Synthetic Metals 114 Ž2000. 155–160
www.elsevier.comrlocatersynmet
Characterization of picric-acid-doped poly žo-toluidine/ and the
picric-acid-doped poly žo-toluidine/ -induced conductive composite of
acrylonitrile–butadiene–styrene
Seddique M. Ahmed, Rahul C. Patil, Masaharu Nakayama, Kotaro Ogura)
Department of Applied Chemistry, Faculty of Engineering, Yamaguchi UniÕersity, 2557 Tokiwadai, Ube 755-8611, Japan
Received 24 August 1999; received in revised form 27 January 2000; accepted 14 March 2000
Abstract
PolyŽ o-toluidine. ŽPOT. in emeraldine base form ŽEB. has been doped with picric acid ŽPA-doped POT. and dinitrophenol
ŽDNP-doped POT.. The conductivities of these doped polymers were found to be 10.52 and 4.82 = 10y3 S cmy1 , respectively. The large
difference in conductivity of these polymers is attributed to the more acidic nature of picric acid ŽPA. than dinitrophenol ŽDNP.. The
disappearance of the peak at ; 650 nm due to EB and the presence of the peak at ; 850 nm due to localized polaron in the absorption
spectra clearly reveal the dopant-induced protonation of POT. This is also supported from the XPS spectrum showing the disappearance
of the peak at ; 398.2 " 0.1 eV due to the imine nitrogen component in EB and the appearance of the peak at ; 402.16 " 0.1 eV owing
to the iminium ion Ž`NHq5. of PA-doped POT.The PA-doped POT is thermally stable up to 1408C and completely dedoped at
; 3008C. The mass spectra recorded simultaneously with the thermal weight loss showed several fragments due to the decomposition of
PA in the temperature region from 1408C to 3008C. The observed high conductivity of PA-doped POT film is attributed to the expanded
coil-like conformation, which was proven with the help of the reduced viscosity measurement. The composite of PA-doped POT with
insulating acrylonitrile–butadiene–styrene copolymer ŽABS. was prepared, and the percolation threshold for this composite was ; 3
wt.%, which was considerably low compared to that for a typical composite, e.g., ; 16 vol.% for polyŽ3-alkylthiophenes. ŽP3ATs.rpolystyrene ŽPSt. composite. q 2000 Elsevier Science S.A. All rights reserved.
Keywords: Picric acid; PolyŽ o-toluidine.; Acrylonitrile–butadiene–styrene
1. Introduction
Electrically conducting polymers have attracted a huge
amount of attention since they were discovered just two
decades ago. Possibly, this could be due to potentialities
for their application to various fields ranging from sensor
and light weight batteries to printing circuit boards in a
micro-circuit technology w1–6x. Many devices would need
a balance of conductivity, processability and stability. The
difficulty in processing conducting polymers, which is one
of the barriers to be conquered in a practical use, has been
considerably mitigated by protonating these polymers, e.g.,
the protonation of polyaniline ŽPANI. by functional organic acids w7x, such as camphor sulfonic w8–10x, dodecyl
)
Corresponding author. Tel.: q81-836-35-9417; fax: q81-836-322886.
E-mail address: [email protected] ŽK. Ogura..
benzene sulfonic w7,11x, and p-phenol-sulfonic acid w12–
14x. Furthermore, some other organic compounds like picric acid ŽPA., whose acidity is comparable to that of
mineral acid w15,16x, can also be employed for the protonation of PANI. The protonation of PANI with PA has been
prepared by the dry method w17x, and the PANI composite
reported to exhibit excellent electrical, optical and mechanical properties w18,19x. So far, however, no reports are
available on the wet protonation of polyŽ o-toluidine. ŽPOT.
with PA. The wet method is interesting since such solution
processability is useful for the fabrication of conducting
composites with different insulating matrixes.
In the present work, efforts have been made to characterize PA-doped POT and to prepare its composite with
insulating acrylonitrile–butadiene–styrene copolymer
ŽABS.. Additionally, dinitrophenol ŽDNP. and nitrophenols ŽNP. were utilized for the protonation of POT in order
to examine the effect of nitro group in governing the
protonation ability of these organic acids.
