Effect of synthesis temperature on the structure, doping level and charge-transport properties of polypyrrole

Synthetic Metals, 52 (1992) 227-239

227

Effect of synthesis temperature on the structure, doping level and charge-transport properties of polypyrrole W e n b i n Liang, J u n t i n g Lei a n d Charles R. Martin* Department of Chemistry, Colorado State University, Fort Collins, CO 80523 (USA)

(Received March 17, 1992; accepted May 18, 1992)

Abstract Polypyrrole perchlorate was chemically synthesized at various temperatures and the resulting polymers were investigated by elemental analysis, Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and conductivity measurements. A correlation between conductivity, doping level and synthesis temperature was observed. The doping level of the polymer is higher when the synthesis is conducted at lower temperature. In addition, polypyrrole synthesized at lower temperature exhibits longer conjugation length, fewer structural defects, higher charge-carrier hopping frequency and higher conductivity. Furthermore, the results of this study indicate that the positive charges of doped polypyrrole are preferentially localized on the nitrogen atoms which are adjacent to the doping anions, as opposed to being uniformly delocalized along the polymer backbone.

Introduction P o l y p y r r o l e (PPY) is o n e o f t h e m o s t e x t e n s i v e l y s t u d i e d of t h e c o n d u c t i v e p o l y m e r s [ 1 - 3 ] . This p o l y m e r is o f i n t e r e s t b e c a u s e it e x h i b i t s b o t h high c o n d u c t i v i t y a n d g o o d e n v i r o n m e n t a l stability. F u r t h e r m o r e , s o m e n e w app l i c a t i o n s of this p o l y m e r , s u c h as in b i o s e n s o r s [4, 5] a n d m i c r o e l e c t r o n i c d e v i c e s [6, 7], h a v e r e c e n t l y b e e n e x p l o r e d . P o l y m e r s with well-defined c h e m i c a l , o p t i c a l a n d e l e c t r o n i c p r o p e r t i e s a r e e s s e n t i a l f o r m o s t of t h e s e a p p l i c a t i o n s . T h e p r o p e r t i e s o f the p o l y m e r , in turn, d e p e n d u p o n the synthetic c o n d i t i o n s e m p l o y e d , s u c h as r e a c t a n t c o n c e n t r a t i o n , r e a c t i o n t e m p e r a t u r e , r e a c t i o n time, p H o f t h e s y s t e m a n d n a t u r e o f i n c o r p o r a t e d c o u n t e r i o n s [8, 9]. U n f o r t u n a t e l y , the effects of t h e s e v a r i o u s s y n t h e t i c p a r a m e t e r s o n the chemical and electronic properties of the resulting polymer have been i n v e s t i g a t e d o n l y s p o r a d i c a l l y [ 8 - 1 1 ], h e n c e , t h e r e is a lack of s y s t e m a t i c c o r r e l a t i o n b e t w e e n the s y n t h e t i c c o n d i t i o n s u s e d a n d t h e p r o p e r t i e s o f the polymer obtained. W e h a v e r e c e n t l y initiated a series of i n v e s t i g a t i o n s a i m e d at s y s t e m a t i c a l l y e x p l o r i n g t h e effects o f s y n t h e t i c c o n d i t i o n s o n t h e s t r u c t u r e a n d p r o p e r t i e s of conducting heterocyclic polymers [10-12]. For example, we recently *Author to whom correspondence should be addressed.

0379-6779/92/$5.00

© 1992- Elsevier Sequoia. All rights reserved

228 investigated the effects of concentrations of the m o n o m e r and oxidant used in the polymerization on the chemical and electronic properties of the resulting p o l y mer [10]. We have since turned our attention to the effect of synthesis t e m p e r a t u r e on the properties of the resulting polymer. These studies have focused on chemically synthesized polypyrrole. We have explored the effects of synthesis t e m p e r a t u r e on doping level, conjugation length, nature and extent of chemical defect sites, and charge distribution in the resulting polymer. We r e p o r t he r e the results of these investigations.

