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Differential anomalous scattering studies of amorphous hbr-doped polyaniline
ELSEVIER
Synthetic
Differential
Anomalous
Scattering
Metals
84 (1997)
Studies
725-728
of Amorphous
HBr-doped
M.J. Winokur” and B.R. Mattesb aDepartment of Physics, University of Wisconsin, Madison WI 53706, bLos Alamos National Laboratory, Los Alamos, NM 87545, USA
Polyaniline
USA
Abstract The local molecular structure ential anomalous scattering in Br-ion site along the polymer and situated, on average, 3.7A Keywords:
Amorphous
thin
of amorphous HBr-doped polyaniline has been probed using conventional and differconjunction with a single chain modelling analysis. Initial results identify a probable chain approximately 3.3A above (or below) the average polyaniline molecular plane from the closest neighboring amine nitrogen.
illms,
polyaniline
and derivatives,
1. Introduction The microscopic structural order of conducting polymers in general and especially within the polyanilines (PANI) exhib i t s a pronounced sensitivity to both the synthesis and processing. Although crystalline and semi-crystalline regions are common structural packings, it is often the structural order within amorphous regions which defines and limits the behavior for both the bulk charge and mass transport. Hence there is a strong impetus to develop techniques which can adequately elucidate the nature of the microscopic order in these amorphous areas. Even though amorphous materials (polymeric or otherwise) lack a clearly identifiable long range periodic order, there is a local structural assembly within the host matrix which produces scattering profiles having systematic intensity modulations and these data can be analyzed using radial (or pair) distribution function (RDF or PDF) analysis [1,2]. Typically this method requires that the entire wide-angle scattering spectrum be recorded, suitably reduced and then Fourier transformed to obtain a weighted sum of partial atom-atom pair correlation functions (PCF’s). For the emeraldine base (EB) form of PANI, composed of just C, N, and H, there are six (i.e., n(n + 1)/2 where n=3) partial pair After doping by halogen acids to form distributions. emeraldine salt (ES) this rises to ten specific terms. Further it is also necessary to differentiate between intrachain and interchain atom pairs. For both amorphous EB and ES samples the scattering [3,4] is always dominated by the well-defined geometry of the aniline monomer; especially at higher momentum transfer (or q). To a lesser extent, and 037967791971%17.00 PII SO379-5779(96)04119-7
0 1997
Ekevier
Science
S.A
All
rights
reserved
X-ray
scattering
at somewhat lower 4, the monomer-to-monomer intrachain and polymer-polymer interchain pair correlations can be resolved and it is possible to obtain semiquantitative agreement between model calculations and experimental data [4]. For EB the intrachain structure is consistent with the theorized three benzenoid to one quinoid alternation of the aniline monomer units along the PAN1 backbone. The interchain structure ordering only weakly resembles that of the reported crystalline phase. Continued improvements in both the experimental x-ray methodology and in the theoretical modelling have begun to yield fully quantitative treatments [5] comparable to neutron-based RDF studies [6] of conventional polymeric materials. PDF analysis of ES includes additional subtlety. For the relatively low-Z HF- and HCl-doped ES samples, the halogen-atom pairs effect only a relatively modest contribution to the x-ray scattering profiles so that detailed PAN1 polymer chain structural analysis is still possible. HBr- and HI-doped samples are more heavily dominated by the halogen ions so that the detailed chain structure is less easily discerned. However Br (as does I) has an absorption edge well-placed in the x-ray spectrum so that at energies close to its K-absorption edge there is a reduction in the real part of the Bratom scattering factor. Scanning at two different x-ray energies, just below the edge, enables the Br scattering factor to be altered so that a difference of the two coherent scattering factors may be scaled and transformed to yield a weighted sum of the four element speci6c pair pair correlation functions [7,8]. This paper presents an initial dii?erential anomalous scattering study of two HBr-doped PAN1 samples. Both films exhibit clear evidence for the presence of Br-
726
M.J. Winokur,
B.R. Mattes /Synthetic
