Segmented hydroxy ether—siloxane copolymers that are chemically cross-linked and have microphase-separated morphologies were prepared. In this series of copolymers, the dimethylsiloxane or soft block was modified by the incorporation of various compositions of diphenylsiloxane. The incorporation of the more polar diphenyisiloxane decreased the solubility parameter difference between the siloxane and hydroxy ether segment, allowing bulk copolymerization. Bulk copolymerizations were realized with soft segments containing 50 wt % or more diphenylsiloxane. Calorimetric, dynamic mechanical, and small-angle X-ray scattering measurements showed that microphase-separated morphologies were achieved at diphenylsiloxane compositions of 50 wt % and less in the siloxane soft segment or block. The mechanical behavior of these copolymers was influenced by the compositional and subsequent morphological changes. © 1989, American Chemical Society. All rights reserved.