The effect of polymorphism on polymer properties: crystal structure, stability and polymerization of the short-chain bio-based nylon 52 monomer 1,5-pentanedi­amine oxalate

The compound 1,5-pentanedi­amine (PDA) is prepared by biological methods using biomass as raw material. The salt of 1,5-pentanedi­amine oxalate (PDA-OXA) was used directly as the monomer for the preparation of a new bio-based nylon 52 material. High-performance polymer materials require initial high-quality monomers, and crystallization is an essential approach to preparing such a monomer. In this work, three crystal forms of PDA-OXA, the anhydrate, dihydrate and trihydrate, were found and the single crystals of two hydrates were obtained. Their crystal structures were determined using single-crystal and powder X-ray diffraction. The thermal behaviors were characterized by thermodynamic analysis, and the lattice energy was calculated to further explore the relationship between the thermal stability and crystal structure. Detailed computational calculations, Hirshfeld analyses and lattice energy calculations were performed to quantify both the contribution of intra- and intermolecular interactions to the supramolecular assembly, as well as the influence on the stability of the structure. The structure–property relationship between the PDA-OXA crystal forms was established. Moreover, the phase transformation mechanism between the crystalline forms of PDA-OXA has been established, and the control strategy of specific crystal forms was developed from the water activity–temperature phase diagram and relevant thermodynamic data. Finally, the influence of the polymorphism of the monomer and the polymerization methods on the properties of the polymer was investigated. The nylon 52 product obtained showed good appearance, high hardness and thermal stability, the polymer made using the anhydrate as the monomer has better thermodynamic properties than that prepared from the dihydrate, indicating practical industrial application prospects.