MICROPHASE SEPARATION IN SUPER-H-SHAPED BLOCK-COPOLYMER COLLOIDS

Block copolymers of the A(3)BA(3) type with a short connector block B exhibit features known in multi-arm star polymers in addition to their block, copolymer nature. We have studied the ordered state morphology and the order-to-disorder transition (ODT) in three model super I-I-s haped block copolymer melts of the A3BA3 type. From the three asymmetr ic nonlinear block copolymers (0.07 < f(B) < 0.35, B is polystyrene (P S)) one is in the homogeneous disordered phase (f(PS) = 0.072), anothe r is in the ordered phase (f(PS) = 0.345), and the third (f(PS) = 0.11 8) undergoes an order-to-disorder transition in a temperature range ac cessible by SAXS and rheology. In the former experiment the ODT has be en identified from the discontinuous changes in the peal; intensity an d width, whereas in the latter it has been identified by the discontin uous drop of the storage modulus. The SAXS results have shown a peculi ar T dependence of the peak position q, which is attributed to the st arlike nature of the system. TEM and SAXS identify the morphology of t he f(PS) = 0.118 Sample as spheres organized in a body-centered-cubic (bcc) lattice with each domain consisting of about 40 connector blocks (PS), whereas in the fps = 0.345 sample, PS cylinders are packed in a hexagonal lattice. At some temperatures in the vicinity of the T-ODT two states with very different viscosities exist and large amplitude d eformation and/or temperature results in a transformation from the hig h (state I) to the low viscosity state (state II). To identify the ass ociated structural changes, we have compared the viscoelastic response of each state to the corresponding structural profiles. We found that the viscoelastic response in state I is controlled by the relaxation of grains whereas in state II it is controlled by the relaxation of in dividual domains in the absence of long range order. At low shear rate s, below the grain relaxation, we identify a flow regime that is a con sequence of the colloidal nature of these block copolymers.