INVESTIGATION ON THE PHYSICAL PROPERTIES OF POLYPYRROLE

Transcription

1 INVESTIGTION ON THE PHYSICL PROPERTIES OF POLYPYRROLE F. Devreux, F. Genoud, M. Nechtschein, J. Travers, G. Bidan To cite this version: F. Devreux, F. Genoud, M. Nechtschein, J. Travers, G. Bidan. INVESTIGTION ON THE PHYS- ICL PROPERTIES OF POLYPYRROLE. Journal de Physique Colloques, 1983, 44 (C3), pp.c3-621-c < /jphyscol: >. <jpa > HL Id: jpa Submitted on 1 Jan 1983 HL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNL DE PHYSIQUE Colloque C3, supplement au n06, Tome 44, juin 1983 page C3-621 INVESTIGTION ON THE PHYSICL PROPERTIES OF POLYPYRROLE F. ~evreux*, F. ~enoud*, M. ~echtschein*, J.P. ~ravers* and G. ida an** Centre dretudes NucZBaires de Grenoble, DBpartement de Recherche Fondamentale, *section de Rgsonance Magne'tique, ER 216, **Laboratoires de Chimie, 85 X, GrenobZe Cedex, France Resume - Nous presentons des resultats experimentaux obtenus sur des m i l l o n s de polypyrrole : proprietes de transport (conductivite) et proprietes magnetiques (RMN, RPE). bstract - We report transport properties and magnetic resonance measurements carried out on pol ypyrrol e samples. Pol ypyrrol e samples have been obtained by chemical (Ch) and electrochemical (ECh) polymerization of pyrrole in different solvents (THF, CH3CN, PC) using LiC1O4 as counter ion and under different conditions (ph, current density,... ) /l/.wereport here experimental results of transport properties and magnetic resonance measurements. Fig. 1 shows the structure of oxidized polypyrrole, the degpee of oxidization corresponds to about one CIOk anion per three pyrrole units. Room temperature DC Fig. 1 - Polypyrrole configuration. conductivity has been measured on all samples by a standard four points method. Electrochemical samples were 100 to 300 u thick films, whereds chemical samples were compacted powder pellets of 500 u thick. Most of the samples show a conductivity o in a 1 to 20 s2-l cm-' range. The temperature dependence of conductivity have been recorded on three typical electrochemical sampl es between 300 and 4 K (Fig. 2). We have used four parallel contacts on 8 nun long and 3 mm wide, films contacts were achievied by using graphite painting (Electrodag 502). The experimental data roughly agree with a three im nsional variable range hopping conduction mechanism: tj.174 a is proportional to ex^[-(^) 1. rticle published online by EDP Sciences and available at

3 JOURNL DE PHYSIQUE I I Ech. THF CH, CN h 0 - i CH3CN 0 * P + C0ll*ClO~ + H'ClOi rb - h - d - t Fig. 2 - Temperature dependence of conductivity for different electrochemical samples. Fig. 3a and 3b show the resistance variations of two electrochemical samples (prepared in THF and in CH,CN) versus an applied electric field , 1 I,,,m a 1 111(1'1 1 - r R (E)/R (0) b 1- s. 9 +?a a :, %, a 220H 0 leal l33r + BOW 496 JOW V 15K + *r + a v v * v * 8'. v. e 49K 30K v Ism 7 0 ' """'I ' ' ""'.I ' ' ' ' * ".' I 1 a,,,#,,i,,,,,,,, 1, to-a lo-. lo-' I E (~olt/cm) 130-' lo-' I E (~olt/cnl) to Fig. 3 - Resistance variations as a function of the electric field. 3a : electrochemical sample prepared in THF I,,I,I jb : " CHBCN To avoid heating effects measurements were made by a pulse method. It turns out that non linear effects have been observed for electric fields as low as 50 mv/cm. The amp1 itude of this non-1 inear effect depends on the sample : it increases with decreasing temperature. In spite of the use of the four contacts method and

4 pulse technics work is still in progress to avoid any misleading interpretation of these non-linear effects. The room temperature spin susceptibility was determined by Schumaker-Slichter method (for a given sample RMN and RPE signals are recorded in the same conditions). For several samples (electrochemical and chemical) the spin number is nearly independent of the preparation, the electron spin concentration is around 5.10-~ by proton, which corresponds to only 5-7 % of the C10, counter ions. The temperature dependence of the ESR signal has been analyzed from 4 to 300 K. very small piece of polypyrrole film was placed into a quartz tube in the inert atmosphere (argon) of a dry box and then sealed under vacuum. Measurements were performed with an X band Varian spectrometer and the temperature was measured with a gold iron chrome1 thermocouple. Fig. 4 shows the temperature dependence of both the linewidth H (half width at half maximum of the absorption signal) and the relative spin number (double integration of the ESR signal) for an electrochemical sample prepared in THF. The linewidth is very small (a 0.3 6). This imp1 ies that highly mobile spins are present in pol ypyrrol e. The?pin susceptibil ity x approximately follows a Curie-Weiss law from 300 to 30 K (X a with Ba 25 K) and then it strongly decreases below 30 K. Qualitatively we havewained the same temperature dependence of the ESR signal for an electrochemical sample prepared in CH3CN. The origin of this maximum of susceptibility is currently under investigation. Fig. 4 - Temperature dependence of the relative spin number : double integration of the ESR signal (electrochemical sample in THF, Klystron power : 60 pw). The proton spin relaxation has been studied at two different frequencies (16 and 80 MHz) versus temperature (in the range K) ; it turns out that at high frequency (80 MHz) the spin lattice relaxation rate is constant over the whole temperature range while, at low frequency (16 MHz) it exhibits a drop below 40 K (Fig. 5).

5 JOURNL DE PHYSIQUE 7 - t % F=16 MHz oaal = 5 ~ o O~ o O ~ "0 0; F= 81 MHz I - T (4 Fig. 5 - Variation of the proton spin lattice relaxation rate versus temperature at two frequencies (16 and 80 MHz). s in polypyrrole sample oxygen has a drastic effect on the magnetic properties of polypyrrole. If a sample is open to air the spin number, detected by ESR measurements, immediatly increases by a factor 3 to 5 while the linewidth increases until 10 to 30 gauss after several days ; initial linewidth may be restored by pumping. In sumnary quantitative studies have been performed on polypyrrole samples. They raise a number of questions about transport and magnetic properties on oxidized polypyrrole : - The temperature dependence of conductivity fit the law exp(- 4)1/4 as early reported /2//S/. Such a temperature variation is consistent with a variable range hopping system but microscopic model does not still exist to describe conduction mechanism in this type of compound. - The spin number observed by ESR is only 5 to 7 % of the charge number, if we assume that a charge transfer corresponds to each perchlorate. This is evidence for a strong spin pairing mechanism, the origin of which is still unknown. The narrowing of the ESR line (%.3 G) provides evidence for highly mobile spins ; at room temperature the nuclear relaxation rate agrees with a F - ~ law / ~ characteristic of a highly one dimensional diffusive spins ; further experiments will specified the diffusion mechanism in these polymer chains. The more striking result is probably the existence of a maximum of the spin susceptibility, versus temperature, as determined by ESR. sharp decrease of the spin susceptibility is observed for T < 30 K. The origin of this transition is under investigation. Further work is currently in progress to account for these features and answer these questions. References 1..F. DIZ et al., J.C.S. Chem. Comm. (1979),6? K. KEIJI KNZW et a1., Synthetic Metals 1 (1979/80) WTNBE et a1., Bull. Chem. Soc. Jpn (1981)