Probing particle structure in waterborne pressure-sensitive adhesives with atomic force microscopy

There is a need to know the nanostructure of pressure-sensitive adhesive (PSA) films obtained from waterborne polymer colloids so that it can be correlated with properties. Intermittent-contact atomic force microscopy (AFM) of an acrylic waterborne PSA film identifies two components, which can be attributed to the polymer and the solids in the serum (mainly surfactant). It is found that when the average AFM tapping force, FavFav, is relatively low, the polymer particles appear to be concave. But when FavFav is higher, the particles appear to have a convex shape. This observation is explained by a height artefact caused by differences in the indentation depths into the two components that vary with the tapping amplitude and FavFav. To achieve the maximum contrast between the polymer and serum components, FavFav should be set such that the indentation depths are as different as possible. Unlike what is found for the height images, the phase contrast images of the PSA do not show a reversal in contrast over the range of tapping conditions applied. The phase images are thus reliable in distinguishing the two components of the PSA according to their viscoelastic properties. At the surface of films dried at room temperature, the serum component is found in localized regions within permanent depression into the film.

Whether particles in an adhesive latex film appear convex or concave in atomic force microscopy depends on the differences in indentation depths between the polymer and serum phases, which are shown to vary with the tapping force. The corresponding phase contrast images (not shown) do not undergo a reversal in contrast.Figure optionsDownload as PowerPoint slide