Competing Surface Fields on the Ordering Transition of Block Copolymer Films

The ordering temperatures of \textit{polystryene-b- polymethylmthacrylate} (PS-$b$-PMMA) thin film diblock copolymers of thickness h$\le $2$L$, supported by \textit{SiO}$_{x}$\textit {/Si} substrates, were examined in compressed CO2 fluid and vacuum environments. The substrate is shown to induce ordering into the film as temperatures above the bulk order-disorder transition. This ordering temperature is estimated to be about 200 degrees above the bulk ODT. We also show that in compressed CO$_{2}$ environments, these films are ordered at temperatures above which they would be ordered in vacuum! This latter observation is of particular significance since compressed CO$_{2}$ is a known plasticizer for both polymers and low molecular weight diluents generally have the effect of lowering the \textit{ODT}, not increasing it.