Experimental Polypyrrole syntheses Pyrrole (Aldrich) was purified by fractional distillation and stored under argon at - 15 °C. Ferric perchlorate (Mallinkrodft) was used without further purification. Distilled water was further purified by passage through a Millipore milli-Q cartridge system. This water was used as the solvent for all solutions. Polypyrroles were synthesized by mixing solutions of pyrrole with solutions of ferric p er ch lor at e [ 1 0 - 1 2 ] . The m o n o m e r solution was 0.1 M pyrrole and the concentration of Fe(ClO4)a was 1.0 M. Both solutions were degassed twice by the f r e e z e - p u m p - t h a w technique and kept under argon prior to use. The syntheses were carried out in the absence of air at t em perat ures of 0, 27 and 45 °C. An e x c e s s of oxidant (molar ratio of Fe(C104)3:pyrrole = 5:1) was employed for all syntheses to ensure complete consumption of pyrrole m on o mer . In a typical experiment, 40 ml of the degassed pyrrole solution was transferred u n d e r argon into a 250 ml, two-necked round-bottomed flask equipped with an argon gas inlet. The solution was kept at the desired t e m p e r a t u r e using a constant t e m p e r a t u r e bath. To the pyrrole solution was added 20 ml of 1.0 M Fe(C104)a solution (also at the desired temperature) via a double-tipped syringe. The mixture was stirred magnetically and the polymerization was allowed to p r o c e e d for 2 h. The m o t h e r liquor was re m o v ed by v a c u u m filtration. The polypyrrole precipitate was washed (under argon) with copious a m ount s of degassed Millipore water and then dried u n der v a c u u m overnight.

X-ray photoelectron spectroscopy (XPS) XPS m e a s u r e m e n t s were p e r f o r m e d using a Hewlett Packard 5950 ESCA s p e c t r o m e t e r with m o n o c h r o m a t i z e d Al Ka radiation. The energy resolution was 0.9 eV. The s p e c t r o m e t e r was calibrated with graphite using the C ( l s ) line at 284.6 eV. The pressure inside the s p e c t r o m e t e r analyzer was maintained in the low 10 -9 Torr range by ion pumps. The polypyrrole samples were taken to the measuring c ha m ber directly without exposure to air. Samples were m o u n t e d onto c o p p e r sample holders using double-sided adhesive tape. The flood gun was not used during the XPS measurements on these doped polypyrroles since these samples are sufficiently conductive to prevent electrostatic charging.

229 Prior XPS investigations have shown that simple Gaussian-shaped peaks are never observed for polypyrroles [13, 14]. The complex XPS peaks were deconvolved into component Gaussian peaks using the 'automatic fitting' program provided with the XPS spectrometer [15]. Information about the chemical structure and composition of the polymer can be obtained from the positions and relative intensities of these component Gaussians [10]. The elemental compositions of the polypyrroles were determined from the areas under the total C(ls), N(ls), Cl(2p) and O(ls) peaks for the various polypyrrole samples. Instnunental sensitivity factors for these elements were first determined by conducting XPS analyses on Bu4NCI4, a reference compound with known stoichiometry [16]. Details of this XPS method for determining the chemical compositions can be found in refs. 10-12. We have shown that compositions obtained via the XPS method agree with compositions obtained via conventional elemental analysis [10, 11 ].

Infrared spectroscopy Infrared measurements were performed using an IBM IR/30 Fourier transform infrared spectrometer. Pressed pellets of the PPY powder samples were ground with KBr powder (Aldrich, IR grade) a n d u s e d for the IR data acquisition. The spectral resolution was set at 4 cm -1 and the signal-tonoise ratio was increased by averaging the signal over 256 scans for each sample.

Conductivity m e a s u r e m e n t s Room temperature conductivities were measured via the four-point method on pressed rectangular pellets (1 × 1 x 10 m m 3) of the various PPY powder samples. The pellets were pressed under a pressure of 6.5 t o n / c m 2 for 2 min. These samples were also used for variable temperature conductivity measurements. In this measurement, a PPY pellet was mounted onto a fourlead probe with a thermocouple in close proximity to the sample (c. 3 mm from sample). The probe was inserted in a long (30 cm) Schlenk tube and sealed under vacuum. The sample was cooled with methanol/liquid nitrogen to - 98 °C in a Dewar flask and allowed to warm up gradually (approximately 0.5 °C/min). The electrical resistance of the sample was monitored during this warming process using the four-point method. The sample was held under a dynamic vacuum of 10 -a Torr during data acquisition.

