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

Timothy P. Lodge, En Wang, Jiahao Zhu, Frank S Bates

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Abstract

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 (fS ≈ 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.

Original languageEnglish (US)
Pages (from-to)6444-6451
Number of pages8
JournalMacromolecules
Volume56
Issue number16
DOIs
StatePublished - Aug 22 2023

Bibliographical note

Funding Information:
This work was supported in part by Infineum, L.L.P. SAXS experiments were performed at the 5-ID-D beamline of the Dupont–Northwestern–Dow Access Team (DND-CAT) at the Advanced Photon Source (APS), Argonne National Laboratory. Use of the APS, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by the Argonne National Laboratory, was supported by the U.S. DOE under contract no. DE-AC02-06CH11357. The authors acknowledge the Characterization Facility, College of Science and Engineering, University of Minnesota, which receives partial support from the NSF through the MRSEC (Award Number DMR-2011401) and the NNCI (Award Number ECCS-2025124) programs.

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© 2023 American Chemical Society

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