5418
J. Phys. Chem. 1993,97,5418-5423
Mechanistic Studies on the Interactions between Poly(pyrro1e) and Organic Vapors John M. Charlesworth,' Ashton C. Partridge, and Neil Garrard Materials Research Laboratory-DSTO, PO Box 50, Ascot Vale 3032, Victoria, Australia Received: May 21, 1992; In Final Form: July 20, I992
Results relating to the application of conducting polymers as chemiresktors for the sensing of vapors are reported. In particular, we describe the use of a piezoelectric microbalance with gold electrodes on which we have electrochemically deposited a thin layer of poly(pyrro1e). Uptake of a selection of common neutral organic vapors, covering a range of physical properties, is then measured a t the same time as measurement of the resistance of a similar thickness of poly(pyrro1e) deposited on a narrow-gap resistance probe. Mass uptake and resistance change are considered together to avoid masking the real trends caused by differences in polymersolvent compatibilities. Analysis of the mass measurements shows that the BET adsorption isotherm is obeyed, and the number of bound layers is approximately four for methanol and water and eight for dichloromethane. The fractional change in resistance varies linearly with fractional mass uptake below about 5% mass change, and the number and type of adsorption sites are the same for the three solvents. Analysis of the specific resistance change, i.e. resistance divided by mass, for a range of solvents reveals a positive correlation with the dielectric constant of the solvent. This can be explained by treating the variable range electron hopping process responsible for the conductivity as an electron-transfer mechanism, for which the electrochemical theory predicts a relationship between the rate constant and the dielectric constant of the medium.
Much work needs to be done in order to explain the mechanism of interaction between conducting polymers and organic vapors The use of conducting polymers as chemiresistorsfor the sensing before attempting to tailor-make a polymer which is chemically of gases and vapors is an active area of interest. In the earliest specific. Furthermore, an important factor which has been reported work it was claimed that electron donating or accepting overlookedin virtually all of the conducting polymer chemiresistor species will decrease or increase, respectively, the conductivity of studies undertaken to date is the variation in vapor sorption by poly(pyrro1e)l and a variety of conducting polymers, including the polymer. It is accepted that molecules can penetrate into the poly(phenylene), poly(thiophene), poly(aniline), and poly(indole), bulk of a conducting polymer during exposure,Is yet in spite of have now been evaluated in the search for selective and sensitive this observation, there have been no data reported relating transducers for the detection of molecules in the vapor p h a ~ e . ~ - I ~ resistance change to the mass of organic vapor taken up by Amines, such as ammonia and hydrazine, have been shown to poly(pyrro1e). Clearly, in order to make comparisons for a given cause a reversible drop in the conductivity of some of these series of polymers and vapors and to developa generalmechanistic polymers at low vapor concentrationsand for short contact times. theory, it is essential that the mass uptake and resistance change Exposure to 1 atm of ammonia increases resistance by a factor are considered together to avoid masking any trends caused by of 30,1°and levels of ammonia down to 0.1 wg cm-3 can be differences in polymer-solvent compatibilities. detected;Iz however, there is a large (>lo3)and irreversible Piezoelectriccrystals have been applied with some succcss as resistance change if poly(pyrro1e) is kept in contact with high a sensitive means of following the interactions between polymers concentrations of ammonia, or ammonia with water vapor, for and vapors. These devices have been in use for around 25 years, extended periods of time (>4 days).lO Nitrogen dioxide, at and it is known that the vibrating crystal undergoes a change in concentrationsin the range 20-200 ppm, can readily be detected its natural oscillation frequency following sorption of molecules by an increase in conductivity, although a poisoning effect of this by a layer of polymeric material upon the crystal surface. molecule is also evident." These two gases represent examples Measuring limits around g for a 15-MHz crystal have been of species which are thought to either compensate the original estimated, and it has been shown that the frequency change is doping of thepolymer,aswithammoniainteractiqgwitha pdoped directly proportional to the mass of vapor sorbed.I6 This system, or enhance the pdoping in the case of nitrogen dioxide. frequency-mass relationshiphas enabled a variety of information Spectroscopicevidence has been presented which shows that in to be determined for those polymers which can be deposited as the UV/vis range there is a lowering of the bipolaron absorption a thin film. For example, using crystals coated with poly(isobuintensity and an increase in the 1c++ transition intensity,4J0 