Polymer Znternational41(1996)463-471 Structure and Some Physical Properties of Electropolymerized Polyacrylonitrile F. M. Reicha Physics Department, Faculty of Science, Mansoura University, Mansoura, Egypt (Received 2 January 1996; revised version received 11 June 1996; accepted 15 July 1996) Abstract: Electropolymerization of acrylonitrile in an aprotic medium leads to the formation of two different structural forms of polyacrylonitrile (PAN). First, PAN having a striated structure and highly cross-linked was condensed on the cathode. This type of material exhibits unique physical properties, namely that its conductivity is about (Qcm)-' and is independent of the frequency. The second type was gathered from the medium, and elemental analysis, optical microscopy, X-ray, UV, IR and other investigations showed that the polymer has an amorphous structure with the presence of interchain cyclization, which spreads as the current density of electropolymerizationincreases. The chemical nature of the chromophore developed in PAN prepared by this method is attributed to conjugation of C z N . The structural changes were accompanied by an increase in conductivity. The conductivity was about (Q cm)-' with an activation energy of 1.48eV for a sample prepared at a current density of 0*417rnA~m-~, increasing to about 8 x lO-'(Qcm)-' with an activation energy of 0.93 eV for a sample prepared at 2.5 mA cm-' or higher. Key words: acrylonitrile,electropolymerization,characterization, conductivity. I NTRO DUCTlO N In the mid-l960s, fibres of outstanding strength were obtained by heating polyacrylonitrile (PAN) through a carefully controlled temperature programme, in a controlled atmosphere, to temperatures in excess of 1000°C. This discovery made PAN an appealing target for investigation into the methods of preparation and thermal degradation, as well as physical and mechanical properties. The properties of the carbon fibres depend upon the PAN from which it is prepared and the thermal treatment to which it is subjected. Some controversies over the structure of PAN have been discussed in comprehensive Differences in the physical properties of PAN homopolymer can be found in publications from different laborat~ries,'~-' as a result of differences in sample p rep arati~ n.'~ -'~ In spite of an enormous number of reports on the methods of PAN preparation, very little attention has been paid to its synthesis by the electrochemical technique.6 The technique reported was performed under carefully controlled physical and chemical conditions and under a very pure argon atmosphere, which would make commercial mass-production difficult. This paper describes the electrochemical polymerization of acrylonitrile (AN) monomer, in an aprotic medium with different electrode potentials, under normal laboratory conditions of temperature and atmosphere. EXPERIMENTAL The electrochemical polymerization was carried out as follows. Two platinum strips (2cm x 3cm) were suspended at a distance of 2.5 cm from each other by platinum wires in an electrolytic solution consisting of the AN monomer (concentration 1.5 M) and a supporting electrolyte (tetramethyl ammonium bromide, concentration 0.05 M) dissolved in the aprotic solvent (dimethylformamide, DMF), and connected to a suitable variable resistor, microammeter and constantcurrent power supply (Gelman Sciences, Inc.). The 463 Polymer International 0959-8103/96/$09.00 0 1996 SCI. Printed in Great Britain F. M . Reicha 464 electrolytic current density was fixed for each preparation period (4h), and ranged from 0.25 to 3.33 mAcm-'. The temperature was held constant at 25°C using a water bath, and measured by a copperconstantan thermocouple. The products are classified into two types: (a) the product deposited on the cathode, called cathodic electropolymerized acrylonitrile (CEPAN); (b) the product collected from the medium, called medium electropolymerized acrylonitrile (MEPAN). CEPAN was collected directly from the cathode, while MEPAN was separated from the electrolytic solution, which was added to redistilled water and collected by filtration, then dried for about 1 week at 40°C in a vacuum oven. The yield was found to be greater than 75% based on total monomer initially present. All chemicals were of Analar Grade (BDH) and used without further purification. From selected CEPAN and MEPAN samples the content of carbon, hydrogen, nitrogen and oxygen was determined by elemental analysis (Cairo University, Egypt). Density measurements of both CEPAN and MEPAN samples were carried out using a density gradient column (Davenport, London) comprising a mixture of n-heptane and carbon tetrachloride, because no swelling takes place between PAN and this mixture.16 Polymer discs were prepared under constant pressure (20toncm-') for 