Quantifying molecular orientation in Poly(p-phenylene terephthalamide) fibers by polarized Raman spectroscopy

Poly(p-phenylene terephthalamide) (PPTA) forms high-performance fibers, and the molecular orientation in the fibers are crucial to their mechanical properties. Here we report a method for quantitative analysis of the molecular orientation in individual PPTA fibers by polarized Raman spectroscopy. Four parallel- and cross-polarized spectra are acquired in a backscattering geometry for each fiber on a confocal Raman spectrometer fitted with a 785 nm laser. The intensity of the aromatic C–C stretching band of the benzene rings at ∼1610 cm⁻¹ is utilized to quantify the molecular orientation in the fiber. The results thus obtained are in good agreement with the crystal orientation in these fibers revealed by wide-angle X-ray diffraction, especially for the ones of low or high degree of orientation. This method is then applied to monitor the orientation development of PPTA fibers in a post-treatment process of thermal stretching, which reveals that the degree of orientation decreases upon heating at a high temperature, and is enhanced significantly after an external stress is applied.