Polyvinylamine-graft-TEMPO Adsorbs onto, Oxidizes, and Covalently Bonds to Wet Cellulose

TEMPO-radical-bearing metal-organic frameworks and covalent organic frameworks for catalytic applications

It is known that 2,2,6,6-tetramethylpiperidinyl-1-oxy (or TEMPO) is a stable, radical-containing molecule, which has been utilized in various areas of organic synthesis, catalysis, polymer chemistry, electrochemical reactions, and materials chemistry. Its unique stability, attributable to its structural features, and molecular tunability allows for the modification of various materials, including the heterogenization of solid materials. Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are porous and tunable because of their ligand or linker portion, and both have been extensively studied for use in catalytic applications. Therefore, synergistically combining the chemistry of TEMPO with the properties of MOFs and COFs is a natural choice and should allow for significant advancements, including improved recyclability and selectivity. This article focuses on TEMPO-bearing MOFs and COFs for use in catalytic applications. In addition, recent strategies related to the use of these functional porous materials in catalytic reactions are also discussed.

High Yield Poly(ethylene-alt-maleic acid) Grafting to Wood Pulp while Minimizing Fiber/Fiber Wet Adhesion

Heating bleached kraft pulps treated with poly(ethylene-alt-maleic acid) (PEMAc) can lead to high yields of carboxylated polymer grafted to fibers. However, in many cases, the cured, dry pulp cannot be effectively repulped (redispersed in water) because the wet strength is too high. Impregnation with PEMAc solutions at pH 4 followed by high temperature (120-180 °C), catalyst-free curing for short times can give fixation yields >85% while maintaining repulpability. The combination of high fixation yields with low wet strength is possible because the extent of curing required for high grafting yields is less than the curing requirement for high wet strength. Two challenges in moving this technology to practicable applications are (1) identifying the optimum laboratory pulp curing conditions and (2) translating laboratory curing conditions to industrial processes. A modeling tool was developed to meet these challenges. The model is based on the observation that for curing conditions giving high fixation yields the wet tensile indices of grafted pulp sheets showed a power-law dependence on the βΓ product where β is the conversion of the succinic acid moieties in PEMAc to the corresponding succinic anhydride groups in the curing step and Γ is the amount of polymer applied to the pulp. For two PEMAc molecular weights and two pulp types, the power-law slopes were 0.6; however, the pre-exponential terms depended upon the specific polymer and pulp type combination. We propose that the relationships between the wet tensile index and βΓ, from polymer-treated, laboratory pulp handsheets, can be used to predict if proposed curing conditions for larger-scale processes will produce a repulpable product.

A recoverable dendritic polyamidoamine immobilized TEMPO for efficient catalytic oxidation of cellulose

Polyamidoamine (PAMAM) dendrimers of G1.0 and 2.0 were synthesized by the repeated Michael addition and ester aminolysis of ethylenediamine and methyl acrylate. Through the reductive amination reaction of primary amines in PAMAM and carbonyl groups in 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl (4-oxo-TEMPO), the water-soluble PAMAM immobilized TEMPO (PAMAM-TEMPO) was successfully prepared. The obtained PAMAM-TEMPO was characterized by Fourier transform infrared spectroscopy (FT-IR) and ultraviolet-visible spectrophotometer (UV-vis). PAMAM-TEMPO was used as catalyst instead of free TEMPO for selective catalytic oxidation of primary hydroxyl groups in cellulose with water as reaction medium. The results showed that the catalytic performance of G1.0 PAMAM-TEMPO with 28.8% TEMPO loading was equivalent to free TEMPO. After salting out the supernatant of oxidation mixture, PAMAM-TEMPO was recovered by extraction with N,N-dimethylformamide and reused for further oxidation cycles. No significant reduction in catalytic performance was found after 4 oxidation cycles. The recovery of PAMAM-TEMPO after each cycle was about 90%. By sonication of oxidized cellulose obtained with G1.0 PAMAM-TEMPO as catalyst, the individualized cellulose nanofibers with approximately 10 nm in diameter were successfully prepared. This is the first report on the use of immobilized TEMPO catalysts comparable to the performance of free TEMPO to oxidize cellulose in water.