tylene), vapor sorption isotherms and other thermodynamic consistent with the hypothesis that during ammonia exposure the information have been measured for hydrocarbons and chlorinated polymer becomes less doped. methanes,17 and we have previously shownl* that the operation of epoxy-coated crystals in the gas phase at vapor concentrations For interactions with neutral molecules, such as alcohols, up to saturation point leads to a dual site adsorption process acetonitrile, chlorinated methanes, and water, the changes in which can be described by the BET equation.I9 electrical properties of poly(pyrro1e) are much less pronounced than for interactions with Lewis acids and bases.5s*s9J4 For In this work we describe the use of a piezoelectric microbalance example, exposure to saturated methanol vapor produces only a with gold electrodes on which we have electrochemicallydeposited 2% increase in the optical transmission of the polymerl4 and an a thin layer of poly(pyrro1e). Uptake of a selection of common approximate 15% changein resistance? The data from the optical neutral organic vapors, covering a range of properties, is then measured at the same time as measurement of the resistance of measurements suggest that partial electron transfer takes place from these organic vapors to the polymer, but this is not nearly a similar thickness of poly(pyrro1e) deposited on a narrow-gap as prominent as the transformation that occurs with ammonia. resistance probe. IntdUCtiOa
0022-3654f 93f 2097-5418$04.00 f0
0 1993 American Chemical Society
Interactions between Poly(pyrro1e) and Organic Vapors
Experimental Section Two-electrode resistance probes were constructed by sandwiching a Mylar film (25 pm thick) between two pieces of 0.1mm-thick p l a t i n ~ m All . ~ three componentswere bonded together with Epon 828 epoxy. Four-electrode resistance probes were constructed in a similar manner. Finally, the probes were encased in Epon 828 epoxy and polished with various grades of silicon carbide paper and diamond paste (to 1Mm) to exposethe platinum electrodes(exposed area 1mm2per electrode). Gold-platedquartz crystals were purchased from Daintree Industries, Melbourne. Prior to immersion in the polymerization solution, the wires making the electrical contact to the gold were painted with Fortolac stop-off lacquer to ensure no electrode reactions took place on these surfaces. Poly(pyrro1e) was deposited on the resistance probes and quartz crystals from a degassed solution 0.1 M in both pyrrole (Aldrich, freshly distilled) and sodium toluenesulfonate (Aldrich) at a constant current of 0.1 mA (galvanostaticmode of an Amel Model 5000 potentiostat/galvanostat). Typically, 10or 16 mC ofcharge was passed during the deposition of poly(pyrro1e) onto the twoelectroderesistance probes or quartz crystals, respectively. Some difficulty was encountered in depositing a film of the required resistance and stability across the inner two electrodes of the four-electrode probe. This was overcome by passing 12-16 mC of charge through each of the two outer electrodesprior to passing further current through all four electrodes connected in parallel. During the final step, the current was periodically interrupted, the resistance between the two inner electrodes measured, and the cycle repeated until the required resistance was obtained. After deposition, all devices were washed with copious quantities of distilled water and placed in a vacuum desiccatorfor a minimum of 3 days prior to use. Even when the same charge was passed through different probes, the final resistances varied over the range 10-500 s2 due to the slight variation in the distancebetween the platinum electrodes. Variations in sensitivity (AR/R) were also found for some electrodes prepared under apparently identical conditions; therefore, to ensure all data could be interrelated, methanol wasselected asan appropriatecalibrant. Thesensitivites of the probes were then standardized by measuring the response to methanol and multiplying by a normalizationfactor determined by the response of an arbitrarily selected electrode. Resistance measurements were performed using the constantcurrent device and a two-electrode probe of the type described by Bartlett et aL7 The output of this device was monitored by an DGH Corp. (PO Box 5638, Manchester, N H 03108) DllOO voltage-to-digital converter. During the four-electrode measurements,an Amel Model 55 l potentiostat/galvanostatwas used to apply a constant current of 1mA acrossthe two outer electrodes, and the potential difference between the two inner electrodes was measured using a DGH DllOO module. Detection of mass changes on the quartz crystals was effected using the circuit described by Bruckenstein and Shay,*othe output of which was connected to a DGH D1601 frequency-to-digital converter. The converters were connected to the serial ports of two independent IBM-compatible computers running programs written in Microsoft QuickBasic. The programs enabled the sampling of the response of the converters at a user-specified time interval and provided a display of the data on the monitor. The final data were saved in a disk file after the completion of each experiment. Further details regarding the experimental arrangement of the quartz microbalance are available in an earlier publication.21 A purposebuilt flow system,constructedusing glass and Teflon, was used to expose the poly(pyrro1e)-coated quartz crystal and resistance probe to various organic vapors. High-purity nitrogen carrier gas was passed through the organic liquid which was contained in a gas bubbler immersed in a temperature controlled water bath. The resulting vapor was then directed through a three-way tap that allowed the vapor to be passed Over the sensors
