Upcycling Polyolefin Blends into High-Performance Materials by Exploiting Azidotriazine Chemistry Using Reactive Extrusion.

The revalorization of incompatible polymer blends is a key obstacle in realizing a circular economy in the plastics industry. Polyolefin waste is particularly challenging because it is difficult to sort into its constituent components. Untreated blends of polyethylene and polypropylene typically exhibit poor mechanical properties that are suitable only for low-value applications. Herein, we disclose a simple azidotriazine-based grafting agent that enables polyolefin blends to be directly upcycled into high-performance materials by using reactive extrusion at industrially relevant processing temperatures. Based on a series of model experiments, the azidotriazine thermally decomposes to form a triplet nitrene species, which subsequently undergoes a complex mixture of grafting, oligomerization, and cross-linking reactions; strikingly, the oligomerization and cross-linking reactions proceed through the formation of nitrogen-nitrogen bonds. When applied to polyolefin blends during reactive extrusion, this combination of reactions leads to the generation of amorphous, phase-separated nanostructures that tend to exist at polymer-polymer interfaces. These nanostructures act as multivalent cross-linkers that reinforce the resulting material, leading to dramatically improved ductility compared with the untreated blends, along with high dimensional stability at high temperatures and excellent mechanical recyclability. We propose that this unique behavior is derived from the thermomechanically activated reversibility of the nitrogen-nitrogen bonds that make up the cross-linking structures. Finally, the scope of this chemistry is demonstrated by applying it to ternary polyolefin blends as well as postconsumer polyolefin feedstocks.

Chemically Fueled Self-Sorted Hydrogels.

Narcissistic self-sorting in supramolecular assemblies can help to construct materials with more complex hierarchies. Whereas controlled changes in pH or temperature have been used to this extent for two-component self-sorted gels, here we show that a chemically fueled approach can provide three-component materials with high precision. The latter materials have interesting mechanical properties, such as enhanced or suppressed stiffness, and intricate multistep gelation kinetics. In addition, we show that we can achieve supramolecular templating, where pre-existing supramolecular fibers first act as templates for growth of a second gelator, after which they can selectively be removed.

Magnetic Control over the Fractal Dimension of Supramolecular Rod Networks.

Controlling supramolecular polymerization is of fundamental importance to create advanced materials and devices. Here we show that the thermodynamic equilibrium of Gd3+-bearing supramolecular rod networks is shifted reversibly at room temperature in a static magnetic field of up to 2 T. Our approach opens opportunities to control the structure formation of other supramolecular or coordination polymers that contain paramagnetic ions.

Re-programming Hydrogel Properties Using a Fuel-Driven Reaction Cycle.

Nature uses catalysis as an indispensable tool to control assembly and reaction cycles in vital non-equilibrium supramolecular processes. For instance, enzymatic methionine oxidation regulates actin (dis-)assembly, and catalytic guanosine triphosphate hydrolysis is found in tubulin (dis-)assembly. Here we present a completely artificial reaction cycle which is driven by a chemical fuel that is catalytically obtained from a "pre-fuel". The reaction cycle controls the dis-assembly and re-assembly of a hydrogel, where the rate of pre-fuel turnover dictates the morphology as well as the mechanical properties. By addition of additional fresh aliquots of fuel and removal of waste, the hydrogels can be re-programmed time after time. Overall, we show how catalytic fuel generation can control reaction/assembly kinetics and materials' properties in life-like non-equilibrium systems.

Devising Synthetic Reaction Cycles for Dissipative Nonequilibrium Self-Assembly.

Fuel-driven reaction cycles are found in biological systems to control the assembly and disassembly of supramolecular materials such as the cytoskeleton. Fuel molecules can bind noncovalently to a self-assembling building block or they can react with it, resulting in covalent modifications. Overall the fuel can either switch the self-assembly process on or off. Here, a closer look is taken at artificial systems that mimic biological systems by making and breaking covalent bonds in a self-assembling motif. The different chemistries used so far are highlighted in chronological order and the pros and cons of each system are discussed. Moreover, the desired traits of future reaction cycles, their fuels, and waste management are outlined, and two chemistries that have not been explored up to now in chemically fueled dissipative self-assembly are suggested.

Surface-Assisted Self-Assembly of a Hydrogel by Proton Diffusion.

Controlling supramolecular growth at solid surfaces is of great importance to expand the scope of supramolecular materials. A dendritic benzene-1,3,5-tricarboxamide peptide conjugate is described in which assembly can be triggered by a pH jump. Stopped-flow kinetics and mathematical modeling provide a quantitative understanding of the nucleation, elongation, and fragmentation behavior in solution. To assemble the molecule at a solid-liquid interface, we use proton diffusion from the bulk. The latter needs to be slower than the lag phase of nucleation to progressively grow a hydrogel outwards from the surface. Our method of surface-assisted self-assembly is generally applicable to other gelators, and can be used to create structured supramolecular materials.