J . Phys. Chem. 1987, 91, 6315-6321 dependence of Dz(m) is estimated by using the self-diffusion coefficients of water as obtained by N M R spin-echo experiments and the self-diffusion coefficients of the magnesium ions as obtained by tracer diffusion experiments in +MgCl2solutions. Reinterpretation of former N M R results leads to the conclusion that in the magnesium hydration phase both the 2H and "0 interaction constants are ca. 25% smaller than the corresponding values obtained in pure water, but do not change with the MgClZ concentration (1 and 4 m ) and the temperature between -10 and 53 'C. Incorporating these details into the two-phase model yields the correlation times T ~ and + ro+ in the hydration phase as a function of the MgC12 concentration. It is concluded that the hydration water reorients anisotropically. The measure of an) , with the isotropy, as depicted in the ratio ( T ~ + / T ~ + changes MgC12 concentration; e.g., the ratio increases from 0.76 ( m 0) to 1.18 (6.44 m ) . The dynamical behavior of the hydration water is interpreted in terms of a diffusion tensor which incorporates the isotropic overall diffusion of the Mg(H20)a2+units and an internal diffusion within the units along the cation-oxygen axis. The two kind of diffusions are characterized respectively by the correlation times TO,(m)and Ti(m). In concordance with the results of neutron diffraction experiments in NiC12 solutions the orientation of the cation-oxygen axis is confined to the bisectrk plane of the water molecule. The tilting angle B(m) between this -+
6315
axis and the axis bisecting the water plane is taken to be variable. In analogy to the concentration dependence of rb(m),the overall correlation time T,,(m) is related to DM,(m),the self-diffusion coefficients of the magnesium ions, according to ~ o V ( m ) / ~ o , ( 0 ) = DMsg(0)/DMg(m). From the hydrodynamic analyses of the viscosity (a) and the self-diffusion (D2)data it is concluded that the radius of the magnesium hydration units is 3.4 f 0.1 A. Hence it is estimated that at 25 'C the overall correlation time in infinitely diluted MgCI, solutions is 36 f 4 ps. In terms of the dynamical model described above it is concluded that the tilting angle /3 is concentration independent. The average value is (0) = 28.1 f 0.7'. This result contrasts with the neutron diffraction results in NiC12 solutions where, up to ca. 1.5 m, /3 increases with the concentration from 0' to 42' and remains constant (42') above 1.5 m NiC12. The internal correlation time q ( m ) does hardly change (2.8 f 0.2 ps.) up to ca. 5 m and increases to 4.0 ps for m = 6.44 m MgC12. Acknowledgment. This work has been carried out under the auspices of the Netherlands Organization for the Advancement of Pure Science (Z.W.O.). We thank Professor Dick Bedeaux for constructive contributions to this study. Registry No. MgC12, 7786-30-3; H20, 7732-18-5; Mg, 7439-95-4; Mg(H20)2+, 19592-06-4.
Electrochemistry of Polyurethanes Containing Tetracyanoquinodimethane Units in the Polymer Backbone Cecil V. Francis: Pal JOO,~ and James Q.Chambers* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996 (Received: May 5. 1987 In Final Form: June 24, 1987)
Polyurethanes, derived from the comonomers, methylenebis(4-phenylene isocyanate), 1,4-toluenediyl diisocyanate, and 1,6-hexamethylene diisocyanate, have been synthesized that contain the tetracyanoquinodimethane group in the polymer backbone. The electrochemistry of these materials has been examined in N,N-dimethylacetamide solution and as thin films in contact with either aqueous or nonaqueous electrolyte solutions. In solution successive electron-transfer steps are observed for the formation of noninteracting radical anion and dianion sites on the polymer chain. As thin films on conducting substrates the behavior varies from partial to full electroactivity depending on the solvent and the polymer structure. In contact with 0.1 M LiCl/H20, films of the aromatic polyurethanes are only partially r e d u d to the radical anion state on the voltammetric time scale, while full electroactivity is seen in acetone solvent. For all the materials, charge transport through the films via the anion/dianion oxidation states is a sensitive function of the solvent, swelling, and flexibility of the polymer chain backbone.
Charge transport through thin films of electroactive polymers, swollen with solvent/electrolyte, and through electrically conducting polymers has been a subject of much interest in recent years.' In the former case charge transport is generally depicted as occurring by a series of electron-exchange reactions between redox sites accompanied by migration of electrolyte to maintain electroneutrality. Many examples of redox polymers that exhibit this behavior have been reported since the initial work on tetrathiafulvalene-substituted polystyrene,z poly(~inylferrocene),~ and p~ly(nitrostyrene).~ It is generally recognized that solvent swelling plays a key role in the charge transport process for electrodes modified with films of redox polymersS5 This has repeatedly been noted by researchers studying electroactive polymer quinone films.&* The solvent provides a fluid medium for the electrolyte migration and for alignment of the redox sites necessary for rapid electron exchange. Counter to this effect is the need for a high concentration of +Presentaddress: Corporate Research Laboratory/3M, P.O. Box 33221, St. Paul, MN 55133-3221 Present address: Department of Chemistry, Kossuth Lajos University, Debrecen, Hungary. f
exchange sites as indicated in theoretical treatments that predict a strong dependence of the overall conductance of redox polymers on the polymer volume f r a ~ t i o n . ~Also, solution pH will dramatically alter the extent of electroactivity of quinone polymer films.s*7910911 It is likely that these pH effects are due to the complex nature of the quinone/hydroquinone redox reactions that are required to transport charge through the swollen polymer films. (1) (2) (3) (4) 3223. (5) (6)
Murray, R. W. Annu. Rev.Mater. Sci. 1984, 14, 145. Kaufman, F. B.; Engler, E. M. J . Am. Chem. SOC.1979, 101, 547. Merz, A.; Bard, A. J. J. Am. Chem. SOC.1978, 100, 3222. Miller, L. L.; Van De Mark, M. R. J . Am. Chem. SOC.1978, 100,
Degrand, C.; Miller, L. L. J . Electroanal. Chem. 1982, 132, 163. Fukui, M.; Kitani, A.; Degrand, C.; Miller, L. L. J. Am. Chem. SOC. 1982, 104, 28. (7) Funt, B. L.; Hoang, P. M. J. Electroanal. Chem. 1983, 154, 229. (8) Joo, P.; Chambers, J. Q.J . Electrochem. SOC.1985, 132, 1345. (9) De Gennes, P. G. Physica 1986, 138A, 206. (10) Miller, L. L.; Zinger, B.; Degrand, C. J . Electroanal. Chem. 1984, 178, 87. (1 1) Inzelt, G.; Chambers, J. Q.; Kinstle, J. F.; Day, R. W. J. Am. Chem. S o t . 1984, 106, 3396.
