Melt-induced transition in thickness and interfacial molecular orientation of poly(ethylene oxide) thin films

Melting behavior of poly(ethylene oxide) (PEO) in thin films was examined by observing the variations of thickness (Delta d) and interfacial molecular orientation using ellipsometry and sum-frequency generation (SFG) spectroscopy, respectively. The thinner PEO films exhibited a smaller ratio of Delta d to the film thickness on melting, indicating a lower degree of crystallinity (chi). For the as-cast films, this thinning effect was more evident on hydrophilic substrates than on hydrophobic substrates. For the annealed films, the thinning effect was less dependent on the substrates. These results imply that the decrease in the chi of PEO with decreasing thickness originates from two factors: reduced chain dynamics at the substrate interface and crystallization confined to a limited space, which are dominant for the as-cast and annealed films, respectively. The SFG measurements detected characteristic signals originating from crystallized PEO at the air/PEO and PEO/substrate interfaces.