3 research outputs found
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>-polyÂ(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>
Free Volume Manipulation and <i>In Situ</i> Oxidative Crosslinking of Amine-Functionalized Microporous Polymer Membranes
Membranes for gas separations are limited by the trade-off
relationship
between permeability and selectivity. In this study, we demonstrate
an in situ thermal oxidative crosslinking strategy
for amine-functionalized polymers using tert-butoxycarbonyl
(tBOC) groups to improve separation performance. The use of labile
tBOC groups offers two major benefits for inducing thermal oxidative
crosslinks: (1) they trigger free radical chain reactions at more
moderate temperatures, preventing polymer backbone degradation pathways
that otherwise occur at elevated temperatures, and (2) they enable
free volume manipulation (FVM) conditions that yield increased free
volume and narrower free volume element size distribution. This thermal
oxidative crosslinking strategy is demonstrated using an amine-functionalized
polymer of intrinsic microporosity (PIM-NH2). The resulting
crosslinked polymer yielded up to a 22-fold increase in H2/CH4 selectivity while retaining 96% of H2 permeability
from pristine PIM-NH2 films. By combining thermal oxidative
crosslinks and FVM, we demonstrate an effective approach to overcome
the traditional permeability–selectivity trade-off and offer
a greater resistance to major performance stability issues like plasticization
and physical aging, making membranes better suited for industrial
applications
Geometric Transformations Afforded by Rotational Freedom in Aramid Amphiphile Nanostructures
Molecular self-assembly in water
leads to nanostructure
geometries
that can be tuned owing to the highly dynamic nature of amphiphiles.
There is growing interest in strongly interacting amphiphiles with
suppressed dynamics, as they exhibit ultrastability in extreme environments.
However, such amphiphiles tend to assume a limited range of geometries
upon self-assembly due to the specific spatial packing induced by
their strong intermolecular interactions. To overcome this limitation
while maintaining structural robustness, we incorporate rotational
freedom into the aramid amphiphile molecular design by introducing
a diacetylene moiety between two aramid units, resulting in diacetylene
aramid amphiphiles (D-AAs). This design strategy enables rotations
along the carbon–carbon sp hybridized bonds
of an otherwise fixed aramid domain. We show that varying concentrations
and equilibration temperatures of D-AA in water lead to self-assembly
into four different nanoribbon geometries: short, extended, helical,
and twisted nanoribbons, all while maintaining robust structure with
thermodynamic stability. We use advanced microscopy, X-ray scattering,
spectroscopic techniques, and two-dimensional (2D) NMR to understand
the relationship between conformational freedom within strongly interacting
amphiphiles and their self-assembly pathways