Self-Healing in Supramolecular Polymers

A Critical Approach to Polymer Dynamics in Supramolecular Polymers

Over the past few years, the concurrent (1) development of polymer synthesis and (2) introduction of new mathematical models for polymer dynamics have evolved the classical framework for polymer dynamics once established by Doi-Edwards/de Gennes. Although the analysis of supramolecular polymer dynamics based on linear rheology has improved a lot recently, there are a large number of insecurities behind the conclusions, which originate from the complexity of these novel systems. The interdependent effect of supramolecular entities (stickers) and chain dynamics can be overwhelming depending on the type and location of stickers as well as the architecture and chemistry of polymers. This Perspective illustrates these parameters and strives to determine what is still missing and has to be improved in the future works.

Dynamic plant-derived polysaccharide-based hydrogels

Plant-derived polysaccharides are widely used to fabricate hydrogels because of their ease of gelation and functionalization, plus exceptional biological properties. As an example, nanocellulose is a suitable candidate to fabricate hydrogels for tissue engineering applications due to its enhanced mechanical and biological properties. However, hydrogels are prone to permanent failure whilst under load without the ability to reform their networks once damaged. Recently, considerable efforts are being made to fabricate dynamic hydrogels via installation of reversible crosslinks within their networks. In this paper, we review the developments in the design of dynamic hydrogels from plant-derived polysaccharides, and discuss their applications in tissue engineering, sensors, bioelectronics devices, etc. The main goal of the paper is to elucidate how the network design of hydrogels can influence their dynamic properties: self-healing and self-recovery. Complementary to this, current challenges and prospects of dynamic plant-derived hydrogels are discussed.

What dielectric spectroscopy can tell us about supramolecular networks⋆

Polymers which can form supramolecular networks are a promising class of materials to provide highly sought-after properties such as self-healing, enhanced mechanical strength, super-stretchability as well as easy recyclability. However, due to the vast range of possible chemical structures it is very demanding to optimize these materials for the desired performance. Consequently, a detailed understanding of the molecular processes that govern the macroscopic properties is paramount to their technological application. Here we discuss some telechelic model systems with hydrogen-bonding end groups and how dielectric spectroscopy in combination with linear oscillatory shear rheology helped to understand the association mechanism on a molecular scale, and verify the model of bond-lifetime renormalization. Furthermore, we analyze a limitation of these H-bonding polymers, namely that there is a trade-off between high plateau modulus and long terminal relaxation time --both cannot be maximized at the same time. Finally, we show how more complex end groups phase separate from the main chain melt and thus lead to a more sophisticated rheological behavior which can overcome that limitation.

Functional materials with self-healing properties: a review

Self-healing materials (SHMs) have been a research hot topic in recent years owing to their greatly improved longevity and safety in practical applications. Recently, research on SHMs has gradually expanded from structural materials to functional materials. Functional materials with self-healing properties (FMSH) require simultaneous repairing not only of the mechanical properties but of the functionalities from damaged cracks or wounds. It is more challenging to introduce both self-healing properties and a particular functionality to materials owing to the difficulties of preparing the materials and their more complex healing mechanism. Herein, we summarize the recent progress that has been made in FMSH, put forward insights from the perspectives of material preparation and healing mechanisms and highlight future developments for FMSH.

Dynamic cell–cell adhesion mediated by pericellular matrix interaction – a hypothesis

Cell–cell adhesion strength, measured as tissue surface tension, spans an enormous 1000-fold range when different cell types are compared. However, the examination of basic mechanical principles of cell adhesion indicates that cadherin-based and related mechanisms are not able to promote the high-strength adhesion experimentally observed in many late embryonic or malignant tissues. Therefore, the hypothesis is explored that the interaction of the pericellular matrices of cells generates strong adhesion by a mechanism akin to the self-adhesion/self-healing of dynamically cross-linked hydrogels. Quantitative data from biofilm matrices support this model. The mechanism links tissue surface tension to pericellular matrix stiffness. Moreover, it explains the wide, matrix-filled spaces around cells in liquid-like, yet highly cohesive, tissues, and it rehabilitates aspects of the original interpretation of classical cell sorting experiments, as expressed in Steinberg's differential adhesion hypothesis: that quantitative differences in adhesion energies between cells are sufficient to drive sorting.

