THERMOREVERSIBLE CYLINDER-SPHERE TRANSITION OF POLYSTYRENE-BLOCK-POLYISOPRENE DIBLOCK COPOLYMERS IN DIOCTYL PHTHALATE SOLUTIONS

We present experimental results of a thermoreversible morphological tr ansition between spheres and cylinders for polystyrene-block-polyisopr ene diblock copolymers (SI) in dioctyl phthalate solutions. Morphologi es were characterized using small-angle X-ray scattering (SAXS), and s pherical and cylindrical states were found above and below the order-o rder transition (GOT) temperature, T-OOT, respectively. T-OOT increase d with an increase of polymer concentration for the range covered in t his study (higher than 70 wt % polymer concentration). The concentrati on dependence of T-OOT was found to be given by the relationship of(ch i(eff)r(C))(OOT)/phi(p)r(C) = A + B/T-OOT, where (chi(eff)r(C))(OOT) d enotes the critical value of the product of chi(eff) and r(C) at the c ylinder-sphere GOT, chi(eff) is the effective interaction parameter be tween styrene and isoprene segments in the presence of solvent, r(C) i s the reduced degree of polymerization of an entire block copolymer, a nd phi(p) is the volume fraction of the block copolymer in the solutio n. A and B are constants characterizing the temperature dependence of the segmental interaction parameter chi(SI) in bulk, where A = -0.0258 and B = 27.9 were evaluated by analyzing SAXS profiles from the disor dered state. The critical values(chi(eff)r(C))(OOT) = 38.4 for the cyl inder-sphere transition and 30.5 less than or equal to (chi(eff)r(C))( ODT) less than or equal to 32.3 for the order-disorder transition (ODT ) were determined from our experimental result on the phi(p) dependenc e of T-OOT and that of T-ODT, respectively. These critical values were compared to the results of some theoretical studies. The SAXS measure ments revealed thermoreversibility of the cylinder-sphere transition b etween 110 and 120 degrees C for an 80 wt % solution.