Abstract
We have investigated the formation of ordered bicontinuous nanostructures in a diblock copolymer composed of a stereoregular block, syndiotactic polypropylene - block-polystyrene (sPP-b-aPS). The temperature-dependent small angle x-ray scattering (SAXS) revealed that the ordered bicontinuous double-diamond (OBDD) structure underwent a thermally reversible order-order transition (OOT) to ordered bicontinuous double gyroid (OBDG) upon heating, and the transition was accompanied with a slight reduction of domain spacing, as demonstrated both experimentally and theoretically. Based on the assumption of volume conservation, the ratio of the lattice parameters of these two bicontinuous structures was derived to be a_G/a_D =1.634, which closely agrees with that prescribed by the assumption of Bonnet transformation for the OOT. The OBDD structure was further confirmed by the reconstruction of 3D image using electron tomography. The thermodynamic stability of the OBDD structure may be attributed to the ability of the configurationally regular sPP block to form helical segments even above its melting point, as the reduction of internal energy associated with the helix formation may effectively compensate the greater packing frustration of sPP blocks in the tetrapod of OBDD relative to that in the tripod of OBDG, making the OBDD the more stable structure at the lower temperature. By blending sPP-b-aPS system with a small amount of sPP homopolymer, a phase transition from the OBDD structure to the hexagonally-packed cylinder (HEX) structure occured on heating. The volume fraction of the homopolymer in the blend affected not only the temperature range of the OOT, but also TOOT. The hyperbolic sPP domain of the OBDD structure is swollen, leading to a reduction of the interfacial curvature and hence a perturbation of sPP and PS conformation. Moreover, the distribution of sPP homopolymer chains in the tetrapod is non-uniform, giving rise to a loss of translational entropy. The loss of these entropies, which becomes important at high temperature destabilizes the OBDD structure and drives transformation into HEX phase. A possible kinetic pathway of the OBDD-to-HEX transition was proposed based on the assumption of volume conservation and constant lattice parameter.