A DFT study on poly(lactic acid) polymorphs

Poly(lactic acid) (PLA) can crystallize in α-, β-, γ- and stereocomplex (sc)- forms. It has been shown that the formation of stereocomplex between poly(l-lactic acid) and poly(d-lactic acid) significantly improve thermal stability and mechanical properties. However the mechanisms of enhancements are still unclear. In this study, we investigate the PLA polymorphs from the first-principles theoretical perspective in order to understand the intermolecular interaction in the crystals. Density functional theory at the level of Perdew-Wang generalized-gradient approximation was applied to optimize PLA crystal unit cells. A comparison of energies in the various unit cells reveals that sc-form is the most energetically favorable form among the four PLA polymorphs. The order of thermodynamically relative stability is that sc-form is 0.3, 1.1, and 1.3 kcal/mol more stable than α-form, β-form, and γ-form, respectively (when using the ultrasoft pseudopotential and a plane-wave basis set with an energy cutoff of 380 eV) or 0.4, 1.1, and 1.3 kcal/mol more stable than α-form, β-form, and γ-form, respectively (when employing the density functional semi-core pseudopotentials and the double numerical plus polarization orbital basis set with a global orbital cutoff of 3.7 Å). In addition to the energetic properties, structural and electronic properties were calculated as well. The theoretical predicted stability rank is in agreement with some reported observations. Such as, sc-form has higher melting point and larger heat of fusion than those of α-form. The enhanced thermal stability of the sc-form compared to the other three homopolymer forms may be attributed to the unique intermolecular non-conventional hydrogen bonding C-H⋯O(C) network in the stereocomplex.