STRUCTURE AND VISCOELASTICITY OF MATCHED ASYMMETRIC DIBLOCK AND TRIBLOCK COPOLYMERS IN THE CYLINDER AND SPHERE MICROSTRUCTURES

A polystyrene-polyisoprene (PS-PI) diblock copolymer (10,000-50,000 g/ mol) and a matched PS-PI-PS triblock (10,000-100,000-10,000 g/mol) wer e employed to study the effect of chain architecture on the rheologica l response of ordered block copolymer melts. Both samples adopt hexago nal microstructures with PS cylinders embedded in a PI matrix; on furt her heating, an order-order transition (OOT) into a cubic array of sph eres takes place prior to the order-disorder transition. Each morpholo gy was verified by SAXS and TEM. Interestingly, at the OOT the low-fre quency elastic modulus of the diblock increased abruptly, whereas that of the triblock decreased. In contrast, the modulus of the cubic phas e was roughly independent of chain architecture. Chain relaxation para llel and perpendicular to the cylinders was probed by measuring the el astic modulus of a macroscopically aligned sample in directions parall el (G'(parallel to)) and perpendicular (G'(perpendicular to)) to the c ylinder orientation. For both materials G'(parallel to) < G'(R) < G'(p erpendicular to), where G'(perpendicular to) is the elastic modulus of a randomly oriented sample. This result is attributed to the ability of the unentangled PS blocks to move along the direction of the cylind er axis, and thus relax the stress in the PI matrix in the parallel al ignment. In each of the three cylindrical orientations the triblock ha d a larger modulus than the diblock, which is attributed to the presen ce of bridging PI blocks that connect distinct PS domains. About 20 de grees below the OOT G'(parallel to) showed a distinct change in its te mperature dependence, which, coupled with SAXS measurements, is indica tive of the onset of an undulation in the cylinder diameter that presa ges the pinching off of cylinders into spheres, as recently predicted by theory. The use of oriented samples also permitted SAXS confirmatio n of an approximate epitaxial relationship between the cylinder and th e sphere unit cells, although a distinct change in the location of the structure factor maximum, q, is noted at the OOT. (C) 1997 John Wile y & Sons, Inc.