Terminal Flow of Cluster-Forming Supramolecular Polymer Networks: Single-Chain Relaxation or Micelle Reorganization?

We correlate the terminal relaxation of supramolecular polymer networks, based on unentangled telechelic poly(isobutylene) linear chains forming micellar end-group clusters, with the microscopic chain dynamics as probed by proton NMR. For a series of samples with increasing molecular weight, we find a quantitative agreement between the terminal relaxation times and their activation energies provided by rheology and NMR. This finding corroborates the validity of the transient-network model and the special case of the sticky Rouse model, and dismisses more dedicated approaches treating the terminal relaxation in terms of micellar rearrangements. Also, we confirm previous results showing reduction of the activation energy of supramolecular dissociation with increasing molecular weight and explain this trend with an increasing elastic penalty, as corroborated by small angle x-ray scattering data.

3D Printing of Supramolecular Polymers: Impact of Nanoparticles and Phase Separation on Printability.

3D printing of linear and three-arm star supramolecular polymers with attached hydrogen bonds and their nanocomposites is reported. The concept is based on hydrogen-bonded supramolecular polymers, known to form nano-sized micellar clusters. Printability is based on reversible thermal- and shear-induced dissociation of a supramolecular polymer network, which generates stable and self-supported structures after printing, as checked via melt-rheology and X-ray scattering. The linear and three-arm star poly(isobutylene)s PIB-B2 (Mn = 8500 g mol -1 ), PIB-B3 (Mn = 16 000 g mol -1 ), and linear poly(ethylene glycol)s PEG-B2 (Mn = 900 g mol-1 , 8500 g mol -1 ) are prepared and then probed by melt-rheology to adjust the viscosity to address the proper printing window. The supramolecular PIB polymers show a rubber-like behavior and are able to form self-supported 3D printed objects at room temperature and below, reaching polymer strand diameters down to 200-300 µm. Nanocomposites of PIB-B2 with silica nanoparticles (12 nm, 5-15 wt%) are generated, in turn leading to an improvement of their shape persistence. A blend of the linear polymer PIB-B2 and the three-arm star polymer PIB-B3 (ratio ≈ 3/1 mol) reaches an even higher structural stability, able to build free-standing structures.

CuAAC-Based Click Chemistry in Self-Healing Polymers.

Click chemistry has emerged as a significant tool for materials science, organic chemistry, and bioscience. Based on the initial concept of Barry Sharpless in 2001, the copper(I)-catalyzed azide/alkyne cycloaddition (CuAAC) reaction has triggered a plethora of chemical concepts for linking molecules and building blocks under ambient conditions, forming the basis for applications in autonomous cross-linking materials. Self-healing systems on the other hand are often based on mild cross-linking chemistries that are able to react either autonomously or upon an external trigger. In the ideal case, self-healing takes place efficiently at low temperatures, independent of the substrate(s) used, by forming strong and stable networks, binding to the newly generated (cracked) interfaces to restore the original material properties. The use of the CuAAC in self-healing systems, most of all the careful design of copper-based catalysts linked to additives as well as the chemical diversity of substrates, has led to an enormous potential of applications of this singular reaction. The implementation of click-based strategies in self-healing systems therefore is highly attractive, as here chemical (and physical) concepts of molecular reactivity, molecular design, and even metal catalysis are connected to aspects of materials science. In this Account, we will show how CuAAC reactions of multivalent components can be used as a tool for self-healing materials, achieving cross-linking at low temperatures (exploiting concepts of autocatalysis or internal chelation within the bulk CuAAC and systematic optimization of the efficiency of the used Cu(I) catalysts). Encapsulation strategies to separate the click components by micro- and nanoencapsulation are required in this context. Consequently, the examples reported here describe chemical concepts to realize more efficient and faster click reactions in self-healing polymeric materials. Thus, enhanced chain diffusion in (hyper)branched polymers, autocatalysis, or internal chelation concepts enable efficient click cross-linking already at 5 °C with a simultaneously reduced amount of Cu(I) catalyst and increased reaction rates, culminating in the first reported self-healing system based on click cycloaddition reactions. Via tailor-made nanocarbon/Cu(I) catalysts we can further improve the click cross-linking reaction in view of efficiency and kinetics, leading to the generation of self-healing graphene-based epoxy nanocomposites. Additionally, we have designed special CuAAC click methods for chemical reporting and visualization systems based on the detection of ruptured capsules via a fluorogenic click reaction, which can be combined with CuAAC cross-linking reactions to obtain simultaneous stress detection and self-healing within polymeric materials. In a similar concept, we have prepared polymeric Cu(I)-biscarbene complexes to detect (mechanical) stress within self-healing polymeric materials via a triggered fluorogenic reaction, thus using a destructive force for a constructive chemical response.

