Ordering transition of block copolymer films under compressed CO2

It is well-known that a bulk, symmetric, A-b-B diblock copolymer forms a lamellar morphology, with period L, below an order- disorder transition temperature (TODT), for XN $<$ 10.5; X is the Flory-Huggins interaction parameter and N is the degree of polymerization of the copolymer. The ordering temperatures of poly(styrene-b-methyl methacrylate) (PS-b-PMMA) thin film diblock copolymers of thickness ca. 2L, supported by SiOx/Si substrates, in vacuum environments, are shown to increase beyond the bulk, having unusually large shifts of the transition temperature with small changes of XN. We find that in compressed CO$_2$ environments, these films are ordered at temperatures where the films are disordered in vacuum (or air) environments. This latter observation is of particular significance because small molecule diluents, including compressed CO$_2$, are known to decrease the TODT of the bulk (enhanced miscibility). We will discuss these results in light of the thin film compressibility and preferential sorption of the individual blocks at the experimental conditions.