Multifunctional Engineering of iPP/TiO2 Nanocomposite Fibers: Coupled Opto-Mechanical, Electronic, and Antimicrobial Property Development via In Situ Dual-Beam Interferometry

Understanding and controlling the hierarchical microstructure of semicrystalline polymer fibers during mechanical processing remains central to materials design. However, quantitative relationships between processing, structure, and multifunctional properties are poorly understood in nanocomposite system. This study investigates the coupled opto-mechanical, electronic, and structural evolution of isotactic polypropylene (iPP) and iPP/TiO2 nanocomposite fibers (2 wt%) during progressive uniaxial drawing using dual-beam polarizing interference microscopy integrated with in situ fiber stretching. A comprehensive methodology was established for extracting refractive indices, birefringence, polarizabilities, and stress-optical coefficients from interference patterns, with density and crystallinity determined via the Lorentz-Lorenz equation and validated by X-ray diffraction. Pure iPP fibers showed systematic evolution during drawing: birefringence increased 200%, crystallinity rose from 39.74 to 50.00%, and the stress-optical coefficient improved 45%. In contrast, anatase TiO2 nanoparticles (2 wt%) fundamentally altered the response, enabling 24% greater extensibility, a dramatic birefringence increase, and exceptional crystallinity of 60.13%, surpassing pure iPP’s apparent limit. The nanocomposite also demonstrated a 75% higher stress-optical coefficient and broad-spectrum antimicrobial activity (52–68% pathogen reduction) via photocatalysis. This provides a powerful strategy for creating multifunctional polymer fibers with superior structural, mechanical, optical, and antimicrobial properties.