11 research outputs found
Synthesis and Rheological Characterization of Star-Shaped and Linear Poly(hydroxybutyrate)
Indium and zinc complexes, [(NNO<sub>tBu</sub>)InCl]<sub>2</sub>(μ-Cl)(μ-OTHMB) (<b>2</b>) and (NN<sub>i</sub>O<sub>tBu</sub>)Zn(CH<sub>2</sub>CH<sub>3</sub>) (<b>3</b>), were
used to produce monodispersed three- and six-armed star-shaped PHBs
using tris(hydroxymethyl)benzene (THMB) and dipentaerythritol as the
chain transfer agents. Reactions catalyzed by complex <b>2</b> were highly controlled, with THMB:catalyst ratios of up to 590:1,
resulting in star-shaped PHBs with predictable molecular weights (<i>M</i><sub>n</sub> = 1.25–219 kDa) and narrow dispersities
(<i>Đ</i> = 1.02–1.08). The zinc-based catalyst, <b>3</b>, was less controlled than the indium analogue but nevertheless
generated moderately syndiotactic PHBs with maximum <i>M</i><sub>n</sub> values of ∼100 kDa. Importantly, <b>3</b> allowed the formation of previously unknown 6-armed star PHBs, allowing
us to compare the effects of the different PHB architectures on the
rheological behavior of the materials. High molecular weight linear
and star polymers were characterized using solution and melt viscoelastic
studies. Zero-shear viscosity of linear PHBs exhibited a power law
relationship with the span molecular weight; however, it scaled exponentially
for star polymers with slightly higher values for the 6-armed star
PHBs. This was attributed to the moderately syndiotactic microstructure
of these polymers. The absence of a district arm retraction relaxation
in the dynamic master curves, and overshoot in the transient viscosity
for the 6-armed star PHBs, are due to the lower entanglement density
and slightly broader molecular weight distribution of these polymers
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]<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
Highly Active Chiral Zinc Catalysts for Immortal Polymerization of β‑Butyrolactone Form Melt Processable Syndio-Rich Poly(hydroxybutyrate)
Highly
crystalline poly(hydroxybutyrate) suffers from high melting
point and entanglement molecular weight. This leads to low melt strength,
limits processing through regular techniques, and precludes many applications.
In this work we report a series of racemic and enantiopure zinc catalysts
supported by variously substituted diaminophenolate ancillary ligands
which form high melt strength PHBs with different molecular weights
and microstructure. These complexes are active for the highly controlled
polymerization of β-butyrolactone (BBL); some can polymerize
2000 equiv of BBL in less than 30 min. Changing the steric bulk of
the ligand forms PHBs of varied syndiotacticity (<i>P</i><sub>r</sub> = 0.75 to 0.55). These are highly robust systems capable
of polymerizing an unprecedented 20000 equiv of BBL in the presence
of 5000 equiv of benzyl alcohol. Thermorheological investigations
reveal that the synthesized PHBs have surprisingly high melt strength
at above the melting point. For processable PHBs, high density of
entanglements and relatively low crystallinity are crucial. We show
that the best PHBs should have high molecular weight and moderate
syndiotacticity
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<sup>–1</sup>
The Role of Nitrogen Donors in Zinc Catalysts for Lactide Ring-Opening Polymerization
The electronic effects
of nitrogen donors in zinc catalysts for ring-opening polymerization
of cyclic esters were investigated. Alkyl and benzyloxy zinc complexes
supported by tridentate diamino- and aminoimino phenolate ligands
were synthesized, and their solid-state and solution structures characterized.
The solution-state structures showed that the alkyl complexes are
mononuclear, while the alkoxy complexes are dimeric with the ligands
coordinated with different denticities depending on the nature of
the ligand donors. The catalytic activities of these compounds toward
the ring-opening polymerization of racemic lactide were studied and
showed that catalysts with secondary and imine nitrogen donors are
more active than analogues with tertiary amines
The Role of Nitrogen Donors in Zinc Catalysts for Lactide Ring-Opening Polymerization
The electronic effects
of nitrogen donors in zinc catalysts for ring-opening polymerization
of cyclic esters were investigated. Alkyl and benzyloxy zinc complexes
supported by tridentate diamino- and aminoimino phenolate ligands
were synthesized, and their solid-state and solution structures characterized.
