Picosecond Acoustics to Probe Interphase Properties of Sub-Micron Polymer Films

The ability to probe mechanical properties on the 5-10 nanometer length scale provides new opportunities to study polymer behavior at interfaces and in sub-micron films. We have utilized ultra-high frequency acoustic waves (30-100 GHz) to probe the through-film longitudinal wavespeed of 100-800Åthick homopolymer films as a function of the polymer/substrate interaction energy. Low energy surfaces are obtained using self-assembled monolayers, and we detect a 30longitudinal wavespeed of polymethylmethacrylate (PMMA) films thinner than 200Åcompared to the bulk. For weak interactions of polystyrene (PS) deposited on Al, we detect no changes in wavespeed compared to bulk values. However, for strongly adsorbed films of poly(2-vinyl-pyridine) (PVP) and poly(methylmethacrylate) (PMMA) deposited on Al we detect up to a 30for films thinner than 200ÅWe attribute these increases in wavespeed to changes in free energy associated with the loop/tail profile at the polymer/substrate interface.