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.
