Unexpected phase behavior of an asymmetric diblock copolymer

We report on measurements of the transmitted depolarized light intensity and on small-angle neutron scattering (SANS) measurements on a compositionally asymmetric poly(ethylene propylene)-poly(dimethylsiloxane) diblock copolymer studied in the bulk. SANS measurements were made both on isotropic and on shear oriented samples. Apart from the disordered phase, three different ordered morphologies were identified as a function of temperature: the body-centered cubic structure at the lowest temperature, a noncubic, slightly birefringent intermediate phase, and a cubic high-temperature structure which may be body-centered cubic. The available data cannot, however, finally prove this. These findings are in contrast to both available theories and to expectations based on analogous experiments on related diblock copolymer systems.