Dissolution and Crystallization of Polyamides in Superheated Water and Concentrated Ionic Solutions

The dissolution and recrystallization of Polyamide 46 (PA46) from superheated (i) water and (ii) concentrated ionic solutions of strong solubilizing mono- and divalent Hofmeister ions are studied, utilizing in situ high-resolution magic-angle spinning (HR-MAS) nuclear magnetic resonance (NMR) supported by wide-angle X-ray diffraction (WAXD), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and gel permeation chromatography (GPC). The samples are sealed in glass capillaries, and employing variable-temperature H-1 HR-MAS NMR spectroscopy, the dissolution process of PA46 as a function of temperature and pressure can be followed in situ. The purpose of such a study is to obtain molecular insight into the dissolution process of these hydrogen-bonded synthetic polymers in the different aqueous solutions. In pure water, at temperatures close to the dissolution of PA46, two distinct H-1 resonances from water are observed. These resonances are associated with water molecules in the vicinity of PA46 and water in the bulk state. On further heating, the signal from water molecules in the vicinity of PA46 dominates. This sudden change in the environment suggests that water molecules, which have escaped the dense hydrogen-bonded network of bulk water, can diffuse into the structure of PA46, triggering dissolution of the polymer. This happens at a temperature that is more than 100 degrees C below the melting temperature of the polymer, notably without hydrolysis as verified by GPC performed prior to and after the dissolution experiments. On cooling, recrystallization of PA46 from, aqueous solution is observed where water molecules are incorporated in the crystal structure. In the presence of salts, such as LiI and Cal(2), weakening of the hydrogen-bonding network of the water molecules occurs. However, above room temperature, independent of the choice of salt, depopulation of the hydrogen bonding between the water molecules occurs, observed as a decrease in the H-1 chemical shift value. The reduced hydrogen bonding in the presence of ions facilitates the dissolution of PA46 at much lower temperatures compared to pure water and ultimately results in the complete suppression of crystallization from solution even at room temperature. Depending on the valency of the cation a more mobile or frozen amorphous state of PA46 is obtained at low temperatures as verified by C-13 HR-MAS NMR, ATR-FTIR, and WAXD.