A series of well-defined poly(n-butyl acrylate)-b-polyacrylonitrile (PBA-PAN) triarm star block copolymers have been synthesized by ATRP. The incorporation of very polar hard segment of PAN was made possible with the aid of the halogen-exchange technique. Phase-separated morphology of cylindrical PAN domains hexagonally arranged in the pBA matrix was observed by atomic force microscopy and small-angle X-ray scattering in ail studied materials. The mechanical and thermal properties of the PBA-PAN triarm star block copolymers have been thoroughly characterized, and their thermoplastic elastomer behavior was studied. it was found that these block copolymers retain a phase-separated morphology until temperatures of at least 250 degrees C. Furthermore, because of chemical cross-linking of the PAN segments that takes places at around 280 degrees C, the triarm star PBA-PANs preserve their elastomeric properties until temperatures of more than 300 degrees C.