NMR crystallography of α-poly(l-lactide)

Abstract

A complementary approach that combines NMR measurements, analysis of X-ray and neutron powder diffraction data and advanced quantum mechanical calculations was employed to study the α-polymorph of L-polylactide. Such a strategy, which is known as NMR crystallography, to the best of our knowledge, is used here for the first time for the fine refinement of the crystal structure of a synthetic polymer. The GIPAW method was used to compute the NMR shielding parameters for the different models, which included the α-PLLA structure obtained by 2-dimensional wide-angle X-ray diffraction (WAXD) at -150 °C (model M1) and at 25 °C (model M2), neutron diffraction (WAND) measurements (model M3) and the fully optimized geometry of the PLLA chains in the unit cell with defined size (model M4). The influence of changes in the chain conformation on the (13)C σ(ii) NMR shielding parameters is shown. The correlation between the σ(ii) and δ(ii) values for the M1-M4 models revealed that the M4 model provided the best fit. Moreover, a comparison of the experimental (13)C NMR spectra with the spectra calculated using the M1-M4 models strongly supports the data for the M4 model. The GIPAW method, via verification using NMR measurements, was shown to be capable of the fine refinement of the crystal structures of polymers when coarse X-ray diffraction data for powdered samples are available.

High-Resolution Solid-State NMR Characterization of Morphology in Annealed Polylactic Acid

“…[10] Reasonable agreement with these data was achieved for PLA-(v) as well as for PLA-(a), for which the ratios 3.1:1.1:1.0 and 2.9:0.9:1.0, respectively, were found in the areas beneath P5-P7 peaks (Table II). [10] Reasonable agreement with these data was achieved for PLA-(v) as well as for PLA-(a), for which the ratios 3.1:1.1:1.0 and 2.9:0.9:1.0, respectively, were found in the areas beneath P5-P7 peaks (Table II).…”

“…[5] Due to the very weak 13 C NMR signal, hundreds of scans need to be accumulated to obtain single-pulse (SP) magic-angle spinning (MAS) 13 C NMR spectra with a reasonable signal-tonoise ratio. [10] This approach revealed a structure with 10 nonequivalent CH 3 , CH, and CO carbons. This drawback can be overcome using the cross-polarization (CP) technique, as in this case recycle delay is determined by the much shorter proton spinlattice relaxation time T 1 ( 1 H), and this technique alone was used in previous NMR studies of PLA.…”

n→π* interactions in poly(lactic acid) suggest a role in protein folding

“…8-12 Recently, data from neutron diffraction and NMR spectroscopy have been used to complement X-ray diffraction data. 13-14 Though refinement of the structural model continues, the overall topology appears to be consistent between studies (Fig. 2A).…”

Characterization of the Slow Molecular Dynamics of Poly(l‐Lactic Acid) in α and α′ Phases, in a Glassy State, and in a Complex with Poly(d‐Lactic Acid) by Solid‐State NMR

“…Later, CSA principle values are used for determinations of dynamics geometry of PLLA in different phases. Recently, Pawlak et al, using density functional theory calculations, determined the orientations of the principal axes of the C(H) and C(O) groups of PLLA . For CH carbon, the σ 11, direction aligns along the C(H)CH 3 bond, the σ 22 direction remains in the plane formed by HC(H)CH 3 , and the σ 33 direction is vertical to the plane constructed by σ 11 and σ 22 , respectively, as depicted in Figure a.…”