Results

and discussion

Chemical composition and doping level The results of the XPS determination of elemental composition [ 10-12] for the various PPY samples are shown in Table 1. The carbon and nitrogen stoichiometries are in good agreement with that expected for polypyrrole. The doping level p, expressed as the perchlorate content in the PPY samples,

230 TABLE 1 Chemical composition of polypyrrole perchlorate samples synthesized at various temperatures Synthesis temperature (°C)

Designation

Composition I

0 27 45

PPY0c PPYeTc PPY45c

C4.ooNo.95(C104)o.350o.48 C4.00N0.94(C104)0.270 0.43 C4.00N0.92(C104)0.2400.48

aAs determined via XPS. See text.

i i '~1

/, ~

i1 .

/J

!

7

"\

,.

I

I

I

404

4~) Binding Energy (eV)

396

Fig. 1. N(ls) XPS core-level spectrum of the polypyrrole perehlorate sample synthesized at 0 °C (PPY0c)increases by almost 50% from p = 0.24 at the highest synthesis temperature (45 °C) to p = 0.35 at the lowest temperature (0 °C). Thus, the data in Table 1 demonstrate that the doping level of polypyrrole can be varied over a moderate range by varying the polymerization temperature. We have recently proposed an alternative XPS method for determining the doping level in polypyrrole [10, 11]. Figure 1 shows a typical N(ls) spectrum for one of our polypyrrole samples. According to Kang et al. [13] and Ingan~is et al. [17], the N(ls) XPS spectrum can be deconvolved into three component Gaussians indicating that three distinct types of nitrogen exist in these polypyrrole samples. The high binding energy component at 401.5 eV has been assigned to positively charged pyrrolylium nitrogen cations [13, 18, 19]. Each positively charged nitrogen (N +) must be compensated by a negatively charged counterion in order to maintain charge neutrality in the polymer. If this is true, the component with a binding energy of 401.5 eV should be equivalent to the doping level of the polymer [10]. In order to test this premise, the ratio of the area under the Gaussian at 401.5 eV to that of the total N(ls) peak area gave the fraction of positively

231

charged N atoms designated the N+/N ratio in Table 2. This Table compares the N+/N ratio with the doping level as determined by the XPS-based elemental analyses [10, 11]. As can be seen in Table 2, the agreement between these two methods for determining the doping level is very good. We have observed analogous results in our previous investigations [10, 11]. The correlation between the fraction of nitrogens that are positively charged (401.5 eV component in Fig. 1) and the doping level has interesting and important implications for distribution of charge along the polymer chain. This correlation suggests that rather than being distributed uniformly across all of the molecular units in the chain, the positive charge is preferentially localized on the nitrogen atoms adjacent to the doping anions. The fact that several forms of nitrogen exist in PPY also supports this conclusion. Analogous conclusions have also been reached by Kang et al. [13] and by Eaves et at. [181. A final point is worth making about the elemental analysis data in Table 1. These data show that excess amounts of oxygen exist in all of the PPY samples. Excess oxygen has been widely observed in PPY samples prepared chemically and electrochemically [10, 13, 20, 21]. Various explanations for this excess oxygen have been proposed. For example, it has been suggested to arise from a surface oxidation reaction, such as formation of a chargetransfer complex with oxygen [21-23l. Since the samples investigated here were synthesized in the complete absence of air, the excess oxygen found here is not due to reaction of the polymer with molecular oxygen. Furthermore, the samples were transferred to the XPS chamber without exposure to oxygen and these samples were evacuated to a high vacuum (10 .9 Torr) prior to measurements. Again, this strongly suggests that the oxygen present in these samples is not from air. We will discuss the origins and nature of this excess oxygen in a later section of this paper. S t r u c t u r a l defects XPS is an effective tool for elucidating structural defects in polypyrrole and other conductive polymers [19 l- We have shown in the previous sections that the N ( l s ) core-level XPS spectrum is asymmetric and that this spectrum can be deconvolved into several components (Fig. 1). The C ( l s ) peak is also asymmetric and can likewise be deconvolved into its components TABLE 2 Comparison of the doping level with the N+/N ratio Sample

p~

N +/N ratio b

PPYoc PPY27c PPY45c

0.35 0.27 0.24

0.31 0.24 0.22

aDoping level as determined via XPS-based elemental analysis. See text. bRatio of the 401.5 eV component to the total area of the N ( l s ) peak. See text.

