Erasing Conformational Limitations in N,N '-1,4-Butanediyl-bis(6-hydroxy-hexanamide) Crystallization from the Superheated State of Water

Aliphatic polyamides consist of regularly distributed amide moieties located in an aliphatic chain, off which the segment length can be varied. The crystallization and hence the. eventual performance of the material can be tailored by changing the aliphatic lengths, and thus the hydrogen bonding density and the directional chemical positioning of the amide motifs. In this paper, N,N'-1,4-butanediyl-bis(6-hydroxy-hexanamide) crystallized either from the melt or from the superheated state of water is investigated. A comparison with N,N'-1,2-ethanediyl-1,2-bis(6-hydroxy-hexanamide) reveals the role of the hydrogen bonding density on the accommodation of water molecules in amide based crystals grown from the superheated state of water. However, wide-angle X-ray diffraction (WAXD), Fourier transform infrared (FTIR), and solid state H-1 and C-13 NMR spectroscopy reveal hydrogen bonding between the amide planes, while aliphatic polyamides and N,N'-1,2-diethyl-bis(6-hydroxy-hexanamide) feature hydrogen bonds that reside within the amide plane only. As a consequence, the role of N,N'-1,4-butanediyl-bis(6-hydroxy-hexanamide) crystals as a model system for polyamide 4Y polymers is questionable. However, the thermodynamic and structural behavior as function of temperature is determined by it balance between thermally introduced gauche conformers and hydrogen bonding efficiencies. These crystals enable a thorough investigation in the effect Of Superheated water on the crystallization of these uniquely hydrogen bonded molecules. Crystallization from the superheated state of water results in denser molecular packing and enhanced hydrogen bonding efficiencies. The induced spatial confinement hinders molecular motion upon heating, and thermodynamically more stable crystals are observed. Although the amide-hydroxyl hydrogen bonded crystals do not favor the accommodation of physically bound water molecules in the lattice, saturation of the amide motifs during crystallization erases conformational restrictions of the planar amide moieties that facilitates maximum hydrogen bonding efficiencies in the eventual lattice.