The effect of immersion time on the spontaneous formation of an aryl diazonium adlayer on exposed carbon fibers of a reinforced polymer (CFRP) composite was investigated. The composite was a standard airframe structure prepared with polyacrylonitrile (PAN-based) carbon fibers (7 μm diam.) in a plain wave pattern. The surface treatment was performed under open circuit conditions in solution of 50 mmol L–1 4-nitrophenyldiazonium tetrafluoroborate (4-NPD) and purified acetonitrile by immersion for times from 6 to 24 h. The blocking effects of the adlayer were assessed using the surface-sensitive redox systems, Fe(CN)63–/4– and O2 reduction. A progressively increasing electroactive surface coverage was observed with increasing immersion time, trending toward a limiting value after 18 and 24 h. The adlayer formed during all immersion times completely blocked electron transfer for Fe(CN)63–/4–, but only immersion times 12 h and longer effectively blocked electron transfer for the smaller, uncharged O2 molecule in 0.5 mol L–1 Na2SO4 with a 70–75% cathodic current attenuation at −1.0 V vs Ag/AgCl. An electroactive surface coverage of 1.79 (±0.12) × 10–9 mol cm–2 was observed, based on the peak charge for the pendent group NO2/NH2 reduction reaction in deaerated 0.5 mmol L–1 Na2SO4 for specimens immersed for 24 h. XPS results are consistent with some multilayer formation after the 24 h immersion, with evidence for a second aryl molecule grafting to the initially bound nitrophenyl admolecule through an azo linkage (–N═N–). This assessment is based on the presence of N 1s peak intensity at 400 (–N═N–) and 406 eV (NO2). The results demonstrate that electron exchange can occur under open circuit conditions forming reactive aryl diazonium radicals that graft to the carbon fiber surface. The blocking effects of the 4-NPD adlayer formed spontaneously after 24 h for both Fe(CN)63–/4– and O2 reduction were superior to that of an adlayer formed electrochemically via potential cycling.
Augustine et al. (Wed,) studied this question.