Nitroxide radical polymers – a versatile material class for high-tech applications

A comprehensive summary of synthetic strategies for the preparation of nitroxide radical polymer materials and a state-of-the-art perspective on their latest and most exciting applications.

Relating Redox Properties of Polyvinylamine-g-TEMPO/Laccase Hydrogel Complexes to Cellulose Oxidation

The structure and electrochemical properties of adsorbed complexes based on mixtures of polyvinylamine-g-TEMPO (PVAm-T) and laccase were related to the ability of the adsorbed complexes to oxidize cellulose. PVAm-T10 with 10% of the amines bearing TEMPO moieties (i.e., DS = 10%), adsorbed onto gold sulfonate EQCM-D sensor surfaces giving a hydrogel film that was 7 nm thick, 89% water, and encasing laccase (200 mM) and TEMPO moieties (33 mM). For DS values >10%, all of the TEMPOs in the hydrogel film were redox-active in that they could be oxidized by the electrode. With hydrogel layers made with lower-DS PVAm-Ts, only about half of the TEMPOs were redox-active; 10% DS appears to be a percolation threshold for complete TEMPO-to-TEMPO electron transport. In parallel experiments with hydrogel complexes adsorbed onto regenerated cellulose films, the aldehyde concentrations increased monotonically with the density of redox-active TEMPO moieties in the adsorbed hydrogel. The maximum density of aldehydes was 0.24 μmol/m², about 10 times less than the theoretical concentration of primary hydroxyl groups exposed on crystalline cellulose surfaces. Previous work showed that PVAm-T/laccase complexes are effective adhesives between wet cellulose surfaces when the DS is >10%. This work supports the explanation that TEMPO-to-TEMPO electron transport is required for the generation of aldehydes necessary for wet adhesion to PVAm.

Redox Properties of Polyvinylamine-g-TEMPO in Multilayer Films with Sodium Poly(styrenesulfonate)

Layer-by-layer (LbL) assemblies of polyvinylamine with grafted TEMPO moieties (PVAm-T) with sodium polystyrenesulfonate (PSS) were prepared on gold-sulfonate surfaces, and the redox properties were measured by cyclic voltammetry. LbL compositions were probed by quartz crystal microbalance (wet) and ellipsometric (dry) film measurements. Approximately 30% of the TEMPO moieties in the LbL assemblies were redox-active when the total TEMPO coverage was varied up to 6 μmol/m², by either varying the TEMPO content in PVAm-T or by varying the number of LbL bilayers. Three non-redox-active PVAm/PSS blocking bilayers were required to prevent the electrode from oxidizing PVAm-T in the exterior LbL layer. This suggests significant intermixing between the layers in the LbL film. In addition to contributing to the small but growing body of work on redox polymers based on grafted TEMPO, this work serves as a reference point for understanding the redox properties of colloidal PVAm-T-laccase complexes in future work.

Superior adsorption performance for triphenylmethane dyes on 3D architectures assembled by ZnO nanosheets as thin as ∼1.5nm

The 3-dimensional hierarchical ZnO flower-like architectures have been synthesized in a Zn(Ac)2·2H2O-Na2SeO3-KBH4-pyridine solvothermal system at 100°C for 24h. The flower-like architecture is assembled from ZnO nanosheets with a thickness of ∼1.5nm, and the flower-like architecture specific surface area is 132m(2)/g. When the ZnO flower-like architecture is used as the adsorbent for acid fuschin (AF), malachite green (MG), basic fuchsin (BF), congo red (CR) and acid red (AR) in water, the adsorption capacities for AF, MG, BF, CR and AR are 7154.9, 2587.0, 1377.9, 85.0 and 38.0mg/g, respectively. Evidently, the as-obtained ZnO flower-like architectures show excellent adsorption performances for triphenylmethane dyes, and the adsorption capacity of 7154.9mg/g for AF is the highest of all adsorbents for dyes. The adsorption mechanism can be attributed to the electrostatic attraction and the formation of ion-association complex between triphenylmethane dyes and ZnO hierarchical flower-like architectures.

