Poly(3-hydroxybutyrate) (PHB) is a semicrystalline biodegradable polymer with chains consisting of methyl (CH3), methylene (CH2), methine (CH) and carbonyl (CO) groups. The 1 H-13 C cross polarization NMR measurements were performed on an as-supplied powder PHB sample at a magic-angle spinning rate of 10 kHz. The measured cross polarization build-up curves and their analysis provided information on the dynamics of 1 H-13 C NMR cross polarization in functional groups with directly bonded hydrogens. The measurements required setting up the Hartmann-Hahn condition, which was inferred from the Hartmann-Hahn matching profiles measured for each functional group. The cross polarization build-up curves displayed an oscillatory course, which indicates the presence of rigid 1 H-13 C spin pairs isolated from the lattice. The frequency of the observed oscillations is directly proportional to the 1 H-13 C dipolar coupling constant, which is related to the C-H distance and its value also reflects the mobility of particular functional groups. The values of dipolar coupling constants were derived from splittings in the Fourier transforms of cross polarization build-up curves. The mobility of particular groups was assessed with the order parameter S calculated using experimental and rigid lattice values of dipolar coupling constants.
Mechanical and thermal properties of composite polymer materials strongly depend on their local structure and molecular dynamics which can be effectively studied by the solid-state nuclear magnetic resonance (NMR) techniques. In the present paper, the influence of formamide (F) and sorbitol (S) plasticizers on molecular motion in thermoplastic starch (TPS) was studied using solid-state NMR spectroscopy and dynamic mechanical thermal analysis (DMTA). DMTA measurements carried out for formamide-(F-TPS) and sorbitol-plasticized (S-TPS) corn starches indicated heterogeneous plasticizer distribution of plasticizer-rich and starch-rich phases within the samples. The single pulse and cross-polarization 13 C NMR spectra measured for both plasticized starches confirmed the amorphous character of their structure and distinctly different chain mobility supported by the values of 13 C spin-lattice relaxation times. The results of the analysis of broad line and magic angle spinning 1 H NMR spectra are in accordance with the results of DMTA measurements, revealing lower mobility of starch chains within S-TPS in comparison to F-TPS. Crosslinking of the starch chains with sorbitol molecules was suggested as being responsible for the lower mobility of the starch chains in S-TPS.
Single-pulse 13 C NMR spectra and spin-lattice relaxation times T 1 ( 1 H), detected indirectly via 13 C carbons, and T 1 ( 13 C) were measured at 31°C for virgin pelletized and annealed polylactic acid (PLA) samples using the magic-angle spinning technique. The structural relaxation resulting in more regular crystals with narrower conformation distribution and increase in the lamellae thickness and crystallinity brought about by annealing at 100°C was deduced from the narrowing of the 13 C NMR lines and proton spin-lattice relaxation times T 1 ( 1 H). The spin-lattice relaxation times T 1 ( 13 C) related to the respective carbons of the -polymorph of PLA are also discussed in the study.
MAS 13 C-NMR measurements were used for the study of morphology and molecular mobility in amorphous quenched and triacetine-plasticized PLA samples and PLA samples which underwent cold crystallization during annealing at 80 and 100 8C. The single pulse MAS 13 C-NMR spectra indicate that plasticizer promotes cold crystallization which results in the decrease of the temperature of crystallization and formation of more perfect crystalline domains. The T 1 ( 13 C) spin-lattice relaxation times show that the presence of plasticizer molecules leads to an increase of local mobility in PLA chains but plasticized PLA after annealing at 100 8C shows more rigid structure. The series of broad line 1 H-NMR spectra performed at temperatures up to 100 8C provided information on the changes in relaxation processes and morphology of the studied samples. The interpretation of the results obtained using the techniques of NMR spectroscopy were supported by WAXD and DSC measurements.
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