Hydrogen-Induced Morphological Changes in Semi-Crystalline Polyamides Investigated by ¹³C Solid-State Nuclear Magnetic Resonance Techniques

The morphological variation of four semicrystalline polyamides─PA6, PA11, PA66, and PA612─under 250 psi hydrogen gas exposure was investigated using ¹³C cross-polarization (CP) and direct-polarization (DP) magic angle spinning (MAS) NMR. Additionally, two-dimensional ¹³C-¹H wide-line separation (2D WISE) NMR provided insight into site-specific molecular dynamics. While all samples exhibited broadly similar segmental mobilities, PA6 showed slightly enhanced mobility at carbonyl sites, whereas PA612 displayed reduced mobility. The average chain mobility followed the trend: PA612 > PA6 > PA11 > PA66. Quantitative ¹³C NMR revealed the presence of a mobile amorphous or interfacial phase, most prevalent in PA11 (16%) and least in PA612 (9%). Initial crystallinity was highest in PA11 (36%) and lowest in PA612 (21%). Hydrogen exposure led to a marked reduction in crystallinity─up to 38% in PA612─followed by partial recovery upon depressurization. Site-specific analysis indicated the lowest crystallinity at carbonyl sites, with the unit end sites (CO and NH sites) showing distinct behavior between one-monomer (PA6 and PA11) and two-monomer (PA66 and PA612) polyamides. These findings suggest that hydrogen preferentially interacts with carbonyl and amide groups. A strong correlation was observed between chain mobility, the degree of crystallinity, and the reduction in the crystalline phase resulting from pressurization with 250 psi H₂ gas. Among the polyamides studied, PA66 exhibited the greatest resistance to hydrogen-induced morphological changes, attributed to its higher crystallinity and reduced chain mobility. This underscores the importance of structural rigidity in enhancing polymer resilience under high-pressure hydrogen environments.