Post-Polymerization Modification to Synthesize Atactic and Isotactic Polyacrylonitrile Copolymers and Related Carbon Fibers

Over 90% of carbon fibers (CFs) are made from polyacrylonitrile (PAN), which is typically prepared as a copolymer through free radical polymerization that yields a broad dispersity (<i>D̵</i> ≥ 2) and little to no control over the stereochemistry in the polymer backbone (∼30% <i>meso</i>-<i>meso</i> (<i>mm</i>) triads). PAN-based CFs are fabricated through a series of high-temperature curing steps on wet-spun fiber precursors that lead to cyclization of the nitrile groups and oxidation, followed by the formation of graphitic crystallites during the final carbonization step. The extent of graphitization is expected to be highly dependent on the stereochemistry of the precursor polymer. Here, we introduce a new approach to synthesize isotactic PAN (iPAN) using a two-step chemical process that starts with a Lewis acid-based stereocontrolled polymerization to make low dispersity (<i>D̵</i> < 1.25) isotactic poly(<i>tert</i>-butylacrylamide) (iPTBAM; <i>M</i>_w ∼ 150 kDa; ∼77% <i>mm</i> triads), followed by a post-polymerization modification step to convert the majority of acrylamide groups to nitriles (up to ∼78%). Atactic PTBAM was also prepared without using a Lewis acid. Conversion to iPAN-TB and aPAN-TB was confirmed using IR and ¹H NMR spectroscopy. Each copolymer, along with a control PAN (<i>M</i>_w ∼ 130 kDa; Sigma-Aldrich), was wet spun into precursor fibers and processed in muffle (250 °C) and tube (1500 °C) furnaces. The results from tensile experiments (pre- and post-muffle furnace) showed iPAN-TB had the highest tensile strength and modulus after heating, nearly six- and four-fold higher, respectively, than the fibers derived from Sigma PAN, while not sacrificing extensibility after heating.