Lattice Disordering and Domain Dissolution Transitions in Polystyrene-block-poly(ethylene-co-but-1-ene)-block-polystyrene Triblock Copolymer Having a Highly Asymmetric Composition

The lattice disordering transition (LDT) and the domain dissolution transition (DDT) of a highly asymmetric polystyrene- block -poly(ethylene- co -but-1-ene)- block -polystyrene (SEBS-8) triblock copolymer with a volume fraction of polystyrene (PS) block of 0.084 have been investigated by small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and rheology. The PS spheres formed in the SEBS-8 sample exhibited a body-centered cubic (bcc) lattice at lower temperatures and underwent disordering in the bcc lattice (so-called LDT) at ∼150 °C. Above this temperature ( T LDT ), spheres in liquidlike short-range order (LSO) with relatively thin interface between the PS domain and the poly(ethylene- co -but-1-ene) (PEB) matrix were detected up to ∼210 °C, above which the spherical domains started to dissolve into the PEB matrix. Finally, the spherical domains were completely dissolved into a homogeneous state at ∼232 °C. The starting and the final dissolution temperatures are referred to as the T DDT and the order-to-disorder transition temperature ( T ODT ). The LDT was verified by the SAXS results that the higher order diffraction peaks from the bcc lattice disappeared above the T LDT, while particle scattering of spheres due to the intraparticle interference as well as the interparticle interference of spheres in LSO was clearly observed between the T LDT and the T DDT . The spheres in LSO were further elucidated by rheology and TEM observation. It was found that a precipitous decrease in storage modulus ( G ‘) and a dramatic change in the Bragg spacing occurred at the same temperature of the T LDT . It was also observed that the slope in the plots of G ‘ versus frequency (ω) and that in the plots of loss modulus ( G ‘ ‘) versus ω in the terminal region were two and one, respectively, at temperatures above the T LDT . This is attributed to the fact that because of the absence of the bcc lattice in long-range order, spheres in LSO do not contribute significantly to the shear moduli in the terminal region. Therefore, even if the terminal behavior observed generally for a homogeneous mixture (namely the slopes in the plots of G ‘ versus ω and G ‘ ‘ versus ω are two and one) is exhibited at a temperature, this temperature is not necessarily above the T ODT . The characteristic domain spacing in LSO did not change much with temperature, but it increased between the T DDT and the T ODT due to the dissolution of spheres.