Lattice disordering and domain dissolution transitions in polystyrene-block-poly(ethylene-co-but-1-ene)-block-polystyrene triblock copolymer having ahighly asymmetric composition

The lattice disordering transition (LDT) and the domain dissolution transit ion (DDT) of a highly asymmetric polystyrene-block-poly(ethylene-co-but-1-e ne)-block-polystyrene (SEBS-8) triblock copolymer with a volume fraction of polystyrene (PS) block of 0.084 have been investigated by small-angle X-ra y 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 la ttice (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) m atrix were detected up to similar to 210 degrees C, above which the spheric al domains started to dissolve into the FEB matrix. Finally, the spherical domains were completely dissolved into a homogeneous state at similar to 23 2 degrees C. The starting and the final dissolution temperatures are referr ed 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 sca ttering of spheres due to the intraparticle interference as well as the int erparticle interference of spheres in LSO was clearly observed between the TLDT and the TDDT The spheres in LSO were further elucidated by rheology an d TEM observation. It was found that a precipitous decrease in storage modu lus (G') and a dramatic change in the Bragg spacing occurred at the same te mperature 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 ") vers us 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 si gnificantly 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 ab ove 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.