Isotropic and anisotropic composition fluctuations close to the order-to-disorder transition in an asymmetric diblock copolymer melt subjected to reciprocating shear fields

An asymmetric poly(ethylene-propylene)-poly(ethylethylene) (PEP-PEE) di-block copolymer was examined near the order-to-disorder transition temperature, T_ODT, by small-angle neutron scattering as a function of shear rate, γ̇. Heating in the absence of shear disorders the material from a hexagonal structure at T_ODT = 155 ± 1°C. Application of a reciprocating simple shear field markedly influences the phase behaviour near the ODT. Application of shear while heating increases T_ODT consistent with the theoretical prediction, T_ODT ∼ γ̇². During fast cooling studies we observe an additional characteristic temperature, T_S, at which the material spontaneously orders, which we interpret as a stability (i.e., spinodal) limit. This result also agrees with the theoretical prediction, τ_S ∼ γ̇^-1/3. Upon cessation of shear at temperatures close to but above the quiescent T_ODT a transient phase is observed before the system macroscopically disorders. The transient structure has a symmetry compatible with spheres arranged on a body centered cubic (BCC) space lattice, or undulating cylinders of the minority component (PEE). Anisotropic (i.e. lattice pinned) fluctuations can also be observed close to, but below, T_ODT upon heating the hexagonal structure under shear free conditions.