Showing papers by "Christoph Weder published in 2016"

Mechanoresponsive Luminescent Molecular Assemblies: An Emerging Class of Materials

Abstract: The possibility to change the molecular assembled structures of organic and organometallic materials through mechanical stimulation is emerging as a general and powerful concept for the design of functional materials. In particular, the photophysical properties such as photoluminescence color, quantum yield, and emission lifetime of organic and organometallic fluorophores can significantly depend on the molecular packing, enabling the development of molecular materials with mechanoresponsive luminescence characteristics. Indeed, an increasing number of studies have shown in recent years that mechanical force can be utilized to change the molecular arrangement, and thereby the optical response, of luminescent molecular assemblies of π-conjugated organic or organometallic molecules. Here, the development of such mechanoresponsive luminescent (MRL) molecular assemblies consisting of organic or organometallic molecules is reviewed and emerging trends in this research field are summarized. After a brief introduction of mechanoresponsive luminescence observed in molecular assemblies, the concept of "luminescent molecular domino" is introduced, before molecular materials that show turn-on/off of photoluminescence in response to mechanical stimulation are reviewed. Mechanically stimulated multicolor changes and water-soluble MRL materials are also highlighted and approaches that combine the concept of MRL molecular assemblies with other materials types are presented in the last part of this progress report.

Supramolecular polymer adhesives: advanced materials inspired by nature

Abstract: Due to their dynamic, stimuli-responsive nature, non-covalent interactions represent versatile design elements that can be found in nature in many molecular processes or materials, where adaptive behavior or reversible connectivity is required. Examples include molecular recognition processes, which trigger biological responses or cell-adhesion to surfaces, and a broad range of animal secreted adhesives with environment-dependent properties. Such advanced functionalities have inspired researchers to employ similar design approaches for the development of synthetic polymers with stimuli-responsive properties. The utilization of non-covalent interactions for the design of adhesives with advanced functionalities such as stimuli responsiveness, bonding and debonding on demand capability, surface selectivity or recyclability is a rapidly emerging subset of this field, which is summarized in this review.

Articular cartilage: from formation to tissue engineering

Abstract: Hyaline cartilage is the nonlinear, inhomogeneous, anisotropic, poro-viscoelastic connective tissue that serves as friction-reducing and load-bearing cushion in synovial joints and is vital for mammalian skeletal movements. Due to its avascular nature, low cell density, low proliferative activity and the tendency of chondrocytes to de-differentiate, cartilage cannot regenerate after injury, wear and tear, or degeneration through common diseases such as osteoarthritis. Therefore severe damage usually requires surgical intervention. Current clinical strategies to generate new tissue include debridement, microfracture, autologous chondrocyte transplantation, and mosaicplasty. While articular cartilage was predicted to be one of the first tissues to be successfully engineered, it proved to be challenging to reproduce the complex architecture and biomechanical properties of the native tissue. Despite significant research efforts, only a limited number of studies have evolved up to the clinical trial stage. This review article summarizes the current state of cartilage tissue engineering in the context of relevant biological aspects, such as the formation and growth of hyaline cartilage, its composition, structure and biomechanical properties. Special attention is given to materials development, scaffold designs, fabrication methods, and template-cell interactions, which are of great importance to the structure and functionality of the engineered tissue.

A critical review of the current knowledge regarding the biological impact of nanocellulose.

Abstract: Several forms of nanocellulose, notably cellulose nanocrystals and nanofibrillated cellulose, exhibit attractive property matrices and are potentially useful for a large number of industrial applications. These include the paper and cardboard industry, use as reinforcing filler in polymer composites, basis for low-density foams, additive in adhesives and paints, as well as a wide variety of food, hygiene, cosmetic, and medical products. Although the commercial exploitation of nanocellulose has already commenced, little is known as to the potential biological impact of nanocellulose, particularly in its raw form. This review provides a comprehensive and critical review of the current state of knowledge of nanocellulose in this format. Overall, the data seems to suggest that when investigated under realistic doses and exposure scenarios, nanocellulose has a limited associated toxic potential, albeit certain forms of nanocellulose can be associated with more hazardous biological behavior due to their specific physical characteristics.

