Structure evolution in all-aromatic, poly(p-phenylene-vinylene)- derived carbon fibers Michael R. Buchmeiser a, b, * , Erna Muks a, b , Roman Schowner a , Erik Frank b , Ulrich Hageroth b , Sabine Henzler b , Johanna Sp € orl b , Antje Ota b , Ronald Beyer b , Alexandra Müller b a Chair of Macromolecular Compounds and Fiber Chemistry, Institute of Polymer Chemistry (IPOC), University of Stuttgart, Pfaffenwaldring 55, D-70550, Stuttgart, Germany b German Institutes of Textile- and Fiber Research (DITF) Denkendorf, K€ orschtalstr. 26, D-73770, Denkendorf, Germany article info Article history: Received 2 July 2018 Received in revised form 23 December 2018 Accepted 24 December 2018 Available online 25 December 2018 abstract Carbon fiber (CF) structure is strongly influenced in terms of CF's crystallinity, crystallite dimensions, orientation, and interlayer spacing by the structure of the CF precursor, including its sp 2 /sp 3 carbon content, and the process parameters for spinning, oxidative stabilization, and carbonization/graphiti- zation. In order to retrieve information about structure formation in all-aromatic CF precursors, poly(p- phenylene-vinylene) (PPV) fibers have been prepared through dry spinning of a sulfinyl-based precursor polymer followed by thermal conversion into PPV. By applying different stretch ratios, different degrees of orientation were realized. Subsequent thermal conversion of these PPV fibers into CFs with and without additional tension during carbonization allowed for following structure formation in the final CFs. Wide-angle X-ray scattering and Raman data were recorded at different stages of carbonization and compared to those of a poly(acrylonitrile)-derived CF as well as to lignin- and cellulose-derived CFs. Structure formation during carbonization was correlated with the sp 2 /sp 3 carbon content of the CF precursors. The all-sp 2 carbon precursor PPV was found to favor crystallite growth in the direction of the CF axis, parallel to the individual graphite planes, and formation of ordered graphitic structures at an earlier stage and to a higher extent than in high-/all-sp 3 carbon precursors. © 2018 Elsevier Ltd. All rights reserved. 1. Introduction In view of dwindling resources, lightweight, fiber-reinforced materials are of increasing interest. One of the most prominent fi- ber materials used in such fiber-matrix composites comprises car- bon fibers (CFs). CFs can be prepared from different sources including poly(acrylonitrile) (PAN), pitch, cellulose, lignin or poly- olefins, to name just a few [1,2]. Their chemical structure is highly dependent on the chemistry of the precursor as well as on the processing conditions. In fact, the differences in crystallinity, crys- tallite size, and orientation and also in carbon structure, which can be, e.g., amorphous, graphitic, or turbostratic, and the sp 2 /sp 3 -car- bon content are decisive for the final properties of CFs. Thus, high Young-modulus, pitch-derived CFs possess a high sp 2 -carbon content and graphitic structure, whereas low Young-modulus, cellulose-derived CFs possess a comparably low sp 2 -carbon con- tent [1,3,4]. PAN-derived CFs contain both sp 2 -carbon and sp 3 - carbon; in these CFs, the so-called turbostratic carbon accounts for their high tensile strength. Finally, poly(methylvinylketone)- derived CFs also show both sp 2 -carbon and sp 3 -carbon [5]. With regard to these differences in structure and mechanical properties, it is still not fully clear how the chemical structure and properties of a certain precursor fiber in terms of sp 2 /sp 3 carbon content, crys- tallinity, and degree of orientation translate into a certain CF structure and how CF structure exactly evolves during thermal processing. In view of the complexity of the process of CF structure formation, we used an all-sp 2 carbon-based precursor for our in- vestigations, i.e., poly(p-phenylene-vinylene) (PPV). We already reported on PPV and its transformation into carbo- naceous materials, e.g., non-stretched films, by thermal processing at 1000, 1400, and 1800  C[6,7]. Structure formation during py- rolysis was investigated by thermogravimetric analysis coupled * Corresponding author. German Institutes of Textile- and Fiber Research (DITF) Denkendorf, K€ orschtalstr. 26, D-73770, Denkendorf, Germany. E-mail address: michael.buchmeiser@ipoc.uni-stuttgart.de (M.R. Buchmeiser). Contents lists available at ScienceDirect Carbon journal homepage: www.elsevier.com/locate/carbon https://doi.org/10.1016/j.carbon.2018.12.096 0008-6223/© 2018 Elsevier Ltd. All rights reserved. Carbon 144 (2019) 659e665