0379-6779r00r$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 3 7 9 - 6 7 7 9 Ž 0 0 . 0 0 2 3 9 - 3
156
S.M. Ahmed et al.r Synthetic Metals 114 (2000) 155–160
2. Experimental
All chemicals used were of analytical reagent grade.
POT was synthesised following the previously described
procedure w18x. The HCl-doped POT was converted to the
base form ŽPOT-EB. by treating it with 3% aqueous
ammonia solution for 3 h, followed by washing with
deionzied water, methanol and diethyl ether. The dried
powdery POT-EB was then mixed with PA or other NPs at
a given molar ratio using an agate mortar and pestle in an
inert atmosphere. The 2% Žwrw. solution of PA-doped
POT in m-cresol was prepared at 508C under ultrasonic
irradiation for 1 h. The resulting viscous deep green solution was further utilized for the preparation of composites.
The composite solutions with different weight ratios of
PA-doped POT and ABS were obtained by mixing required quantities of both polymers at room temperature.
The conductivities of acid-doped POT and its composite
with ABS were measured by using a comb shaped Pt
microelectrode. After casting the composite solution on the
microelectrode and removing the solvent Ž m-cresol. at
508C, the conductivity was monitored by the two-probe
method using a Hokuto Denko galvanostat Žmodel HA501.. The viscosity data of PA-doped POT in m-cresol
solution were obtained with an Ostwald type viscometer
ŽSibata, No. 3. at 24 " 18C.
Thermogravimetricrmass analyses were performed with
a Jeol MS-TGrDTA ŽModel 220. under He atmosphere. A
given amount of PA, POT-EB and PA-doped POT was
placed in a sample holder, and the temperature was changed
from 408C to 6508C at the programmed heating rate of
58Crmin.
UV-visible absorption spectra of PA, POT-EB and PAdoped POT in m-cresol solution were recorded on a
Shimadzu double beam spectrophotometer ŽModel 2200..
Fourier transform infrared spectroscopic ŽFTIR. measurements of POT-EB, PA and PA-doped POT in KBr pellets
were carried out using a Shimadzu FTIR ŽType 8100 M.
spectrometer. Transmission electron micrographs ŽTEM.
of the PA-doped POT composite film were taken with a
Hitachi apparatus ŽH 8000. in which the sample film was
directly cast on carbon-coated copper grid. X-ray photoelectron spectroscopic ŽXPS. analysis of the PA-doped
POT film was carried out with a FISONS, ESCALB 210
apparatus using an Al K a X-ray source radiation Ž1486.6
eV photons.. The X-ray source was run at 15 kV and 20
mA. The energy resolution was better than 0.01 eV at the
pass energy of 20 eV, and the vacuum in the analysis
chamber was about 10y6 Pa.
3. Results and discussion
3.1. UV-Õisible spectra of PA, POT-EB and PA-doped
POT
Fig. 1 represents the absorption spectra of PA Ža.,
POT-EB Žb. and PA-doped POT Žc. in m-cresol solution.
Fig. 1. UV-visible absorption spectra of Ža. PA, Žb. POT-EB, and Žc.
PA-doped POT in m-cresol solution.
POT-EB shows a well defined peak at ; 650 nm ŽFig. 1b.
that is characteristic of emeraldine base. Upon the protonation of POT-EB with PA, the absorption due to the emeraldine base vanishes, and a new peak at ; 860 nm and a
shoulder at ; 430 nm appear ŽFig. 1c., indicating the
formation of localized polaron and radical cation in the
emeraldine salt, respectively w20,21x. This suggests that PA
operates as a protonating agent to induce the internal
conversion of POT-EB to the emeraldine salt. The PA
alone shows only the peak at ; 320 nm due to p–p )
transition ŽFig. 1a.. This absorption is enhanced for PAdoped POT, owing to the additional contribution of phenyl
rings from the POT chain ŽFig. 1c..
