Miktoarm star polymers as templates for mesoporous phenolic materials: Enhancing microphase separation and expanding phase diagrams

In the field of soft matter, designing different amphiphilic chemical structures could lead to the formation of ordered microphase-separated arrangements. Unlike linear block copolymers, miktoarm star polymers have been extensively utilized and inherently enhance the conformational asymmetry value (ε) in the microphase separation system through their branched chemical structure, allowing control over the interfacial curvature between microphase-separated domains and facilitating the formation of unique self-assembled structures, offering the potential to expand phase diagrams. In this study, we synthesized a common linear PEO-b-PCL diblock copolymer via ring-opening polymerization (ROP) and also prepared an AB₃-type block copolymer, PEO-b-PCL₃, by linking PEO to a branched PCL₃ block through a combination of ROP and click reactions with the similar volume fraction of PEO segment. Resol-type phenolic resin was synthesized by step-growth polymerization and used as the matrix, which was mixed with different weight fractions of linear PEO-b-PCL and miktoarm-type PEO-b-PCL₃ block copolymers as templates. FT-IR analysis of these blends at room temperature revealed that, under the similar volume fraction, the miktoarm-type star polymer forms the higher fraction of intermolecular hydrogen bonds with the phenolic resin in the PCL₃ block segment, and no crystallization was observed. Variable-temperature SAXS analysis demonstrated that upon thermal cross-linking, the blends exhibited improved stability and enhanced ordered microphase-separated structures. By heating to pyrolysis temperatures to remove the template, the resulting structures were characterized by using TEM and SAXS analyses. The systems constructed by two different templates creating mesoporous materials with different ordered structures such as lamellar, hexagonal columnar, spherical, BCC packing, and even the rare short-range order Frank-Kasper phase. Through phase diagram construction, we found that even with the similar weight fraction, the ordered mesoporous structures formed using the two different type of templates displayed significant differences. Additionally, the results indicated that when the branched segment of the miktoarm-type polymer is located at the core of the micelle, the interfacial curvature decreases. Overall, this study reveals the existence of the unique short-range order Frank-Kasper phase in systems with low χ values using wet-brush blending and validates the feasibility of employing miktoarm-type polymers as templates through analysis of molecular interaction and morphological characterization. This expands the phase diagram in the direction of tuning conformational asymmetry, providing a novel approach for designing innovative or specialized ordered mesoporous materials.