Thermal Properties of Polylactides with Different Stereoisomers of Lactides Used as Comonomers

To investigate the effect of the stereoisomeric composition of lactides as comonomers on the thermal behavior of polylactide (PLA), a series of PLAs, including poly(L-co-D-lactides), poly(L-co-meso-lactides), and poly(L-co-D-co-meso-lactides), were synthesized by the random copolymerization of lactide stereoisomeric mixtures. The thermal behavior of the PLAs was investigated by differential scanning calorimetry in detail. The glass transition temperature (T-g), melting temperature (T-m), and melting enthalpy (Delta H-m) of poly(L-co-D-lactides) and poly(L-co-meso-lactides) increased with increasing stereochemical purity (SCP). At the same SCPs, T-g, T-m, and Delta H-m of poly(L-co-D-lactides) were slightly higher than those of poly(L-co-meso-lactides) because meso-lactide differed from d-lactide in terms of their contributions to the disorder degree of PLA molecular chains. The relationship of T-g or Tm with the stereoisomeric composition of lactides as comonomers was established for poly(L-co-D-co-meso-lactides) on the basis of the experimental data and used to design and prepare PLAs with different T-g and T-m values.