30min, and then kept in a vacuum oven at 40°C for 24 h prior to density measurements. Relative viscosity measurements for MEPAN were carried out for 0.5(w/v) solutions in DMF at 30°C using an Ubbelohde viscometer. Infrared spectra for both CEPAN and MEPAN samples were recorded on a Mattson 5000 FTIR spectrometer by embedding 20mg of the polymer powder in 200mg KBr, followed by compression of 40mg quantities of this mixture to make pellets. UV spectra for MEPAN samples were recorded on a Unicam UV2-100 UV/visible spectrometer V3-32 using a constant concentration of 2 x i0-4gmi-1 for all samples, with dimethylsulphoxide,(DMSO) as solvent. X-ray diffraction studies of both CEPAN and MEPAN were carried out on a Phillips PM992/05 PW 1840 diffractometer using Cu-Ka irradiation. The degree of polymer homogeneity of MEPAN samples was checked by thin layer chromatography (TLC) using silica gel G F 254 (Merck) with benzene and acetone solution in a ratio of 9 :1. Electrical conductivity measurements were performed at temperatures ranging from room temperature to 423 K, by using a two-probe technique.' The sample discs (0.1 cm thick and 1.25 cm diameter) were made by compressing the powdered sample at a constant pressure of 10 ton cm-' for 10min. The common circular area of diameter 8.0mm on the disc surfaces was silvercoated to achieve better electrode contact. To avoid N surface charge effects, another ring, 2.0mm wide, on one face close to the disc edge was also coated and connected to earth. The specimen was kept in a thermostatically controlled chamber stable to & 0.1"C. The temperature of the specimen was measured by a thermocouple kept close to the specimen. The chamber was evacuated to a pressure lower than 10-3torr to minimize moisture effects. RESULTS AND DISCUSSION Cathodic electropolymerized acrylonitrile (CEPAN ) It is worth mentioning that CEPAN materials prepared with different electrolytic current densities usually have the same physical and chemical properties. The polymers formed were found to be insoluble in common organic solvents indicating that they have a highly cross-linked structure. The average density of ~ 3WC, CEPAN samples was 1.194 + 0 . 0 0 5 g ~ m - at which is higher than that reported for PAN prepared by free-radical polymerization in DMF (1-1685 and 1.1776g~m-~)'','~or in NaCNS (1.1863g ~ m - ~ ) . ' ' , ' ~ This difference in density may be attributed to the high molecular packing due to cross-linking. Molecular packing in PAN is mostly due to the strong dipoledipole interaction between the nitrile groups of the adjacent chain^,^ which increases in the case of crosslinking. Optical microscopy studies of CEPAN showed that the condensed film exhibited a striated structure. Figure 1 shows a typical fibril structure of CEPAN prepared at a current density of 0.417mA cm-'. X-ray diffraction studies indicate that CEPAN samples have a partially crystalline structure independent of the current density used for preparation. Figure 2 represents the X-ray diffraction spectrum of samples prepared at a current density of 0-417mA cm-'. X-ray peaks appeared at 28 values of 39.1", 42.2", 46.2" and 54.6" after using an average moving point filter for 10 scans. These values are quite different from that reported for PAN prepared by free-radical polymerization (28 = 16.5" and 29.5").8,10*16,'8This difference in X-ray diffraction may be ascribed to the high cross-linking and/or the presence of conjugated structure. . Electrical conductivity measurements of CEPAN showed that DC conductivities are less than (0cm)- ', while AC measurements exhibited no response to frequency in the range 10 to 105Hz. Likewise there was no observed temperature effect from room temperature to 423 K. These results clearly indicate that the C=N groups, even the unconjugated groups, form a highly cross-linked structure. Elemental analysis of CEPAN samples prepared at different electrolytic current density showed approximately the same carbon, hydrogen and nitrogen perPOLYMER INTERNATIONAL VOL. 41, NO. 4, 1996 465 Structure and properties of PAN Fig. 1. Optical micrograph of CEPAN prepared at electrolytic current density of 0 . 4 1 7 m A ~ m -(magnification ~ x 1200). centages (C 67.86, H 5.69 and N 26.27 (9942%)). These values are in agreement with those reported16 for PAN treated up to 200°C, and coincide with those calculated for pure acrylonitrile (calculated percentage, C 67-91,H 5.709, N 25.40). IR spectra of these samples showed no significant difference with current density. Figure 3 illustrates the identical IR spectrum of CEPAN samples. This spectrum is comparable with that reported6 for electropolymerized PAN, and that having amorphous structure.