Amphiphilic miktoarm star copolymers can self-assemble into micelle-like aggregates in nonselective solvents: a case study of polyoxometalate based miktoarm stars

Abstract

It is well-known that amphiphilic star-shaped copolymers can self-assemble in selective solvents to form complicated micellar constructs as a synergistic result of both the topological constraints and relative volume fractions of the arms. Although the association phenomena of amphiphilic stars have been observed in nonselective solvents, both the structural detail and formation mechanism of these associates are not clear yet. Moreover, these experimental observations are controversial with respect to molecularly dispersed starlike copolymers in nonselective solvents as is popularly believed. To clarify these issues, we have synthesized a series of polyoxometalate-based polystyrene-poly(ethylene glycol) (PS-PEG) miktoarm star supramolecular copolymers (SEW-1-5) by coupling a Keggin-type polyoxometalate of K₄[α-SiW₁₂O₄₀] with 1,2,3-triazolium bridged block copolymers of PS_n-b⁺-PEG_mI⁻ (n=17, 26, 39, 57, 81; m=45) through ionic exchange reactions, respectively. TEM imaging, contact angle and ¹H NMR studies reveal that SEW-2-5 self-assemble in chloroform, THF, and toluene to create micellelike aggregates ranging from cylinder to sphere with a PS corona and a PEG core, while for SEW-1, reverse bilayers are captured with a PEG corona and a PS core. Among these aggregates, the Keggin clusters of [α-SiW₁₂O₄₀]⁴⁻ localize at the core-corona interfaces between PS and PEG. In terms of solvent quality, chloroform, THF, and toluene are only slightly poorer for PEG than that for PS with a relative order of chloroform<THF<toluene. These unexpected aggregates originate from the topological constraints of the chemically different arms of PS and PEG in the miktoarm stars, where the weak incompatibility between the PS and PEG arms is intensified appropriately. The presence of the reverse bilayered structures of SEW-1 is due to the magnified steric hindrance of the PEG₄₅ arm with decreasing the molecular weight of the PS₁₇ arm. However, to the best of our knowledge, these are the first examples clearly indicating that miktoarm star copolymers can self-assemble in common good solvents or slightly selective solvents to generate micellelike aggregates. This scenario is not only in sharp contrast to the intuitively considered behavior of unimolecular miktoarm stars in nonselective solvents, but also rather different from the conventional self-assembly behavior of amphiphilic star copolymers in selective solvents.