Breaking the Hydrogen Bond Barrier Reversibly: Toward Ultradrawable Polyamides.

Abstract

In polyamides, hydrogen bonding and conformations of amide motifs are strongly influenced by pH, ions and their concentration, and water molecules and their structure. To fulfill the physical requirements for ultradrawing of polyamide 6, we first complete our fundamental insight into the role of water, ions, and polyamide 6 crystal structures on the concept of reversible shielding of hydrogen bonds. The reversible shielding depends on a complementary superchaotropic effect of anions and the kosmotropic effect of cations, locally affecting the structuring and interactions of water. We show that in the presence of large halogen anions, specifically polyiodides, crystallization from the random coil state or during crystallographic reorganization is suppressed by hydrophobic hydration. Among the cations, hydrated lithium and calcium cations promote the formation of polyiodides, specifically I₃ ^-. The small size of lithium cations entails high diffusivity with water molecules, retrospectively effectively shielding the hydrogen bonding in the crystals. Upon reorganization of the conformationally distorted β phase upon heating and close to the boiling point of water, ions promote gel formation. The gel can be extruded and shaped, e.g., into monofilaments at 85 °C, and at room temperature, it can be stretched to a draw ratio of 25 to secure chain orientation. After immersion in water to remove the ions and restore the amide-amide hydrogen bonds, postdrawing and drying render high anisotropy, oriented chain crystals of high perfection, and tensile modulus and strength up to ∼19 × 10³ and ∼1140 MPa, respectively. The process holds potential in achieving extended chain crystals desired for ultimate mechanical properties.