Controlling the Relaxation Dynamics of Polymer Networks by Combining Associative and Dissociative Dynamic Covalent Bonds.

Dynamic polymer networks offer a promising solution to key challenges in polymers such as recyclability, processability, and damage repair. However, the trade-off between combining facile processability, fast self-healing, and high creep resistance remains a major obstacle to implementation. To overcome this, two very distinct dynamic covalent chemistries, Diels-Alder and transesterification, is combined in a single network. The resulting dual dynamic networks offer an unprecedented set of properties and control over the relaxation times. The system decouples the relaxation dynamics of the network from the spatial motifs, and the tuning of the ratio between chemistries enables to control of the relaxation dynamics over six orders of magnitude. Taking advantage of this control, the composition and rheological behavior is optimized to drastically improve the resolution for extrusion-based additive manufacturing of dynamic covalent networks. Additionally, two well-defined and separated stress relaxation peaks are observed at compositions close to 50% of each dynamic chemistry, accentuating the double character of the system's relaxation dynamics. This atypical situation, enables to preparation of self-healing materials with negligible creep, and with shape-memory properties solely leveraging the two distinct relaxation dynamics, instead of the glass transition temperature or the melting point.

Diels-Alder Network Blends as Self-Healing Encapsulants for Liquid Metal-Based Stretchable Electronics.

Two dynamic covalent networks based on the Diels-Alder reaction were blended to exploit the properties of the dissimilar polymer backbones. Furan-functionalized polyether amines based on poly(propylene oxide) (PPO) FD4000 and polydimethylsiloxane (PDMS) FS5000 were mixed in a common solvent and reversibly cross-linked with the same bismaleimide DPBM. The morphology of the phase-separated blends is primarily controlled by the concentration of backbones. Increasing the PDMS content of the blends results in a dilute droplet morphology at 25 wt %, with a growing size and concentration of droplets and the formation of two separate PPO- and PDMS-rich layers at 50 wt %. Further increasing the PDMS content to 75 wt % leads to larger droplets and a thicker layer of the secondary phase. The hydrophobic PDMS phase creates a barrier against water, while the more hydrophilic PPO phase enhances the resistance against oxygen diffusion. Lowering the maleimide-to-furan stoichiometric ratio resulted in a decrease in cross-link density and thus more flexible and stretchable encapsulants. Changes in the stoichiometric ratio also affected the phase morphology due to resulting changes in phase separation and network formation kinetics. Lowering the stoichiometric ratio also resulted in enhanced self-healing properties of 96% at room temperature as a consequence of the increased chain mobility in the blended networks. The self-healing blends were used to encapsulate liquid metal circuits to create stretchable strain sensors with a linear electro-mechanical response without much drift or hysteresis, which could be efficiently recovered by 90% after the damage-healing cycles.

Variable sensitivity multimaterial robotic e-skin combining electronic and ionic conductivity using electrical impedance tomography.

Electronic skins (e-skins) aim to replicate the capabilities of human skin by integrating electronic components and advanced materials into a flexible, thin, and stretchable substrate. Electrical impedance tomography (EIT) has recently been adopted in the area of e-skin thanks to its robustness and simplicity of fabrication compared to previous methods. However, the most common EIT configurations have limitations in terms of low sensitivities in areas far from the electrodes. Here we combine two piezoresistive materials with different conductivities and charge carriers, creating anisotropy in the sensitive part of the e-skin. The bottom layer consists of an ionically conducting hydrogel, while the top layer is a self-healing composite that conducts electrons through a percolating carbon black network. By changing the pattern of the top layer, the resulting distribution of currents in the e-skin can be tuned to locally adapt the sensitivity. This approach can be used to biomimetically adjust the sensitivities of different regions of the skin. It was demonstrated how the sensitivity increased by 500% and the localization error reduced by 40% compared to the homogeneous case, eliminating the lower sensitivity regions. This principle enables integrating the various sensing capabilities of our skins into complex 3D geometries. In addition, both layers of the developed e-skin have self-healing capabilities, showing no statistically significant difference in localization performance before the damage and after healing. The self-healing bilayer e-skin could recover full sensing capabilities after healing of severe damage.

Fast Self-Healing at Room Temperature in Diels-Alder Elastomers.

