Study on the Brill transition of polyamide 6 with different crystal forms using low- and high-frequency Raman spectroscopy†

Abstract

Polyamide 6 (PA6) in its α and γ-forms was studied from 30 to 220 °C using Raman spectroscopy in the low- and high-wavenumber regions. Quantum chemical calculations were employed to assist with band assignments. In the low-wavenumber region, a peak at approximately 100 cm⁻¹, attributable to a mixed mode of methylene lateral motion and amide group stretching, was observed. Additionally, a new band at approximately 60 cm⁻¹ was observed and assigned to molecular chain torsions in the α-form. Both bands indicated that molecular chain rotation occurs prior to the Brill transition at approximately 130 °C. In the high-wavenumber region, bands at approximately 1126 cm⁻¹ and 1060 cm⁻¹ indicated a simultaneous weakening of C–C stretching modes in the trans conformation at the same temperature, consistent with observations in the low-wavenumber region.