Adhesive and Reinforcing Properties of Soluble Cellulose: A Repulpable Adhesive for Wet and Dry Cellulosic Substrates

This work reports, for the first time, the excellent performance of an aqueous alkaline solution of cellulose as an adhesive for wet and dry cellulosic substrates. Uniaxial tensile tests of filter paper and sulfite writing paper strips bonded with this adhesive (5% cellulose and 7% NaOH aqueous solution) show that failure never occurs in the joints but always in the pristine substrate areas, except in butt joint samples prepared with sulfite paper. Tensile test also shows that paper impregnated with cellulose solution is stronger than the original substrate. X-ray microtomography and scanning electron microscopy reveal that dissolved cellulose fills the gaps between paper fibers, providing a morphological evidence for the mechanical interlocking adhesion mechanism, while scanning probe techniques provide a sharp view of different domains in the joints. Additionally, bonded paper is easily reconverted to pulp, which facilitates paper reprocessability, solving a well-known industrial problem related to deposition of adhesive aggregates (stickies) on the production equipment.

Polyvinylamine: a tool for engineering interfaces

With the highest content of primary amine functional groups of any polymer, polyvinylamine (PVAm) is a potent tool for the modification of macroscopic and nanoparticle surfaces. Based on the free radical polymerization and subsequent hydrolysis of N-vinylformamide, PVAm is prepared as linear polymers (0.8 kDa to >1 MDa), microgels, macrogels, and copolymers. The amine groups serve as reaction sites for grafting PVAm to surfaces and for the preparation of derivatives. Coupling low-molecular-weight molecules and oligomers gives PVAm-X, where X includes hydrophobes, carbohydrate oligomers, proteins, TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), phenylboronic acids, and fluorocarbons. This contribution highlights the use of PVAm and PVAm-X to modify solid surface properties. Where possible, the PVAm properties and applications as an interfacial agent are compared to those of linear polyethylenimine, polyallylamine, and chitosan.

Laccase complex with polyvinylamine bearing grafted TEMPO is a cellulose adhesion primer

Polyelectrolyte complexes formed between laccase and polyvinylamine with grafted TEMPO moieties, PVAm-T, adsorb onto cellulose, causing oxidation. All evidence supports the view that aldehyde groups on oxidized cellulose condense with primary amine groups, giving a grafted layer of PVAm-T complexed with laccase. The grafted PVAm-T serves as a primer layer promoting wet cellulose-to-cellulose adhesion in the presence of PVAm adhesive. The cellulose modification occurs at ambient temperatures and pH 5. The adhesion improvements with mixtures of PVAm-T and laccase are remarkable because both components are macromolecular, which should inhibit the ability of the TEMPO to act as a shuttle between the enzyme and the primary hydroxyl groups on cellulose. It is proposed that PVAm-bound oxoammonium ions exchange with neighboring TEMPO moieties, providing a mechanism for the transfer of oxidation activity from immobilized enzyme to the cellulose surfaces.

Microgel adhesives for wet cellulose: measurements and modeling

Nanostructured adhesive layers were prepared by adsorbing and/or grafting polyvinylamine (PVAm) onto carboxylated poly(N-isopropylacrylamide) (PNIPAM) microgels that were then assembled between layers of wet oxidized cellulose. The wet delamination force was measured as functions of PVAm content, PVAm molecular weight, coverage (mass adhesive/joint area), and the distribution of carboxyl groups in the PNIPAM microgels. The use of microgels is attractive because simple physical adsorption onto the cellulose surfaces before lamination gives much higher adhesive content and strength compared to the corresponding adsorbed linear PVAm. Wet adhesion increased with PVAm content in the microgels and the quantity of microgels in the joint whereas adhesion was independent of PVAm molecular weight. Physical adsorption of the PVAm onto/into the microgels gave the same adhesion as covalently coupled PVAm. Finally, the roles of microgel diameter, elasticity, and coverage were simulated by a simple peel adhesion model in which the microgels were treated as ideal springs.