Reprocessable Polyurethane Foams Using Acetoacetyl-Formed Amides.

Like any other thermosetting material, polyurethane foams (PUFs) contain permanent cross-links that hinder their reprocessability and make their recyclability a tedious and environmentally unfriendly process. Herein, we introduce acetoacetyl-formed amides, formed by the reaction of isocyanates with acetoacetate groups, as dynamic units in the backbone of PUFs. By extensive variation of the foam composition, optimum parameters have been found to produce malleable foams above temperatures of 130 °C, without the requirement of any solvent during the foaming process. The PU cross-linked material can be compression-molded at least three times, giving rise to PU elastomers and thus maintaining a cross-linked network structure. Characterization of the original foams shows comparable properties to standard PUFs, for example, having a density of 32 kg/m3, while they show similar chemical and thermal properties upon reprocessing to strong PU elastomers, exhibiting Tg ranging from -42 to -48 °C. This research provides a straightforward method to produce thermally reprocessable PUFs as a promising pathway to address the recycling issues of end-of-life foams.

Fluorescent supramolecular hierarchical self-assemblies from glycosylated 4-amino- and 4-bromo-1,8-naphthalimides.

An investigation into the self-assembly of two 4-amino- and a 4-bromo-1,8-naphthalimide (Nap) based structures (1-3) possessing an appended glycan unit, from protic polar media, is presented. The results demonstrate the formation of complex hierarchical luminescent aggregates, wherein the morphologies, sizes and spherical structures were highly dependent on both the media and the Nap structure. Upon cleaving the native glycosidic bond, using an enzyme, the structure/morphology of the self-assembly of 3 in buffered solution was significantly transformed.

Light-Responsive Arylazopyrazole Gelators: From Organic to Aqueous Media and from Supramolecular to Dynamic Covalent Chemistry.

Versatile photoresponsive gels based on tripodal low molecular weight gelators (LMWGs) are reported. A cyclohexane-1,3,5-tricarboxamide (CTA) core provides face-to-face hydrogen bonding and a planar conformation, inducing the self-assembly of supramolecular polymers. The CTA core was substituted with three arylazopyrazole (AAP) arms. AAP is a molecular photoswitch that isomerizes reversibly under alternating UV and green light irradiation. The E isomer of AAP is planar, favoring the self-assembly, whereas the Z isomer has a twisted structure, leading to a disassembly of the supramolecular polymers. By using tailor-made molecular design of the tripodal gelator, light-responsive organogels and hydrogels were obtained. Additionally, in the case of the hydrogels, AAP was coupled to the core through hydrazones, so that the hydrogelator and, hence, the photoresponsive hydrogel could also be assembled and disassembled by using dynamic covalent chemistry.

Smart Air-Water Interfaces with Arylazopyrazole Surfactants and Their Role in Photoresponsive Aqueous Foam.

A new light-switchable azo-surfactant arylazopyrazole tetraethylene glycol carboxylic acid (AAP-E4) was used as a molecular building block to functionalize macroscopic foams. AAP-E4 was studied in the bulk solution with UV/vis spectroscopy and at the interface with sum-frequency generation (SFG) as well as tensiometry. Additional foaming experiments were performed with a dynamic foam analyzer to study the role of AAP-E4 surfactants at the ubiquitous air-water interface as well as within macroscopic foam. In the bulk, it is possible to switch the AAP-E4 surfactant reversibly from trans to cis configurations and vice versa using 380 nm UV and 520 nm green light, respectively. At the interface, we demonstrate the excellent switching ability of AAP-E4 surfactants and a substantial modification of the surface tension. In addition, we show that the response of the interface is strongly influenced by lateral electrostatic interactions, which can be tuned by the charging state of AAP-E4. Consequently, the electrostatic disjoining pressure and thus the foam stability are highly dependent on the bulk pH and the charging state of the interface. For that reason, we have studied both the surface net charge (SFG) and the surface excess (tensiometry) as important parameters that determine foam stability in this system and show that neutral pH conditions lead to the optimal compromise between switching ability, surface excess, and surface charging. Measurements on the foam stability demonstrated that foams under irradiation with green light are more stable than foams irradiated with UV light.

A Modular System for the Design of Stimuli-Responsive Multifunctional Nanoparticle Aggregates by Use of Host-Guest Chemistry.

A self-assembly approach for the design of multifunctional nanomaterials consisting of different nanoparticles (gold, iron oxide, and lanthanide-doped LiYF4 ) is developed. This modular system takes advantage of the light-responsive supramolecular host-guest chemistry of ß-cyclodextrin and arylazopyrazole, which enables the dynamic and reversible self-assembly of particles to spherical nanoparticle aggregates in aqueous solution. Due to the magnetic iron oxide nanoparticles, the aggregates can be manipulated by an external magnetic field leading to the formation of linear structures. As a result of the integration of upconversion nanoparticles, the aggregates are additionally responsive to near-infrared light and can be redispersed by use of the upconversion effect. By varying the nanoparticle and linker concentrations the composition, size, shape, and properties of the multifunctional nanoparticle aggregates can be fine-tuned.