0022-3654/87/2091-6315$01.50/0 0 1987 American Chemical Society
6316
The Journal of Physical Chemistry, Vol. 91,No. 24, 1987
For films of the tetracyanoquinodimethane adipate polymer that we have studied extensively,l’ the pH dependence of the charge transport process was correlated with pK values of the reduced redox sites. There are few systematic studies that address the role of the polymer structure, aside from the nature of the redox sites, on the charge-transportprocess. As shown by the behavior of viologen electron relay sites in Nation films,l* in clay modified ele~trodes,’~ and as part of a polymer backbone in a redox p ~ l y m e r , ~the ~.’~ environment of the electroactive group and its mobility at the modified interface will often dictate the electrochemical response. In the present work, we have varied the polymer interface at the molecular level by the synthesis of polyesters, polyurethanes, and copolymers containing TCNQ redox sites incorporated into a polymer chain of variable flexibility. The synthetic methods are based on the work of Day, who first synthesized polymers containing neutral TCNQ units.1s-’6 Here we report the synthesis --[--TCNQ--MDI--I,--
--[--TCNQ--TDI--],--
1
2
--[--TCNQ--HMDI--I,-3 and characterization of the polyurethanes 1-3 derived from the comonomers methylenebis(4-phenyl isocyanate) (MDI), 1,4toluene diisocyanate (TDI), and 1,dhexamethylene diisocyanate (HMDI). These specific diisocyanates, which are widely used in the synthesis of commercial polyurethanes and are readily available, were chosen for the variation of structural features they impart to polymers. The MDI comonomer is a known “stiffening agent” that tends to make polymers rigid, while HMDI will act more like a plasticizer to add flexibility to a polymer chain.”J8 The comonomer TDI will form polymers with properties intermediate between those of MDI and HMDI. As described below, there are interesting differences between the charge-transport process through thin films of 1-3 and 4, the TCNQ polymer of Day, under a variety of conditions.
- -[- -TCNQ- -adipate- -I,,- 4
Experimental Section Chemicals. Analytical grade LiCl, NaCl, KC1, RbC1, CsCI, NaClO,, NaOH, CaCl,, and BaCl, (all from Fisher) were used without further purification; NaH2P04, N a 2 H P 0 4 (Fisher), Et4NC104,Et4NCl, and Me4NCl (Eastman) were recrystallized and dried under vacuum prior to use. All solvents, including methanol, ethanol, 1-propanol, 1-butanol, acetonitrile (99+%, Aldrich), reagent grade acetone, and doubly distilled water, were purged with either argon or nitrogen before electrochemical experiments. Dimethylacetamide (DMA) and dimethyl sulfoxide (DMSO) were distilled under reduced pressure and stored over 3A molecular sieves. The diols, 1,4-bis(2’-hydro~yethoxy)benzene~~ 5, and 2,5-bis(2’-hydroxyethoxy)-7,7,8,8-tetracyanoq~inodimethane,~~ 6, were prepared by published procedures and recrystallized from acetonitrile. The diisocyanates (Kodak), methylenebis(4-phenylene isocyanate) (MDI), 1,Ctoluenediyl diisocyanate (TDI), and 1,6-hexamethylene diisocyanate (HMDI), were distilled under reduced pressure and stored in a refrigerator before use. Dibutyltin (12) Gaudiello, J. G.; Ghosh, P. K.; Bard, A. J. J . Am. Chem. SOC.1985, 107,3027. (13) White, J. R.; Bard, A. J. J . Electroanal. Chem. 1986, 197, 233. (14) Bookbinder, D. C.; Wrighton, M. S. J . Am. Chem. SOC.1980, 102, 5123. (15) Day, R. W. Ph.D Thesis, University of Tennessee, Knoxville, 1984. (16) Day, R. W.; Inzelt, G.; Kinstle, J. F.; Chambers, J. Q.J. Am. Chem. SOC.1982, 104, 6804. (17) Saunders, J. H.; Frisch, K. C. Polyurethanes: Chemistry and Technology, Part I; Interscience: New York, 1962. (18) Odian, G. Principles of Polymerization, 2nd ed.; Wiley-Interscience: New York, 1981; p 108. (19) Fujita, Y. Japanese Patents Nos. 7407146 and 7448938, 1976. (20) Hertler, W. R. J. Org. Chem. 1976, 41, 1412.
