pH sensitive poly(acrylonitrile-co-acryhc acid) having similar to 50 mol % acrylic acid with block type structure (AA50B) was synthesized by controlled dosing method of free radical polymerization. The polymer was converted into fibers by wet solution spinning technique in DMF-water system. The resulting block type copolymer could generate a domain type morphology with segregated domains of acrylonitrile and acrylic acid on heat-setting. The drawing ratio and heat-setting temperature had a significant effect on the formation of these domains and their stability. The domain formation was more pronounced when the fibers could be drawn to higher draw ratios during coagulation or heat-set at higher temperature. The stability of the fibers, which is influenced by domain formation, was lowest (at few cycles of transitions) when the fibers were heat-set at 100 degrees C, while it is improved significantly to more than 50 cycles as the heat-setting temperature was increased to 150 degrees C. The coagulation conditions, drawing and the heat-setting also greatly influenced the mechanical properties, transition behavior, and retractive stresses of the responsive fibers. The tenacity improved by 6.6 times in swollen state and 1.4 times in deswollen state,, while the retractive stresses during deswelling were significantly increased to about 4.7 times. However, the increased heat-setting temperature was also found to have a negative,, effect on the equilibrium swelling values as well as the response rate. The effect of heat-setting on chemical structure of the copolymer was also investigated. (c) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 109: 37923803,2008.