BCC Grain Formation Triggered by Miscibility Jump on Temperature Drop

Abstract

Formation of a body-centered cubic (BCC) structure and grain growth process triggered by segregation-power jump on a temperature drop was studied by small-angle X-ray scattering (SAXS), rheology, and differential scanning calorimetry (DSC). Polystyrene-<i>block</i>-poly­(ethylene-<i>alt</i>-propylene) (SEP) was dissolved in tridecyl-2,2,4-trimethyl hexanoate (salacos913) that was a practically neutral good solvent for both of polystyrene (PS) and PEP block chains at temperature above 84 °C (<i>T</i>_sol), while it was highly selective (good for PEP) below <i>T</i>_sol. Spherical microdomains in a short-range liquid-like order were formed above <i>T</i>_sol; the system was in the so-called “lattice disordered state”, designated as disordered sphere. The solution was annealed at a temperature (130 °C) above <i>T</i>_sol for 10 min and successively subjected to a temperature drop across <i>T</i>_sol. The system stayed in the lattice-disordered state for a certain induction period. During this induction period, stronger segregation power at the lower temperature increased the domain spacing, whereas a storage shear modulus (<i>G</i>′) showed liquid-like behavior (<i>G</i>′<i>∝ ω</i>²) at low frequencies (ω < 0.2 s^–1) in a terminal zone and a shoulder at ω ∼ 1 s^–1. The shoulder shifted toward the smaller ω region, arising from dissociation of the PS block from the solvent. Once BCC lattice structures of spherical microdomains formed, grains eventually grew in size up to ca. 2.5 μm with a large size-distribution as revealed by the 2d-SAXS with spot-like scatterings, whereas <i>G</i>′ in the terminal region increased, arising from the increase in correlation length of the spherical microdomains. Eventually, the <i>G</i>′ showed plateau at lower frequencies at ω < 0.2 s^–1, indicating that the BCC lattice of spheres with long-range order (grain stuructures) was percolated throughout the solution. The number of the grains still continued to increase at the cost of spherical microdomains in the lattice disordered state, which caused the further increase in <i>G</i>′ at the plateau until the end of the ordering process of the BCC structure