Francis et al. dilaurate, T-12, catalyst for formation of the urethane bond, was dried over 3A molecular sieves. Synthesis of Poly[bis(oxyethyleneoxybenzene)] urethanes. The following preparation of the MDI polymer is typical. In a dry 150-mL flask containing 2.504 g of MDI (0.01 mol) dissolved in 10 mL of DMSO was added 5 mL of a DMSO solution containing 0.06 mL of T-12 and 1.982 g (0.01 mol) of compound 5 in a dropwise fashion. After addition was complete, the solution was warmed to 70 OC for 2 h and then stirred at 30 OC for 17 h. The product was precipitated by pouring the solution into 80 mL of methanol to give 2.9 g (65% yield) of a white solid. The material was soluble in DMSO and DMA. The intrinsic viscosity in the former solvent was 0.12 dL/g. Gel permeation chromatography (p-Styragel columns) using DMA as the mobile phase at 60 OC indicated that products with molecular weights in range 10000 to 20000 were produced. The infrared spectrum of the solid (KBr pellet) contained strong bands for the urethane linkage at 1730 cm-I (-C(0)O-) and 3290 cm-’ (-NH-). Synthesis of Poly[2,5-bis(hydroxyethoxy)-7,7,8,8-tetracyanoquinodimethane-methy lenebis( 4-phenylene isocyanate)], 1. The procedure for the preparation of 1 was essentially identical with the above procedure for the synthesis of the polyurethanes derived from 5 except that DMSO solvent was replaced by DMA and 6 was used in place of 5. After precipitation in methanol and drying at 60 OC for 4 h, a blue-black, lustrous solid was obtained: yield, 60%; IR (KBr) 3300, 2970, 2440, 2220, 2180, 1725, 1605, 1560,1530,1510,1415,1310,1260,1220,1155,1120, 1080, 1015, 920, 820, 755, 515 cm-’; DSC TdWmp,207 OC. The UV-vis spectrum of 1 in acetonitrile featured bands at 412 and 433 nm; in DMA the bands are shifted to 450 and 510 nm. Spectra of reduced films of 1 in contact with 0.1 M LiCl displayed bands at 640 and 720 nm characteristic of the TCNQ2- dimer dianion and the TCNQ’- radical anion, respectively.21 ESCA spectra of reduced films of 1 contained prominent peaks with binding energies of 493 and 484.5 eV, which can be assigned to tin lines. Anal. Found: C, 63.0; N, 13.0; H, 5.7. Calcd (assuming 10% T-12): C, 64.1; N, 13.0; H, 4.6. Synthesis of Poly[2,5-bis(hydroxyethoxy)-7,7,8,8-tetracyanoquinodimethane-2,4-toluenediyldiisocyanate] , 2 . The reaction of 0.3244 g of 6 (0.001 mol) and 0.2105 g of TDI (0.0012 mol) in DMA in the absence of catalyst yielded a blue-black solid: yield, 5.6%; IR (KBr) 3340,2960, 2930, 2860, 2240, 2230, 2185, 1720, 1615, 1540, 1420, 1385, 1305, 1265, 1215, 1080,875,810, 210 620,485 cm-I; UV-vis (CH,CN) 413,432 nm; DSC Tdecomp, OC. Anal. Found: C, 60.0; N, 17.0; H, 3.7. Calcd (based on the --TCNQ--TDI-- repeat unit of 2): C, 60.0; N, 17.0; H, 3.7. Synthesis of Poly[2,5-bis(hydroxyethoxy)-7,7,8,8-tetracyanoquinodimethane-l,6-hexamethylenediisocyanate], 3 . To a rapidly stirred solution of 0.1890 g of HMDI (0.001 mol) in 2 mL of nitrobenzene under nitrogen was slowly added 0.3289 g of 6 (0.001 mol) dissolved in 8 mL of nitrobenzene. After the initial exotherm had subsided, the reaction mixture was heated to 115-120 OC for 2 h and then cooled to room temperature and filtered to give a black solid (yield, 21.2%). The filtrate was poured over methanol to precipitate another fraction (yield, 51.3%): IR (KBr) 3320, 2940, 2870, 2235, 2210, 2195, 1725, 1615, 1555, 1530, 1485,1390, 1345, 1310, 1265, 1225, 1150, 1040,930,840, 785,575 cm-I; UV-vis (CH,CN) 416,430 nm; DSC Tdecomp, 220 “ C . The fractions gave identical electrochemical responses (see below), although the product obtained from the initial fraction was less soluble in DMA. Elemental analysis indicated that the soluble fraction was low molecular weight material. Anal (soluble fraction) Found: C, 57.7; N, 16.0; H, 5.5. Calcd (for 3, n = 2): C, 57.9; N, 16.5; H, 5.2. Procedures. The polymer-modified electrodes were prepared by evaporation of a few drops of a DMA/acetone solution (1:2, V/V)containing ca. 0.1 mg/mL of 1-3 on the platinum electrode surface in a desiccator under vacuum. The electrodes were than baked at 110 “C for 2-3 min unless stated otherwise. The cyclic (21) Inzelt, G.; Day, R. W.; Kinstle, J. F.; Chambers, J. Q. J . Elecrroana/. Chem. 1984, 161, 147.
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6317
Polyurethanes Containing Tetracyanoquinodimethane voltammograms were obtained with a BAS 100 electrochemical analyzer. Generally a Ag/AgCl reference electrode was used. The potential of this electrode was +0.05 V vs a commercial SCE in 0.1 M LiCl. Other electrochemical procedures have been described elsewhere.21j22ESCA spectra were obtained with a PHI Model 5100 ESCA spectrometer, ESR spectra with a Varian ES109 spectrometer, visible spectra with a Cary 171 spectrometer, and GPC chromatograms with a Waters Model GPCl liquid chromatograph equipped with a R I detector.
