Disulfide-Cross-Linked Tetra-PEG Gels.

The preparation of polymer gels via cross-linking of four-arm star-shaped poly(ethylene glycol) (Tetra-PEG) precursors is an attractive strategy to prepare networks with relatively well-defined topologies. Typically, Tetra-PEG gels are obtained by cross-linking heterocomplementary reactive Tetra-PEG precursors. This study, in contrast, explores the cross-linking of self-reactive, thiol-end functional Tetra-PEG macromers to form disulfide-cross-linked gels. The structure of the disulfide-cross-linked Tetra-PEG gels was studied with multiple-quantum NMR (MQ-NMR) spectroscopy and small-angle neutron scattering (SANS) experiments. In line with earlier simulation studies, these experiments showed a strong dependence of the relative fractions of the different network connectivities on the concentration of the thiol-end functional Tetra-PEG macromer that was used for the synthesis of the networks. Disulfide-cross-linked Tetra-PEG gels prepared at macromer concentrations below the overlap concentration (c = 0.66c*) primarily feature defect connectivity motifs, such as primary loops and dangling ends. For networks prepared at macromer concentrations above the overlap concentration, the fraction of single-link connectivities was found to be similar to that in amide-cross-linked Tetra-PEG gels obtained by heterocomplementary cross-linking of N-hydroxysuccinimide ester and amine functional Tetra-PEG macromers. Since disulfide bonds are susceptible to reductive cleavage, these disulfide-cross-linked gels are of interest, e.g., as reduction-sensitive hydrogels for a variety of biomedical applications.

Correction to "Ductile, High-Lignin-Content Thermoset Films and Coatings".

[This corrects the article DOI: 10.1021/acssuschemeng.3c03030.].

Pyroelectric Polyelectrolyte Brushes.

Piezo- and pyroelectric materials are of interest, for example, for energy harvesting applications, for the development of tactile sensors, as well as neuromorphic computing. This study reports the observation of pyro- and piezoelectricity in thin surface-attached polymer brushes containing zwitterionic and electrolytic side groups that are prepared via surface-initiated polymerization. The pyro- and piezoelectric properties of the surface-grafted polyelectrolyte brushes are found to sensitively depend on and can be tuned by variation of the counterion. The observed piezo- and pyroelectric properties reflect the structural complexity of polymer brushes, and are attributed to a complex interplay of the non-uniform segment density within these films, together with a non-uniform distribution of counterions and specific ion effects. The fabrication of thin pyroelectric films by surface-initiated polymerization is an important addition to the existing strategies toward such materials. Surface-initiated polymerization, in particular, allows for facile grafting of polar thin polymer films from a wide range of substrates via a straightforward two-step protocol that obviates the need for multistep laborious synthetic procedures or thin film deposition protocols. The ability to produce polymer brushes with piezo- and pyroelectric properties opens up new avenues of application of these materials, for example, in energy harvesting or biosensing.

Ductile, High-Lignin-Content Thermoset Films and Coatings.

In the context of transitioning toward a more sustainable use of natural resources, the application of lignin to substitute commonly utilized petroleum-based plastics can play a key role. Although lignin is highly available at low cost and presents interesting properties, such as antioxidant and UV barrier activities, its application is limited by its low reactivity, which is a consequence of harsh conditions normally used to extract lignin from biomass. In this work, the use of glyoxylic acid lignin (GA lignin), rich in carboxylic acid groups and hence highly reactive toward epoxy cross-linkers, is presented. GA lignin, which is directly extracted from biomass via a one-step aldehyde-assisted fractionation process, allowed the preparation of thermoset films and coatings via a simple reaction with sustainable poly(ethylene glycol) diglycidyl ether and glycerol diglycidyl ether cross-linkers. This allows one to prepare freestanding films containing up to 70 wt % lignin with tunable mechanical properties and covalently surface-attached coatings containing up to 90 wt % lignin with high solvent resistance. Both films and coatings display antioxidant properties and combine excellent UV barrier activity with high visible transparency, which is attractive for applications in sustainable food packaging.

Supramolecular Polymer Brushes Grown by Surface-Initiated Atom Transfer Radical Polymerization from Cucurbit[7]uril-based Non-Covalent Initiators.

Polymer brushes are densely grafted, chain end-tethered assemblies of polymers that can be produced via surface-initiated polymerization. Typically, this is accomplished using initiators or chain transfer agents that are covalently attached to the substrate. This manuscript reports an alternative route towards polymer brushes, which involves the use of non-covalent cucurbit[7]uril-adamantane host-guest interactions to surface-immobilize initiators for atom transfer radical polymerization. These non-covalent initiators can be used for the surface-initiated atom transfer radical polymerization of a variety of water-soluble methacrylate monomers to generate supramolecular polymer brushes with film thicknesses of more than 100 nm. The non-covalent nature of the initiator also allows facile access to patterned polymer brushes, which can be produced in straightforward fashion by drop-casting a solution of the initiator-modified guest molecules onto a substrate that presents the cucurbit[7]uril host.

