Capillary wave dynamics of thin liquid polymer films

Molecular Probe Diffusion in Thin Polymer Films: Evidence for a Layer with Enhanced Mobility Far above the Glass Temperature

We studied experimentally the influence of interfaces on the dynamics in thin polymer films at temperatures far above the glass temperature (T_g + 80 °C). Polyisoprene (PI) was employed as a model system. We examined glass substrate supported films with thicknesses (d) spanning the range from 10 μm to 10 nm that correspond to d/R_g from 400 to 1, where R_g is the polymer radius of gyration. We employed fluorescence correlation spectroscopy (FCS) to monitor the translational diffusion of small fluorescent tracer molecules, dispersed at nanomolar concentrations in the PI matrix. In thick films, a single diffusion process correlated to the bulk segmental dynamics of the matrix polymer was present. However, when the film thickness was smaller than the normal dimension of the FCS observation volume, a second, faster diffusion process appeared, reflecting enhanced segmental dynamics near the free surface. Our results provide direct experimental evidence for the existence of a layer with enhanced mobility near the free surface of supported PI films at temperatures as high as 80 °C above the bulk T_g.

Dynamics of ultra-thin polystyrene with and without a (artificial) dead layer studied by resonance enhanced dynamic light scattering

Using non-invasive, marker-free resonance enhanced dynamic light scattering, the dynamics of capillary waves on ultrathin polystyrene films' coupling to the viscoelastic and mechanical properties have been studied. The dynamics of ultrathin polymer films is still debated. In particular the question of what influence either the solid substrate and/or the fluid-gas interface has on the dynamics and the mechanical properties of films of glass forming liquids as polymers is in the focus of the present research. As a consequence, e.g., viscosity close to interfaces and thus the average viscosity of very thin films are prone to change. This study is focused on atactic, non-entangled polystyrene thin films on the gold surface. A slow dynamic mode was observed with Vogel-Fulcher-Tammann temperature dependence, slowing down with decreasing film thickness. We tentatively attribute this relaxation mode to overdamped capillary waves because of its temperature dependence and the dispersion with a wave vector which was found. No signs of a more mobile layer at the air/polymer interface or of a "dead layer" at the solid/polymer interface were found. Therefore we investigated the influence of an artificially created dead layer on the capillary wave dynamics by introducing covalently bound polystyrene polymer brushes as anchors. The dynamics was slowed down to a degree more than expected from theoretical work on the increase of density close to the solid liquid interface-instead of a "dead layer" of 2 nm, the interaction seems to extend more than 10 nm into the polymer.

Foundation of correlation ellipsometry

An experimental strategy for the detection of fluctuation dynamics at interfaces based on a combination of photon correlation spectroscopy (PCS) with a nulling ellipsometry scheme is investigated theoretically. The intensity description of ellipsometry measurements is generalized to PCS time correlation functions. The nulling ellipsometry procedure is applied for every lag time t of the correlation functions, to extract the dynamics connected to the coherent signal which contains the interface dynamics. The classical ellipsometry parameters Δ and tanΨ are generalized to functions and tan [capital Psi, Greek, tilde]_Q(t). A suitable Siegert relation is derived and employed to show that either field correlation functions or intensity correlation functions after baseline subtraction can be used as the starting point for the nulling ellipsometry procedure.