The Journal of Physical Chemistry, Vol. 97, No. 20, I993 5419 TABLE I: Specinc Resistance Changes for Poly(pyrrok)
Exposed to Sdvent Vapors
compound water
methanol ethanol acetone 1-propanol 1-butanol
2-methyl-1-propanol 2-butanol
2-methyl-2-propanol 2-propanol dichloromethane chloroform ethyl acetate diethyl ether benzene hexane
MARIRAhP 2.7 2.0 1.9 1.5 1.3 1.1 1.o 0.9 0.8 0.7
0.4 0.2 0.1 0.0 0.0 0.0
c
78.5 32.6 24.3 20.7 20.1 17.1 17.7
15.8 10.9
18.3 9.1 4.8 6.0 4.3 2.3 1.9
I,(eV) 12.6 10.8
10.5
9.7 10.1 10.0 NDb ND ND 10.2 11.4 11.4 10.1
p(D) 1.85
1.70 1.69 2.88
1.68
1.66 ND
ND
ND 1.66 1.60 1.01 1.78
9.6
1.15
9.2
0.00
10.2
0.00
MARIRAM = fractional change in resistance divided by fractional change in mass. ND = data not available.
-- IO.-"
Figure 1. Representative transient responses resulting from replicate cycling between (a) dry nitrogen and (b) dry nitrogen containingmethanol vapor at a concentration of 0.88 mg L-'at 22 OC. The bottom curve showsthe fractionalchangein massof a 0.2-pm-thick film ofpoly(pyrro1e) electropolymerized on the gold electrodes of a IO-MHz piezoelectric crystal, and the top curve shows the fractional change in conductivity of the same material electropolymerized in a layer approximately 1 pm thick on platinum electrodes in a four-point probe configuration.
or vented straight to the atmosphere. Finally, diluent gas (nitrogen) was introduced into the vapor stream after the tap but prior to the glass tube containing both sensors. Sensor responses were recorded at ambient temperature (22 f 2 "C). Appropriate gas-phaseconcentrationsof the various compounds were obtained by varying the temperature of the water bath and the flow rates, via calibrated Porter F150 flow meters, of the organic-vapor and diluent streams. Initially, the response of the sensors to various concentrations of the same vapor were obtained by measuring the response to one concentration and flushing the sensors with nitrogen, followed by recording the response to the next higher concentration. This method, however, leads to scatter in the experimental data, which was significantlyreduced by measuring the response to increasing concentrations without any intervening flusheswith nitrogen. A completelist of all theorganic compounds (analytical grade or better) used in this study is presented in Table I.
Results and Discussion The reproducibilityand responsefor repeated switchingbetween dry nitrogen and methanol vapor using a four-point probe and 10-MHz crystal are shown in Figure 1. These data indicate that resistance and mass equilibration occur over approximately the
5420 The Journal of Physical Chemistry, Vol. 97, No. 20, 1993
same period of time, although an allowance must be made for the differences in the thickness of the two films. The adsorptiondesorption process is highly reversible for this vapor, and poly(pyrro1e)-coatedcrystals and four-point probes could be used for several weeks without any significant loss in sensitivity. The use of two-point resistance probes over extended periods of time resulted in a slight but significant gradual increase in base line resistance which was thought to be due to a loss of adhesion to the electrode,causing an increase in surface resistance. The fourpoint probe resistance measuring method was therefore considered to offer more reliable and absolutevalues because any contribution from the contact resistance was avoidedSZ2 Examination of the surface of electrochemically grown poly(pyrrole) using electron microscopy has shown that a nodular or “cauliflower” structure is observed (for example, ref 8). This makes the modeling of diffusion behavior difficult becauseexisting theories rely on either a bounded planar or bounded spherical surface.23Nevertheless,the transient data for resistancechanges of poly(pyrro1e) during uptake of methanol have been interpreted in terms of some of these conventionalmodels, including a simple Langmuir site binding modeL8 As a first approximation it will be assumed that the layer of poly(pyrro1e) behaves as a uniform sheet, in which case the rate of uptake of mass during exposure to vapors is generally described by Fick’s equation for diffusionZ3 as follows
Charlesworth et al. I
I
1.00
-8 F
.75
v
+
5
.50
.25
0
0
10
20 t”2
30
($2)
Figure 2. Normalized mass uptake measurements for poly(pyrro1e) obtained by using a homologous series of solvents (R-OH,(1) R = Me, (2) R = Et, (3) R = n-Pr and, (4) R = n-Bu), and also the least-squaresfitted diffusion behavior using eq 1 with n = 100 (dashed lines).
