The Role of Superheated Water on the Crystallization of N,N′-1,2-Ethanediyl-bis(6-hydroxy-hexanamide): Implications on Crystallography and Phase Transitions

Abstract

A symmetrical, hydrogen bonded low molecular weight molecule N,N′-1,2-ethanediyl-bis(6-hydroxy-hexanamide), crystallized from melt or from the superheated state of water, is examined. Thermodynamic and structural changes during phase transitions are followed by DSC, time-resolved X-ray techniques and polarized optical microscopy. Considering the hydrogen bonding motifs present in this bisamide-diol, it is selected as a model compound for crystalline domains present in semicrystalline polyamides. By studying this model compound it was moreover aimed to elucidate the specific role of water molecules that are likely to reside in the crystals obtained from the superheated state of water. On heating the melt crystallized sample, the observed crystalline transitions are not the same as observed in polyamides. However, similar to polyamides the origin of the transition is due to the electron exchange between the hydrogen bonding moieties and conformational changes in the aliphatic sequences. At low temperatures (below 22 °C) non-trans conformations in the central diamine methylene moieties induce a different triclinic structure, having unit cell parameters close to monoclinic, with potential existence of intersheet hydrogen bonding. Crystallization from superheated water entails remarkable differences in the physical behavior. A metastable crystalline structure, obtained from the superheated state of water and having relatively large interchain and intersheet distances, transforms into another hydrogen bonded crystal via sequential temperature cycles. When compared with the melt crystallized sample the crystal obtained after sequential temperature cycles show considerable difference in the crystal-to-crystal phase transition while melting remains the same. In combination with the increased crystal-to-crystal transition temperature, an expansion along the c-axis suggests a stabilizing effect of rigid hydroxylic protons that contribute to the unit cell parameters.

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

“…Upon cooling, crystallization occurs above the boiling point of water, where the continuous electron exchange between the water molecules and the amide motifs leads to the entrapment of water molecules within the crystal lattice . Since polyamides are semicrystalline in nature, the use of a low molecular weight symmetrical bisamide-diol model compound was proposed in our earlier work, evading the complications of an amorphous component, to investigate the location of water molecules and its implications on hydrogen bonding within the crystalline domains. , …”

“…Polyamides feature variations in hydrogen bonding densities by changing the length of the aliphatic segments that are connected by amide motifs in either a symmetrical or anti-symmetrical way. − In this study, it is aimed to investigate the balance between the molecular motion in the aliphatic sequences on both sides of the amide moieties and the stability and effect of water−amide interactions. Another bisamide-diol, namely, N , N ′-1,4-butanediyl-bis(6-hydroxy-hexanamide) (Figure ), is introduced and compared with the previously reported N , N ′-1,2-ethanediyl-bis(6-hydroxy-hexanamide), , both representing the crystalline domains in the polyamide XY family. Differences in hydrogen bonding density can be classified by means of the ratio between the number of aliphatic CH 2 groups and the number of hydrogen bonding amide and hydroxyl groups.…”

Self-assembling process of Oxalamide compounds and their nucleation efficiency in bio-degradable Poly(hydroxyalkanoate)s

“…Upon cooling from melt (205 °C), two exothermic peaks at 192.9 °C and ~ 7.1 °C are observed. The phase transition at 192.9 °C is attributed to crystallization from melt, whereas the low temperature exothermic transition, 7.1 °C, is attributed to reorganization of the crystalline state to achieve the equilibrium state 34 . The recorded phase transitions and respective enthalpy of melt and water crystallized samples are summarized in Table S1 (see Supplementary Section ).…”

Tailoring the crystallization behavior of poly(L-lactide) with self-assembly-type oxalamide compounds as nucleators: 1. Effect of terminal configuration of the nucleators

“…The oxalamide group consists of two amide groups next to each other in reverse fashion, which allows the formation of two hydrogen bonds per oxalamide motif, resulting in an increased tendency to crystallize into sheet-like structures [25]. The formation of hydrogen bonds is regarded as the driving force for self-organization (i.e.…”