Subcritical CO 2 Assisted Polymer Surface Engineering at Low Temperatures
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Subcritical CO2 Assisted Polymer Surface Engineering at Low Temperatures Yong Yang and Ly James Lee Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, U.S.A. ABSTRACT Polymer-based Micro/Nano Electro Mechanical Systems (MEMS/NEMS) have attracted a great deal of interest from industries and academia. The common polymer processing methods involve either organic solvents or temperatures above the glass transition temperature (Tg), which is undesirable, particularly for biomedical applications. On the basis of different properties near polymer surfaces from those in the bulk, we introduce subcritical fluids (particularly carbon dioxide, CO2) into polymer surfaces to manipulate the polymer properties at the nanoscale so that we can achieve low temperature surface engineering. In this study, polymer surface dynamics under CO2 were addressed using atomic force microscopy (AFM) and neutron reflectivity (NR). Monodispersed nanoparticles were deposited onto the smooth polymer surface and then embedded into the surface by annealing the sample at the pre-specified temperatures and CO2 pressures. The embedding of nanoparticles in the proximity of the surface was measured using AFM, and thus the surface Tg profile could be determined. It was revealed that there is a rubbery layer of up to a hundred nanometers thick at the surface where the Tg is lower than that in the bulk and CO2 dramatically reduced the surface Tg. NR studies also show that CO2 can enhance chain mobility at the polymer surfaces below the polymer bulk Tg. These results indicated that even low concentrated CO2 greatly could enhance polymer chain mobility below the Tg of the CO2–plasticized polymers. The thickness of the rubbery layer can be controlled by tuning either temperatures, or CO2 pressures, or both, which makes it possible to engineer polymer surfaces at low temperatures. Guided by the CO2 enhanced polymer surface dynamics, we developed a novel CO2 bonding technique to succeed in low temperature bonding of polymers at the micro/nanoscales. This CO2 bonding technique has been applied to seal polymeric nanofluidic biochips and construct well-defined three-dimensional (3D) biodegradable polymeric tissue scaffolds. INTRODUCTION The demand for high-precision polymer-based miniature devices (e.g., biomedical devices) has been growing rapidly in recent years. When polymers are processed at the nanoscale, viscosity and surface tension become major limiting factors. Common polymer processing techniques involve either organic solvents or a temperature beyond the Tg. Although solvents can be used to lower the viscosity, they are detrimental to many biomolecules and surface wetting may still lead to processing difficulties. In addition, they tend to deform microstructures [1]. Recent research [2-7] of polymer thin films revealed that properties (e.g., Tg) near the polymer surface are different from those in the bulk. The competition between the polymer-free surface and the polymer-substrate interactions deter
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