In Situ Quantification of Polymorphism and Brill Transition Study in PA11 During Thermal Cycles by Raman Spectroscopy and WAXS

ABSTRACT Understanding and monitoring the microstructure of polyamide 11 (PA11) is essential to link structure–property relationships for Type IV hydrogen tank liners, where thermal and pressure cycles govern performance. In this study, in situ Raman spectroscopy and WAXS are combined with Differential Scanning Calorimetry (DSC) analysis to quantify phase fractions during thermal cycles. Two complementary Raman criteria are established: C₁, based on the 1,067/1,107/1,120 cm −1 C‐C stretching bands, tracks the α′ crystalline fraction, while C₂, derived from the 1,640 cm −1 ν(C=O) band, quantifies the overall hydrogen‐bonded order (α′ + δ′). WAXS deconvolution quantifies the fraction of α′, δ′, and amorphous contributions and serves as a reference for validation. Both techniques consistently identify the Brill transition near 95°C: By WAXS with the merging of α′(200) and α′(210/010) reflections, while Raman detects an inversion of the 1,440 cm −1 CH 2 band Raman shift. The results confirm the transient role of the smectic δ′ phase during the α′ → δ reorganization and show that α′ reorganizes more slowly than δ′ during thermal cycling. The proposed Raman criteria provide a robust, non‐destructive framework for in situ quantification of ordered and pseudo‐ordered phases, useful for monitoring long‐term PA11 microstructure and mechanical performance in hydrogen storage applications.