232

/..,/ S /

I 290

;,

\ "~. \

I 286

I 282

Binding Energy (eV~

Fig. 2. C(ls) XPS core-level spectrum of PPY0c. TABLE 3 Percentages of deprotonated nitrogens and carbonyl carbons as determined by XPS a Sample

> C= N -

> C= O

PPY0c PPY27c PPY45c

3.0 4.5 8.0

5.5 7.0 9.2

aDetermined by taking the ratio of the Gaussian at either 397.5 eV (N) or 288.0 eV (C) to the total peak area. See text.

(Fig. 2). T h e r e f o r e , Figs. 1 a n d 2 s h o w t h a t t h e r e are s t r u c t u r a l irregularities a n d d e f e c t s involving C a n d N a t o m s in t h e p o l y m e r . W e identify a n d q u a n t i f y a n u m b e r o f t h e s e c h e m i c a l d e f e c t sites below.

Deprotonated nitrogen T h e r e is a clearly r e s o l v e d low b i n d i n g e n e r g y s h o u l d e r at 3 9 7 . 5 eV in t h e N ( l s ) XPS s p e c t r u m (Fig. 1). This s h o u l d e r h a s b e e n a t t r i b u t e d to d e p r o t o n a t e d , u n c h a r g e d , imine-like n i t r o g e n s ( > C - - - - N - - ) [13, 14, 17). T h e p e r c e n t a g e o f N a t o m s p r e s e n t as this imine-like n i t r o g e n c a n b e o b t a i n e d b y taking the ratio o f t h e a r e a u n d e r t h e 3 9 7 . 5 eV G a u s s i a n t o the a r e a u n d e r the entire N ( l s ) p e a k . T h e s e d a t a a r e s h o w n in T a b l e 3. As i n d i c a t e d in the Table, t h e q u a n t i t y o f this d e f e c t site i n c r e a s e s w i t h s y n t h e s i s t e m p e r a t u r e . F u r t h e r m o r e , t h e q u a n t i t y o f this d e f e c t site is v e r y small (3% of t h e total N) in t h e m a t e r i a l s s y n t h e s i z e d a t 0 °C. T h e c o n c e n t r a t i o n o f this d e f e c t site is i m p o r t a n t b e c a u s e t h e s e d e f e c t s i n t e r r u p t c o n j u g a t i o n [141. T h e d a t a in T a b l e 3 s u g g e s t t h a t t h e p o l y m e r s y n t h e s i z e d at 0 °C s h o u l d b e m o r e c o n d u c t i v e t h a n t h e m a t e r i a l s s y n t h e s i z e d at h i g h e r t e m p e r a t u r e s . As we shall see, this is, indeed, t h e case.

233

Carbonyl Another type of structural defect in PPY is carbonyl [ 11, 2 4 - 2 6 ]. Carbonyls can be formed by over-oxidation of polypyrrole [24, 25] or as the product of chain termination by nucleophilic attack by H20 on the pyrrole rings [11, 26]; thus, carbonyl defects may exist at E-carbon positions in the middle of a chain or at the chain ends. In both cases it is obvious that the conjugation is disrupted at pyrrole rings which contain carbonyl defects. The infrared spectrum of the polypyrrole sample synthesized at the highest temperature (45 °C) shows a weak absorption at 1710 cm -~ (Fig. 3(A)); this band has been attributed to carbonyl [24, 27]. This band is completely absent in the spectrum for the material synthesized at 0 °C (Fig. 3(B)). These data suggest that the quantity of carbonyl defects incorporated into the polymer varies with synthesis temperature. This relationship between quantity of carbonyl defects and synthesis temperature is corroborated and quantified by the XPS data. A typical C ( l s ) XPS spectrum is shown in Fig. 2. As indicated in Fig. 2, the asymmetric C(ls) peak can be deconvolved into four component Gaussians. As discussed in detail in our previous papers [10-12], the predominant part of the highest binding energy component (288.0 eV) in

Ij i i

l 1800

1600

1400 :200 1000 Wavenumber (cm-l) Fig. 3. IR spectra of polypyrrole samples: (A) PPY4sc and (B) PPYoc-

234 the C(ls) spectra can be attributed to carbonyl carbons [28, 29]. Thus, by taking the ratio of the area under the 288.0 eV Gaussian to the total C(ls) area, an estimate [10, 11] of the quantity of carbonyl defects can be obtained (Table 3). As shown in the Table, the percentage of carbonyl defects increases from 5.5% for polypyrrole synthesized at 0 °C to 9.2% for PPY synthesized at 45 °C. Again, because carbonyl interrupts conjugation, these data (and the analogous IR data) suggest that the material synthesized at lower temperature should be more conductive.

