Aqueous Photoiniferter Polymerization of Acrylonitrile

Abstract

Polyacrylonitrile (PAN) is a key industrial polymer for the production of carbon fiber for high-strength, lightweight composite material applications, with an estimated 90% of the carbon fiber market relying on PAN-based polymers. Traditionally, PAN synthesis is achieved by conventional radical polymerization, resulting in broad molecular weight distributions and the use of toxic organic solvents or surfactants during the synthesis. Additionally, attempts to improve polymer and processing properties by controlled radical polymerization methods suffer from low monomer conversions and struggle to achieve molecular weights suitable for producing high-performance carbon fiber. In this study, we present an aqueous photoiniferter (aqPI) polymerization of acrylonitrile, achieving high monomer conversion and high PAN molecular weights with significantly faster kinetics and dispersity control when compared to traditional methods. This approach allows for the unprecedented control of polymer properties that are integral for downstream processing for enhanced carbon fiber production.

Figure 1

a) Schematic representation of the photoiniferter polymerization process. b) Example reaction conditions explored in this study.

Figure 2

a) AN monomer conversion kinetics with constant illumination of UV light at 27 °C in 60 wt % ZnCl₂ (blue circles), 50 wt % NaSCN (red squares), EC (black diamonds), and DMSO (green triangles). Data displayed as mean ± standard deviation, n = 3 per time point. b) Monomer conversion kinetics with interrupted illumination at 27 °C and 60 wt % ZnCl_2(aq) as solvent. c) Theoretical (black) and experimental (red triangles) molecular weights and dispersities (blue squares) of the reaction kinetics as a function of monomer conversion. M_n,th = [M]₀/[CCPA]₀ × MW_M × p + MW_CCPA, where [M]₀, [CCPA]₀, MW_M, p, and MW_CCPA represent initial monomer concentration, initial CCPA concentration, molar mass of the monomer, conversion, and molar mass of CCPA. d) Representative GPC profiles of the purified polymer at several reaction times following polymerization at 27 °C with 60 wt % ZnCl_2(aq) as solvent.

Figure 3

a) GPC traces of high molecular weight PAN prepared by aqPI polymerization with constant illumination of UV light for 24 h, at 27 °C in 60 wt % ZnCl₂ or DMSO as solvent with target degree of polymerization (DP) provided_. b) Representative image of PAN DP 9.5k after 12 h of polymerization, demonstrating the high viscosity achieved during polymerization. c) Polymerization kinetics of AN at 27 °C in 60 wt % ZnCl₂ using PAN DP 4,000 as a macroCTA. d) PAN DP 1,000 macroCTA and chain extended poly(AN-b-AN) DP 5,000 GPC traces.

Figure 4

XPS spectra of PAN samples following a) direct precipitation and washing with deionized water and b) redissolution in DMSO and precipitation in deionized water.