COMPOSITIONAL DEPENDENCE OF THE ORDER-DISORDER TRANSITION IN DIBLOCK COPOLYMERS

The bulk morphologies and microphase separation behavior were studied in seven low molecular weight poly(styrene-co-isoprene) diblock copoly mers with polystyrene volume fractions ranging from 0.21 to 0.76. Thei r order-disorder transitions (ODT) were investigated using small-angle X-ray scattering and rheological measurements as a function of the de gree of polymerization (N), composition, and temperature. The resultan t (chi)N versus composition plot has been compared with theories for t he ODT in diblock copolymers. Near compositions of 50 vol %, our exper iments agree with the fluctuation theories of both Fredrickson-Helfand and Muthukumar. At other compositions the critical (chi)N for the ODT is smaller than that predicted by the available theories. This discre pancy increases as the copolymer composition deviates further from 50 vol %. In addition, transitions from one ordered morphology to another , order-order transitions, exhibit only a slight dependence on the com position.