Carbon black functionalized with hyperbranched polymers: synthesis, characterization, and application in reversible CO2 capture

Abstract

The functionalization of carbon black by grafting of hyperbranched polymers from the surface via self-condensing atom transfer radical polymerization (SC-ATRP) is reported. The surfaces of the pristine carbon black were modified with ATRP initiators using two different methods. The first method uses acid oxidation to place COOH groups on the carbon black surface, followed by the esterification to give ATRP initiators bound to the carbon black surface. Alternatively, ATRP initiators bearing both azido and bromine groups were placed directly on the carbon black surface by nitrene chemistry. The hyperbranched poly(p-chloromethylstyrene) (PCMS) was grafted from the ATRP initiator modified carbon black surfaces using the inimer of p-chloromethylstyrene, and CuCl/CuCl₂/N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA) as the catalytic system in N,N-dimethylformamide (DMF). In addition, a one-pot two-step method was developed to graft crosslinked polymers from the carbon black surfaces. The polymerization process was well controlled and the fraction of the grafted polymers on carbon black could be adjusted by changing the polymerization time. The resulting samples were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The hyperbranched polymers on the carbon black surfaces were quaternized to give an ammonium group on the polymer, and a chloride counterion was subsequently exchanged to give a hydroxide counterion. Under these basic conditions, the ATRP initiators should be attached to the surface using nitrene chemistry rather than acid oxidation, to avoid the hydrolysis of the ester groups that link the grafted polymers to the carbon black surface. The ammonium hydroxide functionalized carbon black materials were utilized for the reversible absorption and desorption of CO₂ from ambient air, providing improved absorption/desorption kinetics compared with the commercially available Excellion membrane.