M(t)/M(-) = n=m
1- (8/r2)x(1/(2n
+ 1)2)exp[-D(2n + 1)2?r2t/4L2](1)
n=O
where M ( t ) is the mass taken up by the film at time f , M ( m ) is the equilibrium mass uptake by the film, L is the film thickness, and D is the diffusion coefficient. The rapidly rising portion of the adsorption curves can be approximated by the following equation:24
M ( t ) / M ( - ) = 2(Dt/?rL2)”2
(2)
It has also been shown that for coated AT quartz crystals the film thickness is related to the frequency by the following expression2s
L = N,P,A.F,/PJY
0
.2
.4
.6
.0
PIP0
Figure 3. Plots of the fractional change in resistance of poly(pyrro1e) exposed to various partial pressures of water, methanol, and dichloromethane in dry nitrogen at 22 O C .
(3)
where Nq is the quartz crystal frequency constant (1.668 X 105 Hz cm), ps is the quartz density (2.69 g cm-j), p c is the density of the coating (1.2 g cm-3), and AFc is the frequency change due to the coating alone. Experimental adsorption results were analyzed by plotting fractional mass increase against tl12. Determination of D was made by least-squares fitting the rising part of the adsorption curve to eq 2, using the film thickness (0.2 pm) calculated from eq 3. Figure 2 shows typical normalized experimental mass uptake measurements obtained by using a homologous series of solvents (R-OH, R = Me, Et, n-Pr, and n-Bu) and also the predicted diffusion behavior using eq 1 with n = 100. The data fit the planar diffusion model well for methanol; however, the goodness of fit decreases as the molecular size of the alcohol increases. The calculated values of D for the above series of alcohols are 2.2 X 10-12,1.3 X 10-12,6.4X lW3,and2.4X 10-13cm2~-l,respectively. Attempts to use other models, including the simple Langmuir site binding model, gave a much poorer fit. Non-Fickian curves are frequently found for the adsorption of penetrants by polymers below their Tg.24Although poly(pyrro1e) is a relatively brittle material, its Tgis not well defined,*6and the present results suggest that the polymer either is relatively porous or has sufficient mobility to enable the chains to relax during the time scale for diffusion. The mass uptake at equilibrium is determined by the ther-
modynamic factors associated with the interactions between polymer and solvent vapor. A recent articleZ7provides a detailed literature review and a discussion on the interpretation of polymersolvent interactions and suggests that the most appropriate rationalization of solvation data is Kamlet’s comparison methodZs in which the magnitude of each specific interaction (H bond donor, acceptor, dipoldipole) is assessed by the assignment of a solvatochromicparameter. Values of these parameters for low molecular weight organic molecules are readily found in the literature; however, the unavailability of this information for poly(pyrro1e) renders the interpretation of the results in this investigation impossible. Figure 3 illustrates the results of measurements for the resistance sensor exposed to various concentrations of water, methanol, and dichloromethane at 22 OC. The fractional change in resistancevaries nonlinearly with partial pressure and does not reach a plateau value as has previously been reported.* The same general type of behavior is observed for the increase in mass as a function of solvent partial pressure, shown in Figure 4, except that the order of solvent sensitivity is not the same as for the resistance increase. Water and methanol do not swell poly(pyrrole) as effectivelyas dichloromethane,yet they both have a larger effect on the resistance of the polymer. Even though both types of sensors respond to a change in vapor pressure, the adsorption isotherms describing the processes may be different since not all the sites which bind solvent molecules
Interactions between Poly(pyrro1e) and Organic Vapors
The Journal of Physical Chemistry, Vol. 97,No. 20, 1993 5421
.4
.l
0
.2
0
.4
.6
.0
PlP, Figure 4. Plots of the fractional change in mass of poly(pyrro1e) exposed to various partial pressures of water, methanol, and dichloromethane in dry nitrogen at 22 O C . The solid lines represent the curves of best fit obtained by nonlinear regression analysis using the BET equation (eq 4). The fitting parameters are as follows: dichloromethane, a, = 9.1 X l e 2 , h = 12.6, n = 7.9; methanol, a, = 3.7 X h = 17.8, n = 4.1; water, a, = 1.4 X l e 2 , h = 7.2, n = 4.1.