* The absorption peaks appearing in the range 1000-1600 cm- indicate the presence of conjugated C=N g r o ~ p s . ~ * *Th *~ e ~band - ~ ~ around 2200 cmgenerally attributed" to the presence of C=N, appeared in these spectra and its relative height to the band at 2240cm-' (which is ascribed to the presence of ' ', 4 3.5 3 2.5 2.5 e -3 v 2 C 1.5 1.5 1 0.5 ' 0 -10 15 20 25 30 35 ze. 0 40 45 50 55 1 0 60 Fig. 2. X-ray diffraction of CEPAN prepared at electrolytic current density of 0-417mAcm-'; dashed curve, as produced; the smooth curve is the average of 10 scans. POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996 C 3 N ) remained constant for all samples prepared under different current densities. Therefore it can be concluded that the general structure of the CEPAN prepared is usually independent of the electrolytic current density used for sample preparation. The structure of this CEPAN sample is highly cross-linked, striated, partially crystalline and has a few cycles of single conjugated C=N bonds. Medium electropolymerized acrylonitrile (MEPAN ) MEPAN products formed at various electrolytic current densities were found to be highly soluble in DMF in contrast to CEPAN. They exhibited colour variations from pale yellow to brown, changing to dark brown with increasing electrolytic current density. Their densities were found to be 1-1676, 1.1681, 1.683 and 1.1689 g ~ m -for~ samples prepared at electrolytic current densities 0.417, 0.83, 1.67 and 2-5mA cm-', respectively. The slight differences in density may be attributed to the degree of cyclization, which increases with applied current density. Moreover, these values of density are lower than the reported values,12'16which may be due to lower molecular weights of MEPAN. Optical microscopy investigations showed no characteristic features for all the samples prepared. Also, X-ray diffractographs of MEPAN indicated an overall amorphous pattern in the region 28 = 5-60", One can conclude from this that at low electrolytic current density this method of preparation can be used for synthesizing amorphous PAN, previously obtained by chemical methods.8*22In order to investigate the chain configuration of this amorphous PAN, the IR spectrum of MEPAN samples prepared at an electrolytic current density of 0.25 mA cm-' was compared with that of 466 F. M. Reicha c C P ! W V ztz 5z a + [L 8 WAVE NUMBER CM-' Fig. 3. IR spectra for CEPAN prepared at current density of 0.417 mAcm-2. samples prepared chemically* (Fig. 4). No significant differences between these two spectra were observed. Bands appearing at 1500-1600 cm-l were attributed to the cyclic by-product^,^^ as were the relatively weak bands around 1420 and 1640cm-'. Most of the bands were typical of those observed previously and were ascribed to linear polymerization of nitrile Samples prepared at high current densities were found to be more conjugated, as described in the discussion of elemental analysis and IR spectra. The changes of weight percentage for carbon, hydrogen and nitrogen in relation to current density are tabulated in Table 1, as are the apparent colours of the products. We note that the percentage of C, H and N remains nearly constant on using a current density up to 2.5 mA ~ m - Furthermore, ~ . the colour of MEPAN gradually changes from pale yellow to dark brown as the current density increases. These results indicate that the formation of chromophore could be attributed to the formation of cyclized rings of C S N groups without liberation of hydrogen from the products (i.e. single conjugation). The IR spectra of the products of MEPAN prepared at different electrolytic current densities are presented in Fig. 5. These spectra of electropolymerized PAN gathered from the medium, even at low current, have characteristic absorption bands as listed in Table 2. This drastic change in IR spectra is very similar to those reported for coloration of radically polymerized acryloWe note that for all nitrile by pyroly~is.~,' these samples the band around 2200 cm- l, generally attributed to C=N, appears in these spectra beside the 1920*21925-27 TABLE 1. Results of elemental analysis and the percentage of the relative intensity +I,,,,) of MEPAN samples prepared at different electrolytic current densities and their colours /,,oo/~(/,200 Current density (mA cm-') 0.417 0.83 1 *67 2.50 C (wt%) 68-10 68.28 67.19 68.22 N H (wt%) (wt%) Calc 0 (wt%) 5.46 5.51 5.49 5.52 25.64 25.63 25.68 25.71 0.60 0.58 0.64 0.55 (%I Apparent colour 33.53 34.58 44.58 45.83 Pale yellow Light brown Brown Dark brown ~2200/~(~22 +012240) 0 POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996 467 Structure and properties of PAN 4000 3600 3;00 2800 2400 2000 2240 cmband. However, the intensity of this 22OOcm-I band shows a systematic increase, while the nitrile bond stretching ( C e N ) shows a systematic decrease