Processing of Polyamides in the Presence of Water via Hydrophobic Hydration and Ionic Interactions

Abstract

In synthetic as well as natural polyamides, hydrogen bonding and conformations of amide motifs are strongly influenced by the presence of ions and their concentration, water molecules, and their structure, as well as the pH of the solution. This concept combined with solubility of synthetic aliphatic polyamides, in particular nylons, in water at elevated temperature and corresponding vapor pressure is evaluated as a new reversible shielding route in the processing of these polymers. So far, reversible shielding has not been feasible due to a lack in controlling desired activation and deactivation of hydrogen bonding at the judicious moments. Here we show that in the presence of large halogen anions, crystallization from the random coil state is suppressed by hydrophobic hydration, where the amorphous state of the fast crystallizing nylons can be maintained even at 20 °C. Small hydrating lithium cations are favored since they strengthen the hydrophobic nature of the anions. Complete deshielding of hydrogen bonding, after processing, is facilitated by simple migration of ions in water that allows recovery of the desired conformation and structure.

Effect of polar solvents on the crystalline phase of polyamides

“…Furthermore it was recently shown that formation of amide-amide hydrogen bonds can be hindered at high temperature by the strong solvation of amide groups by water molecules, inhibiting crystallisation and/or allowing dissolution of crystallites, as observed in waterimmersed Nylon-4,6 at a temperature of about 200 C [29][30][31]. The mobility of hydrogen bonds induced in the crystalline phase at high temperature by superheated water was also discussed in analogy with the Brill transition, which takes place in PA6 and consists in reorientations of hydrogen bonds within crystallites [32].…”

Self‐Organization of Poly(ethylene oxide) on the Surface of Aqueous Salt Solutions

“…Several mechanisms have been proposed including preferential ionic hydration, water dipole orientation, electrostatic effects, osmotic effects and direct interaction between ions and macromolecules through van der Waals forces . A qualitative classification of ions considers two groups: kosmotropes, anions located to the left of chloride, are well hydrated anions proposed to increase the ordering of water molecules and tend to salt macromolecules out of solution and to stabilize folded conformations, and chaotropes, to the right of chloride, which are poorly hydrated, reduce the structuring of water and promote solubility and unfolding of macromolecules . Systematic studies of the role of specific ion effects in polymeric systems focused initially on aqueous PEO solutions and the influence on its phase behavior .…”

“…[ 9 ] A qualitative classifi cation of ions considers two groups: kosmotropes, anions located to the left of chloride, are well hydrated anions proposed to increase the ordering of water molecules and tend to salt macromolecules out of solution and to stabilize folded conformations, and chaotropes, to the right of chloride, which are poorly hydrated, reduce the structuring of water and promote solubility and unfolding of macromolecules. [ 10 ] Systematic studies of the role of specifi c ion effects in polymeric systems focused initially on aqueous PEO solutions and the infl uence on its phase behavior. [ 8,11 ] More recently other polymeric systems have also been investigated such as poly(2-oxazoline)s, [ 12 ] poloxamers, [ 13 ] and polyamides.…”

Processing polyamides with superheated water

“…16 While hydrolytic degradation was a concern in these processes, it was shown to be manageable for short residence times at 200 8C and may not pose a significant problem in melt processing methods such as extrusion or injection molding. 18 Finally, water has also been used to aid in montmorillonite dispersion for PA6 nanocomposites during extrusion. 19,20 While shH 2 O is known to dramatically affect the stability of PA crystal structures, its use as a processing aid for common commercial PAs has not been fully explored.…”