The solution-state structures showed that the alkyl complexes are
mononuclear, while the alkoxy complexes are dimeric with the ligands
coordinated with different denticities depending on the nature of
the ligand donors. The catalytic activities of these compounds toward
the ring-opening polymerization of racemic lactide were studied and
showed that catalysts with secondary and imine nitrogen donors are
more active than analogues with tertiary amines
Air- and Moisture-Stable Indium Salan Catalysts for Living Multiblock PLA Formation in Air
We
introduce an air- and moisture-stable hydroxy-bridged indium
salan complex as a highly active and controlled catalyst for the ring-opening
polymerization of cyclic esters in air. The reversible activation
of this complex with linear and branched alcohols leads to immortal
polymerization, allowing the controlled formation of block copolymers
in air. It is the only reported example of a living catalyst that
remains controlled after multiple exposures to ambient air at high
temperatures. Although the prevalent catalyst for ring-opening polymerization,
tin octanoate, is robust, it does not promote controlled polymerization.
Our indium catalyst is exceptional in being both robust and controlled
Hydrothermal Gelation of Aqueous Cellulose Nanocrystal Suspensions
We
report the facile preparation of gels from the hydrothermal
treatment of suspensions of cellulose nanocrystals (CNCs). The properties
of the hydrogels have been investigated by rheology, electron microscopy,
and spectroscopy with respect to variation in the temperature, time,
and CNC concentration used in preparation. Desulfation of the CNCs
at high temperature appears to be responsible for the gelation of
the CNCs, giving highly porous networks. The viscosity and storage
modulus of the gels was shown to increase when samples were prepared
at higher treatment temperature. Considering the wide natural abundance
and biocompatibility of CNCs, this simple, green approach to CNC-based
hydrogels is attractive for producing materials that can be used in
drug delivery, insulation, and as tissue scaffolds
Effect of Extreme Wettability on Platelet Adhesion on Metallic Implants: From Superhydrophilicity to Superhydrophobicity
In order to design
antithrombotic implants, the effect of extreme wettability (superhydrophilicity
to superhydrophobicity) on the biocompatibility of the metallic substrates
(stainless steel and titanium) was investigated. The wettability of
the surface was altered by chemical treatments and laser ablation
methods. The chemical treatments generated different functionality
groups and chemical composition as evident from XPS analysis. The
micro/nanopatterning by laser ablation resulted in three different
pattern geometry and different surface roughness and consequently
wettability. The patterned surface were further modified with chemical
treatments to generate a wide range of surface wettability. The influence
of chemical functional groups, pattern geometry, and surface wettability
on protein adsorption and platelet adhesion was studied. On chemically
treated flat surfaces, the type of hydrophilic treatment was shown
to be a contributing factor that determines the platelet adhesion,
since the hydrophilic oxidized substrates exhibit less platelet adhesion
in comparison to the control untreated or acid treated surfaces. Also,
the surface morphology, surface roughness, and superhydrophobic character
of the surfaces are contributing factors to platelet adhesion on the
surface. Our results show that superhydrophobic cauliflower-like patterns
are highly resistant to platelet adhesion possibly due to the stability
of Cassie–Baxter state for this pattern compared to others.
Our results also show that simple surface treatments on metals offer
a novel way to improve the hemocompatibility of metallic substrates
CO<sub>2</sub>‑Switchable Cellulose Nanocrystal Hydrogels
A carbon-dioxide-switchable (CO<sub>2</sub>-switchable) hydrogel
was prepared by adding imidazole to a suspension of cellulose nanocrystals
(CNCs). Sparging of CO<sub>2</sub> through the imidazole-containing
CNC suspension led to gelation of the CNCs, which was reversible by
subsequent sparging with nitrogen (N<sub>2</sub>) to form a low-viscosity
CNC suspension. The gelation process and the properties of the hydrogels
have been investigated by rheology, ζ potential, pH, and conductivity
measurements, and the gels were found to have interesting and reversible
tunable mechanical properties. The present work describes a straightforward
way to obtain switchable CNC hydrogels without the need to functionalize
CNCs or add strong acids or bases. These CO<sub>2</sub>-responsive
CNC hydrogels have potential for application in stimuli-responsive
adsorbents, filters, and flocculants