  For the first time a Fddd network structure is found in a
poly(styrene-b-isoprene) (S-I) diblock copolymer melts by using small-angle
X-ray scattering and transmission electron microscope, as predicted by Tyler et
al. by using calculations with self-consistent field theory for diblock
copolymer melts. The phase diagram of the S-I exhibits the sequence of
Disorder- Fddd-Lamellae with decreasing temperature. The measured unit cell
parameters ratios (a,b,c) are (1:2.02:3.46), which agrees with the result of
the theoretical calculation.