PAN1 nearest neighbor pair distances averaging close to 3.3A. Comparisons with model calculation are also shown. Within a single Br-atom/PAN1 chain picture we identify at least one likely halogen anion site along the PAN1 chain. This site selection is consistent with recent results for both HF and HCl-doped ES [5]. With improved counting statistics and further refinements in the modelling analysis a far more detailed description of the local halogen structural environment is achievable. 2. Theory
The general form of total coherent scattering intensity from N atoms in a system is given by,
where q = y sin 0 is the scattered photon momentum transfer, r;j is i,j pair spacing and fi is the ith atom scattering factor. This expression can be rewritten in terms of a normalized intensity, jcoh, to yield a total structure function, H(q), given by
H(q) = voh - (f “Mf )” where
(2)
“) = Cy=“=,cif:(q) is the self-scattering term, represents the mean scatterer and (f) = CL1 cifdq) ci is the element specific fractional concentration, H(q) can be related to the total pair correlation function, G(r)) through G(r)
(f
=
1+ 1
M q H(q)
2++p0
sin(qr)dq
Jo
where po is the mean density. At two photon energies, En,,, and Efar, below the absorption edge, the specific atomic scattering factor, nominally f(q, E) = fo(q) - f’(E) + if”(E), becomes and fn = fo(q) - f; + f;‘. Ignorfn = fo(q) - f:, + if: ing the relatively small f” terms allows the difference structure function, AH(q), to be written as AH(q) = (f);+(q)
- (fMMQ)
2CBrrn(fA
- f;)
and this can be transformed to give, in this case, the weighted Br partial pair correlation function, AG(r), 1 O3 = 1+ ~ q AH(q) si4qr)& . (5) 2n2rp0 J o In addition to eqn. 4, it is also possible to express H(q) (and similarly AH(q)) directly in terms of a weighted sum of partial structure functions, eij (q), according to
Metals
84 (I 997) 725- 728
where Cij is an assumedisotropic root-mean-square deviation of the rij spacing and n;j is the relative number of i, j atom pairs of a prospective real space model. 3. Experimental
Details
A more comprehensive discussion of both the PAN1 film preparation and experimental methodology is given in ref. 4. All films studied were fully amorphous. The nascent EB films were immersed in 0.1 M, 1.0 M and 4.0 M aqueous HBr solutions for over 48 hours to give three ES samples. Refinement of the elastic/inelastic coherent scattering ratios at high-q yields the approximate Br- concentration, y=Br-/N, to be close to 0.34, 0.42, and 0.5 for these three samplesrespectively. Data was acquired at the X7A beamline at the National Synchrotron Light Source operated by Brookhaven National Laboratory. A monochromatic beam of x-rays (1 x 10 mm2 in cross-sectional area) was incident on tightly clamped multilayer stacks of the PAN1 films. Typical samples consisted of 5 to 25 stacked layers of the various PAN1 films. All samples were mounted in a He-filled canister using a symmetrical transmission geometry in combination with standard e-26’scans. Scattering data was acquired at 20 angles ranging from 5” to 130’ in 0.05 and O.lOA-1 steps using three different wavelengths, X=0.5020, 6.9214 (fA=7.0) and 0.9377 A (f;=3.6) where the latter two wavelengths are approximately 20 eV and 250 eV below the nominal 13.477 keV Br K-edge. The anomalous scattering factors were approximated using the CromerLiberman scheme [9]. The scattered photons were collimated using a pair of rectangular slits adjusted to give a 3x12 mm2 aperture at the energy dispersive detector orifice. Data collection times were extended somewhat at higher q to compensate for the reduction in the scattering intensity and the increasing proportion of Compton scattering. Standard corrections for beam intensity fluctuations, detector dead time, sample geometry, attenuation, partial polarization, background and incoherent Compton scattering [lo] have been effected. Normalization of the experimental data was accomplished by using the analytical expression of Krogh-Moe [ll]. The displayed g-space data has not been smoothed in any way so that the point-to-point fluctuations of these curves are representative of the statistical uncertainties.
AG(r)
H(q) =
(6)
~CCCiYf~(q)f~(q)eij(q) \JI
i
j
in which the double sum is over all elemental constituents and ejj (q) =
nij
exp(-.!Jfjq2)
Sin(Wij)
Vij
4. Results
and
Conclusions
Figure 1 displays the individual q-weighted structure functions (qSF’s) for an as-cast film (y=O) and the three ES films prepared. For the y=O.34 and 0.42 samples, both conventional (X=O.502OA) and anomalous scans were acquired. These spectra show subtle changes in the overall oscillatory positions and line shapesfor q 5 SA-i with increasing y. In particular the position the first two intensity maxima, originally near 1.6 and 3.5 A-i, shift to higher q. Significant changes in the shape
MJ.
