Lamellar Mesophases in Polyacrylonitrile: A Synchrotron Small-Angle X-ray Scattering Study Andreas F. Thu 1 nemann* Max Planck Institute of Colloids & Interfaces, Am Mu ¨ hlenberg, 14476 Golm, Germany W. Ruland Fachbereich Chemie und Zentrum fu ¨ r Materialwissenschaften, Philipps-Universita ¨ t Marburg, Germany Received August 4, 1999; Revised Manuscript Received January 20, 2000 ABSTRACT: The formation of lamellar structures in polyacrylonitrile (PAN) fibers was investigated in the temperature range from 25 to 500 °C using synchrotron small-angle X-ray scattering. It was found that, in addition to a first lamellar mesophase showing up at about 200-300 °C, a second lamellar mesophase appears in an inert atmosphere at temperatures higher than about 350 °C. The former was attributed to the cyclization, the latter to the aromatization reaction of PAN. The periods of the first (R) and the second () lamellar phase are about 13 and 5 nm, respectively. Both the R and the  phase show preferred orientation. The average angle between the fiber axis and the normals of the lamellae is about 15°. Homopolymer and copolymer fibers differ considerably in the kinetics of developing the R phase. 1. Introduction Polyacrylonitrile fibers are used widely for textiles and as reinforcement fibers. The pyrolysis of PAN fibers results in black orlon, a dark and noninflammable product. This represents the first stage in the formation of carbon fibers and is an important industrial process consisting of a number of chemical reactions and structural reorganizations. Reviews on the thermal conversation of PAN have been given by Wo ¨hrle, 1 Olive ´ and Olive ´, 2 Jain and Abhiraman, 3,4 Jain et al., 4 and Bashir. 5 A simplified reaction scheme is given in Figure 1. The conversation of PAN fibers to carbon fibers starts with an exothermic cyclization reaction in the range of 200-300 °C during which adjacent nitrile groups form conjugated CN sequences. The conjugated sequences are restricted to a length of about three to six rings. At the end of the cyclization reaction, about 20% of the nitrile groups remain unchanged. At a temperature of about 360 °C, the dehydration of the fibers to a polymer with naphthiridine moieties begins. These planar ladder chains cross-link to form a two-dimensional network at temperatures in the range from 400 to 700 °C. During the aromatization, hydrogen- and nitrogen-rich frag- ments split off and extensive polymer structures with sp 2 carbon atoms are formed. The nitrogen content decreases from 26% (PAN fibers) to 2-5% (carbon fibers). The appearance of a lamellar structure during the first stages of the heat treatment of PAN fibers has been reported by Tyson, 6 Fillery and Goodhew, 7 Hinrichsen, 8 Warner, 9 Warner et al., 10 and Walenta and Fink. 11 It was assumed that the formation of the lamellar struc- ture on heating is due to ordered and disordered domains already present in the pristine fibers, similar to the effect observed in the rayon fibers. 12 However, detailed X-ray studies on the structure of PAN fibers carried out recently 14 have shown that PAN fibers have a uniform pseudohexagonal structure and that, in particular, a semicrystalline structure can be excluded. This observation is corroborated by the fact that pristine fibers do not show any meridional synchrotron small- angle X-ray scattering (SAXS) maxima. The aim of the present work is to obtain more information on the lamellar structure and its formation using time- and temperature-dependent synchrotron SAXS. 2. Theoretical Section The following treatment uses largely the symbols already defined in earlier publications. 15-17 We consider the 2D SAXS intensity distribution obtained with * Corresponding author. E-mail: andreas@mpikg-golm.mpg.de. Figure 1. Schematic representation of the reaction stages during the thermal transformation of PAN to carbon fibers. 2626 Macromolecules 2000, 33, 2626-2631 10.1021/ma991298k CCC: $19.00 © 2000 American Chemical Society Published on Web 03/08/2000