The tensile and elastic behavior of triblock copolymers containing uniform aramide (T Phi B) hard end-segments (HS) and poly(tetramethylene oxide) (PTMO, M-n = 2900 g/mol) soft segments (SSs) was studied. The molecular weight of the copolymer was varied by changing the length of the soft mid-segment; by extending the PTMO2900 with terephthalate units, the SS length was increased from 2900 g/mol to 21,000 g/mol and concurrently the aramide concentration decreased from 18 to 3 wt %. The mechanical properties were investigated by means of tensile testing, stress relaxation (SR) experiments, and cyclic tensile set (TS) tests. The E-modulus Was found to increase with increasing aramide content. The low molecular weight copolymers were brittle whereas the high molecular weight copolymers displayed large fracture strain values. The transition from brittle to ductile seemed to Occur at a triblock copolymer molecular weight of 6600 g/mol. A strain-induced crystallization was observed at strains above 250%, and both the fracture strain and stress were found to be highly dependant on the molecular weight of the copolymer. Cyclic tensile experiments showed that the materials had low TS values up to the Strain hardening g point. On the other hand, the SR data at 10% strain seemed to be little dependant on the molecular weight. The higher molecular weight copolymers did not display lower SR values than their low molecular Weight counterparts. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 111: 1780-1785, 2009