OBSERVATION OF A REVERSIBLE THERMOTROPIC ORDER-ORDER TRANSITION IN A DIBLOCK COPOLYMER

We report the observation of a reversible thermotropic transition betw een the lamellar and cylindrical morphologies in a polystyrene-poly(et hene-co-butene) diblock copolymer. The nature of the intermediate stru ctures which form as the transition progresses has been examined using two-dimensional small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Results show that the transformation from t he lamellar to the cylindrical morphology proceeds in two steps. Initi ally, fluctuations along the interface joining the components of the c opolymer grow in amplitude until the lamella transforms into a sheet o f evenly spaced cylinders; cylinders which form in adjacent lamellae a re usually in poor register with one another. This intermediate struct ure subsequently anneals into the hexagonal packing of cylinders chara cteristic of the cylindrical morphology. The reverse transition from c ylinders to lamellae proceeds without producing any intermediate signa ture detectable by SAXS.