3.2. FTIR spectra of POT-EB, PA and PA-doped POT
In Fig. 2, the FTIR spectra of POT-EB Ža., PA Žb. and
PA-doped POT Žc. are shown, giving a clear evidence for
the protonation of POT-EB. The peak appearing at 3390
cmy1 in Fig. 2a is attributed to the `NH stretching of
POT-EB, and that at ; 3105 cmy1 in Fig. 2b is due to the
hydroxyl group of PA. The absence of peak at ; 3105
cmy1 in the case of PA-doped POT ŽFig. 2c. suggests that
PA is effectively dissociated, and the emeraldine base is
doped with PA to form electrically conductive emeraldine
salt. Moreover, the peaks at 1599 and 1493 cmy1 are
ascribed to quinoid and benzenoid rings of POT-EB, respectively ŽFig. 2a.. These wavenumbers are shifted to
1560 and 1485 cmy1 , respectively, for PA-doped POT
ŽFig. 2c.. Such red shift indicates the delocalization of
quinoid and benzenoid structures to the semiquinoid form
Žpolaron. in the polymer chain, confirming the protonation
of the emeraldine base. The ratio in spectral intensity of
benzenoid to quinoid form was lower for PA-doped POT
than for POT-EB, revealing again the conversion of benzenoid ring to semiquinoid. In fact, the intensity of quinoid
should be larger than that of benzenoid in the case of
PA-doped POT since the peak at 1560 cmy1 should be
piled up with that of NO 2 group appearing at ; 1558
cmy1 w17x. The peaks at ; 1632 and 1605 cmy1 appear-
S.M. Ahmed et al.r Synthetic Metals 114 (2000) 155–160
157
ing moisture, and the other major fragments Ž mrz 30, 62,
91, 199 and 229. are due to PA. However, the intensities
of these fragments were relatively low, and, therefore, one
can suggest that these are caused by the excess PA, which
has not been incorporated in the polymer backbone. Although the 1 to 2 molar ratio of EB to PA is the exact
value required to give completely protonated polymer, it is
true that there were unreacted POT and PA and partially
protonated POT in the reaction mixture w20x. The two
minor fragments at mrz 77 and 106 are related to POT
oligomers. On further elevation of temperature to 2008C,
the intensity of the fragment observed at mrz 199, corresponding to C 6 H 3 N2 O6 , was significantly enhanced, indicating that picrate ions were ready to be detached from the
polymer backbone ŽFig. 3 t 2 .. At this temperature, however, there is no indication of degradation of the polymer
backbone. More interestingly, at 3008C the mass spectra
do not show any single fragment resulting from the decomposition of PA, suggesting that the acid was completely
dedoped from the polymer. Oxygen atoms generated by
Fig. 2. FTIR spectra of Ža. POT-EB, Žb. PA, and Žc. PA-doped POT. The
measurements were performed in the transmission mode as KBr tablets.
ing as a doublet are due to the asymmetric stretching
vibration of NO 2 group, whereas the peak at 939 cmy1 is
attributed to C`N stretching of the aromatic nitro compound.
3.3. TG r MS spectra of PA, POT-EB and PA-doped POT
PA possessed one step decomposition pattern Ž160–
2358C.. On the other hand, POT-EB was thermally stable
up to 3008C, and then degraded rapidly to attain a steady
state at ; 6008C. The thermal behaviour of PA-doped
POT described an intermediate characteristics between PA
and POT-EB. It was found that PA-doped POT is stable up
to 1408C and completely dedoped at ; 3008C, and the
total weight loss was about ; 45%.
The mass spectra obtained simultaneously during the
thermal analysis provided some useful information about
the degradation path of PA-doped POT. In Fig. 3, MS
spectra obtained at different temperatures Ž t 1 , t 2 , t 3 . are
exhibited. At 1108C, a sharp fragment of water molecule
Ž mrz 18. is obviously owing to the ability of PA adsorb-
Fig. 3. MS spectra of PA-doped POT taken at different temperatures.
158
S.M. Ahmed et al.r Synthetic Metals 114 (2000) 155–160
the degradation of PA may combine with carbon and
hydrogen atoms from the decomposition of benzene rings
to form CO 2 and H 2 O Ž mrz 18, 44.. The appearance of
small fragments at mrz 272 and 296 suggests the initiation of full-scale polymer degradation.
