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May 12, 2004 Low-Density Polymer Thin Film Formation in Supercritical Carbon DioxideT. Koga1, Y.-S. Seo1, J.L. Jerome1, S. Ge1, M.H. Rafailovich1, J.C. Sokolov1, B. Chu1, O.H. Seeck2, M. Tolan3 and R. Kolb4 We have developed a method for producing stable low-density polymer films using supercritical carbon dioxide (scCO2). Two different molecular weight polystyrene films with various thicknesses were exposed to scCO2 along the density fluctuation ridge in a phase diagram. The swollen structures could be then frozen via the flash evaporation of CO2 without forming additional large voids. X-ray reflectivity data clearly showed that exposure to scCO2 could be used to produce uniform low-density films about 2Rg thick or less, where Rg is radius of polymer gyration.
scCO2 has long been known to be a “green” medium for polymer chemistry or material science. Unfortunately, its potential benefits have not been fully exploited since only a limited class of polymers can be dissolved in it. Recently, we have shown that a wide variety of polymer thin films can swell by as much as 30 to 60% when exposed to scCO2 within a narrow temperature and pressure regime, known as the “density fluctuation ridge,” even when the bulk miscibility of the films with scCO2 is poor. In addition, we found that the in situ film quality, i.e. density, roughness, and film thickness, could be frozen via the flash evaporation of CO2. We report x-ray reflectivity (XR) measurements in which the density profile of the vitrified films is measured directly and correlated with other properties, such as the dielectric constant, index of refraction, and surface glass transition.
Two kinds of polystyrene (PS), corresponding to molecular weights (Mw) of 2.0×105 and 6.5×105 g/mol, were used in this study. Thin films spun cast on cleaned Si substrates were placed in a high pressure chamber containing scCO2 at T = 36ºC and P = 8.2 MPa, i.e., the ridge condition, for two hours and then quickly depressurized to atmospheric pressure within 10 seconds. The films were then characterized by x-ray reflectivity at NSLS beamline X10B. As shown in Figure 1, we found that the density of the films, which is obtained from a dispersion value in the x-ray refractive index, decreased from 23% to 13% by varying the thickness of both PS films. Furthermore, the data could be collapsed on a universal curve when the initial film thickness (L0) was scaled by Rg (Fig. 1 inset). Hence, when confined as a thin film on a surface, thickness and molecular structure influence the density of the materials.
The reduction in density has several consequences affecting the material properties of the films. First, there is a decrease in its optical index of refraction (or dielectric constant). Figure 2 shows the refractive indices (nf) of the PS (Mw = 6.5×105 g/mol) films for the unexposed and two exposed films measured using spectroscopic ellipsometry. From the figure we can clearly see that the nf values for the exposed films decreased with decreasing Lo values over the wide wavelength range. Second, there is a decrease in the glass transition temperature (Tg). In the Figure 2 inset, we plot the modulation amplitude vs. T for unexposed and exposed PS thin films obtained using the Shear Modulation Force Microscopy technique. From the figure we can see that the discontinuity in the plot, which corresponds to Tg, occurs at T = 100 ºC for the unexposed film. The Tg value for the exposed film decreases by 10 ºC compared to that of the unexposed PS film. BEAMLINE FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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Increased miniaturization in the electronics industry has
produced a need for highly uniform ultrathin polymer coatings with
well-controlled optical and dielectric properties. Often the
thickness of these films is commensurate with a few radii of
gyration (Rg) of the polymer, and tight clearances impose very
strict tolerances on surface roughness. This high degree of
confinement therefore makes it difficult to satisfy the stringent
uniformity requirements if we modify the electronic properties using
standard gas foaming methods or blending it with other polymers or
inorganic additives. Here we show a novel approach for producing
stable, uniform, low-density polymer films using supercritical
carbon dioxide (scCO2). 