Results and Discussion Synthesis of TCNQ Polyurethanes. The synthesis of polyurethanes via the solution polycondensation reaction of a dialcohol ~.~~ Lyman and a diisocyanate has been well s t ~ d i e d . ~ ' J Following and Sorenson, DMSO was chosen HO-R-OH
+ OCN-R'-NCO
-
--[R-OCONH-R'-NHC021n-
-
1
I
E/Volt
(1)
as the solvent for the preparation of polyurethanes from 1,4bis(2'-hydroxyethoxy)benzene by reaction with MDI, TDI, and HMDI. Viscosity and GPC data indicated that products with molecular weights greater than lo4 were obtained in these "practice" polymerizations. However, when these procedures were followed with the TCNQ diol, 2,5-bis(2'-hydroxyethoxy)-7,7',8,8'-tetracyanoquinodimethane20 (5) darkly colored solids were obtained that did not display the expected TCNQ electrochemistry or absorption bands at ca. 410 and 430 nm. Since reduction of the TCNQ units by the solvent (DMSO) was suspected, and indicated by absorption bands indicative of the radical anion and dianion in the reaction mixture, other solvents were used for the preparation of the TCNQ polyurethanes. The TCNQ-MDI polyurethane was prepared in DMA with dibutyltin dilaurate (T-12) as a catalyst and low temperatures (60 "C) to minimize side reactions. Under these conditions a product was obtained that displayed a prominent = 412 and 432 nm and the T C N Q elecabsorption with A, trochemistry. The material as used in these studies contained a significant amount of the catalyst, ca 5-lo%, as indicated by the elemental analyses and ESCA spectra of the reduced polymer films. Similar procedures using the T-12 catalyst and TDI or HMDI monomers gave blueblack or black metallic looking products that, while displaying the expected TCNQ electrochemistry, exhibited complex visible absorption spectra. Accordingly, the TDI and H M D I T C N Q polymers were prepared by performing the condensation reaction in the absence of catalyst. For TDI the solvent was DMA, and for H M D I nitrobenzene (at 120 "C) was empl~yed.~~?~~ All of the TCNQ-containing materials prepared by these procedures displayed absorption bands at ca. 410 and 430 nm in saturated acetonitrile solutions and were weakly paramagnetic in the solid state presumably due to residual radical anion sites. Typically a single line ESR spectrum with a peak width of 10 G and a gvalue of 2.0030 f 0.0005 was obtained on either a powder sample or a film cast from DMA/acetone solution. The polyurethanes prepared by these procedures have polymeric properties, i.e., viscosity and GPC behavior, very similar to the TCNQ adipate material synthesized by Day.26 As such they are probably low-molecular-weight linear polymers, or oligomers, with molecular weights in the range of 10000 f 5000 K. Polyurethanes 1-3 showed a major decomposition endotherm starting at temperatures above 200 "C assigned to an "uncapping" of the urethane bond.27 (22) Inzelt, G.; Day, R. W.; Kinstle, J. F.; Chambers, J. Q.J . Phys. Chem. 1983,87,4592. (23) Backus, J. K. In Polymerization Processes; Schildkneckt, C. E., Ed.; Wiley-Interscience: New York, 1977; Chapter 17. (24) Lyman, D. J. J . Polym. Sci. 1960, 45, 49. (25) Beachell, H. C.; Peterson, J. C. J. Poly. Sci., Part AI 1969, 7, 2021. (26) Day, R. W.; Karimi, H.; Francis, C. V.; Kinstle, J. F.; Chambers, J. Q.J . Polym. Sci: Part A: Polym. Chem. 1986 24, 645. (27) Wicks, 2.W., Jr. Prog. Org. Coatings 1975, 3, 73.
v s Ag/AgCI
Figure 1. Cyclic voltammogram of 1in 0.1 M Et,NClO,/DMA; sweep rate, 30 mV/s; current axis, 1 pA/div. TABLE I: Cyclic Voltammetric Half-Wave Potentials and Peak Separations for Solutions of TCNQ Compounds in DMA, 0.1 M NEtdClOI E,,:/-,' Epla - Epic, E1/2-1/2-,a Ep2a- Ep2', compd V mV V mV lb 0.08 62 -0.34 171 2b 0.07 67 -0.35 1I1 3b 0.09 83 -0.38 257 TCNQ diolc 0.01 60 -0.52 65
'vs Ag/AgCl quasi-reference electrode. 'Sweep rate, 50 mV/s.
Sweep rate, 30 mV/s.
Solution Electrochemistry. All three T C N Q polyurethanes were soluble at the 0.1 mg/mL level in DMA and yielded cyclic voltammograms showing the expected successive reduction waves for the formation of radial anion and dianion sites (EE behavior) on the polymer chain. This behavior indicates that there is minimal interaction between the reduced anionic sites on a given polymer chain.28 The HMDI TCNQ polymer, 3, isolated in the insoluble fraction (see Experimental Section), was less soluble than the MDI and TDI polymers. A cyclic voltammogram of 1, which is typical of the three polyurethanes, is shown in Figure 1 and potential data are collected in Table I. The E l I 2values for the TCNQOI- waves for all three polymers are ca. 70 mV positive of the E l l 2 of the TCNQ diol and are close to the value for unsubstituted TCNQ. The positive shift from the TCNQ diol value may reflect an interaction with the diiiocyanate comonomer linking units. For all three polymers the second wave due to the TCNQ-I2couple is quasi-reversible exhibiting relatively large peak potential separations at slow sweep rates. Compound 3, the HMDI TCNQ polymer, also exhibits quasi-reversible behavior for the first wave. The most noticeable difference between the voltammetry of the TCNQ polymers and the diol is the attenuated peak current due to the much smaller diffusion coefficients of the polymers. This effect is most clearly manifested in the chronocoulometric Cottrell slopes. For the electrode reaction --[--TCNQ--I,--
+ ne--
--[--TCNQ---I,--
(2)
where it is assumed that all the acceptor sites are reduced, the diffusion coefficient can be estimated from the Cottrell slope, eq 3, using eq 4.28 In this equation A is the electrode area, C,, is S = dQ/dt1I2 = 2nFAD112C*/?r1/2
(3)
Dpolym112 = S(T'/~/~FAC,,)M,
(4)
the polymer concentration in g mL-', and M , is the formula weight of the polymer repeat unit. Others have used this approach to estimate the molecular weights of fully electroactive polym e r ~ . Chronmulometric ~ ~ , ~ ~ ~ ~ data and D values for the TCNQ (28) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J . Am. Chem. SOC.1978, 100, 4248.