3.4. Electrical conductiÕity, Õiscosity and XPS measurements
As the optical absorption and FTIR spectra give a
strong support for the protonation of POT by PA as
mentioned above, it is essential to measure the conductivity of PA-doped POT and related polymers. The electrical
conductivities of PA and POT-EB were too small to be
measured by the method used. The values of 5.00 = 10y1 5
and 4.10 = 10y1 2 S cmy1 are often cited for the conductivities of PA w17x and POT-EB w22x, respectively. The
conductivity of PA-doped POT was found to be 10.52 S
cmy1 . On the other hand, when POT is incorporated with
DNP, o-NP or p-NP instead of PA, the conductivities of
these polymers become considerably low: 4.81 = 10y3
ŽDNP-doped POT., 4.85 = 10y5 Ž p-NP-doped POT., 4.04
= 10y5 S cmy1 Ž o-NP-doped POT., and the conductivity
drops down about ; 3–5 orders of magnitude compared
to that of PA-doped POT. Hence, it follows that the
conductivities of doped POT depends considerably on the
acidity of acids used. In the case of PA, the proton is
easily released from the hydroxyl group due to the reduced
electron density on the oxygen atom. The mesomeric effect
of the aromatic system containing three nitro groups is
thus responsible for high acidity, which is comparable to
that of mineral acid w15,16x. As the number of nitro groups
become smaller, the release of proton from the hydroxyl
group becomes less efficient, and the acidities Žp K a . decreases in the sequences: PA Ž0.29. 4 DNP Ž4.11. 4 p-NP
Ž7.15. , o-NP Ž7.23.. This conclusion is in agreement with
the result obtained by MacDiarmid et al. w23,24x in which
the conductivity of PANI emeraldine base is highly affected by the acidity of dopant.
Fig. 4. Reduced viscosity Ža. and conductivity Žb. of PAyrEBHq
m-cresol solutions as a function of molar ratio of PA to EB. The
concentration of POT-EB was always 3.93=10y3 M.
Scheme 1.
It is interesting to note that the electrical conductivity of
PA-doped POT Ž; 10.52 S cmy1 . is about three orders of
magnitude higher than that of HCl-doped POT Ž; 10y2 S
cmy1 . w15x. Such an enhancement of conductivity is probably caused by the formation of more delocalized state for
PA-doped POT than HCl-doped POT w15x. In fact, MacDiarmid et al. w14,25x have demonstrated that the conformational change in polymer chain results in the significant
change in conductivity of polymer.
Recently, we have reported that POT doped with camphor sulfonic acid is soluble in m-cresol Ž8 wt.%., and the
cast film shows a conductivity of ; 28 S cmy1 w26x. Such
a high conductivity is attributed to the attainment of the
expanded coil-like conformation. In the present study, a
plot of reduced viscosity vs. molar ratio of EB to PA
revealed the similar observation to the case of POT with
camphor sulfonic acid w26x. As can be seen in Fig. 4a, the
observed high conductivity Ž10.52 S cmy1 . of PA-doped
POT film prepared at the molar ratio of 2 ŽPA. to 1
ŽPOT-EB. is attributed to the change in the molecular
conformation from coil to expanded coil-like. On the other
hand, the reduced viscosity of PAyrEBHq in m-cresol
increases with an increase in the molar ratio, and reaches a
maximum at the ratio of 2 to 1 ŽFig. 4b., indicating the
complete protonation of POT to produce polysemiquinone
radical cation. Hence, the structure of the protonated polysemiquinone radical tetramer unit present in PAyrEBHq
solution can be displayed in Scheme 1.
The initial increase in reduced viscosity up to the molar
ratio of 2 to 1 suggests the change in the polymer conformation from coil to expanded coil. This transformation is
increasing favorable as the hydrodynamic volume of the
polymer increases, i.e., with increasing the viscosity of the
polymer solution w14,25x. The expanded coil-like conformation especially facilitates the delocalization of electron
through the polymer backbone, leading to the enhancement
of conductivity of the polymer. The slight decrease in the
reduced viscosity at the molar ratio of 3.2 to 1 and 4 to 1 is
consistent with the property observed commonly in polyelectrolytes w27x.
The presence of expanded coil-like conformation can be
also proven from the absorption spectra showing in Fig. 5
where they were recorded in the range between 400 and
1400 nm. Free carrier tail around ; 1000 nm is observed
for the mixtures of PA and EB with the molar ratios of 4
to 1 and 2 to 1 ŽFig. 5a and b., demonstrating that the
S.M. Ahmed et al.r Synthetic Metals 114 (2000) 155–160
Fig. 5. Electronic spectra of PA-doped POT at various ratios of PA to EB
in m-cresol solution. The molar ratio of PA to EB: Ža. 4, Žb. 2 and Žc.
