Difference in the micro-dynamics mechanism between aromatic nylon and aliphatic nylon during water absorption: spectroscopic evidence

The difference in the micro-dynamics mechanism between aromatic nylon and aliphatic nylon during water absorption was studied to explore the reason for the significantly smaller decline of barrier performance of poly(m-xylene adipamide) (MXD6) film than that of polyamide 6 (PA6) film under high humidity. Attenuated total reflection (ATR) FTIR, combined with scaling moving-window 2D correlation spectroscopy (scaling-MW2D) and generalized 2D correlation analysis, was used. The scaling-MW2D confirmed that the water absorption of MXD6 comprises two processes, designated process I (2.0–12.0 min) and process II (12.0–27.0 min). According to the sequential order of the functional groups’ movement obtained from the generalized 2D correlation analysis, processes I and II undergo three and two steps, respectively. Process I is the process of water plasticizing MXD6, which primarily consists of hydrophilic groups forming hydrogen bonds with water molecules and the hydration of hydrophobic groups. Process II is the strong crystallization of MXD6 induced by water molecules, entailing the generation of the double hydrogen bond and the rearrangement of hydrophilic groups into the crystal lattice. However, for PA6, the results confirmed that its water absorption only had a single process with two steps, which was actually the plasticization of PA6 macromolecular chains by water molecules. Therefore, the reason for the barrier performance decline of the MXD6 film being significantly smaller than that of the PA6 film is that water molecules induced a large amount of new crystalline regions in the MXD6 matrix during plasticization.