Crystal structure, thermodynamics, and crystallization of bio-based polyamide 56 salt†
Bio-based polyamide 56 is a new type of bio-based polyamide that is regarded as a most promising material for the textile industry. High-quality polymer materials require first high-quality monomers. In this report, we investigated the solid–liquid equilibrium of polyamide 56 salt (an ionic solid compound used to prepare polyamide 56), thermodynamic parameters of the dissolution process, and the crystallization process, and screened out a favorable crystallization system to obtain a high-quality product. The dissolution was shown to be endothermic and not a spontaneous process; entropy was the overall driving force for the dissolution. The crystallization process and the effect of impurities were investigated. Cooling crystallization in pure water was beneficial to the nucleation and crystal growth of polyamide 56 salt, and is a good choice from the perspective of economy and the stability of the product. Additionally, the crystal structural characteristics and thermal behavior were systematically characterized by single crystal X-ray diffraction (SSRD), powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). It was found that there were two crystal forms of polyamide 56 salt, dihydrate and anhydrate. The dihydrate was a stable crystal form, and the anhydrous form could be converted into the dihydrate by absorbing moisture. The temperature 183.7 °C was the onset of 1,5-pentanediamine cation deprotonation and adipic acid anion protonation rather than their degradation. These contributions provide a potential industrial preparation strategy for high-quality polyamide 56 salt, as well as laying a foundation for its polymerization.