Small-angle neutron scattering (SANS) measurements have been performed on a disordered block copolymer from deuterated polystyrene (dPS) and self-adhesive poly(isooctyl acrylate) (POA) in order to elicit the effective Flory-Huggins chi, which carries the essence of the copolymer phase behavior. The copolymer sample for the measurement was prepared by blending two polydisperse dPS-b-POAs of different molecular weights, where the overall average size of the blend was low enough to ensure to be in the mean-field region but high enough to have discernible scattering intensities. The SANS profiles for the copolymer were fitted to Leibler's scattering function for a polydisperse copolymer system described by Schulz-Zimm distribution. The resultant chi as a function of inverse temperature was shown to have a strong entropic contribution and a weak enthalpic contribution. By adopting Sanchez-Balasz or ten Brinke-Karasz-type simple analysis for specific interactions, it was found that the entropically dominated chi for dPS-b-POA arises from the steric hindrance of long alkyl side groups of POA. (C) 2012 Elsevier Ltd. All rights reserved.