In Situ AFM Observation of the Movements of Isolated Isotactic Poly(methyl methacrylate) Chains in a Precursor Film of an Oligo(methyl methacrylate) Droplet Spreading on Mica.

Atomic force microscopy (AFM) is a powerful tool to observe polymer chains at the molecular level. In this study, we show that the movements of isolated linear polymer chains in a precursor film of a droplet of an oligomer spreading on a substrate could be visualized in situ at the molecular level by AFM for the first time. The system was an isotactic poly(methyl methacrylate) (it-PMMA) solubilized in an oligo(MMA) matrix (it-PMMA/oligo(MMA) = 1/10000 wt/wt) spreading on mica under high humidity. Due to the limited resolution of the AFM instrument, condensed linear polymer chains could not be visualized, but a small amount of it-PMMA chains that were solubilized as isolated chains in the oligo(MMA) matrix could be visualized in the precursor film, the contrast of which came from a large difference in glass transition temperature (Tg) of it-PMMA and oligo(MMA). The it-PMMA chains in the precursor film spread in the radial direction of the droplet with vigorously changing chain conformations. The spreading rate of it-PMMA chains under 72% RH was ~ 1/30 of the spreading rate of the oligo(MMA) matrix, which was estimated based on the decrease in volume of the macroscopic droplet. The spreading of the it-PMMA chains and droplet strongly depended on humidity and was suppressed with the decrease in humidity, most likely due to the increase in friction with the substrate. The difference in spreading rate of it-PMMA and oligo(MMA) further increased under low humidity. The dynamic molecular information of a precursor film by AFM should help to elucidate the wetting dynamics on a substrate.