Winokur,
B.R Manes/SsntheticlLietals
84 (199i’)
725-728
727
24 h=0.502,h
5
-
4
3 ?IE
2
Ir’ ly=O.O
1 I 2
Fig. 1. q-weighted
I 4
I 6
structure
I 8
I 10
functions
12
I 14
1 16
for the samples.
of the third LLpeak,” centered about 5.8A-I, are also quite distinct. These variations are indicative of changes in both the intrachain and interchain components of the various pair correlations. The changes with respect to varying wavelength at fixed y are hard to discern although there is, for instance, a systematic increase in the intensity of the qSF data at 5.5 A-r as f’ diminishes. Better statistics in the ES data, especially at the higher q range, would improve these comparisons. The respective G(r)% are shown in Fig. 2. For distances below 3 A the spectra are heavily dominated by the aniline monomer atom pair construction and fourier truncation effects. At higher r distinct systematic variations can be discerned in the curve comparisons. With respect to increasing y, there are numerous signatures in the data indicative of structural evolution: the rapid G(r) rise in the 3 to 4 A region shifts to lower r and becomes more pronounced, the distinct y=O sample G(r) dip at 4.OA diminishes, the 5.3 A peak (or peak pair) develops, and the overall oscillatory behavior gradually increases. In contrast to an earlier RDF study of HBr-doped PAN1 [12], these data show no evidence for any appreciable covalent Br-PAN1 bonding. Since Br uptake is accompanied by both changes in the intrachain and interchain correlation functions, modelling is required to further interpret these results. Figure 3 contains the calculated PCF and qSF curves in combination with the experimental data from the y=O.5 sample. The fit, especially in the G(r) comparison, is quite satisfactory. The actual PAN1 chain conformation used here is identical to that obtained recently from a full refinement of a fully amorphous HCldoped sample. Thus the only refmed parameters in this modelling are those associated with the positioning of two Br- ions (per four aniline units). It is also neces-
0 b
I
I
I
I
I
2
4
6
8
10
12
r (4
Fig. 2. Weighted samples.
pair
correlation
'
functions
for the
sary to arbitrarily choose a consistent interchain G(r) profile. The pair correlations in this interchain profile are similar those in the analogous curves obtained during structural rehnements of both HF- and HCl-doped ES data. A view of the actual single chain construction is displayed in the Fig. 3 inset. In this case the Br- ion is situated, on average, some 3.3 A above the overall PAN1 molecular plane and at approximately 3.7 A from the nearest amine nitrogen. Although the existence other possible sites cannot be dismissed, this site is comparable to the halogen anion sites found for both HF- and HCl-doped ES. The significance of the relative Br-atom pair contributions, especially in the region spanning from 5.0 to 12.0 A-i (and most relevant for a single PAN1 chain structural construction) are also evidenced in Fig. 3. In general, the sharp oscillatory features that develop in the 5 to 7 A-i region (for the qSF’s) and the new aforementioned G(r) changes arise primarily from Br-PAN1 pair correlations. The Br-atom partial pair correlation functions and partial structure functions, as obtained from the two anomalous scattering data sets (at y=O.34 and 0.42), are displayed sequentially in Fig. 4 in direct comparison with the equivalent curves calculated from small variations in the y=O.5 base model. The limited statistics of these current data sets are evidenced by the large point-to-point fluctuations, especially at higher-q. Still there is modest agreement between experimental and calculated profiles in the 2 to 5 A-’ and 3 to 7 A regions of the respective qSF and G(r) plots. This Br site selection clearly (and qualitatively) reproduces the
728
M.J. Winokur, I
I
/
lntrachain 2.5 -
non-Br . Nomlnal
,
