The Full Phase Behavior for Block Copolymers in Solvents of Varying Selectivity

Abstract

The phase behavior of six poly(styrene- b -isoprene) (SI) diblock copolymers has been mapped out in the styrene-selective solvents di- n -butyl phthalate (DBP), diethyl phthalate (DEP), and dimethyl phthalate (DMP). The polymer molecular weights were chosen to make the melt order−disorder transition (ODT) experimentally accessible, and the styrene compositions f ranged from 0.23 to 0.70, to access the full range of melt morphologies. For each polymer a phase diagram was constructed, with polymer volume fractions, φ, ranging from 0.01 to 1.0 and temperatures, T, from 0 to 250 °C. Phase assignments were based on small-angle X-ray scattering (SAXS), and the ODTs and order−order transitions (OOTs) were located by a combination of SAXS, rheology, and static birefringence. The critical micelle temperatures (cmt) in dilute solution were determined by dynamic light scattering. In this manner the full “phase cube” was mapped out in each solvent, enabling generation of phase maps (φ, f ) at constant T and ( f, T ) at constant φ. The solvents range from slightly to strongly selective, in the sequence DBP, DEP, and DMP, and in each case the selectivity diminishes with increasing T . This property gives rise to a plethora of thermally induced OOTs, and several solutions exhibit four distinct equilibrium phases upon heating. In addition to the eight phases well established for SI copolymers in the melt (a body-centered-cubic (bcc) array spheres of styrene or isoprene, hexagonally packed cylinders of styrene or isoprene, gyroid with isoprene or styrene matrix, lamellae, disordered), broad regions of lamellae + cylinder coexistence and face-centered-cubic (fcc) isoprene spheres were observed. The sequence of phases could be broadly understood in terms of changes in spontaneous interfacial curvature arising from differential swelling of the two microdomains. For a given polymer and solvent, the ODT varied smoothly with φ from the melt value down toward the dilute solution critical micelle temperature (cmt); at about φ ≈ 0.2 the ordered phases gave way to a solution of micelles. In some cases solutions near φ ≈ 0.2 exhibit reentrant ODTs, as they evolved from a solution of micelles to an fcc (and/or bcc) lattice, to a solution of chains, upon heating. The origins of these various phenomena are discussed, and the results are compared and contrasted with other measurements on SI copolymers in the literature.