NMR crystallography of alpha-poly(L-lactide)

作者：Pawlak Tomasz; Jaworska Magdalena; Potrzebowski Marek J^*

来源：Physical Chemistry Chemical Physics, 2013, 15(9): 3137-3145.

DOI：10.1039/c2cp43174b

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 alpha-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 alpha-PLLA structure obtained by 2-dimensional wide-angle X-ray diffraction (WAXD) at -150 degrees C (model M1) and at 25 degrees 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 C-13 sigma(ii) NMR shielding parameters is shown. The correlation between the sigma(ii) and delta(ii) values for the M1-M4 models revealed that the M4 model provided the best fit. Moreover, a comparison of the experimental C-13 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.

• 出版日期2013