Hydrogen bonding and crystalline structure of bio-based PA56

PA56 is a newly commercialized bio-based polyamide with excellent thermal, mechanical and processing properties. However, its hydrogen bonds and crystalline structures are not well investigated yet. In this study, PA56 films with gamma form and alpha-like form crystalline phases were successfully prepared by melt-crystallization and solution casting, respectively. The hydrogen bond and crystalline structure were systematically studied with solid-state 13C NMR, variable temperature-FTIR and dynamic oscillatory rheological tests. 13C NMR and variable temperature-FTIR results indicate that the hydrogen bond strength of PA56 in alpha-like form crystal is higher than that in gamma-form, while both are lower than that of PA66 in alpha-form. The chemical shifts of carbons in diamine units are highly dependent on the crystalline forms due to the molecular chain of PA56 adopts different conformations in gamma-form and alpha-like form. The curve-fitting of variable temperature-FTIR spectra reveals that the melt crystallization results in gamma-form regardless of the crystal form before melt. Dynamic oscillatory rheological tests demonstrate that the stronger hydrogen bond interactions endow alpha-like form of PA56 a higher dynamic storage modulus and complex viscosity than gamma-form.

Automated summary

The study successfully prepared PA56 films with gamma form and alpha-like form crystalline phases, and systematically studied their hydrogen bond and crystalline structure through solid-state 13C NMR, variable temperature-FTIR, and dynamic oscillatory rheological tests.