In-situ birefringence mapping of PET/TiO₂ nanocomposite fibers by dual-channel fourier-transform polarizing interferometry: Drawing evolution and multi-factor environmental degradation

This study integrates dual-channel polarizing interference microscopy with Fourier transform phase demodulation and in-situ uniaxial drawing to simultaneously track both principal refractive indices and birefringence as continuous functions of draw ratio for blank PET and PET/TiO₂ (2.5 wt%) nanocomposite fibers. Environmental durability was assessed via accelerated aging protocols simulating UV-A irradiation, controlled humidity conditioning (30–90% RH), and repeated laundering (up to 50 cycles). TiO₂ nanoparticle incorporation extended the maximum draw ratio from 1.25 to 1.35 and raised maximum birefringence from Δn = 0.176 to 0.193, confirming enhanced nucleation density, molecular alignment, and delayed chain rupture. Under UV irradiation, PET/TiO₂ retained 92.6% of baseline drawability versus 89.6% for blank PET, attributed to UV screening and elevated crystallinity. Above 70% RH, both systems exhibited near-identical hydrolytic degradation (~9.6% loss), indicating kinetics governed by chemical hydrolysis rather than photocatalytic mechanisms. Laundering imposed the most severe degradation (~11.1–11.2% loss after 50 cycles), establishing a severity hierarchy of laundering > humidity > UV irradiation. Across all conditions, birefringence reduction correlated quantitatively with mechanical property loss, validating Δn as a sensitive, non-destructive diagnostic of cumulative structural degradation in PET-based nanocomposite fibers.