Abstract: This study employed bulk melt polycondensation to synthesize a series of odd-even polyamide PA5X (X=6, 8, 10, 12, 14) materials using bio-based 1, 5-pentanediamine and dicarboxylic acids of varying chain lengths as monomers. The effects of dicarboxylic acid chain length on the condensed-phase structure and properties of these materials were systematically investigated. The study revealed that increasing the carbon chain length X of the dicarboxylic acid enhances the molecular chain conformation mobility of PA5X, facilitating crystallization and the formation of more complex single-crystal morphologies. Concurrently, the strength of single-crystal hydrogen bonds also increases with X. In dilute solutions, longer-chain PA5X readily forms highly ordered monoclinic α-crystals (independent of solvent) . Performance studies indicate that the water absorption rates of PA56 and PA58 significantly increase over time, reaching 2.41% and 1.53% respectively after 30 days. In contrast, the water absorption rates of longer-chain polyamides such as PA510, PA512, and PA514 show smaller increases. It is hoped that the above research will provide theoretical support for the development of high-performance polyamides.