From Hard Spheres to Soft Spheres:  The Effect of Copolymer Composition on the Structure of Micellar Cubic Phases Formed by Diblock Copolymers in Aqueous Solution

Abstract

Small-angle X-ray scattering (SAXS) was used to determine the packing of micelles in cubic phases formed by poly(oxyethylene)−poly(oxybutylene) (EB) diblock copolymers in water. SAXS with large amplitude oscillatory shear was used to identify structures formed by the highly asymmetric molecules E96B18, E184B18, E315B17, and E398B19 (where the subscripts denote the number of repeats). At a constant temperature (20 °C) and concentration (10 wt %), we find that the two copolymers with shorter hydrophilic blocks form face-centered cubic (fcc) gels, whereas the two copolymers with longer corona-forming blocks form body-centered cubic (bcc) gels. SAXS also confirmed that the sols formed at lower copolymer concentrations are micellar liquids. Our results for the gel structures are in accord with the observation that micelles with relatively short coronas behave as hard spheres, and pack in a fcc structure, whereas micelles with large coronas act like soft spheres and pack in a bcc array. This is confirmed by assembling a phase diagram as a function of copolymer asymmetry and concentration using results from the four copolymers discussed here and a series of gels of other EB diblocks studied previously by us.

Three-dimensional periodic complex structures in soft matter: investigation using scattering methods

“…As the bcc packing exhibit a smaller compacity of 0.68 (instead 0.74 for fcc/hcp), its occurrence may at first sight be surprising. In fact, this packing occurs in the presence of a more long-range potential in addition to the hard-sphere potential [26][27][28]. A physical interpretation has been proposed by Kamien & Ziherl [29] in the case of dendrimers in terms of an additional repulsive potential driven by the conformational entropy of the chains located at the surface of the particles.…”

“…This interpretation is, in fact, quite general and can be extended for many kind of particles where long enough chains are present on their surface. For example, for block-copolymer micelles with a shell made of EO chains, fcc packing is formed for short EO chains and bcc for longer ones [27].…”

Origin of the Thermoreversible fcc-bcc Transition in Block Copolymer Solutions

“…Fifth, previous workers established the validity of this approach in aqueous polyethyleneoxidepolybutyleneoxide (PEO-PBO) systems [11,18].…”

“…Consequently, analytical theory and computer simulations have been highly refined [2 -7]. This transition has recently been observed in block copolymer micelles [8][9][10][11][12][13], which are model soft colloids, analogous to highly branched star polymers. In contrast to star polymers, however, the micellar functionality can be tuned with temperature, providing direct access to the thermoreversible fcc/bcc transition.…”

Structural and Mechanical Hysteresis at the Order-Order Transition of Block Copolymer Micellar Crystals

“…A recent report of the hysteresis in this system using viscosity and grazing-incidence SANS shows persistent structures near a solid surface and the complexity of the phase diagram [19]. A thermoreversible order-order transition (OOT) between face-centered cubic (FCC) and body-centered cubic (BCC) phases has been observed previously for block copolymer solutions of Pluronics ® [6,20,21] and other block copolymer solutions and melts [22][23][24][25][26].…”