Arylazopyrazole Photoswitches in Aqueous Solution: Substituent Effects, Photophysical Properties, and Host-Guest Chemistry.

Getting the green light! Substituted arylazopyrazoles (AAPs) have been investigated as supramolecular photoswitches in aqueous solution. Selective photostationary states (PSSs) and improved binding affinities to ß-cyclodextrin have been determined. The experimental findings are supported by results from DFT calculations.

Controlling Complex Stability in Photoresponsive Macromolecular Host-Guest Systems: Toward Reversible Capture of DNA by Cyclodextrin Vesicles.

An effective and universal method for delivering structurally diverse biomolecules in vivo would greatly benefit modern drug therapy, but has yet to be discovered. Self-assembled supramolecular complexes containing vesicles of amphiphilic cyclodextrin and linker molecules with an azobenzene guest unit and a charged functionality have been established as nanoscale carriers for proteins and DNA, making use of multivalent electrostatic attraction. However, light-induced cargo release is only feasible up to a maximum net charge of the biomacromolecules. Herein, it is shown that it is possible to fine-tune macromolecular complex stability and size by addition of a competitive guest molecule that acts as a stopper, partly blocking the vesicle surface. The superior performance of arylazopyrazoles in photoisomerization compared to azobenzenes, which enables a lower surface charge density of the vesicles in the photostationary state, is also demonstrated. Both strategies allow reversible supramolecular aggregation of high molecular weight DNA (2 and 4.8 kbp).

Self-assembly of colloidal molecules that respond to light and a magnetic field.

This communication reports a novel method to prepare Janus particles with light-responsive arylazopyrazole (AAP) polymer caps, which can be reversibly cross-linked to chain-like colloidal oligomers in the presence of cyclodextrin (CD) functionalized superparamagnetic nanoparticles. The resulting colloidal molecules are light-responsive and can be controlled by an external magnetic field.

Near-infrared photoswitching of cyclodextrin-guest complexes using lanthanide-doped LiYF4 upconversion nanoparticles.

This communication reports a new type of supramolecular cyclodextrin-guest complexes using cyclodextrin coated upconversion nanoparticles as hosts and monovalent and divalent azobenzenes and arylazopyrazoles as guests. A potentially biocompatible photocontrol of the interaction by isomerization of the azobenzene or arylazopyrazole was achieved by laser irradiation at 980 nm and a very low light intensity of 0.22 W cm-2.

Arylazopyrazoles as Light-Responsive Molecular Switches in Cyclodextrin-Based Supramolecular Systems.

A simple and high yield synthesis of water-soluble arylazopyrazoles (AAPs) featuring superior photophysical properties is reported. The introduction of a carboxylic acid allows the diverse functionalization of AAPs. Based on structural modifications of the switching unit the photophysical properties of the AAPs could be tuned to obtain molecular switches with favorable photostationary states. Furthermore, AAPs form stable and light-responsive host-guest complexes with ß-cyclodextrin (ß-CD). Our most efficient AAP shows binding affinities comparable to azobenzenes, but more effective switching and higher thermal stability of the Z-isomer. As a proof-of-principle, we investigated two CD-based supramolecular systems, containing either cyclodextrin vesicles (CDVs) or cyclodextrin-functionalized gold nanoparticles (CDAuNPs), which revealed excellent reversible, light-responsive aggregation and dispersion behavior. To conclude, AAPs have great potential to be incorporated as molecular switches in highly demanding and multivalent photoresponsive systems.

Supramolecular surface adhesion mediated by azobenzene polymer brushes.

Surface immobilised polymer brushes containing azobenzene units were prepared using a combination of microcontact chemistry and surface-initiated atom transfer radical polymerisation (SI-ATRP). These brushes were investigated using AFM, XPS and UV/vis spectroscopy. It was shown that two surfaces bearing azobenzene brushes can be glued together in the presence of a ß-cyclodextrin polymer and hold as much as 700 ± 150 g cm(-2).

Rewritable Polymer Brush Micropatterns Grafted by Triazolinedione Click Chemistry.

Triazolinedione (TAD) click reactions were combined with microcontact chemistry to print, erase, and reprint polymer brushes on surfaces. By patterning substrates with a TAD-tagged atom-transfer radical polymerization initiator (ATRP-TAD) and subsequent surface initiated ATRP, it was possible to graft micropatterned polymer brushes from both alkene- and indole-functionalized substrates. As a result of the dynamic nature of the Alder-ene adduct of TAD and indole at elevated temperatures, the polymer pattern could be erased while the regenerated indole substrate could be reused to print new patterns. To demonstrate the robustness of the methodology, the write-erase cycle was repeated four times.