Optically responsive supramolecular polymer glasses

Abstract: The reversible and dynamic nature of non-covalent interactions between the constituting building blocks renders many supramolecular polymers stimuli-responsive. This was previously exploited to create thermally and optically healable polymers, but it proved challenging to achieve high stiffness and good healability. Here we present a glass-forming supramolecular material that is based on a trifunctional low-molecular-weight monomer ((UPyU)3TMP). Carrying three ureido-4-pyrimidinone (UPy) groups, (UPyU)3TMP forms a dynamic supramolecular polymer network, whose properties are governed by its cross-linked architecture and the large content of the binding motif. This design promotes the formation of a disordered glass, which, in spite of the low molecular weight of the building block, displays typical polymeric behaviour. The material exhibits a high stiffness and offers excellent coating and adhesive properties. On account of reversible dissociation and the formation of a low-viscosity liquid upon irradiation with ultraviolet light, rapid optical healing as well as (de)bonding on demand is possible.

A Thermo- and Mechanoresponsive Cyano-Substituted Oligo(p-phenylene vinylene) Derivative with Five Emissive States

Abstract: Multiresponsive materials that display predefined photoluminescence color changes upon exposure to different stimuli are attractive candidates for advanced sensing schemes. Herein, we report a cyano-substituted oligo(p-phenylene vinylene) (cyano-OPV) derivative that forms five different solvent-free solid-state molecular assemblies, luminescence properties of which change upon thermal and mechanical stimulation. Single-crystal X-ray structural analysis suggested that tolyl groups introduced at the termini of solubilizing side-chains of the cyano-OPV play a pivotal role in its solid-state arrangement. Viewed more broadly, this report shows that the introduction of competing intermolecular interactions into excimer-forming chromophores is a promising design strategy for multicolored thermo- and mechanoresponsive luminescent materials.

The Role of Mass and Length in the Sonochemistry of Polymers

Abstract: The ultrasound-induced cleavage of macromolecules has become a routine experiment in the emerging field of polymer mechanochemistry. To date, it has not been conclusively proven whether the molecular weight of a polymer or its contour length is the determining factor for chain scission upon ultrasonication. Here we report comparative experiments that confirm unequivocally that the contour length is the decisive parameter. We utilized postpolymerization modifications of specifically designed precursor polymers to create polymers with identical chain length but different molecular mass. To demonstrate the universality of the findings, two different polymer backbones were utilized—poly(styrene) and poly(norbornene imide alkyne)—whose molecular weights were altered by bromination and removal of pendant triisopropylsilyl protecting groups, respectively. Solutions of the respective polymer pairs were subjected to pulsed ultrasound at 20 kHz and 10.4 W/cm2 in order to investigate the chain scission trends. The e...

Elucidating the Potential Biological Impact of Cellulose Nanocrystals

Abstract: Cellulose nanocrystals exhibit an interesting combination of mechanical properties and physical characteristics, which make them potentially useful for a wide range of consumer applications. However, as the usage of these bio-based nanofibers increases, a greater understanding of human exposure addressing their potential health issues should be gained. The aim of this perspective is to highlight how knowledge obtained from studying the biological impact of other nanomaterials can provide a basis for future research strategies to deduce the possible human health risks posed by cellulose nanocrystals.

Epoxy Resin-Inspired Reconfigurable Supramolecular Networks

Abstract: With the goal to push the mechanical properties of reconfigurable supramolecular polymers toward those of thermoset resins, we prepared and investigated a new family of hydrogen-bonded polymer networks that are assembled from isophthalic acid-terminated oligo(bisphenol A-co-epichlorohydrin) and different bipyridines. These materials display high storage moduli of up to 3.9 GPa, can be disassembled upon heating to form melts with a viscosity of as low as 2.1 Pa·s, and fully reassemble upon cooling. We show that the new polymers can readily be reconfigured, reprocessed, or recycled and that the reversible (dis)assembly makes them useful as hot-melt adhesives that permit debonding on demand.

Shape Memory Composites Based on Electrospun Poly(vinyl alcohol) Fibers and a Thermoplastic Polyether Block Amide Elastomer.

Abstract: The present study aimed at developing new thermally responsive shape-memory composites, that were fabricated by compacting mats of electrospun poly(vinyl alcohol) (PVA) fibers and sheets of a thermoplastic polyether block amide elastomer (PEBA). This design was based on the expectation that the combination of the rubber elasticity of the PEBA matrix and the mechanical switching exploitable through the reversible glass transition temperature (Tg) of the PVA filler could be combined to create materials that display shape memory characteristics as an emergent effect. Dynamic mechanical analyses (DMA) show that, upon introduction of 10–20% w/w PVA fibers, the room-temperature storage modulus (E′) increased by a factor of 4–5 in comparison to the neat PEBA, and they reveal a stepwise reduction of E′ around the Tg of PVA (85 °C). This transition could indeed be utilized to fix a temporary shape and recover the permanent shape. At low strain, the fixity was 66 ± 14% and the recovery was 98 ± 2%. Overall, the dat...