BR Mattes/SyntheticMetals
I
I
All atom pairs pairs excluded Expt. G(r) Interchain G(r)
725-728
I
-
,, ,,, O 0.0 57 ,E 3 4 -1.0 9 a z ~ 1.0
2.0 z 0
84 (1997)
1.5-
All atom pairs Expt. y=O.42 AG(r) 0 0.0 g4 1.o
l.O-
,,., Expt. y=O.34 AG(r)
-1 .o
,
0.0’o 2.0
,
,
,
4.0
6.0
0
/ 2.0
8.0
4.0
6.0
6.0
q (A")
69
2.0
4.0
6.0
6.0
10.0
0.0
r A
Fig. 4. Comparison of the q-weighted structure functions and the partial Br PCF’s derived from the experimental data and the single chain PAN1 model. his contributions in the initial stages of this work. Support of the X7A beamline by Brookhaven National Laboratory (under the DOE) is also acknowledged. References
2.0
4.0
6.0
8.0
10.0
12.0
14.0
d-’ > Fig. 3. Top: A comparison between the experimentally derived weighted PCF and that obtained from a single chain PAN1 model superimposed on a nominal interchain G(r) “background”. Bottom: Comparisons of the model and data in q-space. Inset: Real space image of the PAN1 model. 3.3 A shoulder, the 4.0 A feature and the double peak behavior in the vicinity of 5.5 A. We note that this halogen site preference would tend to disrupt PAN1 chainto-chain overlap and hinder hopping transport within amorphous HBr-doped PANI. The choice of an appropriate interchain PCF is still rather arbitrary but this curve suggests, at this preliminary stage, the presence of identifiable Br-PAN1 pairs, again at spacings near 3.3 A, with respect to a second neighboring chain. More sophisticated modelling techniques are necessary to better address this aspect. Despite the ambiguities in the data, these PDF techniques provide a direct probe of the local structure and continued retinements will further improve the ability to resolve the structural environment within conducting polymers at this critically important length scale. Acknowledgements
The financial support by NSF Grant No. DMR-9305289 is gratefully acknowledged. We also thank J. Maron for
[l] B. E. Warren, X-ray Diffraction (Addison-Wesley, Reading, Mass., 1969). [2] L. E. Alexander, X-ray Diffraction Methods in Polymer Science (Wiley, Interscience, New York, 1969). [3] M. Laridjani et al., Macromolecules 25, 4106 (1992); M. Laridjani, J. P. Pouget, A. G. MacDiarmid, and A. J. Epstein, J. Phys. 12, 1003 (1992). [4] J. Maron, M. J. Winokur, and B. R. Mattes, Macromolecules 28, 4475 (1995). [5] M. J. Winokur and B. R. Mattes, submitted. [6] B. Rosi-Schwartz and G. R. Mitchell, Polymer 35, 5398 (1994); B. Rosi-Schwartz and G. R. Mitchell, Polymer 35, 3139 (1994). [7] Y. Waseda, Novel Application of Anomalous X-ray Scattering
for
StTuctuTal
Characterization
of Dis-
ordered Materials (Springer-Verlag, Belin Heidelberg, 1984), Vol. 204. [8] E. Schultz, H. Bertagnolli, and R. Frahm, J. Chem. Phys. 92, 667 (1990); H. Cai, R. Hu, T. Egami, and G. C. Farrington, Sol. State Ionics 52, 333 (1992); J. R. Fishburn and S. W. Barton, Macromolecules 28, 1903 (1995). [9] D. T. Cromer and D. Liberman, J. Chem Phys. 53, 1891 (1970). [lo] D. T. Cromer and J. B. Mann, J. Chem Phys. 4’7, 1893 (1967). [ll] J. Krogh-M oe, Acta Cystallogr. 9, 951 (1956). [12] B. K. Annis, A. H. Narten, A. G. MacDiarmid, and A. F. Richter, Synth. Met. 22, 191 (1988).
Synthetic
Differential
Anomalous
Scattering
Metals
84 (1997)
Studies
725-728
of Amorphous
HBr-doped
M.J. Winokur” and B.R. Mattesb aDepartment of Physics, University of Wisconsin, Madison WI 53706, bLos Alamos National Laboratory, Los Alamos, NM 87545, USA
Polyaniline
USA
Abstract The local molecular structure ential anomalous scattering in Br-ion site along the polymer and situated, on average, 3.7A Keywords:
Amorphous
thin
of amorphous HBr-doped polyaniline has been probed using conventional and differconjunction with a single chain modelling analysis. Initial results identify a probable chain approximately 3.3A above (or below) the average polyaniline molecular plane from the closest neighboring amine nitrogen.
