A Comparison of the Rheological and Mechanical Properties
of Isotactic, Syndiotactic, and Heterotactic Poly(lactide)
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Abstract
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]<sub>2</sub>(μ-Cl)(μ-OEt) (<b>1</b>) and (<i>RR</i>/<i>RR</i>)-[(ONNO)In(μ-OEt)]<sub>2</sub> (<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><sub>w</sub>) 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><sub>e</sub>) 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><sub>a,flow</sub>) 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