LWM Figure 5. Plots of the fractional change in resistance against fractional change in mass for poly(pyrro1e) exposed to methanol and water vapor in dry nitrogen at 22 O C .
.04
will necessarily influencethe conductivity in an equivalent manner. For example, those molecules which are bound to the dopant (p-toluenesulfonate) may be expected to exert a different influence on the charge carrying capacity than those bound to the polymer backbone. There are several types of adsorption isotherms which have been used to describe the uptake of vapors by inorganic solidsrg and organic polymemZ4 The Langmuir isotherm has been used previously to explain the behavior of methanol interacting with poly(pyrrole).8 This model assumes that the molecules of the vapor are bound at discretepoints and each site holdsone molecule, to give a monomolecular layer at maximum solvent uptake. The adsorption of gaseousiodineby poly(thiophene) films and powders has been studied,l5and unusual behavior was reported, including a discontinuity in slope. The phenomenon was explained in terms of electromechanical instability caused by the formation of a Schottky diode at the film-substrate interface. The Brunauer-Emmett-Teller (BET) isotherm is generally the most flexible model since it is able to take into consideration the formation of adsorbed layers more than one molecule thick. The following expression has been derived for an adsorption equilibrium involving n successive layerslg
a,hP[ 1 - (n U'
+ 1 ) P + nP+']
(l-P)[l +(h-l)P-hP"+']
(4)
where u is the amount of substance adsorbed, a, is the amount adsorbed when all sites in a monolayer are occupied, P is p / p o , po is the saturated vapor pressure of the adsorbate, and h is approximately equal to exp[-(El - E,,)/RT]. E1 - E, is the differencein the heats of adsorption obtained when the first layer and the nth layer are adsorbed. In the case of a monolayer (i.e., n = 1) the equation reduces to the Langmuir isotherm. Nonlinear regression analysis of the mass uptake results using eq 4 leads to the curves shown in Figure 4. In all cases the goodness of fit is excellent, and n is calculated to be 4.1 for methanol and water and 7.9 for dichloromethane. Figure 5 shows a plot of fractional change in resistance against fractional mass uptake for methanol and water. These two solventschangethe resistance in a manner directly proportional to their mass adsorbed. It must therefore be assumed that because the number of adsorbed layers increases as the vapor pressure increases, each layer of molecules has the same influence on the resistance up to the maximum thickness of four molecules. The resistance versus
'
.03
5
5 .02
, '
00
.05
.x,
.E
20
25
N/M
Figure 6. Plots of the fractional change in resistance against fractional change in mass for poly(pyrro1e) exposed to dichloromethane vapor in dry nitrogen at 22 O C .
mass change measurements for dichloromethane adsorbed in poly(pyrro1e) are shown in Figure 6. This relationship is also linear up to a mass increase of approximately 5%, above which the rate of change of resistance with vapor adsorption is very much less. Since up to approximately eight molecules of this solvent can be adsorbed per binding site, it follows that somewhere beyond the first few layers the interaction of the sorbed molecules with the electron-carryingregion of the polymer is significantly reduced. Evidence that the binding sites for all three solvents are the same comes from an examinationof the parameter a,, defined in eq 4. Using these least-squares-derivedparameter values (see Figure 4), the calculated ratio of the masses of single layers of dichloromethane,methanol, and water is l.W.0.41:O.16. Provided that the same number and type of sites are involved, then this ratio should be the same as the ratio of molecular weights of the species, Le. 1.00:0.38:0.21,which is indeed approximately the case. The results for solvents covering a range of physical properties are listed in Table I. These measurementswere obtained at vapor concentrations in the linear region of the mass uptake versus resistance plots. A strong correlation is apparent between the dielectric constant of the compound and the specific resistance change, measured by the fractional change in resistance divided by the fractional change in mass. This ratio was used because the change in resistance is small when the vapor interacts with the polymer (typically
