Interphase-Resolved Performance in PA6/TiO₂ Nanocomposite Fibers: Four-Phase Geometry Linking Structure to Mechanical and UV Protection

Melt-spun PA6/TiO₂ fibers with TiO₂ modified by silane coupling agents KH550 and KH570 at 0, 1.6, and 4 wt% provide a practical testbed to address three fiber-centric gaps: transferable interphase quantification, interphase-resolved indications of compatibility, and a reproducible kinetics–structure–property link. We implement a documented SAXS/DSC/WAXS workflow to partition the polymer into four components on a polymer-only basis—crystal (c), partitioned into crystal-adjacent (RAF-c), interfacial rigid amorphous (RAF-i), and mobile amorphous fraction (MAF). From Porod invariants we obtain the specific interfacial area Sᵥ, and define Γᵢ (polymer-only RAF-i expressed per composite volume). Upon filling, Γi increases while RAF-c decreases, leaving the total RAF approximately conserved. Under identical cooling, DSC shows increases in crystallization peak temperature and half-time, indicating enhanced heterogeneous nucleation together with growth that becomes increasingly diffusion-limited under interphase confinement. At 4 wt% loading, KH570 vs KH550 exhibits higher α-phase orientation but a lower α/γ ratio. At the macroscale, storage modulus and tenacity increase whereas elongation decreases; this trend is consistent with orientation-driven stiffening accompanied by a reduction in the mobile amorphous fraction and stronger interphase constraints on chain mobility. Knitted fabrics achieve a UV protection factor (UPF) of at least 50. Taken together, the SAXS-derived pair (Sv,Γi) provides transferable interphase quantification and, together with WAXS and DSC, yields a reproducible link from interfacial geometry to kinetics, structure, and properties, revealing two limiting regimes—orientation-dominated and phase-fraction-dominated.