Dependence of the crystal structure on the d-units amount in semi-crystalline poly(lactic acid)

The study investigates the impact of the D-lactic acid units content on the crystallinity and crystal structure of commercial poly(lactic acid) (PLA) grades, which are copolymers of poly(L-lactic acid) (PLLA) containing a minor amount of D-units. As the D-units content increases, a detectable decrease in crystallinity was observed along with a simultaneous rise in mobile amorphous fraction (MAF) and a reduction in rigid amorphous fraction (RAF). The percentage of D-units was found not to significantly affect RAF thickness, suggesting that the D-units are not completely excluded from the crystals. The inclusion of D-units as defects in the PLA crystal lattice was confirmed by XRD analysis, which disclosed that the crystal phase gets gradually richer of D-units as the crystallization time evolves. FT-IR analysis proved that the incorporation of D-units in the crystal phase is promoted by the formation of local CH3···O=C interactions, similar to those massively active between PLLA and poly(D-lactic acid) (PDLA) in the stereocomplex. The establishment of these interactions leads to a contraction of the interplanar distances and a decrease in the crystal cell volume with increasing the crystallization time and the D-units percentage. In summary, the study proves that for PLA copolymers containing a D-units percentage at least up to about 8 %, D-units are included in the crystal lattice.