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

Abstract

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 imageFT‐IR spectra of a PDLLA film, NO‐PLLA, and BO‐PLLA.

Influence of chain structure on crystal polymorphism of poly(lactic acid). Part 2. Effect of molecular mass on the crystal growth rate and semicrystalline morphology

“…Conversely, a varied stereoregularity leads to variations in T g values somewhat higher than experimental error. At parity of molecular mass, the glass transition of PLA decreases with the increase of D-lactic acid content, in agreement with literature data that rationalize this decrease as due to the effect of D-LA/L-LA ratio on the helical conformation of PLA chains [2,[33][34][35]. Table 2 also reports equilibrium melting temperatures (T m o ) of the various PLA grades.…”

Thermally Grafting Aminosilane onto Kenaf-Derived Cellulose and Its Influence on the Thermal Properties of Poly(Lactic Acid) Composites

“…Referring to the thermal characteristics of PLA with different L-and D-lactide contents, with T m1 at 150 o C and T g at 60 o C, it is confirmed that the current PLA used is semicrystalline, with at least 95% L-lactide (Urayama et al 2003). Thus, the occurrence of double melting endotherms as shown by DSC applies for the current research.…”

Biobased additive plasticizing Polylactic acid (PLA)

“…It should be a nontoxic substance miscible with the polymer, thus creating a homogeneous blend. Also, the plasticizer should not be prone to migration as this would cause the material to regain the brittleness of pure PLA [17] . Thus, good miscibility between PLA and plasticizers is essential.…”