Effects of film thickness and molecular weight on the interfacial dynamics of atactic poly(methyl methacrylate)

We have investigated the surface vibrational dynamics at the polymer-vacuum interface of atactic poly(methyl methacrylate) thin films supported on SiO _x/Si(100) as a function of film thickness and molecular weight. The highly surface-sensitive and nondestructive technique of inelastic helium atom scattering probes the vibrational dynamics at the true polymer-vacuum interface due to the lack of helium atom penetration into the film. For higher molecular weight samples (350 kg/mol), scattering spectra differ between films of thicknesses of fractions of the bulk radius of gyration compared with thicker films due to substrate interactions, A difference in the line shape of the scattering spectra is also present for lower molecular weight samples (60 kg/mol) compared with the thick, higher molecular weight samples. The differences in scattering spectra indicate a reduction of interfacial surface vibrational motion for both the thin, high molecular weight, and low molecular weight films as compared to thick, high molecular weight films. Our experiments demonstrate that dynamics at the interface of polymer thin films are unchanging until the films are thin enough for substrate interactions to influence vibrations at the true interface.