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.