11 research outputs found

    Synthesis and Rheological Characterization of Star-Shaped and Linear Poly(hydroxybutyrate)

    No full text
    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)

    No full text
    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)

    No full text
    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

    No full text
    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

    No full text
    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

    No full text
    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

    No full text
    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

    No full text
    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

    No full text
    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

    No full text
    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
    corecore