Despite being primarily categorized as non-autonomous self-healing polymers, we demonstrate the ability of Diels-Alder polymers to heal macroscopic damages at room temperature, resulting in complete restoration of their mechanical properties within a few hours. Moreover, we observe immediate partial recovery, occurring mere minutes after reuniting the fractured surfaces. This fast room-temperature healing is accomplished by employing an off-stoichiometric maleimide-to-furan ratio in the polymer network. Through an extensive investigation of seven Diels-Alder polymers, the influence of crosslink density on self-healing, thermal, and (thermo-)mechanical performance was thoroughly examined. Crosslink density variations were achieved by adjusting the molecular weight of the monomers or utilizing the off-stoichiometric maleimide-to-furan ratio. Quasistatic tensile testing, dynamic mechanical analysis, dynamic rheometry, differential scanning calorimetry, and thermogravimetric analysis were employed to evaluate the individual effects of these parameters on material performance. While lowering the crosslink density in the polymer network via decreasing the off-stoichiometric ratio demonstrated the greatest acceleration of healing, it also led to a slight decrease in (dynamic) mechanical performance. On the other hand, reducing crosslink density using longer monomers resulted in faster healing, albeit to a lesser extent, while maintaining the (dynamic) mechanical performance.

Assisted damage closure and healing in soft robots by shape memory alloy wires.

Self-healing soft robots show enormous potential to recover functional performance after healing the damages. However, healing in these systems is limited by the recontact of the fracture surfaces. This paper presents for the first time a shape memory alloy (SMA) wire-reinforced soft bending actuator made out of a castor oil-based self-healing polymer, with the incorporated ability to recover from large incisions via shape memory assisted healing. The integrated SMA wires serve three major purposes; (i) Large incisions are closed by contraction of the current-activated SMA wires that are integrated into the chamber. These pull the fracture surfaces into contact, enabling the healing. (ii) The heat generated during the activation of the SMA wires is synergistically exploited for accelerating the healing. (iii) Lastly, during pneumatic actuation, the wires constrain radial expansion and one-side longitudinal extension of the soft chamber, effectuating the desired actuator bending motion. This novel approach of healing is studied via mechanical and ultrasound tests on the specimen level, as well as via bending characterization of the pneumatic robot in multiple damage healing cycles. This technology allows soft robots to become more independent in terms of their self-healing capabilities from human intervention.

An Interdisciplinary Tutorial: A Self-Healing Soft Finger with Embedded Sensor.

In the field of soft robotics, knowledge of material science is becoming more and more important. However, many researchers have a background in only one of both domains. To aid the understanding of the other domain, this tutorial describes the complete process from polymer synthesis over fabrication to testing of a soft finger. Enough background is provided during the tutorial such that researchers from both fields can understand and sharpen their knowledge. Self-healing polymers are used in this tutorial, showing that these polymers that were once a specialty, have become accessible for broader use. The use of self-healing polymers allows soft robots to recover from fatal damage, as shown in this tutorial, which increases their lifespan significantly.

Processing of Self-Healing Polymers for Soft Robotics.

Soft robots are, due to their softness, inherently safe and adapt well to unstructured environments. However, they are prone to various damage types. Self-healing polymers address this vulnerability. Self-healing soft robots can recover completely from macroscopic damage, extending their lifetime. For developing healable soft robots, various formative and additive manufacturing methods have been exploited to shape self-healing polymers into complex structures. Additionally, several novel manufacturing techniques, noted as (re)assembly binding techniques that are specific to self-healing polymers, have been created. Herein, the wide variety of processing techniques of self-healing polymers for robotics available in the literature is reviewed, and limitations and opportunities discussed thoroughly. Based on defined requirements for soft robots, these techniques are critically compared and validated. A strong focus is drawn to the reversible covalent and (physico)chemical cross-links present in the self-healing polymers that do not only endow healability to the resulting soft robotic components, but are also beneficial in many manufacturing techniques. They solve current obstacles in soft robots, including the formation of robust multi-material parts, recyclability, and stress relaxation. This review bridges two promising research fields, and guides the reader toward selecting a suitable processing method based on a self-healing polymer and the intended soft robotics application.

The Influence of the Furan and Maleimide Stoichiometry on the Thermoreversible Diels-Alder Network Polymerization.

