An anionically polymerized polystyrene-block-poly(n-pentyl methacrylate) (PS-b-PnPMA) was fractionated by interaction chromatography (IC), and the closed-loop phase behavior of the mother block copolymer and its fractions with different chemical compositions was investigated by depolarized light scattering and polarized optical microscopy. With increasing elution time the average chain size (or the average molecular weight) of the block copolymer decreased slightly, and the average chemical composition changed steadily from a PS volume fraction of 0.583 to 0.403, thereby producing five fractions collected over the different ranges of elution time. This is caused by the difference of relative interaction strength between the stationary phase and the PnPMA chains of PS-b-PnPMA, in which PnPMA chains tend to be adsorbed onto the stationary phase while PS chains behave oppositely. Over the range of accessible temperatures studied (110-250 degrees C) a variety of phase transitions were observed, including fully ordered state, a closed-loop (consisting of the lower disorder-to-order transition and upper order-to-disorder transition), and fully disordered state. The original, unfractionated block copolymer was disordered over the entire temperature range. The variety of observed phase transitions obtained from the fractionated block copolymers is assumed to be governed by the sensitivity of the closed-loop phase behavior to average chain size, average chemical composition, and the compositional broadness.