Role of interfacial interactions on the anomalous swelling of polymer thin films in supercritical carbon dioxide

Abstract

It has recently been shown that thin polymer films in the nanometer thickness range exhibit anomalous swelling maxima in supercritical CO 2 (Sc-Co 2 ) in the vicinity of the critical point of CO 2 . The adsorption isotherm of CO 2 on carbon black, silica surfaces, porous zeolites, and other surfaces, is known to exhibit anomalous maxima under similar CO 2 conditions. It is believed that because CO 2 possesses a low cohesive energy density, there would be an excess amount of CO 2 at the surfaces of these materials and hence the CO 2 /polymer interface. This might cause excess CO 2 in the polymer films near the free surface, and hence the swelling anomaly. In addition, an excess of CO 2 would reside at the polymer/substrate and polymer/CO 2 interfaces for entropic reasons. These interfacial effects, as have been suggested, should account for an overall excess of CO 2 in a thin polymer film compared to the bulk, and would be responsible for the anomalous swelling. In this study, we use in situ spectroscopic ellipsometry to investigate the role of interfaces on the anomalous swelling of polymer thin films of varying initial thicknesses, h 0 , exposed to Sc-CO 2 . We examined three homopolymers, poly(1,1 0 -dihydroperflurooctyl methacrylate) (PFOMA), polystyrene (PS), poly(ethylene oxide) (PEO), that exhibit very different interactions with Sc-CO 2 , and the diblock copolymer of PS-b-PFOMA. We show that the anomalous swelling cannot be solely explained by the excess adsorption of CO 2 at interfaces. V V C 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45:

Swelling measurement of polymers in high pressure carbon dioxide using a spectroscopic reflectometer

“…The mirage effect results in an apparent increase in the measured swelling ratio and is clearly observed near the critical point, where refractive index of CO 2 is sensitive to small changes in temperature [34]. At the same time, the density of CO 2 is also sensitive to changes in temperature or pressure near its critical point, which induces a fluctuation in the density of CO 2 and anomalous swelling of polymers [26][27][28][29][30]. Note that the anomalous swelling is not an apparent increase, because it has been observed even when the mirage effect was not believed to affect swelling measurements [27][28][29].…”

“…Compared to in situ spectroscopic ellipsometry [11,26,30,33,34], the effect of the widow during measurements can be easily removed. Ellipsometry measures the changes in the polarized state caused by reflection using polarized light to obtain the thickness and refractive index.…”

“…Similarly, X-ray reflectivity can also be used for in situ measurements, but measurable conditions are relatively limited compared to NR [32]. In situ spectroscopic ellipsometry has been used to measure the thickness of thin films [11,26,30,33,34]. In ellipsometry, polarized light passes through the birefringent windows of high-pressure vessels, and the complex window effect must be corrected.…”

“…Near the critical point of CO 2 (31.1 • C and 7.38 MPa), polymer thin films exhibit unique anomalous swelling [26][27][28][29][30], where the density fluctuation of supercritical fluids is believed to induce significant swelling. Koga and coworkers [27][28][29] reported that the anomalous swelling of polymers is strongly correlated with the parameter obtained by dividing the sample thickness by the radius of gyration (R g ), and is clearly observed in films of which the thickness is less than several times of R g .…”

Conversion of sustained release omeprazole loaded buccal films into fast dissolving strips using supercritical carbon dioxide (scCO2) processing, for potential paediatric drug delivery

“…This could be due to the fact that scCO2 dried these films in a far more efficient manner and the resulting films were most probably more porous than the original ones. They fell apart as soon as they came in contact with water as they were 'charged' to have a high affinity for water due to a higher specific surface area (Li et al, 2007;Pham et al, 2003).…”