0.4.
polymer is in expanded coil like conformation. Contrary to
this, for the mixture with the molar ratio of 0.4 to 1, no
free carrier tail was observed ŽFig. 5c., but two prominent
peaks at ; 660 and 860 nm instead. These peaks are due
to the emeraldine base and localized polaron, respectively.
Hence, in the mixture of EB with lower content of PA,
POT is partly protonated and the polymer chains exist in a
more coil-like conformation.
Furthermore, PA-doped POT was characterized by XPS.
The imine nitrogen component, centered at binding energy
; 398.2 " 0.1 eV w28–30x in the emeraldine base, disappeared completely. The peak located at ; 402.16 " 0.1 eV
was assigned to the generated iminium ions Ž`NHq 5. of
the PA-doped POT solid film. The presence of about equal
amounts of imine and amine nitrogen Žpeak centered at
; 398.6 and ; 402.16 eV. was consistent with the intrinsic oxidation state of the polymer wratio Ž`NH`.r5N`.
; 1.x w31x.
3.5. Preparation of conductiÕe ABS by the incorporation of
PA-doped POT
In Fig. 6, the conductivity of the composite is plotted
vs. the weight percentage of PA-doped POT in the ABS
159
Fig. 7. TEM image of the PA-doped POTrABS composite with 20 wt.%
of PA-doped POT. The operating voltage was 10 kV.
matrix, showing the percolation threshold at ; 3%. This
percolation threshold is considerably lower than, e.g., that
Ž; 16 vol.%. reported w32x for percholate-doped polyŽ3-alkylthiophenes. ŽP3ATs.rpolystyrene ŽPSt. composite. Earlier, Cao et al. w7x have proven that in the composite with
counter ion-induced conducting polymer, the conductivity
turns on at the concentration that is at least an order of
magnitude below the classical percolation threshold Ž; 16
vol.%.. Their value is in good agreement with our result
described above. Such a low threshold is attributed to
conducting channels formed in the composite through
which electron can travel easily leading to the observed
conductivity. Fizazi et al. w33x have studied the conducting
polyŽ3-octylthiophene. ŽP3OT. doped with iodine in
polyethyene ŽPE. gel, and found an evidence about the
connected conducting paths, but with no indication of
percolation threshold. They discussed their results in terms
of adsorption of the P3OT onto the PE gel network, which
subsequently organizes the P3OT into connected paths.
This view is consistent with our result obtained here.
The TEM image of the composite containing 20 wt.%
of PA-doped POT as shown in Fig. 7. As seen in this
figure, ABS composite with this content of PA-doped POT
denotes fine rod-shape particles of size ranging from 20 to
80 nm embedded in insulating ABS. These particles are
seen to be placed very closed to each other, and electrons
can hop easily on it, thereby responsible for the conduction
in this composite.
4. Conclusions
Fig. 6. Plot of electrical conductivity of the PA-doped POTrABS composite vs. weight % of PA-doped POT in ABS matrix.
The organic NPs such as PA and DNP were found to be
useful for the protonation of POT. The protonating ability
of these acids depends on the number of nitro groups, and
the trinitrophenol Ži.e., PA. revealed the highest conductivity among these NPs. The doping of POT with PA was
proven with the help of various techniques. PA-doped POT
160
S.M. Ahmed et al.r Synthetic Metals 114 (2000) 155–160
is thermally unstable above 1408C, and the thermally
processing with other insulating matrix is not profitable,
but the solution casting is highly promising. The blend of
PA-doped POT with ABS showed low threshold value of
; 3 wt.%.
Acknowledgements
S.M. Ahmed gratefully acknowledges the financial support from the Japanese Ministry of Education for his PhD
program.
References
w1x K. Ogura, H. Shiigi, M. Nakayama, A. Fujii, J. Electrochem. Soc.
145 Ž1998. 3351.
w2x B. Wessling, Synth. Met. 93 Ž1998. 143.
w3x Y. Cao, G.M. Treacy, P. Smith, A.J. Heeger, Appl. Phys. Lett. 60
Ž22. Ž1992. 2711.
w4x A. Tsumura, H. Koezuka, T. Ando, Appl. Phys. Lett. 49 Ž1986.
