Improving adhesion between polyimide surface and carbon nanotube arrays for strain-sensing devices

Multiwall carbon nanotubes (MWCNTs) deposited on a polyimide film substrate are an excellent choice for developing strain and pressure sensing devices. However, the reactivity of the polyimide surface is limited, and the MWCNT/substrate adhesion may play a significant role in the performance of the sensor. Therefore, the present work reports on the functionalization of the surface of a polyimide film (Kapton) using two chemical treatments, aiming to improve the strain transfer with acid-oxidized MWCNTs. The first one is a hydrolysis treatment in potassium hydroxide with subsequent acidification in hydrochloric acid. The second treatment consists in a mixture of sulfuric and nitric acids. X-ray photoelectron spectroscopy revealed the generation of oxygen-containing functional groups by both treatments. The one based on HNO3/H2SO4 increased the average surface roughness of the polyimide by 84%. MWCNT/Kapton adhesion was evaluated by a novel technique that uses digital image processing to quantify the remaining MWCNTs after a peeling test. This technique showed stronger adhesion for the treatment based on HNO3/H2SO4 in the first peeling test. The improved adhesion resulted in strain-sensing devices with moderately higher gauge (sensitivity) factor, which held under cyclic loading. The (undesired) influence of the thermal sensitivity of these strain gauges was also reduced by ∼14% upon acid treatment.