Alternating Lamellar Structure of Triblock Copolymers of the ABA Type

Alternating lamellar structures of poly(2-vinylpyridine-b-styrene-b-2-vinylpyridine) triblock copolymers of the ABA type with different compositions were studied by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and small-angle neutron scattering (SANS). The volume fractions of polystyrene (S), ϕ_s, of the copolymers range from 0.28 to 0.58, whereas the chain lengths of two end block polymers, poly(2-vinylpyridine) (P), are always the same for each sample. It was confirmed that all the solvent-cast films had alternating lamellar structures, and the measured microdomain spacings were in good agreement with the theoretical prediction by Helfand and Wasserman irrespective of the compositions. SANS studies revealed that the middle block polymers (S) are elongated along the direction normal to the lamellar interface, but they are shrunk along the direction parallel to the interface, so that the volume occupied by the middle block polymers are the same as those in the unperturbed states, and the degree of deformation decreases with increasing ϕ_s.