Discovery and characterization of new crystal forms of bio-based nylon 4F salt

Abstract

Nylon 4F is a fully bio-based nylon material. Its raw materials 1,4-tetramethylenediamine (TMDA) and 2,5-furandicarboxylic acid (FDCA) can be prepared by biological methods using biomass. The high-performance polymer materials require high-quality monomers. In this report, we found and obtained three novel crystal forms of nylon 4F salt: form I is anhydrous; form HI and form HII are monohydrates. Their crystal structures were determined and analyzed by single crystal X-ray diffraction (SCXRD) and powder X-ray diffraction (PXRD). Meanwhile, we investigated the solid–liquid equilibrium of nylon 4F salt in different proportions of ethanol–water mixtures at different temperatures and correlated the solubilities with temperature and the solvent ratio using the (CNIBS)/Redlich–Kister model, modified Apelblat model and van't Hoff equation. It was proved that the solubility of nylon 4F salt increases with the increase of temperature and decreases with the increase of ethanol content. Besides that, the thermal behavior and phase transformation process were investigated in detail. Form HII, prepared by a quenching method, was an unstable crystal form and easily transformed into form HI during storage. Form I and form HI were enantiotropic in water–ethanol solution, and the phase diagram of the phase transformation was determined. In order to ensure that each crystal form can be synthesized under control, the control strategy was developed from the phase transformation relationship and relevant thermodynamic data. The lattice energy and crystal density were calculated to explain the relative stability of the different forms.