Dielectric Relaxation of Polyacrylonitrile in Its Pristine and Cyclized Stage Andreas F. Thu 1 nemann* Max Planck Institute of Colloids & Interfaces, Am Mu ¨ hlenberg, 14476 Golm, Germany Received August 31, 1999; Revised Manuscript Received November 29, 1999 ABSTRACT: The dielectric behavior of polyacrylonitrile (PAN) in its pristine and cyclized stages was investigated. A PAN homopolymer displays a strong secondary relaxation at an activation energy of E a ) 111 kJ/mol in the pristine stage and a weak relaxation at Ea ) 48 kJ/mol in the cyclized stage. By contrast, a PAN copolymer with itaconic acid, which is used in carbon fiber production, displays a strong secondary relaxation at Ea ) 134 kJ/mol in the pristine stage and two relaxations with activation energies of Ea ) 24 kJ/mol for the lower frequency relaxation and Ea ) 9 kJ/mol for the higher frequency relaxation. No relaxations were found for either polymer in the aromatized stage. The relaxation spectra were interpretated in terms of the Havriliak-Negami (HN) function. 1. Introduction PAN fibers are widely used for textiles and as precur- sors in carbon fiber production. It is known that the cyclized structure of PAN is an important intermediate step in the formation of carbon fibers, and therefore, there are a large number of publications concerning these reaction and the corresponding changes of the physical and structural properties during carbon fiber production. Some reviews on this subject have been written by Wo ¨hrle, 1 Olive, 2 Abhiraman, 3,4 and Bashir. 5 The present work deals with the investigation of their dielectic properties at the pristine, cyclized, and aro- matized stages of their transformation. The dielectric method used was broadband ac dielectric relaxation spectroscopy in the frequency range of 10 1 -10 6 Hz and within a temperature range of 100-370 K. 2. Experimental Section Dielectric relation measurements were performed with a Hewlett-Packard impedance analyzer (4284A) in the frequency range of 10 1 -10 6 Hz. The temperature of the samples, which were in a nitrogen atmosphere, was controlled by a QUATRO temperature controler produced by Novocontrol. Film samples were prepared from PAN fibers which were dissolved in dimethyl formamide (HPLC grade, Aldrich), and then the solution was cast onto a glass plate. The solvent was removed within 4 h at room temperature and for a further 24 h at 50- 70 °C in an air stream. To remove traces of solvent, the films were stored for 1 week in a vacuum chamber (10 -3 mbar) at 100 °C. By using this procedure, films were obtained with thicknesses in the range of 50-100 μm. The films could be removed from the supporting glass sheets easily. For film preparation two types of PAN fibers were used. The first type was a PAN homopolymer (<0.7% methyl acrylate) produced by Hoechst (Dolanit). The homopolymer fibers have initial tensile moduli of about 17 GPa, extensions at a break of about 8%, and diameters of about 16 μm. The second type was a PAN copolymer fiber produced by Sigri, which contains 2-6% methyl acrylate as well as itaconic acid. The copolymer fibers have an initial tensil modulus of about 11 GPa, extensions at a break of 17%, and diameters of about 14 μm. The cyclized PAN films were prepared at 230 °C for 10 h in nitrogen. Aromatized filmes were prepared from cyclized films by heating them in nitrogen to 450 °C, with a heating rate of 5 K/min. 3. Results and Discussion PAN homo- and copolymer films were investiated by frequency and temperature-dependent dielectric relax- ation spectroscopy at different stages of transformation. These stages were the pristine and cyclized. Brief surveys of the dielectric behavior of the materials were given by three-dimensional representations of the imagi- nary part of the permittivity, which is the loss DK ǫ′′- (ω,T). Films of the pristine homo- and copolymer show one relaxation within the temperature range of 300 and 450 K. In both cases the relaxation strength was high and the contribution of the electric conductivity in- creases with increasing temperature and decreasing frequency. The strength of this relaxation reduces for partially cyclized PAN and a new relaxation process near 280 K appeared, which was due to cyclized regions. For the fully cyclized homopolymer a broad region of a relaxation was observed in a temperature range of 320- 450 K (see Figure 1a). By contrast, two relaxations were found for the copolymer (see Figure 1b). We used the Havriliak-Negami function 7 for a quantitative inter- pretation of the spectra. In the lower frequency region of 10-1000 Hz, and at higher temperatures, the elec- tric conductivity made a significant contribution to the ǫ′′(ω) spectra. This was described by an additive con- ductivity term: 13 The conductivity contribution σ 0 describes the direct current conductivity. The exponent s is related to the charater of the charge transport. A scaling exponent s > 0 is typical for nonohmic charge transport and the influence of polarization close to the electrode and on inner surfaces. A question of interest was whether the relaxation processes were cooperative or thermally activated. Usually a cooperative relaxation process of polymers (R-relaxation, glass transition) can be de- scribed by the WLF equation 8 * E-mail: andreas@mpikg-golm.mpg.de. ǫ l ′′(ω) ) σ 0 ǫ 0 ω 1-s (1) 1790 Macromolecules 2000, 33, 1790-1795 10.1021/ma991494r CCC: $19.00 © 2000 American Chemical Society Published on Web 02/10/2000