The effects of irreversible polymer adsorbed layers induced by CO2 annealing on recrystallization/dewetting of ultrathin PEO films

The effects of CO$_{2}$ annealing on melting/recrystallization processes of spin-cast poly (ethylene oxide) (PEO) ultrathin films (20, 50 and 100 nm) prepared on Si substrates were investigated. In-situ neutron reflectivity results showed that all PEO thin films melt at a pressure as low as P$=$2.9MPa and at T$=$48$^{\circ}$C which is below the bulk melting temperature (T$_{m}$). The films were then subjected to quick depressurization to atmospheric pressure, resulting in the non-equilibrium amorphous state of the saturating polymer with CO$_{2}$, and the recrystallization or dewetting process was induced during the continuous evaporation process at given temperatures below T$_{m}$. Detailed structural characterization using grazing incidence X-ray diffraction, atomic force microscopy, and polarized optical microscopy revealed two unique aspects of the CO$_{2}$-treated PEO films as compared to PEO thin films prepared via thermal annealing: the flat-on lamellar orientation, where the molecular chains stand normal to the film surface, is formed within the entire film regardless of the film thickness and the evaporation temperature; the dewetting kinetics for the 20 nm thick films is much slower than that of the thicker films. We will discuss the origins in the presentation.