincinnati, OH 45239-8707. 0003-2700/92/0364-1140$03.00/0
acridinium esters as CL labels for antibodies and other molecules seeks to exploit the characteristicallylow detection limits and wide dynamic ranges associated with the reaction. The CL reaction of acridinium esters is typically initiated by the addition of alkaline hydrogen peroxide. The hydrogen peroxide dissociates to form hydroperoxyl anion (H02-)which attacks the acridinium ring structure. The attack resulta in multiple bond cleavage to produce N-methylacridone in the excited state? Relaxation of the acridone to the ground state is via emission of light at 430 nm.2 Use of CL reactions in assay of clinical importance has been accomplished in a variety of confiiations. Initial work with CL immunoassay used static systems.' Recently, Wilson and co-workers have demonstrated that flow injection analysis (FIA) used in association with CL immunoassays offers advantages of greater speed and reproducibility. In our laboratory, we have been investigating analytical advantages which arise from electrogeneration of CL emission from several systems. This work has involved CL systems traditionally triggered electrochemically (Ru(bpy),2+)6as well as electrogeneration of CL emission from systems where CL emission has been traditionally triggered by chemical addition (lumin01).'8 CL immunoassay has a requirement that some reagent must be added to initiate the detection reaction (as opposed to radioactive or fluorescent tags where no such reaction is involved). This solution addition must be made in a reproducible way to minimize between-run variation, and the reagent volume added must be small to minimize the effects of dilution. Our research with acridinium esters is concerned 0 1992 American Chemical Society
