Large scale manufacturing of inexpensive plastic parts for electronics relies on centrifugal effects that force liquid into the nooks and crannies of a spinning mold. Designers and engineers assume that thin films of polymer prepared with this method of “spin casting” behave like their bulk counterparts, but David Barbero and Ullrich Steiner at the University of Cambridge in the UK suggest this may not always be the case.
In Physical Review Letters, Barbero and Steiner describe an electro-optical cell for measuring the viscosity of polystyrene films in thickness. They cast a film on one plate of a capacitor and rest the other plate, which is transparent, slightly above the film to allow the surface of the film to move freely. With a camera-equipped microscope, Barbero and Steiner look through the transparent electrode and measure surface waves on the film, which are excited with an electric field across the capacitor.
The time it takes to excite surface wave modes provides a measure of the film’s viscosity. They find that spin-cast films have a viscosity lower than the bulk value by as much as several orders of magnitude, depending on the weight of the polymer.
Barbero and Steiner argue that the rapid solvent evaporation during casting prevents the polymer chains from reaching the strongest possible equilibrium entanglement, resulting in the increased mobility of the chains in the less-entangled melt. Since long annealing times are needed to recover the bulk properties, their results carry a caution for experiments and applications based on thin-film polymers. – Jessica Thomas