Small-angle neutron scattering study of a cylinder-to-sphere order-order transition in block copolymers

The pathway and kinetics of the order-order transition from shear-aligned c ylindrical microdomains to spheres arranged on a bcc lattice in a highly as ymmetrical block copolymer are monitored using small-angle neutron scatteri ng (SANS). Shear-aligned hexagonally closed-packed cylindrical microdomains were heated to the spherical microdomain region under quiescent conditions , and the microstructure evolution associated with the order-order transiti on was followed by SANS. The data indicate a rapid decrease in the cylindri cal microdomain order and a gradual rise in the signatures associated with the spherical microdomains, thereby suggesting the existence of an intermed iate state where the sample is poorly ordered. The duration of this interme diate state increases with increasing distance from the order-order transit ion and is consistent with previous rheological measurements on unaligned w ormlike cylinders. The results of the scattering measurements are compared to the predictions of recent theories describing the cylinder-to-sphere tra nsition.