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

Abstract

Crystalline structure and molecular dynamics in α and α′ crystals of poly(L-lactide) (PLLA) and PLLA/poly(Dlactide) (PDLA) stereocomplex (sc) crystals have been investigated by the temperature-variable FTIR and solid-state 13 C 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(CH 3 ), 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 (T g ), the spin−lattice relaxation times of PLA with different crystalline structures increase in the order of amorphous ≈ α′ < α < sc.

Influence of chain structure on crystal polymorphism of poly(lactic acid). Part 2. Effect of molecular mass on the crystal growth rate and semicrystalline morphology

“…The two crystal modifications have a similar chain packing, but α′-crystals have conformational disorder and a loose chain-packing manner, which classifies this modification as a conformationally disordered crystal (condis crystal) or mesophase [16,21,22]. The two crystal forms grow at different temperatures, as α′-crystals develop at lower temperatures, whereas α-crystals prevail upon crystallization at high temperatures [9].…”

Chemical Properties of Poly(lactic Acid)

“…Another metastable α 0 -form is attained from the stereoregular PLA melt-crystallined at a low crystallization temperature T c (,100 C), whereas the α-form is yielded at higher T c (.120 C) (Zhang et al, 2005a). Pan et al (2011) found that the α-form PLA showed spectral splitting (see Figure 4.12). The α-form PLA spit into a few new peaks when cooled to 2140 C: 23006 cm 21 (CH 3 asymmetric stretching), 2964 cm 21 (CH 3 symmetric stretching), 1777 and 1749 cm 21 (CQO stretching), 1468 and 1443 cm 21 (CH 3 asymmetric bending), 1396 and 1381 cm 21 (CH 3 symmetric bending), 1222 cm 21 (CaOaC asymmetric bending and CH 3 asymmetric rocking), 1144 cm 21 (CH 3 asymmetric rocking), and 1053 cm 21 (CaCH 3 bending).…”

“…Recently, Pan et al (2011) conducted a study using the FT-IR technique to investigate the crystalline structure of PLLA and PLLA/PDLA stereocomplex. PLA tends to form various crystal polymorphisms depending on the crystallization conditions.…”

“…Intensities of FT-IR spectra and their second derivatives in the wavenumber ranges 1500À1325 and 975À890 cm 21 were magnified for clarity. ABS 5 absorbance (adapted from Pan et al, 2011). it showed that the corresponding peak for CQO stretching is about 10 cm 21 lower, while CH 3 asymmetric/symmetric stretching and CH symmetric stretching is reduced by 4À6 cm 21 . It can, therefore, be assumed that weak hydrogen bonds form in CaHaaaaOQC in the amorphous PLA crystal (Zhang et al, 2005b).…”

Injection molding and characterization of polylactide stereocomplex

“…Poly(lactic acid) (PLA) or polylactide is considered a promising ecofriendly polymer material that has the potential to resolve many environmental issues because of its biorenewability and biodegradability [1,2]. PLA exists in three isomeric forms: L-lactide (semicrystalline), D-lactide (semi-crystalline), and DL-lactide (amorphous) [3].…”