Phase behaviour near the order–disorder transition (ODT) of 58 model hydrocarbon diblock copolymers, representing four different systems, is summarized. Six distinct ordered-state microstructures are reported, including hexagonally modulated lamellae (HML), hexagonally perforated layers (HPL) and a bicontinuous cubic morphology with Ia3d space group symmetry. Two non-classical parameters, ε and text-decoration:overlineN, control the occurrence and distribution of these phases, in addition to the classical variables ƒ and χN, where ƒ, χ and N are the composition, segment–segment interaction parameter and degree of polymerization, respectively. ε accounts for differences in the conformational and volume-filling characteristics of each block. Conformational asymmetry, ε≠ 1, produces an asymmetric phase diagram around ƒ= 1/2. The importance of fluctuation effects are inversely related to the magnitude of text-decoration:overlineN, a type of Ginzburg parameter that is proportional to N. As text-decoration:overlineN decreases, the bicontinuous Ia3d phase appears adjacent to the ODT. Development of this cubic phase can be rationalized based on chain-packing frustration near the lamellar hexagonal state. Apparently the Ia3d cubic state is stabilized by fluctuations since it disappears when text-decoration:overlineN becomes large. These findings provide new insights into the origins of phase complexity in condensed soft matter.

High molecular weight block copolymers can be viewed as macromolecular surfactants when blended with thermodynamically incompatible homopolymers. This Letter describes the formation of polymeric bicontinuous microemulsions in mixtures containing a model diblock copolymer and two homopolymers. Although we attribute development of this equilibrium morphology to the effects of fluctuations, mean-field theory provides a quantitative strategy for preparing the bicontinuous state at blend compositions near an isotropic Lifshitz point.

Small-angle neutron scattering experiments have revealed an epitaxial relationship between the hexagonal cylinder phase, and a bicontinuous cubic phase with $\mathrm{Ia}\overline{3}d$ space group symmetry, in a poly(styrene)-poly(2-vinylpyridine) diblock copolymer mixture. Proximity to the order-disorder transition and an inelastic low frequency rheological response suggest that the cubic phase is stabilized by fluctuations. These results identify block copolymers as model compounds for investigating the thermodynamics and dynamics of complex "soft" condensed matter.