Synthesis and Thermorheological Analysis of Biobased
Lignin-<i>graft</i>-poly(lactide) Copolymers and Their Blends
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Abstract
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<sup>–1</sup>