Molecular dynamics of poly(l-lactide) biopolymer studied by wide-line solid-state 1H and 2H NMR spectroscopy

The molecular dynamics of poly(l-lactide) (PLLA) biopolymer was characterized through analyses of 1H and 2H NMR line-shapes and spin-lattice relaxation times at different temperatures. At low temperatures (e.g. 90 K), the methyl group rotation is dominant leading to a significant reduction in the proton second moment. Fast methyl group reorientation occurs at ca. 130 K. In additional to the fast methyl group rotation, hydroxyl groups start to reorient as the temperature increases further, eventually leading to the breakdown of the segments of the biopolymer chains above its glass transition temperature Tg of 323 K. The analyses of the 2H NMR line-shapes indicate that both the methyl and hydroxyl reorientations can be described by the so-called cone model, in which the former has three equilibrium positions with θC-D=70.5∘ and φ=120° while the latter one exhibits two equilibrium positions with θO-D=78∘ and φ=180°.