Diblock copolymer thin films: Compressed fluid induced order

We show that liquid and super critical CO$_{2}$ induces long-range order into thin film, symmetric, polystyrene-b-polymethylmethacrylate (PS-b-PMMA) diblock copolymers, supported by SiO$_{x}$/Si substrates, in a temperature range where both bulk and thin films of these materials remain ordered. Under vacuum conditions the substrate induces order the thin films at temperatures significantly above the bulk order-disorder transition temperature. Ordering occurs at $\chi $N=7.94, where $\chi $ is the Flory-Huggins interaction parameter and N is the degree of polymerization, which is below the bulk value of ($\chi $N)$_{bulk}$=10.5. In the presence of CO$_{2}$, the transition shifts to yet lower values of $\chi $N, indicating a more significant degree of incompatibility between the PS and PMMA components under these solvent conditions. This behavior is in contrast to bulk systems, where the PS-b-PMMA systems of $\chi $N$>$10.5 become phase mixed in the presence of liquid or super critical CO$_{2}$.