Francis et al.
6318 The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 TABLE I 1 Chronocoulometry of TCNQ Polyurethanes in 0.1 M TEAP, DMA at 24 OC concn, Cottrell slope," diffn coeff, compd mg/mL c/s cm2/s 4.0 x 1 0 - 7 b 1 0.32 6.5 X 10" 3.2 X 10" 7.7 x 10-8c 2 0.27 3 ca. 0 . 1 ~ 1 x 10" 4 x 10-6' 6.4 x 10-5 4.1 X 10" 5 0.47
t
1
TA
1
-
"Average of three values, dQ/dt'/Z for E +0.30 -0.30 V vs Ag/ AgCI. bMW of polymer repeat unit = 600, 10% T-12 impurity assumed. CMWof polymer repeat unit = 500. dConcentrationestimated from peak current in saturated solution.
/ /
4
( v / m V s-')''
Figure 2. Variation of voltammetric peak current with sweep rate: (A) 7.9 mg of 6,(B) 17.9 mg of 1 in ca. 30 mL of 0.1 M Et,NCIO,/DMA; I p axis, 4 pA/div.
polymers in DMA are collected in Table 11. The D values for 1-3 are approximately an order of magnitude smaller than the value for the T C N Q diol. This is consistent with the expected In (diff coeff) = constant - a In (MW)
20 30 Tims/min
40
50
Figure 3. Cathodic peak current vs. break-in time for films of 1 in contact with 0.1 M LiCI/H,O; sweep rate, 30 mV/s; baking time: (A) 0, (B) 5, (C) 10,(D) 15 min.
./It
2
IO
(5)
dependence on the polymer molecular weight given by eq 5, where a can vary from 0.5 for a flexible polymer in a &solvent to 2 for rigid rods32or systems with extensive entanglement^.^^ The data of Table I1 and the GPC behavior of the poly[bis(oxyethyleneoxypheny1ene)lurethanes described above suggest that a is in the range for a rodlike polymer; however, strict application of eq 5 is not warranted since simple Mark-Houwink relations of this type are not obeyed for polymers with molecular weights less than ca. 50 K.34 Diffusion-controlled behavior in DMA solvent is also indicated for the T C N Q polyurethanes by the sweep rate dependence of the voltammetric peak currents; see Figure 2. Peak currents were found to be proportional to the square root of the sweep rate as expected for a diffusion-controlled process. For equal concentrations (wt/vol) of polymer and monomer, the slope of the plot of peak current vs (sweep rate)'/* was significantly greater for the monomer than for the polymer in accord with the chronocoulometric results. The positive intercept on the peak current axis in Figure 2 may be due to concentration of a monolayer or less of adsorbed polymer on the electrode surface.3s Chronocoulometric Anson plots of Q vs. also gave positive intercepts on the Q axis after subtraction of the charge for a blank solution. (29) Funt, B. L.; Hsu, L. C.; Hoang, P. M.; Martenot, J. P. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 109. (30) Funt, B. L.; Hsu, L. C.; Hoang, P. M. J . Polym. Sci., Polym. Chem. Ed. 1981, 19, 203. (31) Smith, T. W.; Kuder, J. E.; Wychick, D. J . Polym. Sci., Po/ym. Chem. Ed. 1976, 14, 2433. (32) Sperling, L. H. Introduction to Physical Polymer Science; Wiley: New York, 1986; p 82. (33) De Gennes, P. G. J. Chem. Phys. 1971, 55, 572. (34) Billmeyer, F. W., Jr. Textbook of Polymer Science, 3rd ed.; WileyInterscience: New York, 1984; p 212. (35) Wopshall, R. H.; Shain, I . Anal. Chem. 1967, 39, 1514.