Supramolecular Polymer Brushes.

Polymer brushes are thin polymer films that consist of densely grafted, chain-end tethered polymers. These thin polymer films can be produced either by anchoring presynthesized chain-end functional polymers to the surface of interest ("grafting to"), or by using appropriately modified surfaces to facilitate growth of polymer chains from the substrate ("grafting from"). The vast majority of polymer brushes that have been prepared and studied so far involved chain-end tethered polymer assemblies that are anchored to the surface via covalent bonds. In contrast, the use of noncovalent interactions to prepare chain-end tethered polymer thin films is much less explored. Anchoring or growing polymer chains using noncovalent interactions results in supramolecular polymer brushes. Supramolecular polymer brushes may possess unique chain dynamics as opposed to their covalently tethered counterparts, which could provide avenues to, for example, renewable or (self-)healable surface coatings. This Perspective article provides an overview of the various approaches that have been used so far to prepare supramolecular polymer brushes. After presenting an overview of the various approaches that have been used to prepare supramolecular brushes via the "grafting to" strategy, examples will be presented of strategies that have been successfully applied to produce supramolecular polymer brushes via "grafting from" methods.

Opportunities and Challenges for Lignin Valorization in Food Packaging, Antimicrobial, and Agricultural Applications.

The exploration of renewable resources is essential to help transition toward a more sustainable materials economy. The valorization of lignin can be a key component of this transition. Lignin is an aromatic polymer that constitutes approximately one-third of the total lignocellulosic biomass and is isolated in huge quantities as a waste material of biofuel and paper production. About 98% of the 100 million tons of lignin produced each year is simply burned as low-value fuel, so this renewable polymer is widely available at very low cost. Lignin has valuable properties that make it a promising material for numerous applications, but it is far from being fully exploited. The aim of this Perspective is to highlight opportunities and challenges for the use of lignin-based materials in food packaging, antimicrobial, and agricultural applications. In the first part, the ongoing research and the possible future developments for the use of lignin as an additive to improve mechanical, gas and UV barrier, and antioxidant properties of food packaging items will be treated. Second, the application of lignin as an antimicrobial agent will be discussed to elaborate on the activity of lignin against bacteria, fungi, and viruses. Finally, the use of lignin in agriculture will be presented by focusing on the application of lignin as fertilizer.

Swelling-Activated, Soft Mechanochemistry in Polymer Materials.

Swelling in polymer materials is a ubiquitous phenomenon. At a molecular level, swelling is dictated by solvent-polymer interactions, and has been thoroughly studied both theoretically and experimentally. Favorable solvent-polymer interactions result in the solvation of polymer chains. For polymers in confined geometries, such as those that are tethered to surfaces, or for polymer networks, solvation can lead to swelling-induced tensions. These tensions act on polymer chains and can lead to stretching, bending, or deformation of the material both at the micro- and macroscopic scale. This Invited Feature Article sheds light on such swelling-induced mechanochemical phenomena in polymer materials across dimensions, and discusses approaches to visualize and characterize these effects.

Fluorinated Poly(aryl piperidinium) Membranes for Anion Exchange Membrane Fuel Cells.

Anion-exchange-membrane fuel cells (AEMFCs) are a cost-effective alternative to proton-exchange-membrane fuel cells (PEMFCs). The development of high-performance and durable AEMFCs requires highly conductive and robust anion-exchange membranes (AEMs). However, AEMs generally exhibit a trade-off between conductivity and dimensional stability. Here, a fluorination strategy to create a phase-separated morphological structure in poly(aryl piperidinium) AEMs is reported. The highly hydrophobic perfluoroalkyl side chains augment phase separation to construct interconnected hydrophilic channels for anion transport. As a result, these fluorinated PAP (FPAP) AEMs simultaneously possess high conductivity (>150 mS cm-1 at 80 °C) and high dimensional stability (swelling ratio <20% at 80 °C), excellent mechanical properties (tensile strength >80 MPa and elongation at break >40%) and chemical stability (>2000 h in 3 m KOH at 80 °C). AEMFCs with a non-precious Co-Mn spinel cathode using the present FPAP AEMs achieve an outstanding peak power density of 1.31 W cm-2 . The AEMs remain stable over 500 h of fuel cell operation at a constant current density of 0.2 A cm-2 .

Chemical Approaches for the Preparation of Bacteria - Nano/Microparticle Hybrid Systems.