Thermoresponsive low-power light upconverting polymer nanoparticles

Abstract: We report highly efficient sensitized triplet–triplet annihilation based upconversion in aqueous suspensions of nanoparticles prepared from 9,10-diphenylanthracene-terminated poly(e-caprolactone) and with platinum octaethylporphyrin as the sensitizer. The particles upconversion characteristics are strongly temperature-dependent. This feature gives insights into the mechanisms enabling the process in the nanoparticle environment, and the specific temperature range in which the photophysical parameters change is suitable for live cell and in vivo temperature sensing.

Azo-Containing Polymers with Degradation On-Demand Feature

Abstract: Molecules comprising aliphatic azo moieties are widely used as radical polymerization initiators, but only a few studies have explored their usefulness as stimuli-responsive motifs in macromolecular constructs. The controlled degradation of azo-containing polymers has indeed remained largely unexplored. Here we present the syntheses of linear azo-containing polyamides and polyurethanes and report on their thermally and optically induced responses in solution and the solid state. We show that the stimuli-induced degradation behavior depends strongly on the nature of the polymer backbone, the state of matter, and in solution, on the nature of the solvent. The stimuli-responsive solid-state properties of the azo-containing materials may be particularly useful. In the case of the polyurethanes studied here, temperature- or light-induced cleavage of the azo motifs led to a controllable decrease in the molecular weight, which, in turn, caused a reduction of the elongation at break, modulus, and strength. The co...

Bioinspired surfaces and materials

Abstract: Over millions of years evolution has optimized the properties of materials via natural selection for many specific purposes. Indeed, natural materials have unique properties which come very close to perfection. Cells, for instance, are able to carry out intricate sequences of chemical reactions that are difficult or impossible to carry out ex vivo, cell membranes are the most complex selective and responsive semipermeable membranes that exist, and animal shells exhibit a clever nanostructure that renders them hard and tough at the same time. In short, materials scientists can learn a lot from nature’s materials. The perfection and performance of nature’s materials not only spark fascination, but also trigger the question as to why certain structures or surfaces exhibit outstanding properties and inspire research towards new materials. While the materials of living nature impressively serve dedicated purposes, they are formed under restricted conditions. For instance, they have to be designed to function under a narrowly defined set of physiological conditions, and can only be composed of building blocks an organism has available. Without these restrictions, material scientists can design entirely new materials or surfaces. Taking the design and function of natural materials as a Groningen University, University Medical Center Groningen, Antonius Deusinglaan 1, 9713 AV Groningen, Netherlands. E-mail: romana.schirhagl@gmail.com b University of Fribourg, Adolphe Merkle Institute, Chemin des Verdiers 4, CH-1700 Fribourg, Switzerland. E-mail: christoph.weder@unifr.ch c Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Zhongguancun East Road 29, 100190, Beijing, China. E-mail: jianglei@iccas.ac.cn d Leibniz Institute of Polymer Research Dresden (IPF), Max Bergmann Center of Biomaterials Dresden (MBC), 01069 Dresden, Germany. E-mail: carsten.werner@tu-dresden.de e ETH-Zurich, Department of Materials, Wolfgang-Pauli-Str. 10, 8093 Zürich, Switzerland. E-mail: marcus.textor@mat.ethz.ch

Tuning the thermo- and mechanoresponsive behavior of luminescent cyclophanes

Abstract: Many cyclophanes have been investigated in dilute solution, where their internal cavities are accessible for supramolecular interactions. However, their photophysical properties in the solid state remain largely unexplored. We here report a new mechano- and thermoresponsive luminescent cyclophane that is comprised of two 9,10-bis(phenylethynyl)anthracene moieties and features two hexaethylene glycol bridges. The compound was found to exhibit a nematic liquid-crystalline phase at elevated temperature. X-ray diffraction patterns confirm that thermal and mechanical treatments induce changes in the molecular assembly, which are the basis for the observed photoluminescent color variations. The stimuli-responsive behavior of the new compound is quite different from that of a previously reported cyclophane with similar structure but shorter bridges. Thus, merely changing the ring size is an effective tool to tailor the stimuli-responsiveness and the phase behaviour of luminescent cyclophanes.

Single-Component Upconverting Polymeric Nanoparticles.

