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 <i>meso</i>-lactide using two families of dinuclear indium catalysts: (<i>RR</i>/<i>RR</i>)-[(NNO)­InCl]₂(μ-Cl)­(μ-OEt) (<b>1</b>) and (<i>RR</i>/<i>RR</i>)-[(ONNO)­In­(μ-OEt)]₂ (<b>2</b>). Isotactic and heterotactic PLAs were also synthesized using known methodologies, and the thermal and rheological behaviors 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 (<i>M</i>_w) were highest for iso-PLAs, followed by hetero and then syndio-PLAs. The viscosities of the heterotactically enriched PLAs were in agreement with literature values reported for atactic PLAs. The molecular weight between entanglements (<i>M</i>_e) 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 (<i>E</i>_a,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

Synthesis and Thermorheological Analysis of Biobased Lignin-<i>graft</i>-poly(lactide) Copolymers and Their Blends

Despite numerous accounts of biobased composite materials through blending and copolymerization of lignin and other polymers, there are no systematic studies connecting the synthetic methodology, molecular structure, and polymer topology with the rheological properties of these materials. In this report lignin-<i>graft</i>-poly­(lactide) copolymers are synthesized via three routes (indium and organocatalyzed “graft-from” methods as well as a “graft-to” method) and the resulting reaction products (shown to include linear PLAs, cyclic PLAs, and star-shaped lignin-<i>graft</i>-PLA copolymers) are investigated using chemical and rheological methods. The topology of the products of the graft-from methods is affected by the initial lignin concentration; polymerizations with low lignin loading generate cyclic PLAs, which can be identified by 10-fold lower viscosities compared to linear PLAs of the same molecular weight. Under higher lignin loadings, star-shaped lignin-<i>graft</i>-PLA copolymers are formed which show viscosities 2 orders of magnitude lower than those of comparable linear PLAs. Rheological studies show that cyclic PLAs lack a well-defined rubber plateau, whereas star-shaped lignin-<i>graft</i>-PLAs lack a significant <i>G</i>′ to <i>G</i>′′ cross-over. The rheological results coupled with thermogravimetric analysis give an indication to the structure of star-shaped lignin-<i>graft</i>-PLA copolymers, which are estimated to contain a small lignin core surrounded by PLA segments with molecular weights from 2.0 to 20 kg mol^–1