illms,
polyaniline
and derivatives,
1. Introduction The microscopic structural order of conducting polymers in general and especially within the polyanilines (PANI) exhib i t s a pronounced sensitivity to both the synthesis and processing. Although crystalline and semi-crystalline regions are common structural packings, it is often the structural order within amorphous regions which defines and limits the behavior for both the bulk charge and mass transport. Hence there is a strong impetus to develop techniques which can adequately elucidate the nature of the microscopic order in these amorphous areas. Even though amorphous materials (polymeric or otherwise) lack a clearly identifiable long range periodic order, there is a local structural assembly within the host matrix which produces scattering profiles having systematic intensity modulations and these data can be analyzed using radial (or pair) distribution function (RDF or PDF) analysis [1,2]. Typically this method requires that the entire wide-angle scattering spectrum be recorded, suitably reduced and then Fourier transformed to obtain a weighted sum of partial atom-atom pair correlation functions (PCF’s). For the emeraldine base (EB) form of PANI, composed of just C, N, and H, there are six (i.e., n(n + 1)/2 where n=3) partial pair After doping by halogen acids to form distributions. emeraldine salt (ES) this rises to ten specific terms. Further it is also necessary to differentiate between intrachain and interchain atom pairs. For both amorphous EB and ES samples the scattering [3,4] is always dominated by the well-defined geometry of the aniline monomer; especially at higher momentum transfer (or q). To a lesser extent, and 037967791971%17.00 PII SO379-5779(96)04119-7
0 1997
Ekevier
Science
S.A
All
rights
reserved
X-ray
scattering
at somewhat lower 4, the monomer-to-monomer intrachain and polymer-polymer interchain pair correlations can be resolved and it is possible to obtain semiquantitative agreement between model calculations and experimental data [4]. For EB the intrachain structure is consistent with the theorized three benzenoid to one quinoid alternation of the aniline monomer units along the PAN1 backbone. The interchain structure ordering only weakly resembles that of the reported crystalline phase. Continued improvements in both the experimental x-ray methodology and in the theoretical modelling have begun to yield fully quantitative treatments [5] comparable to neutron-based RDF studies [6] of conventional polymeric materials. PDF analysis of ES includes additional subtlety. For the relatively low-Z HF- and HCl-doped ES samples, the halogen-atom pairs effect only a relatively modest contribution to the x-ray scattering profiles so that detailed PAN1 polymer chain structural analysis is still possible. HBr- and HI-doped samples are more heavily dominated by the halogen ions so that the detailed chain structure is less easily discerned. However Br (as does I) has an absorption edge well-placed in the x-ray spectrum so that at energies close to its K-absorption edge there is a reduction in the real part of the Bratom scattering factor. Scanning at two different x-ray energies, just below the edge, enables the Br scattering factor to be altered so that a difference of the two coherent scattering factors may be scaled and transformed to yield a weighted sum of the four element speci6c pair pair correlation functions [7,8]. This paper presents an initial dii?erential anomalous scattering study of two HBr-doped PAN1 samples. Both films exhibit clear evidence for the presence of Br-
726
M.J. Winokur,
B.R. Mattes /Synthetic
PAN1 nearest neighbor pair distances averaging close to 3.3A. Comparisons with model calculation are also shown. Within a single Br-atom/PAN1 chain picture we identify at least one likely halogen anion site along the PAN1 chain. This site selection is consistent with recent results for both HF and HCl-doped ES [5]. With improved counting statistics and further refinements in the modelling analysis a far more detailed description of the local halogen structural environment is achievable. 2. Theory
The general form of total coherent scattering intensity from N atoms in a system is given by,
where q = y sin 0 is the scattered photon momentum transfer, r;j is i,j pair spacing and fi is the ith atom scattering factor. This expression can be rewritten in terms of a normalized intensity, jcoh, to yield a total structure function, H(q), given by
H(q) = voh - (f “Mf )” where
(2)
“) = Cy=“=,cif:(q) is the self-scattering term, represents the mean scatterer and (f) = CL1 cifdq) ci is the element specific fractional concentration, H(q) can be related to the total pair correlation function, G(r)) through G(r)
(f
=
1+ 1
M q H(q)
2++p0
sin(qr)dq
Jo
where po is the mean density. At two photon energies, En,,, and Efar, below the absorption edge, the specific atomic scattering factor, nominally f(q, E) = fo(q) - f’(E) + if”(E), becomes and fn = fo(q) - f; + f;‘. Ignorfn = fo(q) - f:, + if: ing the relatively small f” terms allows the difference structure function, AH(q), to be written as AH(q) = (f);+(q)
- (fMMQ)
2CBrrn(fA
- f;)
