Circular Upcycling of Bottlebrush Thermosets.

The inability to re-process thermosets hinders their utility and sustainability. An ideal material should combine closed-loop recycling and upcycling capabilities. This trait is realized in polydimethylsiloxane bottlebrush networks using thermoreversible Diels-Alder cycloadditions to enable both reversible disassembly into a polymer melt and on-demand reconfiguration to an elastomer of either lower or higher stiffness. The crosslink density was tuned by loading the functionalized networks with a controlled fraction of dormant crosslinkers and crosslinker scavengers, such as furan-capped bis-maleimide and anthracene, respectively. The resulting modulus variations precisely followed the stoichiometry of activated furan and maleimide moieties, demonstrating the lack of side reactions during reprocessing. The presented circularity concept is independent from the backbone or side chain chemistry, making it potentially applicable to a wide range of brush-like polymers.

Study of the Interactions of Zwitterions and Carbon Nanotubes by Nonlinear Rheology in an Aqueous Environment.

Aqueous nanocomposite solutions of P(NIPAM) and P(NIPAM- co- N-(3-Sulfopropyl)- N-(methacryloxyethyl)- N,N-dimethylammonium betaine), a zwitterionic monomer with carbon nanotubes (CNT) as filler, were synthesized and characterized rheologically. While the influence of P(NIPAM) content and CNT content can be considered to be relatively minor, the introduction of a zwitterionic monomer (Zw) into the polymer leads to clear rheological traces of strong interactions between zwitterionic moieties and surface moieties on the CNTs, namely, a significantly lower nonlinearity limit and a lower modulus at high Zw contents and a higher modulus at intermediate contents due to adsorption of zwitterionic moieties on the CNT surface as well as a significantly lengthened time for the sample to adjust itself to the applied deformation, suggesting that the adsorbed polymer chains need to reorganize themselves significantly to accommodate to the applied strain γ0.

Solvent-free, supersoft and superelastic bottlebrush melts and networks.

Polymer gels are the only viable class of synthetic materials with a Young's modulus below 100 kPa conforming to biological applications, yet those gel properties require a solvent fraction. The presence of a solvent can lead to phase separation, evaporation and leakage on deformation, diminishing gel elasticity and eliciting inflammatory responses in any surrounding tissues. Here, we report solvent-free, supersoft and superelastic polymer melts and networks prepared from bottlebrush macromolecules. The brush-like architecture expands the diameter of the polymer chains, diluting their entanglements without markedly increasing stiffness. This adjustable interplay between chain diameter and stiffness makes it possible to tailor the network's elastic modulus and extensibility without the complications associated with a swollen gel. The bottlebrush melts and elastomers exhibit an unprecedented combination of low modulus (∼100 Pa), high strain at break (∼1,000%), and extraordinary elasticity, properties that are on par with those of designer gels.

Hydrogen bonding in aprotic solvents, a new strategy for gelation of bioinspired catecholic copolymers with N-isopropylamide.

Copolymers of N-isopropylacrylamide (NIPAM) and dopamine methacrylate can establish a reversible, self-healing 3D network in aprotic solvents based on hydrogen bonding. The reactivity and hydrogen bonding formation of catechol groups in copolymer chains are studied by UV-vis and (1) H NMR spectroscopy, while reversibility from sol to gel and inverse as well as self-healing properties are tested rheologically. The produced reversible organogel can self-encapsulate physically interacting or chemically bonded solutes such as drugs due to thermosensitivity of the used copolymer. This system offers dual-targeted and controlled drug delivery and release-by slowing down release kinetics by supramolecular bonding of the drug and by reducing diffusion rates due to modulus increase.

Graphene oxide/carbon nanotube composite hydrogels-versatile materials for microbial fuel cell applications.

Carbonaceous nanocomposite hydrogels are prepared with an aid of a suspension polymerization method and are used as anodes in microbial fuel cells (MFCs). (Poly N-Isopropylacrylamide) (PNIPAM) hydrogels filled with electrically conductive carbonaceous nanomaterials exhibit significantly higher MFC efficiencies than the unfilled hydrogel. The observed morphological images clearly show the homogeneous dispersion of carbon nanotubes (CNTs) and graphene oxide (GO) in the PNIPAM matrix. The complex formation of CNTs and GO with NIPAM is evidenced from the structural characterizations. The effectual MFC performances are influenced by combining the materials of interest (GO and CNTs) and are attributed to the high surface area, number of active sites, and improved electron-transfer processes. The obtained higher MFC efficiencies associated with an excellent durability of the prepared hydrogels open up new possibilities for MFC anode applications.

