Solution and Bulk Structures of Asymmetric PEP-PS-PEP′ Triblock Copolymers

The self-assembly of four polystyrene (S) and poly(ethylene-alt-propylene) (EP) block copolymers was studied in squalane, an EP-selective solvent. The polymers were prepared by sequential living anionic polymerization of isoprene and styrene, followed by catalytic hydrogenation of the diene block(s). The four polymers had comparable total molar masses (ca. 90 ± 6 kg/mol) and volumetric compositions (f_S ≈ 0.25 ± 0.01) but different architectures. Specifically, a diblock SEP(26-70), a symmetric triblock EPSEP(30-24-30), and two asymmetric triblocks EPSEP′(8-26-62) and EPSEP′(15-28-52) were prepared, where the numbers denote block molar masses in kg/mol. Micellization in dilute solution (volume fraction ϕ ≤ 0.01) was studied by dynamic light scattering and small-angle X-ray scattering. All four polymers assembled into spherical micelles, and, as expected, the symmetric triblock formed the smallest particles. The results suggest that the asymmetric triblocks form micelles with a two-layer corona, the inner layer being enriched in the smaller endblock. Upon increasing concentration (ϕ = 0.1 and 0.3), SEP(26-70) and EPSEP(30-24-30) micelles packed onto well-defined body-centered cubic lattices, also as expected. However, surprisingly, the asymmetric triblocks did not adopt a lattice at these volume fractions but rather retained a liquid-like packing. This remarkable effect of architecture was confirmed by linear viscoelastic measurements, which revealed striking differences between the asymmetric triblocks on one hand and the “conventional” architectures on the other. This behavior is speculated to arise from a different inter-micellar potential, stemming from the two-layer corona. At still higher concentrations, all four polymers adopted hexagonal packings, before transitioning to lamellae for ϕ ≥ 0.8.