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

Abstract

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.

Solid state 13C‐NMR study of a plasticized PLA/PHB polymer blend

“…Well‐resolved peaks at 172.1, 171.5, and 169.8 ppm in CO resonance, 70.3 and 69.3 ppm in CH resonance, and 17.5 ppm in CH 3 resonance are observed in the CP MAS 13 C‐NMR spectrum measured for the plasticized PLA/PHB/TAC polymer blend. These peaks together with the shoulders at 170.2 and 16.3 ppm are in a good agreement with the peak positions observed in CP 25 or in SP MAS 13 C‐NMR spectra for the crystalline α‐form of PLA. The peak at 21.4 ppm observed in both spectra belongs to the carbons in crystalline regions of PHB and it is evident that plasticization of the PLA/PHB blend also supports the formation of PHB crystalline domains.…”

“…In addition to the peaks and shoulders present at positions related to the crystalline regions of PHB, the SP MAS 13 C‐NMR spectrum for plasticized PLA/PHB/TAC also shows shoulders related to the carbons in crystalline regions of PLA . Moreover, the amplitude of the peak related to the CH 3 groups of PHB is higher than that for PLA/PHB.…”

“…All spectra in this figure display clearly resolved lines related to the carbons of the respective groups with the chemical shifts listed in Table . The spectrum of PLA‐q displays three broad Gaussian lines with widths of 247 (CO), 275 (CH), and 245 Hz (CH 3 ), which are characteristic of amorphous PLA . Despite melting and quenching, all resonance lines of the SP MAS 13 C‐NMR spectrum for PHB‐q are composed of narrow and broad components arising from the crystalline and amorphous domains, respectively .…”

“…The magic angle spinning (MAS) 13 C‐NMR spectra of amorphous PLA display three broad Gaussian resonance lines related to carbons in carbonyl (CO), methine (CH), and methyl (CH 3 ) functional groups. Each resonance in the 13 C‐NMR spectrum of semicrystalline PLA represents a superposition of the broad singlet arising from amorphous domains and several sharp narrow lines assigned to carbons in crystalline domains . Based on NMR crystallography, the crystal structure for the crystalline α‐polymorph of PLA was refined and it was found that the solid‐state 13 C‐NMR spectrum of PLA consists of 10 lines related to nonequivalent CH 3 , CH, and CO carbons .…”

Solid State 13C Nuclear Magnetic Resonance Study of Morphology and Molecular Mobility in Quenched Poly(3-hydroxybutyrate)

“…The broader lines for the PHBq sample can be explained by the broader chain conformation distribution within amorphous and crystalline domains after quenching of the virgin PHB. The width of the Lorentzian line related to the crystalline regions depends on the regularity of the chains inside the crystal lattice and within the crystal lattice close to the lamellae surface [6]. Smaller crystallites have a larger relative number of nuclei in the surface layer, which have slightly distorted environment reflected in the Lorentzian line broadening.…”