Effect of Polymer Film Baking. The electrochemical response of a multilayer adsorbed polymer film on an electrode substrate can be significantly different from that of the diffusing species in solution. If the film remains intact on the electrode, surface electrochemical waves corresponding to many equivalent monolayers of redox sites can be 0btained.l" Owing to the high concentration of redox sites in the polymer film, increased voltammetric currents can result if there is a mechanism for facile ion and electron transport through the polymer matrix. For the TCNQ polyurethanes of the present study, the stability and the persistence of films on the electrode substrate and the concomitant electrochemical response were markedly dependent on the conditions of the electrode preparation and pretreatment. After evaporation of the casting solvent and degassing in vacuum, the electrodes were usually subjected to a baking step. Unbaked electrodes dissolved in nonaqueous solvents after a few potential cycles through the TCNQ/TCNQ'- oxidation states. In contact with aqueous electrolytes, unbaked electrodes attained maximum peak currents after several potential cycles, typically less than five cycles for films of ordinary thicknesses. On the other hand, electrodes that had been oven-baked required longer cycling times to reach maximum electrochemical response. Figure 3 shows the effect of baking time on the voltammetric peak current for a film of 1 baked at 120 OC. The number of potential cycles required to attain the maximum response increases with both the temperature and time of baking. The break-in time in Figure 3 represents the total elapsed time after initiation of the potential sweep at 30 mV/s for a set of electrodes of the same film thickness. Thus approximately 100 cycles were required for the film baked at 120 O C for 15 min to reach a constant value of the peak current. In addition, the maximum peak current attained for a given film at a given sweep rate and the total charge under the voltammetric peak decreased with baking time. The above considerations demand that, for comparison purposes, the polymer film casting/baking procedure must be consistent in a series of experiments. Conditions close to those of curve B in Figure 3 were employed in the present study. The intensity of the residual ESR signal of a polyurethaneTCNQ film on a platinum electrode was also a function of baking time, decreasing approximately 50% after 25 min of baking at 120 OC. Similar behavior was observed for the TCNQ polyester prepared by Day. A possible explanation for the effect of baking on the polymer films is that a radical induced cross-linking reaction occurs in the condensed film. This is consistent with the observation that the solubility of the baked, and presumably cross-linked films, is much less than the solubility of unbaked films. It is further consistent with the decreased peak currents observed for the baked electrodes. It is also likely that baking facilitates the removal of casting solvent thus promoting interchain interactions and the formation of a more compact film. Electrochemistry of TCNQ Polyurethane Films in Contact with Aqueous Solutions. The electrochemical behavior of the TCNQ polymer films is dependent on the nature of the polymer, the solvent, and the electrolyte. In the following discussion, the MDI polyurethane, 1, will be compared to the polyester, 4, and then
Polyurethanes Containing Tetracyanoquinodimethane
r
I
I
A.
The Journal of Physical Chemistry, Vol. 91, No. 24, 1987 6319
r
1
I
t o
Figure 4. Cyclic voltammograms of films of 1 in contact with (A) 0.1 M LiCI, (B) 0.1 M Et,NCIO, aqueous solutions;sweep rate, 30 mV/s; current axis: (A) 5 pA/div, (B) 2.5 pA/div.
t
1
I
1 -0.2
-03
-04
/
I
-
E / V o l t vs S.C E
E / V o l t vs S.C. E.
-0 I
A
-05
-06
€ / V o l t v s Ag/AgCI
Figure 5. Change in absorbance at 735 nm for a film of 1 in contact with 0.1 M LiCl as a function of electrode potential; 0.01 absorbance unit/div.
differences between the TCNQ polyurethanes, 1-3, will be considered. In contact with 0.1 M MCl ( M = Li’, Na’, K’, Rb+, or Cs’) or 0.05 M MCI2 ( M = Ca2+or Ba2+) solution, films of 1 give rise to a single diffusion-controlled voltammetric wave that generally exhibits a pronounced “break-in” pattern (see Figure 4). For the alkali metal cation series, the peak potentials for films of 1 become more negative in the sequence Cs’ to Li+. The decrease is small, ca 60 mV, indicating only a slight dependence on the ionic radii of the cations. The voltammograms in the presence of 0.1 M RbCl and CsCl showed a split reduction wave indicative of possible dimer formation similar to that found for 4.8*2’,22However, most of the voltammetry in aqueous solution was performed in LiCl electrolyte solution where only a single diffusion-controlled peak voltammogram was observed. For the MDI polymer, there is no clear indication of a second wave corresponding to the TCNQ-12- process in the voltammograms at potentials more positive than ca -0.8 V. (While Figure 4 shows only currents at potentials greater than -0.5 V, more negative potential sweeps gave only monotonically increasing currents into the background region.) This is clearly shown by the spectroelectrochemicl experiment presented in Figure 5 where the absorbance due to the radical anion site at 735 nm2*is given as a function of potential. The absorbance values used for the construction of this plot were steady state and presumably equilibrium values obtained after moderately long (ca 5 min) electrolyses at each potential. The plateau reached in this plot indicates the lack of further reduction of the radical anion at negative potentials. It should be noted that, while the plot of log [(Abs,,, - Abs)/Abs] vs. E was linear, the slopes were greater than the Nernstian value of 60 mV/decade. This is an indication that different electron-exchange sites with different energies exist in the polymer film, perhaps of a dimeric nature.22 The plot of
Figure 6. Cyclic voltammograms of films of 3 (A) and 2 (B) in contact with 0.1 M LiCI/H,O; sweep rate, 30 mV/s; current axis: (A) 10 pA/div, (B) 2.5 pA/div.