as the current density increases. Table 1 shows, for various samples, the percentage of the relative intensity of the enamine band around 2200cm-’, estimated by the ratio I,,,,/(~ I,,,, + I,,,,), which is the ratio of the heights of the absorption C=N to the combined heights of the two bands, 2200 and 2240 cm- l. It can be seen that this ratio increases with increasing current TABLE 2. Characteristic IR absorption bonds attributed to electropolymerized AN IR bands Tentative assignment (absorption modes) (cm-’) 3 525-3 360 2 940 2 240 2 200 1640 1560 1 520 1450 1420 1385 1170 990 770 stretching (NH) stretching (CH) (H 7 = y H ) stretching (C=N) stretching (C=NH) stretching (C=C) stretching (C=N), stretching (C=N) bending (CH,) bending in-plane (CH) (H 7 = T H) bending (CH,) or bending (CH) stretching (C-N) bending out-of-plane (CH) (H 7 = y H) bending out-of-plane (CH) (H 7 =T H) POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996 1800 1600 1400 l;OO 1630 800 7 600 density up to 2.5mAcm-’ then remains nearly constant. In most previous works, the decrease in the intensity of the 2240cm-’ band was attributed19 to the decrease in the concentration of CEN groups, which are not converted to C=N or other chemical structures. The appearance of a band near 2200cm-’ could be explained” on the basis of the formation of cyclic structure of conjugated PAN. Therefore, one can conclude that this ratio reflects the degree of increase of interchain cyclization during the electropolymerization of AN at various current densities. The polymer formed reacts with trace amounts of 0, (present during electropolymerization) to give a pyrridone-type structure,19 which has a brown colour. This is consistent with our observation that, as the current density increases, the sample colour turns from pale yellow to light brown then to brown and finally to dark brown. The UV spectra (Fig. 6 ) for MEPAN samples prepared at various current densities confirm this conclusion. The UV spectra show that the absorption intensity increases with increasing preparation current density, reaching a constant value at a current density of about 2.5 mA cm-’. The absorption peak at 4.05eV is characteristic of all samples prepared at different current densities. Also, it can be noted that some absorption peaks near to the characteristic one (4.05 eV) appeared for samples prepared at current densities above 2.5rnAcm-’. This may be a result of byproducts due to additional monomer reaction pathways F. M . Reicha 468 w 7 I, I 4 3 I YO 'r /f C r a 11 s 111 . . _._- I t t 1 n C ~ .. . Wavenumbers Fig. 5. IR spectra of MEPAN prepared at current densities: (a) 0*417rnA~m-~; (b) 0 . 8 3 3 m A ~ m - ~(c) ; 1 * 6 7 m A ~ r n -(d) ~; 2*5rnA~rn-~. POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996 469 Structure and properties of PAN 5.0-4.8 - 4.61.4- i90200 20 40 tio m 300 20 40 m 80 W a n length 400 20 LO m 80 m M 40550 (nm) Fig. 6. UV spectra for MEPAN prepared at current densities: (a) 0-417rnAcm-’; (b) 0.833mAcrn-’; (c) 1*67rnAcm-’; (d) 2.5 d c m - ’ ; (e) 3-33mA cm-’. The peaks below 250nm are artifacts related to the limitations of the detector. and/or further reaction of the polymer. Therefore one can conclude that as the current density used for electropolymerization increases, the interchain cyclization spreads over the polymer chain due to conjugation of CEN, until the current density reaches about 2.5 mA cm-’, above which side reactions may take place. It is interesting to note that the samples prepared at current densities ranging from 0.25 to 2.5 mA cm-’ possess a homogeneous structure, while those prepared at current densities above 2.5 mA cm-’ show nonhomogeneity as indicated by TLC. The samples prepared at current densities 0-417 and 2.5 mA cm-’ were chosen for investigation by TLC. The results indicated that both samples have homogeneous structure. TLC showed mean retardation factors of 0.58 and 0.26 for the two samples, respectively. The retardation factor RP depends upon molecular weight and/or the dipole moments. The ratio of the intrinsic viscosities of both samples was found to be 1 : 1.02, which shows a slight increase in molecular weight for samples prepared at current density of 2.5 mA cm-’ over those prepared at 0.417 mA cm-’. This indicates that the molecular weight is not the major contributing factor. The main change in R, could be related to the dipole moment as POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996 follows. The high value of the dipole moment (3.9 debye)28 of C=N causes strong attraction or repulsion (according to the orientation) of other medium molecules (for example, acetone in the present case) which also possess a high dipole moment. A strong and rather specific interaction between pairs of C E N groups is