Obtaining and interpreting images of waterborne acrylic pressure-sensitive adhesives by tapping-mode atomic force microscopy

Abstract

The first atomic force microscopy (AFM) images of waterborne acrylic pressure-sensitive adhesives (PSAs) are presented along with details of their optimum scanning conditions. Driving this work is a huge practical need for information about the surface morphology of waterborne PSAs, which are deposited from colloidal dispersions to yield highly tacky, soft surfaces. These surfaces present contradictory requirements for tapping-mode AFM. Whereas soft surfaces require light tapping to avoid surface damage, tacky surfaces require energetic tapping to enable the tip to lift off of the surface. We have made a systematic study of the effects of several key parameters: the cantilever spring constant; the free amplitude of oscillation (Ao); the setpoint value (dsp); and the setpoint ratio (rsp = dsp/Ao), which we have re-defined for a soft surface to account for the indentation depth. Amplitude-distance curves were obtained from the PSA surfaces to evaluate the tip's indentation depth. Reliable images are obtained when these parameters are known and optimized. While the "true" surface of the film is actually rather smooth, images of the sub-surface particle morphology are best obtained with a stiff cantilever (spring constant of 48 N/m) and a large Ao (about 135 nm). Setting rsp close to unity minimizes the indentation of the tip and the resultant surface deformation.