Self-Healing in Supramolecular Polymers.

Adaption and self-healing are two major principles in material science, often coupled with the placement of supramolecular moieties within a material. Proper molecular design can enable self-healing within such materials, displaying enormous potential in technology and application. Here, basic physicochemical aspects as well as new material developments in the field are described, published after a recent review in Macromolecular Rapid Communications in 2013.

"Click"-Triggered Self-Healing Graphene Nanocomposites.

Strategies to compensate material fatigue are among the most challenging issues, being most prominently addressed by the use of nano- and microscaled fillers, or via new chemical concepts such as self-healing materials. A capsule-based self-healing material is reported, where the adverse effect of reduced tensile strength due to the embedded capsules is counterbalanced by a graphene-based filler, the latter additionally acting as a catalyst for the self-healing reaction. The concept is based on "click"-based chemistry, a universal methodology to efficiently link components at ambient reaction conditions, thus generating a "reactive glue" at the cracked site. A capsule-based healing system via a graphene-based Cu2 O (TRGO-Cu2 O-filler) is used, acting as both the catalytic species for crosslinking and the required reinforcement agent within the material, in turn compensating the reduction in tensile strength exerted by the embedded capsules. Room-temperature self-healing within 48 h is achieved, with the investigated specimen containing TRGO-Cu2 O demonstrating significantly faster self-healing compared to homogeneous (Cu(PPh3 )3 F, Cu(PPh3 )3 Br), and heterogeneous (Cu/C) copper(I) catalysts.

Adoptive transfer of allogeneic regulatory T cells into patients with chronic graft-versus-host disease.

BACKGROUND AIMS: Mouse models indicate that adoptive transfer of regulatory T cells (Treg) may suppress graft-versus-host-disease (GvHD) while preserving graft-versus-leukemia reactions. We aimed to develop a protocol for the efficient isolation and in vitro expansion of donor-derived Treg and to establish the proof-of-concept for the clinical application of ex vivo-generated Treg preparations in five patients with otherwise treatment-refractory chronic GvHD (cGvHD). METHODS: Allogeneic Treg were isolated from unstimulated leukapheresis products of the corresponding human leukocyte antigen-matched donors by use of clinical-grade magnetic-activated bead sorting. To increase the amount and purity, Treg were cultivated for 7-12 days and infused after a median time of 35 months after allogeneic hematopoietic cell transplantation. RESULTS: Final products contained Treg with a median purity of 84.1% CD4(+)CD25(high)CD127(low)FOXP3(+)of CD45(+) cells and a mean quantity of 2.4 × 10(6) Treg per kg body wt. All isolated cell products showed in vitro suppressive activity. On transfusion, two of five patients showed a clinical response with improvement of cGvHD symptoms. The other three patients showed stable cGvHD symptoms for up to 21 months. In four of five patients, increased counts of Treg were detectable on Treg transfusion, immunosuppressive treatment could be reduced and suppression of CD69 activation marker expression on T-effector cells was observed. However, one patient had development of malignant melanoma and another patient had Bowen skin cancer 4 months and 11 months after Treg transfusion, respectively. CONCLUSIONS: We demonstrate a feasible and reproducible approach of isolating functional Treg in high quantity and purity for clinical application and show opportunities and risks of adoptive Treg transfer into patients with cGvHD.