Hydroxyl The data presented above clearly show that carbonyl groups are present in these polypyrrole chains. However, the concentrations of these defect sites (5-9% , Table 3) cannot account for the large quantities of extraneous oxygen seen in the elemental analysis data (Table 1). Hence, there must be another O-containing functionality in these polymers. We have recently carried out a series of infrared spectroelectrochemical investigations aimed at understanding the source and identity of this extraneous oxygen in polypyrrole [30, 31]. We have found that this excess oxygen exists predominantly as --OH groups that are covalently bonded to the pyrrole rings [30]. These - O H groups result from the reaction of the nascent PPY chains with water present in the solvent. In order to see the absorption bands associated with - O H in the infrared spectrum for polypyrrole, the polymer must be electrochemically undoped [30]. We accomplished this in our previous investigations [30] by electrochemically synthesizing ultrathin films of the polymer on an IR-transparent electrode. It would, however, be very difficult to accomplish this electrochemical reduction with the chemically synthesized polypyrrole powders investigated here. However, XPS can be used to confirm that --OH is present in these polymers. Furthermore, the XPS data can provide an estimate of the concentrations of these - O H defects. As indicated above the C(ls) XPS spectra were deconvolved into four component Gaussians (Fig. 2). In addition to the carbonyl carbons at 288.0 eV and the a and fl ring carbons at 283.8 and 284.9 eV [13, 15, 20, 21, 32], there is an additional component with a binding energy of 286.5 eV. This component can be attributed to a combination of carbon atoms which are adjacent to a positively charged N atom [18, 19, 33] and carbon atoms which are sigma bonded ot oxygen [14]. This 286.5 eV component accounts for about 20% of the total C(ls) area. Furthermore, the contribution from the carbons bonded to positively charged nitrogens can be accounted for via the N(ls) XPS data in Table 2. After this correction is made, we find that 5-10% of the carbon atoms in these samples are sigma bonded to oxygen. If the amount of the carbon atoms existing as carbonyl is also taken into account (Table 2), the combined amount of carbon atoms which are bonded to oxygen (as carbonyl or as sigma bonded) is about 10-15%. This estimate of excess oxygen (about 0.4 to 0.6 oxygen per ring) is in reasonable agreement with the elemental analysis data shown in Table 1. Therefore, in

235 agreement with our previous studies, these data indicate that the majority of the extraneous oxygen found in polypyrrole is present as covalently bound hydroxyl. The O(ls) XPS data corroborate these C ( l s ) spectral assignments and the assessment of the extraneous oxygen in polypyrrole. A typical O(ls) core-level XPS spectrum is shown in Fig. 4. As indicated in Fig. 4, the O(ls) XPS spectra can be deconvolved into two component Gaussians. However, the main component has a peak width at half-height (pwhh) of 1.9 eV. This is significantly wider than a typical Gaussian XPS peak. (Typically 1.3 to 1.5 eV, e.g., the O(ls) peak in Bu4NC104 has a pwhh of 1.4 eV.) This broad, main component at 531.5 eV can be assigned to an overlap of the carbonyl oxygen and the oxygen from the perchlorate doping anions [33, 34]. The minor component, with a binding energy of 534.0 eV, can be attributed to the oxygens sigma bonded to carbons (i.e. carbons bonded to - O H ) [14, 34]. The ratio of the area of this 534.0 eV component to the total O area is 0.13 for PPY45c, 0.15 for PPY2~c and 0.16 for PPY0c- These data translate to 4.5, 5.6 and 7.5% of the C atoms being sigma bonded to O. This estimate of the - O H content is in excellent agreement with the estimate obtained from the C(ls) spectra (see above).