Reversible Self-Healing Carbon-Based Nanocomposites for Structural Applications

Reversible Hydrogen Bonds (RHB) have been explored to confer self-healing function to multifunctional nanocomposites. This study has been carried out through a sequence of different steps. Hydrogen bonding moieties, with the intrinsic ability to simultaneously perform the functions of both hydrogen donors and acceptors, have been covalently attached to the walls of carbon nanotubes. The epoxy matrix has been modified to adapt the formulation for hosting self-healing mechanisms. It has been toughened with different percentages of rubber phase covalently linked to the epoxy precursor. The most performant matrix, from the mechanical point of view, has been chosen for the incorporation of MWCNTs. Self-healing performance and electrical conductivities have been studied. The comparison of data related to the properties of nanocomposites containing incorporated functionalized and nonfunctionalized MWCNTs has been performed. The values of the electrical conductivity of the self-healing nanocomposites, containing 2.0% by weight of functionalized multiwalled carbon nanotubes (MWCNTs), range between 6.76 × 10-3 S/m and 3.77 × 10-2 S/m, depending on the nature of the functional group. Curing degrees, glass transition temperatures, and storage moduli of the formulated multifunctional nanocomposites prove their potential for application as functional structural materials.

Arbitrarily 3D Configurable Hygroscopic Robots with a Covalent-Noncovalent Interpenetrating Network and Self-Healing Ability

Soft materials that can reversibly transform shape in response to moisture have applications in diverse areas such as soft robotics and biomedicine. However, the design of a structurally transformable or mechanically self-healing version of such a humidity-responsive material, which can arbitrarily change shape and reconfigure its 3D structures remains challenging. Here, by drawing inspiration from a covalent-noncovalent network, an elaborately designed biopolyester is developed that features a simple hygroscopic actuation mechanism, straightforward manufacturability at low ambient temperature (≤35 °C), fast and stable response, robust mechanical properties, and excellent self-healing ability. Diverse functions derived from various 3D shapes that can grasp, swing, close-open, lift, or transport an object are further demonstrated. This strategy of easy-to-process 3D structured self-healing actuators is expected to combine with other actuation mechanisms to extend new possibilities in diverse practical applications.

Anilinium Salts in Polymer Networks for Materials with Mechanical Stability and Mild Thermally Induced Dynamic Properties

Dynamic nucleophilic exchange of quaternary anilinium salts has been incorporated into rehealable and malleable polymeric materials that can be activated under mild (60 °C) thermal stimulus. The mechanism of dynamic exchange between quaternary anilinium salt and free aniline was assessed in small-molecule model experiments. The dynamic exchange was found to be dissociative in nature, due to the indirect S_N2 mechanism, where initially the bromide anion attacks the anilinium salt to generate an alkyl bromide which undergoes subsequent attack by a free aniline group. A quaternary anilinium-based cross-linker was synthesized to act as dynamic linkages in the polymer network. Cross-linked polymeric materials showed thermoresponsive rehealing and malleability properties at 60 °C along with being resistant to irreversible creep under ambient conditions. The use of anilinium salts enables dynamic exchange to occur with significantly milder thermal stimulus than other comparable materials, while maintaining mechanical stability.

Synthesis of self-healable waterborne isocyanate-free poly(hydroxyurethane)-based supramolecular networks by ionic interactions

A novel synthetic strategy to isocyanate-free supramolecular polyhydroxyurethanes based on ionic interactions is described.

Tuning thermoresponsive network materials through macromolecular architecture and dynamic thiol-Michael chemistry

Synthesis of precision polymers crosslinked with dynamic thiol-Michael adducts is developed, and the materials are characterized to determine structure–property relationships.