Dependence of the crystal structure on the d-units amount in semi-crystalline poly(lactic acid)
Giovanna Molinari, Paola Parlanti, Elisa Passaglia, Federica Aiello, Mauro Gemmi, Andrea Lazzeri, Maria Cristina Righetti
Abstract
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 CH 3 ···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.