and this can be transformed to give, in this case, the weighted Br partial pair correlation function, AG(r), 1 O3 = 1+ ~ q AH(q) si4qr)& . (5) 2n2rp0 J o In addition to eqn. 4, it is also possible to express H(q) (and similarly AH(q)) directly in terms of a weighted sum of partial structure functions, eij (q), according to
Metals
84 (I 997) 725- 728
where Cij is an assumedisotropic root-mean-square deviation of the rij spacing and n;j is the relative number of i, j atom pairs of a prospective real space model. 3. Experimental
Details
A more comprehensive discussion of both the PAN1 film preparation and experimental methodology is given in ref. 4. All films studied were fully amorphous. The nascent EB films were immersed in 0.1 M, 1.0 M and 4.0 M aqueous HBr solutions for over 48 hours to give three ES samples. Refinement of the elastic/inelastic coherent scattering ratios at high-q yields the approximate Br- concentration, y=Br-/N, to be close to 0.34, 0.42, and 0.5 for these three samplesrespectively. Data was acquired at the X7A beamline at the National Synchrotron Light Source operated by Brookhaven National Laboratory. A monochromatic beam of x-rays (1 x 10 mm2 in cross-sectional area) was incident on tightly clamped multilayer stacks of the PAN1 films. Typical samples consisted of 5 to 25 stacked layers of the various PAN1 films. All samples were mounted in a He-filled canister using a symmetrical transmission geometry in combination with standard e-26’scans. Scattering data was acquired at 20 angles ranging from 5” to 130’ in 0.05 and O.lOA-1 steps using three different wavelengths, X=0.5020, 6.9214 (fA=7.0) and 0.9377 A (f;=3.6) where the latter two wavelengths are approximately 20 eV and 250 eV below the nominal 13.477 keV Br K-edge. The anomalous scattering factors were approximated using the CromerLiberman scheme [9]. The scattered photons were collimated using a pair of rectangular slits adjusted to give a 3x12 mm2 aperture at the energy dispersive detector orifice. Data collection times were extended somewhat at higher q to compensate for the reduction in the scattering intensity and the increasing proportion of Compton scattering. Standard corrections for beam intensity fluctuations, detector dead time, sample geometry, attenuation, partial polarization, background and incoherent Compton scattering [lo] have been effected. Normalization of the experimental data was accomplished by using the analytical expression of Krogh-Moe [ll]. The displayed g-space data has not been smoothed in any way so that the point-to-point fluctuations of these curves are representative of the statistical uncertainties.
AG(r)
H(q) =
(6)
~CCCiYf~(q)f~(q)eij(q) \JI
i
j
in which the double sum is over all elemental constituents and ejj (q) =
nij
exp(-.!Jfjq2)
Sin(Wij)
Vij
4. Results
and
Conclusions
Figure 1 displays the individual q-weighted structure functions (qSF’s) for an as-cast film (y=O) and the three ES films prepared. For the y=O.34 and 0.42 samples, both conventional (X=O.502OA) and anomalous scans were acquired. These spectra show subtle changes in the overall oscillatory positions and line shapesfor q 5 SA-i with increasing y. In particular the position the first two intensity maxima, originally near 1.6 and 3.5 A-i, shift to higher q. Significant changes in the shape
MJ.
Winokur,
B.R Manes/SsntheticlLietals
84 (199i’)
725-728
727
24 h=0.502,h
5
-
4
3 ?IE
2
Ir’ ly=O.O
1 I 2
Fig. 1. q-weighted
I 4
I 6
structure
I 8
I 10
functions
12
I 14
1 16
for the samples.
of the third LLpeak,” centered about 5.8A-I, are also quite distinct. These variations are indicative of changes in both the intrachain and interchain components of the various pair correlations. The changes with respect to varying wavelength at fixed y are hard to discern although there is, for instance, a systematic increase in the intensity of the qSF data at 5.5 A-r as f’ diminishes. Better statistics in the ES data, especially at the higher q range, would improve these comparisons. The respective G(r)% are shown in Fig. 2. For distances below 3 A the spectra are heavily dominated by the aniline monomer atom pair construction and fourier truncation effects. At higher r distinct systematic variations can be discerned in the curve comparisons. With respect to increasing y, there are numerous signatures in the data indicative of structural evolution: the rapid G(r) rise in the 3 to 4 A region shifts to lower r and becomes more pronounced, the distinct y=O sample G(r) dip at 4.OA diminishes, the 5.3 A peak (or peak pair) develops, and the overall oscillatory behavior gradually increases. In contrast to an earlier RDF study of HBr-doped PAN1 [12], these data show no evidence for any appreciable covalent Br-PAN1 bonding. Since Br uptake is accompanied by both changes in the intrachain and interchain correlation functions, modelling is required to further interpret these results. Figure 3 contains the calculated PCF and qSF curves in combination with the experimental data from the y=O.5 sample. The fit, especially in the G(r) comparison, is quite satisfactory. The actual PAN1 chain conformation used here is identical to that obtained recently from a full refinement of a fully amorphous HCldoped sample. Thus the only refmed parameters in this modelling are those associated with the positioning of two Br- ions (per four aniline units). It is also neces-
0 b
I
I
I
I
I
2
4
6
8
10
12
r (4
Fig. 2. Weighted samples.
