Crystalline structure of polyamide 12 as revealed by solid-state 13C NMR and synchrotron WAXS and SAXS

The crystalline structure of polyamide-12 (PA12) was studied by solid-state 13 C nuclear magnetic resonance (NMR) as well as by synchrotron wide- and small-angle X-ray scattering (WAXS and SAXS). Isotropic and oriented PA12 showed different NMR spectra ascribed to γ- and γ'-crystalline modifications, respectively. On the basis of the position of the first diffraction peak, the isotropic γ-form and the oriented γ'-form were shown to be with hexagonal crystalline lattice at room temperature. When heated, the two PA12 polymorphs demonstrated different behaviors. Above 140 °C, the isotropic γ-PA12 partially transformed into α-modification. No such transition was observed with the oriented γ-PA12 phase even after annealing at temperatures close to melting. A γ'-γ transition was observed here only after isotropization by melting point. Various structural parameters were extracted from the WAXS and SAXS patterns and analyzed as a function of temperature and orientation: the degree of crystallinity, the d-spacings, the Bragg's long spacings, the average thicknesses of the crystalline (l c ) and amorphous (l a ) phases, and the linear crystallinity x cl within the lamellar stacks.

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