Combining Ellipsometry and AFM To Probe Subnanometric Precursor Film Dynamics of Polystyrene Melts

Abstract

We investigate the evolution over time of the space profiles of precursor films spreading away from a droplet of polymer, in the poorly explored pseudo-partial wetting case. We use polystyrene melt droplets on oxidized silicon wafers. Interestingly, the film thicknesses measured by ellispometric microscopy are found in a 0.01 to 1 nm range. These thicknesses were validated by atomic force microscopy (AFM) measurements performed on the textured film obtained after a quench in temperature. From this, an effective thickness is obtained and compares well to the thicknesses measured by ellipsometry, validating the use of an optical method in this range of thickness. Ellipsometric microscopy provides a height resolution below the ångström with lateral resolution, image size and framerate well adapted to spreading precursor films. From 1 this, we demonstrate that precursor films of polystyrene consist of polymer chains with a surface density decreasing to 0 away from the droplet. We further find that the polymer chains follow a simple diffusive law with diffusion coefficient independent of density. This demonstrate that polystyrene chains spread independently in precursor films in pseudo partial wetting condition. This behavior differs significantly from the case of chains spreading in total wetting, for which the diffusion coefficient was found in the literature to depend on surface density or thickness.

Rouse 2D Diffusion of Polymer Chains in Low Density Precursor Films of Polybutadiene Melts

“…In the present paper, we study the wetting of polybutadiene melts on oxidized silicon wafers, which experience conjoining interactions that tend to squeeze the precursor film (Π < 0): a motionless droplet coexists with the precursor film. This regime is called pseudopartial wetting and was evidenced for high polarizability polymers. ,− We will show that depositing polybutadiene droplets on such surfaces is a way to prepare precursor films of low density, which smoothly decay over an extended lateral region. We will characterize the precursor film profiles and dynamics by ellipsometry and show that the chains in the film obey a diffusion equation.…”

“…Atomic force microscopy showed that the precursor films have no texture. 43 The typical spreading of a precursor film of PBd 1,4 around a sessile droplet of radius r d ≃ 100 μm at short times is shown in Figure 1. A few minutes after the deposition of the droplet, the contact line stops, while a film spontaneously spreads around the droplet.…”

“…Deposition method, experimental setup, and data analysis were detailed in a previous study. 43 We ensured that the volatility of the polymer melts can be neglected and the absence of volatile pollutes by checking that nothing transfers to the surface from a drop kept slightly above it. Atomic force microscopy showed that the precursor films have no texture.…”

“…The polymer thickness h may be smaller than a molecular size, and in that case the polymer layer is in a nondense state. As demonstrated in the literature, the bulk value of the refractive index of polybutadiene still can be used. , Noise reduction is achieved by taking advantage of the quasi circular shape of the droplets: thicknesses are angularly averaged over 10° angles centered to the droplet center. Deposition method, experimental setup, and data analysis were detailed in a previous study .…”

“…As demonstrated in the literature, the bulk value of the refractive index of polybutadiene still can be used. , Noise reduction is achieved by taking advantage of the quasi circular shape of the droplets: thicknesses are angularly averaged over 10° angles centered to the droplet center. Deposition method, experimental setup, and data analysis were detailed in a previous study . We ensured that the volatility of the polymer melts can be neglected and the absence of volatile pollutes by checking that nothing transfers to the surface from a drop kept slightly above it.…”