Microstructure and Thermal Properties of Polylactides with Different L ‐ and D ‐Unit Sequences: Importance of the Helical Nature of the L ‐Sequenced Segments

A series of polylactides (PLA) with different stereo sequences are prepared by the copolymerization of L‐lactide and DL‐lactide. It is confirmed that the glass transition temperature (Tg) of the PLA decreases with decreasing optical purity of the lactate units (%ee) according to the Fox's equation. Analysis of the FT‐IR spectra of these PLA samples reveals that the absorbance at 1 265 cm−1 (δCH + νCOC) decreases with increasing L‐content while the absorbance at 1 210 cm−1 (νasCOC + rasCH3) increases with increasing L‐content. These changes in absorbance are reasonably correlated with the randomness and helical nature of the L‐sequenced segments involved in PLA. Namely, the PLA chains with higher L‐content comprise a higher number of short helical blocks that are made of several L‐lactate units. This difference in helical nature causes the opposite dependences of Tg and density on the L‐content of PLA; i.e., the increased Tg and decreased density with increasing L‐content. FT‐IR spectra of a PDLLA film, NO‐PLLA, and BO‐PLLA. magnified image FT‐IR spectra of a PDLLA film, NO‐PLLA, and BO‐PLLA.