Non-equilibrium self-assembled structures in thick PS-b-PMMA copolymer films

Phenomena associated with the order–disorder transition (ODT) of block copolymers have been studied by optical light microscopy, SAXS, SEM, TEM and DSC. Observations have been made on almost symmetric polystyrene-block-poly(methyl methacrylate) samples of three molecular weights and their mixture. We observed non-equilibrium supermolecular structures several microns in diameter in the bulk of thick PS-b-PMMA films (ca. 100 μm thickness) prepared by vacuum drying of films cast from a non-selective solvent (after a short-term annealing above the Tg). Apparent LDOT (lower disorder-to-order transition) behaviour is observed for samples with non-equilibrium morphology surviving from solution as deduced from SAXS 1/Im vs 1/T and the full width at half-maximum vs 1/T plots. The measurements point to complex behaviour near the ODT, but homogenization of samples upon long-term annealing well above the Tg temperature call into existence common stacks of lamellae observable in SEM images of microphase-separated samples. This verifies the opinion that the observed apparent LDOT behaviour of samples II, III and II + III is associated with the frozen non-equilibrium morphology surviving from solution. This is confirmed by SAXS measurement on a homogenized sample displaying the expected UDOT behaviour. It has been demonstrated that self-assembled structures prepared by vacuum drying of films cast from a non-selective solvent are non-equilibrium structures and their successive ordering is difficult due to a relative narrow temperature interval between Tg and degradation temperature. The conditions under which BCP films are prepared thus have a pronounced effect on the microstructure and microphase ordering process.