A series of model tri-block copolymers having a chain sequence of polycaprolactam-b-poly(p-phenylene terephthalamide)-b-polycaprolactam with varied block length were designed, synthesized and characterized. The optimization and harmonization of the rigid and flexible block length lead to copolymers showing meltability. Blends of nylon-6 with the tri-block copolymer showed much better phase stability than that of normal blends. The preliminary results suggest that the proposed approach may be practicable to prepare melt-processable molecular composite. (C) 2000 Published by Elsevier Science Ltd.