Carbon-Supported Copper Nanomaterials: Recyclable Catalysts for Huisgen [3+2] Cycloaddition Reactions.

Highly disperse copper nanoparticles immobilized on carbon nanomaterials (CNMs; graphene/carbon nanotubes) were prepared and used as a recyclable and reusable catalyst to achieve Cu(I) -catalyzed [3+2] cycloaddition click chemistry. Carbon nanomaterials with immobilized N-heterocyclic carbene (NHC)-Cu complexes prepared from an imidazolium-based carbene and Cu(I) show excellent stability including high efficiency at low catalyst loading. The catalytic performance evaluated in solution and in bulk shows that both types of Cu-CNMs can function as an effective recyclable catalysts (more than 10 cycles) for click reactions without decomposition and the use of external additives.

Self-healing polymers via supramolecular forces.

As polymers and polymeric materials are "the" smart invention and technological driving force of the 20th century, the quest for self-healing or self-repairing polymers is strong. The concept of supramolecular self-healing materials relies on the use of noncovalent, transient bonds to generate networks, which are able to heal the damaged site, putting aspects of reversibility and dynamics of a network as crucial factors for the understanding and design of such self-healing materials. This Review describes recent examples and concepts of supramolecular polymers based on hydrogen bonding, π-π interactions, ionomers, and coordinative bonds, thus convincingly discussing the advantages and versatility of these supramolecular forces for the design and realization of self-healing polymers.

Improving Kinetics of "Click-Crosslinking" for Self-Healing Nanocomposites by Graphene-Supported Cu-Nanoparticles.

Investigation of the curing kinetics of crosslinking reactions and the development of optimized catalyst systems is of importance for the preparation of self-healing nanocomposites, able to significantly extend their service lifetimes. Here we study different modified low molecular weight multivalent azides for a capsule-based self-healing approach, where self-healing is mediated by graphene-supported copper-nanoparticles, able to trigger "click"-based crosslinking of trivalent azides and alkynes. When monitoring the reaction kinetics of the curing reaction via reactive dynamic scanning calorimetry (DSC), it was found that the "click-crosslinking" reactivity decreased with increasing chain length of the according azide. Additionally, we could show a remarkable "click" reactivity already at 0 °C, highlighting the potential of click-based self-healing approaches. Furthermore, we varied the reaction temperature during the preparation of our tailor-made graphene-based copper(I) catalyst to further optimize its catalytic activity. With the most active catalyst prepared at 700 °C and the optimized set-up of reactants on hand, we prepared capsule-based self-healing epoxy nanocomposites.

Qualitative sensing of mechanical damage by a fluorogenic "click" reaction.

A simple and unique damage-sensing tool mediated by a Cu(i)-catalyzed [3+2] cycloaddition reaction is reported, where a fluorogenic "click"-reaction highlights physical damage by a strong fluorescence increase accompanied by in situ monitoring of localized self-healing.

Click chemistry promoted by graphene supported copper nanomaterials.

A facile and robust approach is provided for the synthesis of highly dispersed copper nanoparticles immobilized onto graphene nanosheets, useful as a recyclable and reusable heterogeneous catalyst with excellent catalytic activity to achieve Cu(I)-catalyzed [3+2] cycloaddition 'click' chemistry.

Insights into the mechanism of polymer coating self-healing using Raman spectroscopy.

Self-healing polymer coatings are an emerging class of smart materials. Upon mechanical damage the material properties may be restored by self-healing, which can be triggered externally, e.g., by an increased temperature. An alternative approach relies on embedding capsules with repair agents into the polymers, the rupture of which is induced by the mechanical damage, and the release of the repair agents triggers the self-repair reaction. The work at hand presents in situ Raman spectroscopic investigations on the reaction dynamics in such self-healing polymer coatings. Analysis of the Raman spectra allows one to assign specific Raman bands characteristic for the progress of the self-healing reaction.