Elucidating Ionic Liquids Functions at the <scp>PPS</scp> /Basalt Interphase: Sizing Versus Modifier Effects

ABSTRACT The performance of fiber‐reinforced polymer composites (FRPCs) is governed by the structure and chemistry of the fiber‐matrix interphases. This study explores the role of imidazolium‐based ionic liquids (ILs) as innovative interfacial agents of basalt fibers (BF) combined with a polyphenylene sulfide (PPS) matrix through two complementary routes: (i) ILs used as fiber sizing agents, that is, at the fiber surface and (ii) ILs incorporation into the PPS matrix. A multi‐scale approach was considered to understand the role of imidazolium ILs on interfacial adhesion and create interphases. As sizing agents, imidazolium‐based ILs formed uniform and continuous coatings on basalt fibers as demonstrated by SEM–EDX and TGA. Deposition efficiency was found to depend primarily on the IL counter‐anion rather than fiber contact time. Micromechanical pull‐out tests, interpreted by Greszczuk and Penn–Lee models, concluded on an improved matrix‐to‐fiber load transfer but also revealed a tendency toward a more brittle interphase. Conversely, incorporating ILs into the PPS matrix produced significant microstructural changes enhancing mechanical performances, fiber wettability, and decreasing crystallinity. Reactive and unreactive ILs as modifiers of PPS increased matrix stiffness (+11%) and dramatically boosted ductility (1% to 237%) yielding a more energy‐dissipative interphase and a significant improvement in interfacial shear strength (IFSS) (+60%). These findings demonstrate that IL chemistry—particularly anion identity—critically governs interphase behavior. This study establishes design principles for engineering next generation IL‐based composites interfaces.