Network formation in graphene oxide composites with surface grafted PNIPAM chains in aqueous solution characterized by rheological experiments.

Poly N-isopropyl acrylamide (PNI) radically polymerized in aqueous solution in the presence of graphene oxide (GO) can significantly change the properties of the resulting solution from a regular polymer solution to a soft solid with a GO content of only 0.176 wt% (3 wt% with respect to PNI). However, these properties require the presence of both grafting and supramolecular interactions between polymer chains and hydrophilic groups on GO (-OH, -COOH), proven by Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), X-ray diffraction and spectroscopy (XRD) and Raman spectra. While very low GO-contents (below 0.05 wt%) only lead to a labile structure, which can be disassembled by shear, higher contents yield composites with solid-like characteristics. This is clearly evident from the rheological behaviour, which changes significantly at a GO content around 0.15 wt%. Intensive shearing destroys the weak network, which cannot reform quickly at lower GO-concentrations, while at intermediate concentrations, restructuring is fast. GO-contents of 0.176 wt% lead to a material behaviour, which almost perfectly recovers from small deformations (creep and creep recovery compliance almost match) but larger deformations lead to permanent damage to the sample.

Tissue-Adaptive Materials with Independently Regulated Modulus and Transition Temperature.

The ability of living species to transition between rigid and flexible shapes represents one of their survival mechanisms, which has been adopted by various human technologies. Such transition is especially desired in medical devices as rigidity facilitates the implantation process, while flexibility and softness favor biocompatibility with surrounding tissue. Traditional thermoplastics cannot match soft tissue mechanics, while gels leach into the body and alter their properties over time. Here, a single-component system with an unprecedented drop of Young's modulus by up to six orders of magnitude from the GPa to kPa level at a controlled temperature within 28-43 °C is demonstrated. This approach is based on brush-like polymer networks with crystallizable side chains, e.g., poly(valerolactone), affording independent control of melting temperature and Young's modulus by concurrently altering side chain length and crosslink density. Softening down to the tissue level at the physiological temperature allows the design of tissue-adaptive implants that can be inserted as rigid devices followed by matching the surrounding tissue mechanics at body temperature. This transition also enables thermally triggered release of embedded drugs for anti-inflammatory treatment.

Bottlebrush Elastomers: A New Platform for Freestanding Electroactuation.

Freestanding, single-component dielectric actuators are designed based on bottlebrush elastomers that enable giant reversible strokes at relatively low electric fields and altogether avoid preactuation mechanical manipulation. This materials design platform allows for independent tuning of actuator rigidity and elasticity over broad ranges without changing chemical composition, which opens new opportunities in soft-matter robotics.

The design of wrinkled microcapsules for enhancement of release rate.

Thermally expandable microcapsules (TEMs) with wrinkled shells are prepared by one-step suspension polymerization, allowing for encapsulation and controlled release of cargos. Wrinkling results from concurrent crosslinking of shell copolymers and vaporization of volatile reagents along with density increase upon polymerization. Through control of the vapor pressure of the reagents and systematic variation of the suspension composition, microcapsules with different degrees of wrinkling are prepared, ranging from locally dimpled to highly crumpled morphologies. The corresponding increase of the surface-to-volume ratio results in increasing release rate of encapsulated oil red dye as a model cargo. As such, in addition to shell thickness and radius, the wrinkleness provides an effective control parameter for adjusting the release rate. The wrinkled microcapsules with a large surface-to-volume ratio may find applications in drug delivery, chemicals scavenging, and self-healing materials.

Weak Hydrogen Bonding Enables Hard, Strong, Tough, and Elastic Hydrogels.

A new type of "rigid and tough" hydrogel with excellent elasticity is designed by dense clustering of hydrogen bonds within a loose chemical network. The resultant hydrogel exhibits a good combination of high modulus (28 MPa), toughness (9300 J m(-3) ), extensibility (800%), and tensile stress (2 MPa). Furthermore, the gel displays good fatigue-resistance and complete and extremely fast recovery of shape and mechanical properties (3 min at 37°C).

Mussel-inspired pH-triggered reversible foamed multi-responsive gel--the surprising effect of water.

Novel covalent gels were prepared by complexation of polymer-bound catechols with NaBH4 at pH ≈ 9. These gels can absorb humidity, which changes the catechol-borate covalent bonds from irreversible to reversible. Furthermore, humidity induces self-healing, proven by rheological data.