the ESR signal intensity as a function of potential at a coated grid electrode also featured an S-shaped response with a slight maximum in a fashion similar to the ESR signal obtained from the TCNQ polyester of Day.22 However, in contrast to the polyester, 4, the ESR signal did not decrease on the cyclic voltammetric time scale at potentials in the region where formation of the dianion is expected to occur. The absence of a second wave in the voltammograms of films of 1 in contact with aqueous electrolyte solutions is a striking difference from the electrochemical behavior of 4. In dilute aqueous 1:l electrolyte solutions, films of the polyester are reduced to the dianion, which then decomposes and/or dissolves from the electrode surface.8 Thus cyclic voltammograms of 4 exhibited two waves on the first sweep segment to potentials more negative than -0.3 V and then greatly diminished peak currents on subsequent cycles. The indication is that the charge transport process through the film via the TCNQ-12- couple for the MDI polyurethane is much slower relative to the process via the TCNQo/couple. This difference in charge-transport rates is more pronounced for 1 than for 4. The two-electron, EE behavior of a TCNQ MDI polymer film that has been electrochemically cycled in contact with an aqueous solution can be immediately restored by transferring the modified electrode to a “good” nonaqueous solvent. For example, transfer to 0.1 M NaC104/acetone solution, where thin-layer behavior indicative of full electroactivity of the polymer film is observed (see below), permits the observation of the TCNQo/- and TCNQ-/2- waves in the first negative-going potential sweep. These experiments strongly suggest that the stiffness of the polymer backbone imparted by the MDI unit and the inability of water to swell the films of 1 play key roles in the charge-transport process. The “tightness” of the TCNQ-MDI films is further shown by their response in contact with aqueous 0.1 M Et4NC104 solutions (Figure 4B). Under these conditions neither films of 1 or of 2 yield cyclic voltammograms with discernible peak currents. This again is in marked contrast to the behavior of 4. Rinsing these films when they are in the neutral state and transferring them to 0.1 M LiCl solutions restores their characteristic electrochemical behavior seen in Figure 4A. It is interesting to compare the voltammetric behavior of films of the H M D I TCNQ polyurethane, 3, where the acceptor sites are connected by a flexible hexamethylene chain, to that of 1 and 2. Figure 6, A and B, shows cyclic voltammograms of a film of 2 and 3 in contact with 0.1 M LiCl. If the potential range is restricted to the region of the TCNQo/- couple, then an ordinary pattern is observed resulting in a stable steady-state voltammogram similar to the behavior of 1. However, when the potential sweep is then extended to the region where the TCNQ-/2- couple should be observed, an irreversible second wave is seen and on the return
Francis et al.
6320 The Journal of Physical Chemistry, Vol. 91, No. 24, 1987
t
I
I
I
S.C.E. Figure 8. Cyclic voltammogram of film of 1 in contact with 0.1 M NaC104/acetone;sweep rate, 30 mV/s; current axis, 5 pA/div. E/Volt
1 E / V o l t vs S C E Figure 7. Cyclic voltammograms of films of 1 (A) and 2 (B) in contact with 0.1 M Et,NCIO,/CH,CN; sweep rate, 30 mV/s; current axis: (A) 10 pA/div, (B) 5 fiA/div.
sweep there is indication of possible loss or destruction of TCNQ sites as evidenced by the decreased currents for the first wave. This behavior is similar to that of the polyester, 4, in dilute aqueous electrolytes.8~21~22 On subsequent cycles, as shown in Figure 6A, the first wave broadens and the second wave is severely diminished and drawn out in the cyclic voltammograms of 3. Interestingly, the second wave will reappear if the film is cycled only in the range of the first wave and then back to the range of the second wave, or if the film is allowed to remain in the solution in the neutral oxidation state for a few minutes. This result suggests that only a surface layer of a film of 3 is electroactive for the TCNQ-1’- couple and that a relaxation process is required to restore the film to an electroactive state after reduction. The voltammogram of the TDI TCNQ polyurethane, 2, shows behavior in a way intermediate between that of 1 and 3; see Figure 6B. For these films, in contact with 0.1 M LiCl solution there is an indication of a second wave in the voltammograms. Another contrast between the electrochemical behaviors of the TCNQ polyurethanes is that films of 3 give well-developed peak currents in the presence of aqueous solutions of Me4NC104,while neither 1 or 2 shows significant electroactivity under similar conditions. In this respect the behavior of 3 resembles that of the polyester 4. This is a further indication that the flexibility of the polyethylene chain in 3 and 4 plays an important role in the charge-transport process. The dependence on the supporting electrolyte suggests, but does not prove, that the chain flexibility permits segmental motion of the polymer chain which allows ion migration through the polymer film. It must be pointed out that the behavior of these TCNQ polymers in contact with aqueous electrolyte solutions is further complicated by a marked dependence on the concentration of the supporting e l e c t r ~ l y t e . At ~ ~ high concentration of electrolyte (e.g., 10 M LiC1) charge transport even through films of 4 is diminished and the second wave in the voltammograms is poorly developed. This has been attributed to changes in the polymer morphology that result at the high electrolyte concentration^.^^*^' Electrochemistry of TCNQ Polyurethane Films in Contact with Nonaqueous Solvents. In contact with nonaqueous solvents, films of 1-3 yield cyclic voltammograms that feature two reversible or quasi-reversible waves characteristic of the TCNQo/- and TCNQ-1’- couples. The behavior of the MDI and TDI polyurethanes in contact with acetonitrile/O. 1 M Et4NCI04shown in Figure 7 is typical of the voltammograms in contact with several solvents. When the baking procedure described above was followed, the films were stable and remained intact on the electrodes ~~~~~
~
(36) Inzelt, G.; Bacskai, J.; Chambers, J. Q.;Day, R. W. J. Elecfroanul. Chem. 1986, 201, 301. (37) Inzelt, G . ; Szabo, L. Elecfrochim. Acta 1986, 31, 1381.
