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.