Micellar Ordering in Concentrated Solutions of Di- and Triblock Copolymers in a Slightly Selective Solvent

Abstract

Micellar ordering in semidilute solutions of polystyrene−polyisoprene diblock and triblock copolymers in the slightly selective solvent di-n-butyl phthalate has been studied using rheology and small-angle X-ray scattering (SAXS). Ordering as a function of temperature has been investigated for a range of polymer concentrations 0.1 ≤ φ ≤ 0.4. For φ < 0.2, the rheological response is liquidlike and SAXS shows that there is no intermicellar order in the liquid; however, the solution viscosity shows a strong maximum near 50 °C. Above a crossover concentration φ ≈ 0.2, ordering of micelles is indicated by the presence of a sharp structure factor peak. The ordered micellar structure, identified as hexagonal for 0.2 ≤ φ ≤ 0.3 and lamellar for φ ≥ 0.3, persists up to an order−disorder transition at T ≈ 40 °C for the diblock and T ≈ 50 °C for the triblock solutions studied. The rheological characteristics of the ordered solutions are reminiscent of those found in ordered block copolymer melts. At higher temperatures, for example approximately 20−30 °C above the ODT for the φ = 0.2 solutions, indications of chain aggregation disappear from the rheological properties; however, some evidence of chain association persists to still higher temperatures in the SAXS profiles. The domain spacing, d, in the ordered solutions obtained from the principal structure factor peak position, shows a crossover at φ ≈ 0.2, in agreement with rheology. At high concentrations, d scales as d ∼ φ-1/3, suggesting a three-dimensional contraction of the microstructure, and thus micelles of finite length. This concentration dependence is opposite to that previously observed for ordered block copolymer solutions in neutral solvents, due to the solvent selectivity. The results are gathered in a rather rich phase diagram for this system.

On the selection of FCC and BCC lattices in poly(styrene-b-isoprene) copolymer micelles

“…Previously the phase diagrams of several SI copolymers, SI(11-32), SI (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21), SI , SI , SI(8-7), SI , and SI(38-14), in both PS and PI selective solvents were constructed as functions of temperature, T, and concentration, φ. 12-14 Based on the (T, φ) phase diagrams at various f, phase maps on two other planes through the phase cube, such as (φ, f ) at constant T and (T, f ) at constant φ, were also determined.…”

“…[11][12][13][14] Spherical micelles were observed at dilute regime for all polymers for which f is in the range 0.23 < f < 0.70, and thus the cubic phases, i.e., fcc or bcc, were formed at higher concentration. In particular the thermotropic fcc to bcc transition was observed in some phase diagrams, and the mechanism and the cause of this transition have been explored recently.…”

Phase Behavior of a Block Copolymer in Solvents of Varying Selectivity

“…9 Previous work in selective solvents has tended to focus on dilute solutions where the selectivity drives micellization. 1,21 In contrast, studies at nondilute concentrations have been few (but with some exceptions 9,[22][23][24][25][26][27] ). Aqueous block copolymer solutions systems have received the most attention, particularly in the case of the ethylene oxide-propylene oxide-ethylene oxide (Pluronic) copolymers.…”

Order−Disorder Transition and Critical Micelle Temperature in Concentrated Block Copolymer Solutions

“…Hamley and co-workers 39 investigated the phase behavior of asymmetric PS-PI and PS-PI-PS block copolymers in the PS-selective solvent di-n-butylphthalate (DBP). The hexagonal cylinder (hex) and lamellar microstructures in the concentration range 0 e φ e 0.4 dissolve into micellar solutions at T ODT .…”