1210.
w5x C.T. Kuo, S.A. Chen, G.W. Hwang, H.H. Kuo, Synth. Met. 93
Ž1998. 155.
w6x J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K.
Mackay, R.H. Friend, P.L. Burn, A.B. Holmes, Nature 347 Ž1990.
539.
w7x Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 55–57 Ž1993. 3514.
w8x B. Sanjai, A. Raghunathan, T.S. Natarajan, G. Raangaraijan, Phys.
Rev. B 55 Ž1991. 10734.
w9x E.R. Holland, S.J. Pomfert, P.N. Adams, A.P. Monkman, J. Phys.:
Condens. Matter 8 Ž1996. 2991.
w10x A.P. Monkman, E. Rebourt, A. Petr, Synth. Met. 84 Ž1997. 761.
w11x Y. Cao, J. Qui, P. Smith, Synth. Met. 69 Ž1993. 187.
w12x S.H. Goh, H.S.O. Chan, C.H. Ong, Polymer 37 Ž1996. 2675.
w13x Y.Z. Wang, J. Joo, C.H. Hsu, A.J. Epstein, Synth. Met. 68 Ž1995.
207.
w14x A.G. MacDiarmid, A.J. Epstein, Synth. Met. 65 Ž1994. 103.
w15x Z.H. Wang, A. Ray, A.G. MacDiarmid, A.J. Epstein, Phys. Rev. B
43 Ž5. Ž1991. 4373.
w16x Y.Z. Wang, J. Joo, C.H. Hus, A.J. Epstein, Synth. Met. 69 Ž1995.
267.
w17x J. Stejskal, I. Sapurina, M. Trchova, J. Prokes, I. Krivka, E. Tobolkova, Macromolecules 31 Ž1998. 2218.
w18x Y. Cao, P. Smith, A.J. Heeger, Synth. Met. 32 Ž1989. 263.
w19x C.O. Yoon, M. Reghu, D. Moses, A.J. Heeger, Synth. Met. 63
Ž1994. 47.
w20x T. Vikki, L.O. Pietila, H. Osterholm, L. Ahjopalo, A. Takala, A.
Toivo, K. Levon, P. Passiniemi, O. Ikkala, Macromolecules 29
Ž1996. 2945.
w21x J. Stejskal, P. Kratochvil, A.D. Jenkins, Polymer 37 Ž1996. 367.
w22x J. Anand, S. Palaniappan, D.N. Sathyanarayana, Synth. Met. 63
Ž1994. 43.
w23x A.G. MacDiarmid, A.J. Epstein, Faraday Discuss., Chem. Soc. 88
Ž1989. 317.
w24x W.S. Huang, A.G. MacDiarmid, A.J. Epstein, J. Chem. Soc., Chem.
Commun. 1784 Ž1987. .
w25x J.K. Avlyanov, Y. Min, A.G. MacDiarmid, A.J. Epstein, Synth. Met.
72 Ž1995. 65.
w26x R.C. Patil, K. Kuratani, M. Nakayama, K. Ogura, J. Polym. Sci.,
Part A: Polym. Chem. 37 Ž1999. 2657.
w27x P.C. Heimenz, Polymer Chemistry, The Basic Concepts, Marcel
Dekker, New York, 1984, pp. 588 and 617.
w28x E.T. Kang, K.G. Neoh, S.H. Khor, K.L. Tan, B.T.G. Tan, J. Chem.
Soc., Chem. Commun. Ž1989. 695.
w29x K.L. Tan, B.T.G. Tan, E.T. Kang, K.G. Neoh, Phys. Rev. B 39
Ž1989. 8070.
w30x K.L. Tan, B.T.G. Tan, E.T. Kang, K.G. Neoh, Chem. Phys. 94
Ž1989. 5382.
w31x A.G. MacDiarmid, J.C. Chiang, A.F. Richter, A.J. Epstein, Synth.
Met. 18 Ž1987. 285.
w32x S. Hotta, S.D.D.V. Rughooputh, A.J. Heeger, Synth. Met. 22 Ž1987.
79.
w33x A. Fizazi, J. Moulton, K. Pakbaz, S.D.D.V. Rughooputh, P. Smith,
A.J. Heeger, Phys. Rev. Lett. 64 Ž18. Ž1990. 2180.