Tuning the Hydrogen Bond Density of Polyamide toward High Barrier Performance and Great Transparency

Polyamide 66 (PA66) is of particular importance in both academic and industrial areas such as automotive, electronics, and machinery. However, its abundance of polar amide groups results in high moisture adsorption, which significantly restricts its barrier performance especially for packaging applications. To address that, we proposed a strategy that enhanced the chain asymmetry and steric hindrance of PA66 via introducing an aliphatic ring to effectively reduce its hydrogen bond density. A series of PA66 copolymers with varying contents of aliphatic rings was designed and prepared via adjusting the content of 4,4′-diaminodicyclohexylmethane (PACM) as well as the dicarboxylic acids with different carbon chain lengths. It was found that the introduction of PACMs not only caused the crystal structure of the copolymer to shift from α-phase to γ-phase but also seriously reduced crystallinity. When combined with longer diacid chains, these changes substantially enhanced the barrier properties of the copolymers, which were further revealed via molecular simulations. The results further revealed that the cyclic structure of PACM weakened the hydrogen bonding strength while complicating gas diffusion pathways, thus improving the copolymer’s barrier performance. These results indicated that the intrinsic strategy of adjusting the chain structure is a promising approach for developing PA66-based materials, thereby demonstrating their potential as high-performance functional components for advanced multilayer packaging films.