Supercritical carbon dioxide induced surface melting/recrystallization process in ultrathin PEO films*

Crystallization of polymeric materials in nanoconfined geometries has attracted considerable attention in the past decade. In this talk, we will show the novel effects of supercritical carbon dioxide as a plasticizer in order to control the melting/crystallization behavior of semicrystalline polymer thin films. Poly(ethylene oxide) (PEO) thin films with thickness of 10nm-100nm were used for this study. In-situ neutron reflectivity technique was utilized to study the swelling behavior of deuterated PEO films in scCO$_{2}$ at T=50\r{ }C, showing the clear evidence of the surface melting phenomenon even below the bulk melting temperature (65\r{ }C). The surface structures before and after exposure at the different CO$_{2}$ process conditions were then investigated in air by using atomic force microscopy and grazing incidence x-ray diffraction. The results clearly showed that scCO$_{2}$-induced re-crystallization from the amorphous state via pressure quench results in various surface crystalline structures, depending on temperature, pressure, quench rates, and the film thickness.