Bacteria represent a class of living cells that are very attractive carriers for the transport and delivery of nano- and microsized particles. The use of cell-based carriers, such as for example bacteria, may allow to precisely direct nano- or microsized cargo to a desired site, which would greatly enhance the selectivity of drug delivery and allow to mitigate side effects. One key step towards the use of such nano-/microparticle - bacteria hybrids is the immobilization of the cargo on the bacterial cell surface. To fabricate bacteria - nano-/microparticle biohybrid microsystems, a wide range of chemical approaches are available that can be used to immobilize the particle payload on the bacterial cell surface. This article presents an overview of the various covalent and noncovalent chemistries that are available for the preparation of bacteria - nano-/microparticle hybrids. For each of the different chemical approaches, an overview will be presented that lists the bacterial strains that have been modified, the type and size of nanoparticles that have been immobilized, as well as the methods that have been used to characterize the nanoparticle-modified bacteria.

Lignin: A Sustainable Antiviral Coating Material.

Transmission of viruses through contact with contaminated surfaces is an important pathway for the spread of infections. Antiviral surface coatings are useful to minimize such risks. Current state-of-the-art approaches toward antiviral surface coatings either involve metal-based materials or complex synthetic polymers. These approaches, however, even if successful, will have to face great challenges when it comes to large-scale applications and their environmental sustainability. Here, an antiviral surface coating was prepared by spin-coating lignin, a natural biomass residue of the paper production industry. We show effective inactivation of herpes simplex virus type 2 (>99% after 30 min) on a surface coating that is low-cost and environmentally sustainable. The antiviral mechanism of the lignin surface was investigated and is attributed to reactive oxygen species generated upon oxidation of lignin phenols. This mechanism does not consume the surface coating (as opposed to the release of a specific antiviral agent) and does not require regeneration. The coating is stable in ambient conditions, as demonstrated in a 6 month aging study that did not reveal any decrease in antiviral activity. This research suggests that natural compounds may be used for the development of affordable and sustainable antiviral coatings.

Uniformly Dispersed Poly(lactic acid)-Grafted Lignin Nanoparticles Enhance Antioxidant Activity and UV-Barrier Properties of Poly(lactic acid) Packaging Films.

Poly(lactic acid) (PLA) represents one of the most widely used biodegradable polymers for food packaging applications. While this material provides many advantages, it is characterized by limited antioxidant and UV-barrier properties. Blending PLA with lignin is an attractive strategy to address these limitations. Lignin possesses antioxidant properties and absorbs UV-light and is a widely available low value byproduct of the paper and pulp industry. This study has explored the use of lignin nanoparticles to augment the properties of PLA-based films. A central challenge in the preparation of PLA-lignin nanoparticle blend films is to avoid nanoparticle aggregation, which could compromise optical properties as well as antioxidant activity, among others. To avoid nanoparticle aggregation in the PLA matrix, PLA-grafted lignin nanoparticles were prepared via organocatalyzed lactide ring-opening polymerization. In contrast to lignin and unmodified lignin nanoparticles, these PLA-grafted lignin nanoparticles could be uniformly dispersed in PLA for lignin contents up to 10 wt %. The addition of as little as the equivalent of 1 wt % of lignin of these nanoparticles effectively blocked transmission of 280 nm UV-light. At the same time, these blend films retained reasonable visible light transmittance. The optical properties of the PLA lignin blend films also benefited from the well-dispersed nature of the PLA-grafted nanoparticles, as evidenced by significantly higher visible light transmittance of blends of PLA and PLA-grafted nanoparticles, as compared to blends prepared from PLA with lignin or unmodified lignin nanoparticles. Finally, blending PLA with PLA-grafted lignin nanoparticles greatly augments the antioxidant activity of these films.

Sustainable polyesters via direct functionalization of lignocellulosic sugars.

The development of sustainable plastics from abundant renewable feedstocks has been limited by the complexity and efficiency of their production, as well as their lack of competitive material properties. Here we demonstrate the direct transformation of the hemicellulosic fraction of non-edible biomass into a tricyclic diester plastic precursor at 83% yield (95% from commercial xylose) during integrated plant fractionation with glyoxylic acid. Melt polycondensation of the resulting diester with a range of aliphatic diols led to amorphous polyesters (Mn = 30-60 kDa) with high glass transition temperatures (72-100 °C), tough mechanical properties (ultimate tensile strengths of 63-77 MPa, tensile moduli of 2,000-2,500 MPa and elongations at break of 50-80%) and strong gas barriers (oxygen transmission rates (100 µm) of 11-24 cc m-2 day-1 bar-1 and water vapour transmission rates (100 µm) of 25-36 g m-2 day-1) that could be processed by injection moulding, thermoforming, twin-screw extrusion and three-dimensional printing. Although standardized biodegradation studies still need to be performed, the inherently degradable nature of these materials facilitated their chemical recycling via methanolysis at 64 °C, and eventual depolymerization in room-temperature water.