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

HL Yu and P Liu and XH Ji and XZ Jiang and B Sun, POLYMERS, 17, 2551 (2025).

DOI: 10.3390/polym17182551

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. This work proposes, for the first time at fiber scale, a four-phase partition into crystal (c), crystal-adjacent rigid amorphous fraction (RAF-c), interfacial rigid amorphous fraction (RAF-i), and mobile amorphous fraction (MAF), and extracts an interfacial triad consisting of the specific interfacial area (S-v), polymer-only RAF-i fraction expressed per composite volume (Gamma(i)), and interphase thickness (t(i)) from SAXS invariants to establish a quantitative interphase-structure-property framework. A documented SAXS/DSC/WAXS workflow partitions the polymer into the above four components on a polymer-only basis. Upon filling, Gamma(i) increases while RAF-c decreases, leaving the total RAF approximately conserved. Under identical cooling, DSC shows the crystallization peak temperature is higher by 1.6-4.3 degrees C and has longer half-times, indicating enhanced heterogeneous nucleation together with growth are increasingly limited by interphase confinement. At 4 wt% loading, KH570-modified fibers versus KH550-modified fibers exhibit higher alpha- phase orientation (Hermans factor f(alpha): 0.697 vs. 0.414) but an similar to 89.4% lower alpha/gamma ratio. At the macroscale, compared to pure (neat) PA6, 4 wt% KH550- and KH570-modified fibers show tenacity enhancements of similar to 9.5% and similar to 33.3%, with elongation decreased by similar to 31-68%. These trends reflect 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, whereas pure PA6 fabrics show only similar to 5.0, corresponding to >= 16-fold improvement. Taken together, the SAXS-derived descriptors (S-v, Gamma(i), t(i)) provide transferable interphase quantification and, together with WAXS and DSC, yield a reproducible link from interfacial geometry to kinetics, structure, and properties, revealing two limiting regimes-orientation-dominated and phase-fraction-dominated.

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