Thermodynamics and kinetics of polyoxyethylene alkyl ether evaporation from inkjet-printed carbon nanotube thin films by vacuum annealing

Carbon nanotubes (CNTs) dispersed in water by sonication with surfactants are used as printing inks. However, surfactants are usually electrical insulators that remain on the prepared CNT films, reducing their conductivity. While evaporation by annealing is an effective removal process, it is usually performed at high temperatures (180 °C) in air, making it inapplicable for printing on common plastic substrates. This work demonstrates a vacuum annealing process for evaporating surfactants. Poly(oxyethylene)₄ lauryl ether (Brij L4) - a nonionic surfactant - was utilized. Its evaporation behaviors (thermodynamics and kinetics) were analyzed by thermogravimetric (TG) analysis and infrared (IR) spectroscopy. The TG measurements confirmed that the evaporation onset temperature under vacuum (51.0 °C) was lower than that in air (148.7 °C). The IR results revealed that ∼100% Brij L4 was removed by annealing under vacuum at 130 °C. The desorption energy of the surfactant from CNTs was calculated from the activation energy results to be about 10 kJ mol^-1; this value is about a half to one order of magnitude smaller than those reported for other surfactants. This energy would allow adequate adsorption interactions for the surfactant to stably disperse the nanotubes, but is sufficiently small to allow easy removal of the surfactant by thermal treatment. The electrical resistance of the vacuum-annealed CNT thin film dramatically decreased compared with that of the film annealed in air. Therefore, this process can be used for fabricating flexible printed electronics with CNTs.