Abstract: Low-power light upconversion is a highly desirable feature for a broad range of applications and new materials enabling this process are sought in both bulk and particulate form. Here, the preparation of upconverting nanoparticles is reported from a methacrylic terpolymer bearing diphenylanthracene and meso-phenoxytris(heptyl)porphyrin pendant groups by a microemulsion technique. The use of a terpolymer in which the upconvering dye molecules are covalently attached mitigates some of the drawbacks of triplet-triplet annihilation upconverting nanoparticles made by other techniques, in particular dye leakage from the nanoparticles, and limited control of the sensitizer and emitter concentration within each nanoparticle. Size and morphology of the new upconverting nanoparticles are investigated by dynamic light scattering and transmission electron microscopy and elucidated their upconverting properties by luminescence spectroscopy.

Deformation‐Induced Color Changes in Melt‐Processed Polyamide 12 Blends

Abstract: Blends of polyamide 12 and small amounts (0.15–1 wt%) of the excimer-forming fluorescent dye 1,4-bis(α-cyano-4-octadecyloxystyryl)-2,5-dimethoxybenzene (C18-RG) are produced by melt-processing. While green monomer fluorescence from well-individualized chromophores is observed at low dye concentration (0.15%), higher dye concentrations lead to aggregation of the dye so that the emission characteristics are dominated by red excimer fluorescence. Upon mechanical deformation of samples with appropriately selected dye content (0.25 wt%), a pronounced mechanochromic effect can be observed, which manifests itself through a mechanically induced transformation from excimer-dominated to monomer-rich emission. The monomer to excimer emission ratio IM/IE is increased by a factor of up to 2 in a step-wise manner when samples are uniaxially deformed past the yield point.

Supramolecular Polymer Adhesives: Advanced Materials Inspired by Nature

Abstract: Due to their dynamic, stimuli-responsive nature, non-covalent interactions represent versatile design elements that can be found in nature in many molecular processes or materials, where adaptive behavior or reversible connectivity is required. Examples include molecular recognition processes, which trigger biological responses or cell-adhesion to surfaces, and a broad range of animal secreted adhesives with environment-dependent properties. Such advanced functionalities have inspired researchers to employ similar design approaches for the development of synthetic polymers with stimuli-responsive properties. The utilization of non-covalent interactions for the design of adhesives with advanced functionalities such as stimuli responsiveness, bonding and debonding on demand capability, surface selectivity or recyclability is a rapidly emerging subset of this field, which is summarized in this review.

Chapter 12:Mechanically Adaptive Nanocomposites Inspired by Sea Cucumbers

Abstract: Sea cucumbers own the fascinating capability to rapidly and reversibly change the stiffness of their dermis. This mechanical morphing is achieved through a distinctive architecture of the tissue, which is composed of a viscoelastic matrix that is reinforced with rigid collagen microfibrils. Neurosecretory proteins regulate the interactions among the latter, and thereby control the overall mechanical properties of the material. This architecture and functionality have been mimicked by researchers in artificial nanocomposites that feature similar, albeit significantly simplified, structure and mechanical morphing ability. The general design of such stimulus–responsive, mechanically adaptive materials involves a low-modulus polymer matrix and rigid, high-aspect ratio filler particles, which are arranged to form percolating networks within the polymer matrix. Stress transfer is controlled by switching the interactions among the nanofibers and/or between the nanofibers and the matrix polymer via an external stimulus. In first embodiments, water was employed to moderate hydrogen-bonding interactions in such nanocomposites, while more recent examples have been designed to respond to more specific stimuli, such as a change of the pH, or irradiation with ultraviolet light. This chapter provides an overview of the general design principles and materials embodiments of such sea-cucumber inspired materials.

Optically upconverting liquid-filled polymeric materials

Abstract: Optically upconverting liquid-filled polymeric materials that are made by curing a curable composition. The materials include a substantially liquid phase that serves to dissolve upconverting chromophores, optionally surfactants and an optionally cross-linked polymer matrix that retains or houses the liquid phase, provides mechanical stability, and offers some protection from oxygen. The optically upconverting liquid-filled polymeric materials have a phase-separated morphology. In preferred embodiments, the domains formed by the two phases are so small that light scattering is largely suppressed, which renders the upconverting liquid-filled polymeric materials largely transparent. The liquid phase provides a high mobility of the dissolved chromophores and the photophysical properties, such as the high upconversion quantum efficiency and the low excitation intensity threshold required to achieve upconverison, are thus more reminiscent of conventional or oxygen-free solutions than polymeric solids. These photophysical properties can be achieved by preparing the polymers under oxygen-free or, in preferred embodiments, under ambient conditions. The design principle introduced here to create upconverting liquid-filled polymeric materials is versatile and general; the liquid phase, the surfactants, the polymer matrix, and the upconverting chromophores can all readily be varied. Importantly, the optically upconverting liquid-filled polymeric materials can be prepared in a one-step process, which makes them preferable over other liquid-containing polymers that enable upconversion.