also possible, which minimizes the possible attraction with a c e t ~ n e ’ ~leading . ~ ~ to an increase in values of R, for samples having more C z N groups without conjugation. This result may therefore be another indication of increasing conjugation with increasing electrolytic current density used for sample preparation. DC conductivity of MEPAN samples The current as a function of the applied field (Vm-’) in the range from lo4 to lO' Vm-l was studied at various temperatures. Within experimental error the current exhibits a linear field dependence. From this ohmic relation the conductivity at each temperature was determined. The temperature variation of DC conductivity for MEPAN samples (prepared at different current density) is shown in Fig. 7. The data clearly show that, in the temperature region investigated, the DC conductivity of 470 F. M . Reicha -3-. -4 - - -5.. .-- ‘5 <!- - -6-- -7-. b I’- 8 - -9-- -10 -- -11 -12 2.2 2.4 2.6 2.8 3.0 3.2 (1WOlT)K 3.4 3.6 3.8 4.0 Fig. 7. Variation of loga as a function of temperature for MEPAN samples prepared at current densities: (a) 0-417mA~rn-~; (b)0~833mAcm-2;(c)1~67mAcm-2;(d)2~5mAcm-2. these samples obeys the Arrhenius relation : o = a. exp(-E/K T ) where cro is the conductivity pre-exponential factor, K is the Boltzmann constant and E is the activation energy for conduction. Each sample is characterized by a single activation energy in the temperature interval used here. The conduction activation energy has been calculated using least-squares fitting of the data to the above relation. The variation of activation energy E with current density is given in Table 3. The activation energy for samples prepared at current density 0.417mA ~ m was - ~ found to be 1.48eV7 which is in good agreement with the earlier reported value of 1-5eV for pyrolysed polyTABLE 3. Values of DC electrical resistivity p at room temperature, activation energy E and preexponential factor a, for MEPAN samples prepared at different current densities Current density (rnA crn-’) 0.41 7 0.833 1.67 2.50 p at 300 K (ncm) 1.5 x 3.4 x 1.3 5.8 x 10’’ 10’0 ~10~ 107 Log Qo E (n-’crn-‘) (ev) 13.12 11.23 9.1 7 7.68 1-48 1a27 1.09 0.93 a~rylonitrile.~’ As the current density increases the conduction activation energy decreases until the current density reaches about 2.5 mA ~ m - ~ but, on further increase in current density there is little change in activation energy values. Extrapolation of the plot of log o against 1000/T gives values of the conductivity pre-exponential factor oo. These are listed in Table 3 together with values of resistivity at room temperature and activation energy E. From Fig. 7 it is concluded that the electronic conduction in these samples is a thermally activated process with a single activation energy in the temperature interval used for measurements. It is noticed from Fig. 7 and Table 3 that E decreases with increasing preparation current density (up to 2-5mA ~ m - ~ ) , and the pre-exponential factor oo, which is of the order of 1013~2-1cm- for samples prepared at current density 0.417 mAcm-2, decreases to about 106C2-’ cm-’ for samples prepared at current density 2*5mAcrn-’ or higher. Also, we note that for all samples prepared at current densities above 2.5 mA ~ m the - ~variation of logo against 1000/T which is not represented in the figure, gives values nearly identical to that prepared at a current density of 2.5 mA cm-’. From the values of resistivity, the preexponential factor and activation energy one can conclude that the samples behave as semi-conducting ’ POLYMER INTERNATIONAL VOL. 41, NO. 4, 1996 Structure and properties of PAN materials. Their conductivities increase as the preparation current density increases, which in turn increases the cyclization in the macrochain. In this way, the electrical conduction of the polymer samples (MEPAN) is due to participation of lone pairs of electrons on the conjugated nitrogen atoms with a-bonds in the macrochain, in which delocalization of electrons is also possible. As conjugation increases the electron mean-free path increases, leading to an increase in conductivity and in the pre-exponential factor, while a decrease in conduction activation energy is possible. CONCLUSIONS The results of the present experiments reveal that MEPAN and CEPAN are different structural types of PAN formed during its synthesis by the electropolymerization technique. The CEPAN samples most commonly studied are highly cross-linked, partially crystalline, have a striated structure and are insulating R-' cm-'). MEPAN in nature (conductivity collected from the solution is completely amorphous, and its conductivity was found to increase as the current density used for the polymerization process increased. 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