Ordering transition of block copolymer films.

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 (T(ODT)) temperature, for chiN < 10.5; chi 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 h </= 2L, supported by SiO(x)/Si substrates, in vacuum environments, are shown to increase beyond the bulk, and estimates of the temperature shifts indicate that small changes of chiN are associated with unusually large shifts of the transition temperature. Further, 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 ODT of the bulk (enhanced miscibility).