MadSci Network: Chemistry |
Chris, I am fairly confident that what you are observing is a manifestation of a phenomenon known as contractility, and that you are observing a physical process, not a chemical process. The PMMA in a film is in a highly crystalline state. The actual polymer chains are in an extended conformation. Where the formic acid drops, it will free up the motions of the molecular chains, which will then move from an extended crystalline to a random coil conformation. In doing so the material will contract away from the point of solvent contact into the bulk. The result will be pits and even holes where the solvent touched, and a slight wall thickening (that you will not notice unless you look really carefully) around the edges of the pits. This sort of phenomenon is discussed in most standard Polymer science textbooks. You will find some discussion, for example, in Mandelkern 'An Introduction to Macromolecules', Springer, 1972., p. 89 and following.
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