vs
upon repeated cycling between the neutral, anion, and dianion oxidation states. For the acetonitrile solutions, multicycle break-in behavior was again observed for virgin film electrodes. Films of the polyester 4 in contact with acetonitrile, on the other hand, do not require multicycle break-in for full electroa~tivity.~~ As indicated by the “diffusion tails” in the peak voltammograms showing a dependence of the current on time-”’, thin-layer behavior is not observed for the conditions of Figure 7. Under similar conditions, films of 4 were fully electroactive and displayed nearly ideal surface electr~chemistry.~~ Furthermore, for films of 4 in contact with acetonitrile/Et4NC10,, the first wave was split into two “0.5-electron” waves as a result of interchain formation of the TCNQ2’- dimer radical anion. The absence of these features in the voltammetric behavior of 1 and 2 is another indication that chain stiffness imparted to the polymers by the MDI and TDI groups has an influence on the charge-transport process. Films of 3 gave two waves in CH,CN, but tended to dissolve from the electrode surface. It is of course reasonable that the charge-transport process is markedly dependent on the solvent and the resulting solvent swelling of the polymer film. Thus, in contact with acetone solutions, cyclic voltammograms of 1 exhibited nearly ideal surface electrochemical behavior indicative of full electroactivity of the material on the electrode surface (Figure 8). This was confirmed by experiments in which a measured amount of 1 was used to prepare the electrodes followed by coulometric integration of the peak voltammograms. For the electrode of Figure 8, approximately 100% of the 6.5 nmol of TCNQ sites (based on the repeat unit formula weight) was found to be electroactive assuming an n value of 1. Electrodes prepared with the same volume of the DMA/acetone stock solution were found to be 44%electroactive in aqueous 0.1 M LiCl before baking and 32% electroactive after a 5-min baking step. The voltammetry of films of the HMDI TCNQ polyurethane in contact with nonaqueous solvents had some features similar to the polyester voltammetry. Figure 9 displays cyclic voltammograms of thin films of 3 with roughly equal thicknesses in contact with alcohol/O.l M LiCl solutions. Half-wave potential and peak separation data are collected in Table 111. Nearly surface electrochemical behavior is seen in the methanol solution, while diffusion-controlled voltammograms are obtained in the higher molecular weight alcohols. (Polymer 3 was somewhat soluble in the alcohol solutions as well as in acetonitrile. This is presumably the cause of the decrease in peak currents observed in successive cycles of the voltammograms in Figure 9.) Interestingly, the first wave in the voltammograms of 3 in contact with MeOH/O.l M LiCl is split, indicating possible dimer formation. The cyclic voltammogram of the polyester, compound 4, obtained under similar conditions* is very similar to that of 3 in Figure 9A. In the higher molecular weight alcohol solvents slower chargetransport rates are observed. (38) Karimi, H.; Chambers, J. Q.J . Electround. Chew. 1987, 217, 313.
6321
E
Volt
vs
Ag/AgCI
Figure 9. Cyclic voltammogram of films of 3 in contact with 0.1 M LiCl in (A) MeOH, (B) EtOH, ( C ) n-PrOH, (D) n-BuOH; sweep rate, 30 mV/s; current axis, 5 bA/div. TABLE III: Cyclic Voltammetric Half-Wave Potentials and Peak Separations for TCNQ Polyurethane Films in Contact with 0.1 M LiCI/Alcohol Solutions compd solvent EIl2I AEpl ElIzZ AE; 1 MeOH -0.03 54 -0.27 30 EtOH 0.05 87 -0.25 77 n-PrOH 0.06 122 -0.28 122 n-BuOH 0.06 120 -0.28 112 3 MeOH 0.05' 30 -0.25 12 EtOH 0.18 123 -0.20 110 n-PrOH 0.13 139 -0.28 157 n-BuOH 0.12 132 -0.28 158 5c MeOH 0.04 64 -0.21 110 EtOH 0.13 72 -0.25 67 n-PrOH 0.13 59 -0.27 68 n-BuOH 0.14 71 -0.28 74 'Potentials vs Ag/AgCI; sweep rate, 30 mV/s. Solution electrochemistry, concentration = ca 1 mM.
'Split wave.
Conclusions The molecular structure of the polymer backbone in 1-4 influences the charge-transport rates through thin films of the respective polymers, and the resulting electrochemical behavior, in several ways. In contrast, the electrochemical response in solution is leveled by diffusion control of the electrode process, which results in similar voltammetry for 1-4 characteristic of the TCNQ units. The present study provides a good example of how sensitive the electrochemical behavior of a thin polymer film is to experimental factors such as solvent, electrolyte, and molecular structure of the polymer. Differences in solvation (solvent swelling) and flexibility of the polymer backbone play important roles in determining the
charge-transport rates through thin films of the TCNQ redox polymers of this study. These factors increase the rates and the accessibility of the TCNQ exchange sites in the polymer films. In aqueous solutions, decreased electroactivity and chargetransport rates are observed for 1-3. This is especially evident for charge transport via the TCNQ-1'- couple, which is nearly absent in the voltammograms of the TCNQ polyurethane modified electrodes obtained in contact with 0.1 M LiCl. Transfer of these electrodes to nonaqueous solvents, however, restores the EE electrochemical behavior of the TCNQ sites. The importance of polymer backbone flexibility is further indicated by comparisons of the electrochemistry of 3 and 4, which possess (CH,), chains between the TCNQ sites, to that of 1 and 2. In MeOH/O.l M LiCl or in the presence of Et4NC10,, the electrochemical behavior of the former polymers indicated the formation of TCNQ,'- dimer anion sites in the reduced films and exhibited surface waves characteristic of facile charge transport. In view of the demonstrated electrolyte effects that have been found for these materials, this is most readily understood in terms of a mechanism in which the polymer flexibility and solvation is necessary for ion migration in the film. Acknowledgment. This work was supported by a grant from the National Science Foundation (CHE-8219210) and by the University of Tennessee. We thank Drs. S. D. Alexandratos, J. F. Kinstle, J. D. Kovac, and G. Inzelt for helpful comments, Eve Frost for obtaining the DSC thermograms, and David Harkins for the ESCA spectra. Registry No. (6)(MDI)(copolymer), 110418-40-1; (6)(MDI)(SRU), 1 10418-44-5; (6)(TDI)(copolymer), 1 104 18-4 1-2; (6)(TDI)(SRU), 110486-02-7; (6) (HMDI) (copolymer), 110418-42-3; (6) (HMDI) (SRU), 110433-11-9.