Conjugation length Tian and Zerbi have recently used the 'effective conjugation coordinate' theory (ECCT) to calculate the vibrational spectra of pristine and doped PPY [35, 36]. They show that ECCT can be used to predict the effects of variation in the extent of delocalization along the polymer chain on the positions and intensities of the infrared bands in PPY. The ECCT analysis of PPY suggests that the relative intensities of the infrared bands at 1550 and 1470 cm -~ (highlighted in Fig. 3) are particularly sensitive to extent of delocalization. We have shown that by taking the ratio of the integrated absorption intensities of the bands at 1550 and 1470 cm-1 (Fig. 3), a qualitative measure of conjugation length can be obtained [10]. We call this ratio I~o/I~4?o; we

/ \

j t+

~

j I 536

\ I 532

L 528

B i n d i n g Ener~9" [eV~

Pig. 4. O(ls) XPS core-level spectrum of PPYoc-

236

have shown that the magnitude of 11s5o/1147o is inversely proportional to the conjugation length [10-12 ].This Vr-IR method for assessing the conjugation length in polypyrrole was applied to polymers synthesized here. I1sso/I147o ratios of 3.3, 4.9 and 7.0 were obtained for the samples synthesized at 0, 27 and 45 °C, respectively. These data show that the conjugation length is longer in the materials synthesized at lower temperatures. Yamaura et al. reached the identicalconclusion concerning the effectof synthesis temperature on conjugation length in electrochemically synthesized P P Y [37]. This conclusion is also supported by the various assessments of quantity of conjugationinterrupting defect sites (Table 3). The material synthesized at the lowest temperature has fewer of these defect sitesand thus has extended conjugation.

Conductivity R o o m - t e m p e r a t u r e conductivity We have d em ons t r at ed that polypyrroles synthesized at lower t e m p e r a t u r e s have higher doping level, fewer structural defects and longer conjugation lengths. This suggests that the materials synthesized at lower t em perat ure will be m o r e conductive. Room-temperature conductivity data are shown in Table 4. As expected, the material synthesized at the lowest t e m p e r a t u r e is significantly more conductive. This inverse relationship between synthesis t e m p er atu r e and conductivity is also observed in our 'template-synthesized' polypyrrole fibers [12] and in electrochemically synthesized polypyrrole film [37, 38]. Again, the data pr e s ent ed above help to explain this synthesis t e m p e r a t u r e - d e p e n d e n t conductivity.

Variable-temperature conductivity Polarons and bipolarons are believed to be the charge carriers in PPY [1-3, 12]. Polarons or bipolarons are localized charges on the PPY chain that constitute boundaries between segments corresponding to different molecular wave functions [39, 40]. The t e m p e r a t u r e d e p e n d e n c e of conductivity in such a system can be described by the variable-range hopping (VRH) model for which the conductivity ~r can be r e p r e s e n t e d by [ 3 9 - 4 1 ]

~r= tro T -1/2 e x p ( - To/T 1/4)

(1)

where O-o= 0.39e2Vo (N(E~)/akB) w2 and To = 1.66[a3/kaN(EF)] w4. Here, N(EF) is the density of states at the Fermi level, a -~ is the decay length of a TABLE 4 Charge transport data for polypyrrole samples Samples

Cr~r (S/cm)

~oa ( × 104)

To"

PPYoc PPY2rc PPY45c

22.3 5.9 4.8

2.0± 0.2 1.2 ± 0.1

16.5 + 0.4 20.0 ± 0.4

~Obtained from plotsof In(T1f2er)vs. T -I/4.

237

localized state, Vo is the hopping attempt frequency and kB and e are the Boltzmann constant and the charge on the electron, respectively. The density of states is closely related to the doping level of the sample and the decay length of a localized state is related to the conjugation length. Samples with longer conjugation lengths (fewer defects and higher degree of structural order) and higher doping level are expected to have larger qo and smaller To values. Therefore, an analysis of the temperature dependence of conductivity can provide useful information about the conduction mechanism, the density of charge carriers and the conjugation length of the polymer system. The temperature-dependent conductivity data for the samples synthesized at 0 and 27 °C are plotted as ln(T1/2o ") versus T -1/4 (eqn. (1)) in Fig. 5. Clearly, In(Tire(r) is linearly related to T - ,/4 (correlation coefficients >10.992). Thus, charge transport in both PPY0c and PPY27c is amenable with the 3D VRH conduction mechanism. The qo and To data obtained from these plots are presented in Table 4. As shown in Fig. 5 and Table 4, the To value for PPYoc is significantly smaller than To for PPY~Tc. The lower To value for the sample synthesized at lower temperature suggests that PPYoc possesses a higher density of states N(Er) and/or a longer conjugation length (a-1). Both the doping level data and the infrared assessment of conjugation length (see above) support this conclusion. PPYoc also shows a larger ao value than PPY2vc (Table 4). This larger ~ro value can be attributed to a higher hopping-attempt frequency in PPY0c, a direct result of the higher doping level in the sample.

