The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

Transition Region and Precursing Films of Polymer Melts

by Dimitar Draganov



The macroscopic wetting we observe every day is controlled by micron and nanoscale
structures and fluid motions near the contact line where the fluid, vapor, and solid meet.
Because the fluid in this region decreases in thickness as we approach the contact line, it must,
at some point, exhibit structural and dynamic behavior unlike those of bulk material. For
polymeric fluids, molecular length scales such as the radius of gyration and the persistence
length are expected to control the thickness at which this transition occurs.
In this thesis, we follow a cascade of structures connecting the bulk meniscus on a
vertical plate (millimeter to micron thicknesses), through Van der Waals (∼ 100 ?A thick)
and molecular scale (∼ 10 ?A thick) films, to the bare surface. Our measurements use small
beam X-ray reflectivity to probe molecular to micron scales and geometric beam blocking
measurements for thicker films and the bulk meniscus. We have measured and compared
two methyl terminated polydimethylsiloxane (PDMS) polymers with viscosities that differ
by a factor of one hundred as they contact the silicon oxide layer on bulk silicon crystals.
Upon contacting the substrate, we observe rapid formation of a meniscus with a larger
than equilibrium contact angle. This is followed by growth of a Van der Waals film over
a time scale of tens of hours for the less viscous material during which time the contact
angle relaxes to equilibrium. For the high viscosity case, the time scale is beyond that of the
measurement. We observe an ultrathin film spreading ahead of the Van der Waals film or
directly out of the bulk meniscus. The shape of the crossover region to this film is consistent
with a theory by Joanny and De Gennes. The ultra thin film appears to have an entangled
structure that extends with decreasing density of order five or six monomer thicknesses from
the substrate. This structure is independent on humidity, polymer chain length, or geometry
of the fluid body feeding the film.