Method to measure the viscosity of nanometer liquid films from the surface fluctuations

Abstract

We describe a method to measure the viscosity of polystyrene liquid films with thicknesses ϳ5 and ϳ80 nm spin-cast on oxide-coated silicon. In this method, temporal evolution of the film surface is monitored and modeled according to the dynamics of the surface capillary waves. Viscosities obtained from the ϳ80 nm films display an excellent agreement with those of the bulk polymer, but those from the ϳ5 nm films are up to 10 6 times reduced. By modeling the data to the Vogel-Fulcher-Tammann relation, we find that the observations are consistent with the thickness dependence of the glass transition temperature previously reported of these films.

Mechanism of fast surface self-diffusion of an organic glass

“…This is possibly the case of IMC because 13.7n * = 5.7 if n * = n = 0.41, matching the order of magnitude of the data [1]. Figure 3 is a plot of log 10 (X bulk /X s ) against n * with data coming from several experiments on different glass formers [1,[27][28][29][30][31][32], where data on X from both bulk and surface were reported in the region around T g . The values of n * were calculated from n and n s , the values of which were either given in the Refs.…”

“…Let us consider other experiments [27][28][29][30][31][32] where measurements of a dynamic variable X(= τ α ,η,1/D,etc.) were made, each by the same technique, in the bulk and at the surface.…”

“…The values of n * were calculated from n and n s , the values of which were either given in the Refs. [27][28][29][30][31][32] [27], (b) lateral force microscopy (LFM) on polystyrene [28], (c) time evolution of surface capillary waves in nonentangled polystyrene [29], (d) LFM on polystyrene [30], (e) scanning viscoelasticity microscopy on polystyrene [31], (f) X-ray Photon Correlation Spectroscopy (XPCS) relaxation rate in dibutyl phthalate [32], (g) self-diffusion in indomethacin [1]. Fig.…”

Crossover to surface flow in supercooled unentangled polymer films

“…[8]. In that work, the morphological evolution of initially flat unentangled short-chain polystyrene (PS) films supported by silica was monitored by atomic force microscopy [23,24]. Due to thermal activation and destabilizing film-substrate interactions, the films roughen spontaneously with the development of surface capillary waves.…”

Thermodynamic Origin of the Vitreous Transition

“…The melting temperatures T m , the experimental glass transition temperatures T g or θ g = (T g − T m )/T m , the VFT temperatures T 01 determined up to temperatures much higher than T g , the VFT temperatures T 02 determined by viscosity or relaxation time measurements in the vicinity of T m , the free-volume disappearance temperatures T 0g = T 0lgs calculated using (7,8) and (18) considering that T g is, in a first approximation, nearly equal to the thermodynamic glass nucleation temperature T* g , the saving energy coefficients ε ls0 (θ = 0) of crystals surviving in the melt far above T g calculated using (19), the saving energy coefficients ε lgs0 (θ = 0) of nascent crystals homogeneously nucleated in the melt near T g calculated using (18), the free-volume disappearance temperatures T 0m = T 01 calculated using (7,8,19,20), T g , and the references [32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63] are given in Table 1.…”