Temperature-Variable FTIR and Solid-State 13C NMR Investigations on Crystalline Structure and Molecular Dynamics of Polymorphic Poly(l-lactide) and Poly(l-lactide)/Poly(d-lactide) Stereocomplex

Crystalline structure and molecular dynamics in α and α′ crystals of poly(l-lactide) (PLLA) and PLLA/poly(d-lactide) (PDLA) stereocomplex (sc) crystals have been investigated by the temperature-variable FTIR and solid-state 13C CP-MAS NMR spectroscopy. The crystal forms of polylactide (PLA) have different band frequencies, correlation field splittings in FTIR spectra and different line shapes, and resonance splittings in solid-state NMR spectra, which become more distinct with cooling to the cryogenic conditions. The well-resolved splittings in NMR resonances of α crystals, attributable to the crystallographically inequivalent sites within crystal unit cell, are considered to be due to the dipolar interactions related to the carbonyl, methyl, and methine groups. The splittings in FTIR bands and NMR resonances are absent in α′ crystals, indicating the disordered conformation and loose molecular lateral packing within their crystal lattices. The significant FTIR frequency shifts of ν(C═O), ν(CH3), and ν(CH) modes during stereocomplex crystallization of PLLA/PDLA blend and the appearance of spectral splittings at cryogenic conditions suggest the coexistence of weak C–H···O═C hydrogen bonds and dipolar interactions between PLLA and PDLA chains in the sc crystals of PLA. Below the glass transition temperature (Tg), the spin–lattice relaxation times of PLA with different crystalline structures increase in the order of amorphous ≈ α′ < α < sc.