Poly(p-phenylene terephthalamide), PPTA, fibers achieve exceptional strength-to-weight ratios through the alignment of crystalline domains along the fiber axis. These domains form during processing, when the polymer adopts a liquid crystalline, LC, state. This work investigates the molecular orientation and dynamics of LC PPTA under shear and extensional flows. A novel Mueller calculus model is developed to directly link molecular polarizability to the intensity collected via polarized resonance Raman spectroscopy. This approach enables in-situ quantification of molecular orientation and dynamics. The model describes the total polarizability as the weighted sum of two distinct polarizability domains: ordered, anisotropic LC regions and disordered, isotropic fluid regions. The limits of the analytical model are evaluated in polar coordinates, assuming the anisotropic polarizability tensor represents a high aspect ratio rigid rod. The model predicts the absence of cross-polarized intensity under two conditions. The first occurs when the polar angle, θ, can vary freely at a fixed azimuthal angle, φ, corresponding to a precessional motion. The second condition occurs when all φ are possible at a fixed θ, corresponding to a tumbling motion. Observed cross-polarized intensity in the LC PPTA system suggests that the molecules maintain a fixed orientation in both θ and φ. This indicates that rigid rod-like PPTA molecules cannot precess or tumble freely in shear flows; instead, they rotate to align toward the shear direction. Applying the model to dynamic shear stress experiments on LC PPTA shows that only 5–10% of rigid rod-like anisotropic domains align tangentially to the shear direction under steady-state conditions. This finding challenges the long-standing assumption that PPTA behaves as a fully aligned rigid rod-like system when the shear stress is independent of time. These results indicate that PPTA exhibits complex orientational behavior and molecular dynamics. This has significant implications for modeling and interpreting the rheology of the LC state.