6

E x

r-.5

4

3

0.24

i

0.25

Fig. 5. Plots of ln(Tlf2~) vs.

i

i

0.26 0.2~ T-I/4 (K- l/4) T -I/4

0.2

for PPYoc (@) and PPY27c (1=t)-

238

Conclusions P o l y p y r r o l e p e r c h l o r a t e s h a v e b e e n s y n t h e s i z e d u n d e r inert a t m o s p h e r e at v a r i o u s t e m p e r a t u r e s . T h e d o p i n g level o f t h e p o l y m e r is f o u n d to i n c r e a s e with d e c r e a s i n g s y n t h e s i s t e m p e r a t u r e . F'r-IR, XPS a n d v a r i a b l e - t e m p e r a t u r e c o n d u c t i v i t y d a t a indicate t h a t p o l y p y r r o l e s s y n t h e s i z e d at l o w e r t e m p e r a t u r e h a v e l o n g e r c o n j u g a t i o n l e n g t h as well a s f e w e r s t r u c t u r a l defects. T h e XPS d a t a s u g g e s t t h a t the p o s i t i v e c h a r g e s a r e n o t u n i f o r m l y distributed a l o n g t h e p o l y m e r chain. T h e s e s t u d i e s h a v e also s h o w n t h a t t h e e x c e s s o x y g e n f r e q u e n t l y o b s e r v e d in PPY is r e d u c e d o n l y slightly b y c a r r y i n g o u t t h e s y n t h e s i s in a n a e r o b i c a t m o s p h e r e ; t h u s t h e s o l v e n t (H20), a n d n o t O2, is s u g g e s t e d t o b e t h e m a i n s o u r c e o f this e x t r a n e o u s o x y g e n [30]. The e x t r a n e o u s o x y g e n is p r e s e n t p r e d o m i n a n t l y as c o v a l e n t l y b o u n d h y d r o x i d e [30]. It is i n t e r e s t i n g t o n o t e t h a t a h y d r o x i d e c o v a l e n t l y b o u n d t o a p y r r o l e ring is a n enol. This enol m i g h t b e e x p e c t e d t o t a u t o m e r i z e to t h e c o r r e s p o n d i n g k e t o n e . W e h a v e s u g g e s t e d t h a t this t a u t o m e r i z a t i o n is s u p p r e s s e d b e c a u s e f o r m a t i o n o f the k e t o n e i n t e r r u p t s c o n j u g a t i o n [30, 31]. Finally, t h e o b s e r v a t i o n t h a t p o l y m e r s s y n t h e s i z e d at h i g h e r t e m p e r a t u r e s c o n t a i n high c o n c e n t r a t i o n s o f d e f e c t sites is n o t surprising. T h e s e d e f e c t s i t e s are i n t r o d u c e d as a result of u n w a n t e d side r e a c t i o n s ; t h e s e r e a c t i o n s a r e essentially o c c u r r i n g in c o m p e t i t i o n with t h e d e s i r e d r e a c t i o n , a - - a c o u p l i n g o f oxidized m o n o m e r units to p r o d u c e a d e f e c t - f r e e p o l y m e r chain. At v e r y low t e m p e r a t u r e s the r a t e s o f t h e s e u n w a n t e d r e a c t i o n s b e c o m e m u c h s l o w e r relative to the rate of the d e s i r e d r e a c t i o n . As a result we p r o d u c e m o r e highly d e f e c t - f r e e p o l y m e r chains at l o w e r t e m p e r a t u r e .

Acknowledgement This w o r k w a s s u p p o r t e d b y the Office o f Naval R e s e a r c h a n d b y t h e Air F o r c e Office of Scientific R e s e a r c h .

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