Unexpected phase behavior of an asymmetric diblock copolymer

We report on measurements of the transmitted depolarized light intensity an d on small-angle neutron scattering (SANS) measurements on a compositionall y asymmetric poly(ethylene propylene)-poly(dimethylsiloxane) diblock copoly mer 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 bod y-centered cubic structure at the lowest temperature, a noncubic, slightly birefringent intermediate phase, and a cubic high-temperature structure whi ch may be body-centered cubic. The available data cannot, however, finally prove this. These findings are in contrast to both available theories and t o expectations based on analogous experiments on related diblock copolymer systems. (C) 1999 American Institute of Physics. [S0021-9606(99)51533-X].