pair
correlation
'
functions
for the
sary to arbitrarily choose a consistent interchain G(r) profile. The pair correlations in this interchain profile are similar those in the analogous curves obtained during structural rehnements of both HF- and HCl-doped ES data. A view of the actual single chain construction is displayed in the Fig. 3 inset. In this case the Br- ion is situated, on average, some 3.3 A above the overall PAN1 molecular plane and at approximately 3.7 A from the nearest amine nitrogen. Although the existence other possible sites cannot be dismissed, this site is comparable to the halogen anion sites found for both HF- and HCl-doped ES. The significance of the relative Br-atom pair contributions, especially in the region spanning from 5.0 to 12.0 A-i (and most relevant for a single PAN1 chain structural construction) are also evidenced in Fig. 3. In general, the sharp oscillatory features that develop in the 5 to 7 A-i region (for the qSF’s) and the new aforementioned G(r) changes arise primarily from Br-PAN1 pair correlations. The Br-atom partial pair correlation functions and partial structure functions, as obtained from the two anomalous scattering data sets (at y=O.34 and 0.42), are displayed sequentially in Fig. 4 in direct comparison with the equivalent curves calculated from small variations in the y=O.5 base model. The limited statistics of these current data sets are evidenced by the large point-to-point fluctuations, especially at higher-q. Still there is modest agreement between experimental and calculated profiles in the 2 to 5 A-’ and 3 to 7 A regions of the respective qSF and G(r) plots. This Br site selection clearly (and qualitatively) reproduces the
728
M.J. Winokur, I
I
/
lntrachain 2.5 -
non-Br . Nomlnal
,
BR Mattes/SyntheticMetals
I
I
All atom pairs pairs excluded Expt. G(r) Interchain G(r)
725-728
I
-
,, ,,, O 0.0 57 ,E 3 4 -1.0 9 a z ~ 1.0
2.0 z 0
84 (1997)
1.5-
All atom pairs Expt. y=O.42 AG(r) 0 0.0 g4 1.o
l.O-
,,., Expt. y=O.34 AG(r)
-1 .o
,
0.0’o 2.0
,
,
,
4.0
6.0
0
/ 2.0
8.0
4.0
6.0
6.0
q (A")
69
2.0
4.0
6.0
6.0
10.0
0.0
r A
Fig. 4. Comparison of the q-weighted structure functions and the partial Br PCF’s derived from the experimental data and the single chain PAN1 model. his contributions in the initial stages of this work. Support of the X7A beamline by Brookhaven National Laboratory (under the DOE) is also acknowledged. References
2.0
4.0
6.0
8.0
10.0
12.0
14.0
d-’ > Fig. 3. Top: A comparison between the experimentally derived weighted PCF and that obtained from a single chain PAN1 model superimposed on a nominal interchain G(r) “background”. Bottom: Comparisons of the model and data in q-space. Inset: Real space image of the PAN1 model. 3.3 A shoulder, the 4.0 A feature and the double peak behavior in the vicinity of 5.5 A. We note that this halogen site preference would tend to disrupt PAN1 chainto-chain overlap and hinder hopping transport within amorphous HBr-doped PANI. The choice of an appropriate interchain PCF is still rather arbitrary but this curve suggests, at this preliminary stage, the presence of identifiable Br-PAN1 pairs, again at spacings near 3.3 A, with respect to a second neighboring chain. More sophisticated modelling techniques are necessary to better address this aspect. Despite the ambiguities in the data, these PDF techniques provide a direct probe of the local structure and continued retinements will further improve the ability to resolve the structural environment within conducting polymers at this critically important length scale. Acknowledgements
The financial support by NSF Grant No. DMR-9305289 is gratefully acknowledged. We also thank J. Maron for
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