Poly(l-lactide): optimization of melting temperature and melting enthalpy and a comparison of linear and cyclic species

Abstract

Twice recrystallized L-lactide was polymerized with a dozen different tin or bismuth catalysts in bulk at 160 °C for 24 h and was annealed at 150 °C afterwards. In two cases T_m values above 197.0 °C were obtained. The parameters causing a scattering of the DSC data were studied and discussed. The samples prepared with SnCl₂, 2,2-dibutyl-2-stanna-1,3-ditholane (DSTL) or cyclic tin(II) bisphenyldioxide (SnBiph) were subject to annealing programs with variation of time and temperatures, revealing that the T_ms did not increase. Hower, an increase of ΔH_m was achieved with maximum values in the range of 93–96 J g⁻¹ corresponding to crystallinities of around 90%. Further studies were performed with once recrystallized L-lactide. Again, those samples directly crystallized from the polymerization process showed the highest T_m values. These data were compared with the equilibrium T_m0 and ΔH_m0 data calculated by several research groups for perfect crystallites. A T_m0 of 213 +/−2 °C and a ΔH_m0 of 106 J g⁻¹ show the best agreement with the experimental data. The consequences of annealing for the thickness growth of crystalites are discussed on the basis of SAXS measurements. Finally, a comparison of cyclic and linear poly(L-lactide)s is discussed.