Hexagonal mesophases between lamellae and cylinders in a diblock copolymer melt

Two intermediate phases have been observed upon heating an asymmetric poly(ethylenepropylene)-poly(ethylethylene) diblock copolymer between lamellar and hexagonal cylinder phases near the order-disorder transition. Phase transitions between ordered morphologies are indicated by the temperature dependence of the dynamic shear moduli, and small angle neutron scattering experiments are used to identify these structures as hexagonally modulated lamellae and layered hexagonal packed channels. The observed wavevectors associated with the interlayer scattering for these structures are incommensurate, leading to an aperiodic structure in which the long range translational order of the layers is destroyed. However, the scattering data show that long range in-plane bond orientational order is present, suggesting an analogy with hexatic B phase in liquid crystals. A Fourier imaging method is used to construct density images for the two structures. This shows modulated lamellae for the low temperature phase while channels are evident in the high temperature phase. A restacking transition occurs on shearing the modulated lamellar phase, but the channel phase cannot support large strain shear deformations and instead transforms into rods. These phases do not occur in the mean-field phase diagram for block copolymers, and we believe they are stabilized by composition fluctuations that are the inevitable result of a finite molecular weight. Furthermore, these fluctuations are anisotropic, a feature which has not been accounted for theoretically.