Role of Superheated Water in the Dissolution and Perturbation of Hydrogen Bonding in the Crystalline Lattice of Polyamide 4,6

Abstract

Here, we demonstrate that water, in the superheated state, is a solvent for polyamide 4,6 (PA4,6) and that the water molecules can strongly influence hydrogen bonding. In the presence of superheated water, the melting temperature of PA4,6 can be suppressed by nearly 100 degrees C. The depression in the melting temperature follows the Flory-Huggins principle. The instantaneous dissolution of the polymer hardly influences the molar mass of the polymer. However, if the polymer is retained in solution above the dissolution temperature for more than 10 min, hydrolysis occurs. These findings suggest that the dissolution of the aliphatic polymer in superheated water is mainly a physical process as opposed to a chemical process. Time resolved X-ray studies show that the dissolution occurs prior to the Brill transition temperature, as reported earlier. Crystals grown from the water solution show a lath-like morphology with interchain and intersheet distances that are similar to the distances obtained for crystals grown from other known solvents. Electron diffraction further confirmed that the crystals grown from superheated water are single crystals, where the chains are perpendicular to the ab-plane. SAXS performed on dried sedimented water grown single crystals showed a lamellar thickness of 6 nm. The lamellar thickness is in accordance with other reported studies on PA4,6, confirming that the single crystals incorporate four repeat units between re-entrant folds with an amide group incorporated in the tight fold. Solid state NMR studies performed on mats of these single crystals showed two different mobilities of the proton associated with the amide groups: a higher mobility linked to the amide protons in the fold and a reduced mobility of the hydrogen bonded amide protons within the crystal. Additionally, the solid state NMR studies on the dried water crystallized single crystals show the presence of water molecule(s) in the vicinity of the amide groups. This was confirmed by infrared studies that conclusively demonstrated the appearance of two new bands arising due to the binding of a water molecule in the vicinity of the amide group (i.e., NH3(+) and COO(-) bands that disappear upon heating at approximately 200 degrees C). Additionally, DSC traces of the water crystallized PA4,6 show an exothermic event in the same temperature region (i.e., in the vicinity of the Brill transition temperature, where the bound water exits from the lattice). Furthermore, this event was corroborated by TGA data.

Efficient one-step melt-compounding of copolyetheramide/pristine clay nanocomposites using water-injection as intercalating/exfoliating aid

“…In the latter phase, H-bonds make it possible for water to solvate the polyamide blocks [18]. HPDSC measurements were also performed at different pressures (20,40,60 and 80 bar, not reported here). Whatever the pressure, the melting point of the PEBA/water phase appears at the same temperature (153.5°C).…”

“…Furthermore, due to their highly polar amide functions, polyamides are hydrophilic materials. PAwater solubility under temperature and pressure conditions has widely been reported in the literature [18][19][20][21][22][23][24]. Considering this peculiar behavior of polyamides and that montmorillonite platelets are separated from each other in water (constituting a colloidal dispersion), attempts have been made to elaborate PA/pristine clay nanocomposites by melt extrusion with the aid of water.…”

Dissolution, hydrolysis and crystallization behavior of polyamide 6 in superheated water

“…Also, the crystallization temperature of PA 6 from aqueous solution decreases compared with that from the melt. The dissolution process of PA 6 in superheated water is a process in which water molecules break the hydrogen bonding between amide groups in PA 6 and then make PA 6 dissolve in superheated water [11,12] . It was reported that the Brill transition temperature (T B ) of PA 6 was 160 C [24,25] , which should be helpful for the PA 6 dissolution in water.…”

“…Rastogi et al demonstrated that Nylon 46 can dissolve rapidly in superheated water at ~200 C, and re-crystallize on cooling [11,12] . A hydrolysis process is hard to avoid when polyamides dissolved in superheated …”

New Solvent for Polyamide 66 and its use for Preparing Single-Polymer Composite Coated-Fabric

“…As discussed earlier, urea has four hydrogen bonding per unit of molecule, even if all the four hydrogen cannot engage in hydrogen-bonding, still has high capability of hydrogen-bonding interactions. On the other hand, PA chains contain amide groups which are the strong hydrogen bonding donor/acceptor groups [19], while FA has only one hydrogen bonding per unit of molecule. Therefore, the mixture of FA/urea will have a higher tendency to penetrate into the PA chains.…”