Atomic force spectroscopy of thermoresponsive photo-cross-linked hydrogel films

Responsive hydrogel thin films are interesting materials as responsive adhesives or as an active matrix in actuators and sensing applications, and thus, knowledge about their structural and micromechanical properties is of high relevance. Using atomic force spectroscopy, temperature-induced structural and adhesive changes of thermoresponsive hydrogel layers with micrometer thickness based on photo-cross-linked N-isopropylacrylamide (NiPAAm) were investigated in the temperature range of 18-50 degrees C. Grafted onto flat surfaces, these hydrogel layers are restricted to a highly anisotropic swelling and deswelling predominantly perpendicular to the substrate surface, which was monitored and evaluated by force spectroscopy during vertical tip approach and retraction. Analyses of the tip penetration depth yielded quantitative information about the degree of swelling. As a second feature, the critical temperature was found to decrease with increasing cross-linking density. Temperature-dependent measurements with hydrophobic and hydrophilic atomic force microscopy (AFM) tips revealed a strong adhesion to the hydrogel layer in the swollen state, which was reduced upon the layer volume collapse. These observations on the micrometer-thick gel network layers are in contrast to previous reports on ultrathin pNiPAAm brushes and monolayers, which show no adhesion in the swollen state but only in the collapsed state. Furthermore, it was found that the hydrophobicity of the hydrogel probed with a hydrophobic tip continuously increases with temperature over a broad range of at least 30 K.