Microphase Separation in Super-H-Shaped Block Copolymer Colloids

Abstract

Block copolymers of the A3BA3 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 H-shaped block copolymer melts of the A3BA3 type. From the three asymmetric nonlinear block copolymers (0.07 < fB < 0.35, B is polystyrene (PS)) one is in the homogeneous disordered phase (fPS = 0.072), another is in the ordered phase (fPS = 0.345), and the third (fPS = 0.118) undergoes an order-to-disorder transition in a temperature range accessible by SAXS and rheology. In the former experiment the ODT has been identified from the discontinuous changes in the peak intensity and width, whereas in the latter it has been identified by the discontinuous drop of the storage modulus. The SAXS results have shown a peculiar T dependence of the peak position q*, which is attributed to the starlike nature of the system. TEM and SAXS identify the morphology of the fPS = 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 TODT two states with very different viscosities exist and large amplitude deformation and/or temperature results in a transformation from the high (state I) to the low viscosity state (state II). To identify the associated 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 individual domains in the absence of long range order. At low shear rates, below the grain relaxation, we identify a flow regime that is a consequence of the colloidal nature of these block copolymers.