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 similar to 150 degrees 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-l-ene) (PEB) matrix were detected up to similar to 210 degrees C, above which the spherical domains started to dissolve into the FEB matrix. Finally, the spherical domains were completely dissolved into a homogeneous state at similar to 232 degrees 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 TLDT and the TDDT 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 TLDT It was also observed that the slope in the plots of G' versus frequency (omega) and that in the plots of loss modulus (G ") versus w in the terminal region were two and one, respectively, at temperatures above the TLDT 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 omega and G " versus omega 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.