POLYMER SCIENCE

Smoothing out Spin-Cast Films

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Science  13 Jul 2001:
Vol. 293, Issue 5528, pp. 175
DOI: 10.1126/science.293.5528.175a

One way to apply thin polymer films to a substrate is spin casting. A solution containing dissolved polymer is sprayed onto a rotating substrate to ensure a full and even coating. Solution processing creates a polymer film with its chains in their relaxed state (which might not be the case if using melt-based processes). Controlling the surface roughness can be critical for the final optical and frictional properties of the film, which may be used as a barrier material or to modify the surface properties of a substrate. Strawhecker et al. examined glassy polymers spun from volatile solvents onto silicon substrates and found that two factors can increase surface roughness. At high evaporation rates, Marangoni instabilities, which are caused by local variations in surface tension, initiate the formation of surface roughness. The authors believe that this phenomenon terminates early on in the drying process, which raises the question of why the low-viscosity film does not heal itself to form a smooth and energetically favorable surface. They find that a second factor, the time it takes for the fluid to spread out and re-cover the surface, is greater than the evaporation time, which can be observed through large contact angles (>15_) for solvents that would normally fully wet the surface. Thus, by controlling the surface evaporation rate, it should be possible to obtain smooth films for most polymers cast from good solvents. — MSL

Macromolecules34, 4669 (2001).

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