“A Comparison of the Rheological and Mechanical Properties of Isotactic, Syndiotactic, and Heterotactic Poly(lactide)”
A series of poly(lactide) (PLA) samples, exhibiting various levels of syndiotactic enrichment, were formed via the ring-opening polymerization of meso-lactide using two families of novel dinuclear indium catalysts, (RR/RR)-[(NNO)InCl]2(μ-Cl)(μ-OEt) (1) and (RR/RR)-[(ONNO)In(μ-OEt)]2 (2). Isotactic and heterotactic PLAs were also synthesized using known methodologies and the thermal and rheological behaviours of these PLAs with different microstructures were compared. Solution rheological studies showed that the values of intrinsic viscosities and hydrodynamic radii as functions of molecular weight (Mw) were higher for the iso- compared to syndio- PLAs with the viscosities of the heterotactically enriched PLAs falling in between and in agreement with literature values reported for atactic PLAs. The molecular weight between entanglements (Me) was greatest for the syndiotactically enriched PLAs, giving rise to the lowest zero-shear viscosity. In addition, hetero- and isotactically enriched PLA had higher flow activation energies (Ea,flow) than syndiotactic variants, implying the inclusion of transient aggregate regions within these polymers due to enhanced L- and D- interactions. Although strain hardening was observed for all types of PLAs, it was more dominant for isotactic PLAs due to stronger L- and D- interactions possibly leading to a small degree of stereocomplex microcrystallites.