Atomic force microscopy as a tool to investigate and use thin polymer films and chain interactions.

摘要

Here we study the interactions between a nanoscale asperity and a variety of surfaces, and how these interactions can be used to evaluate or modify surfaces. An Atomic Force Microscope (AFM) tip was used as the model nanoscale asperity for this investigation.; The first investigation allowed us to observe the chain extension of polydimethylsiloxane (PDMS), as well as measure the hydrogen bonding between the polymer and silanol groups present on the AFM tip. The chain extension was modeled using the Langevin equation, which together with the hydrogen bond force can describe macroscopic adhesion on grafted polymer layers.; In the second investigation an AFM tip coated with a small amount of PDMS liquid was scanned on a variety of low (methyl and perfluoromethyl functional alkylsiloxanes supported on silica) and high (silica-Si/SiO₂) energy surfaces. This measurement reveals the capillary bridging and liquid spreading behavior of the PDMS on these surfaces. We modeled the results using the Young-Laplace equation and discovered that we could use liquid bridging behavior to measure contact angle. It is also possible to create simple, sub-micron surface features with this technique. We describe the example of precision deposited liquid drops, which can then be cured into elastomeric spherical caps.; With understanding of these model systems it is possible to investigate an unknown system for the purpose of improving adhesion or release. We evaluated how unreactive silicone oils added to a silicone elastomer formulation effect its surface properties. We measured the thickness of the oil layers that formed on these surfaces and correlate the film thickness with macroscopic friction. The friction on these surfaces decreases with increasing oil layer thickness until the layer is 3–6 nm thick, at which point a low plateau friction is reached. The molecular weight of the added oil also has an effect. Oils smaller than the mesh size of the elastomer network remain trapped within the bulk, and oils larger than the mesh size diffuse to the surface. However, very high molecular weight oils can become kinetically trapped within the network.