In recent work, the thermoreversible Diels-Alder reaction between furan and maleimide functional groups has been studied extensively in the context of self-healing elastomers and thermosets. To elaborate the influence of the stoichiometric ratio between the maleimide and furan reactive groups on the thermomechanical properties and viscoelastic behavior of formed reversible covalent polymer networks, a series of Diels-Alder-based networks with different stoichiometric ratios was synthesized. Differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA) and dynamic rheology measurements were performed on the reversible polymer networks, to relate the reversible network structure to the material properties and reactivity. Such knowledge allows the design and optimization of the thermomechanical behavior of the reversible networks for intended applications. Lowering the maleimide-to-furan ratio creates a deficit of maleimide functional groups, resulting in a decrease in the crosslink density of the system, and a consequent decrease in the glass transition temperature, Young's modulus, and gel transition temperature. The excess of unreacted furan in the system results in faster reaction and healing kinetics and a shift of the reaction equilibrium.

Substituent effect on the thermophysical properties and thermal dissociation behaviour of 9-substituted anthracene derivatives.

The chemical structure and location of substituents on anthracene derivatives influence the electron balance of the aromatic system, thus determining the wavelengths at which light is absorbed, which results in the photochemically induced dimerization or monomerization. Here, the thermal dissociation kinetics of 7 photodimers of 9-substituted anthracene derivatives are studied using a combination of spectroscopic and calorimetric techniques in the condensed state and compared to scarce literature data on thermal dissociation of other anthracene derivatives. The length and chemical structure of the substituent chains have a clear impact on the melting temperatures of the anthracene derivatives and corresponding photodimers. The crystallinity of the photodimers and monomers in turn influences the thermal dissociation kinetics. The thermal dissociation behaviour and previously published photochemistry data are related to the electronic effects of the substituents by means of the Hammett parameters. Stronger electron-withdrawing effects result in larger red shifts of the maximum wavelength λmax for the photodimerization of the anthracene derivatives. It is also shown that for the studied substitutions on the 9-position of anthracene, the higher the magnitude of the electronic effect - both electron-donating and electron-withdrawing - the faster the thermal dissociation kinetics and thus the lower the thermal stability. The strong electronic effects of the substituents on the thermal and photochemical reactivity of the anthracene derivatives and their photodimers allow tuning of the thermal or photochemical responsiveness, e.g. for polymer networks.

Thermal dissociation of anthracene photodimers in the condensed state: kinetic evaluation and complex phase behaviour.

Thermally and photochemically reversible functional groups, such as photodimers of anthracene derivatives, offer interesting stimuli-responsive behaviour. To evaluate their potential for application in reversible polymer networks, accurate kinetic parameters and knowledge of their thermophysical behaviour are required. Accurate kinetic studies of the thermal dissociation of the photodimers in the condensed state, thus without the influence of solvents on their reactivity, is still lacking. A methodology was set up to accurately evaluate the chemical reaction kinetics and complex phase behaviour during the thermal dissociation of photodimers into their corresponding monomers. Temperature-controlled time-resolved FTIR spectroscopy was used to determine the reaction progress, while non-isothermal DSC measurements were used to study the thermophysical changes, resulting from the thermal dissociation reaction. The thermal dissociation behaviour in the condensed state is more challenging than in the solution state due to the crystallinity of the dimers, stabilizing the dimers and thus slowing down the initial dissociation rates. Distinctly different sets of kinetic parameters were found for the dissociation from the molten and the crystalline state. For experiments performed below the melting temperature of the photodimer, the reaction rate changes abruptly as the dimer is partly dissociated and partly dissolved into the formed monomer. This methodology provides an accurate assessment of the reaction kinetics with detailed knowledge about the complex phase behaviour of the mixture of the anthracene photodimer and monomer during thermal dissociation.

UV-Curable Biobased Polyacrylates Based on a Multifunctional Monomer Derived from Furfural.

The controlled polymerization of a new biobased monomer, 4-oxocyclopent-2-en-1-yl acrylate (4CPA), was established via reversible addition-fragmentation chain transfer (RAFT) (co)polymerization to yield polymers bearing pendent cyclopentenone units. 4CPA contains two reactive functionalities, namely, a vinyl group and an internal double bond, and is an unsymmetrical monomer. Therefore, competition between the internal double bond and the vinyl group eventually leads to gel formation. With RAFT polymerization, when aiming for a degree of polymerization (DP) of 100, maximum 4CPA conversions of the vinyl group between 19.0 and 45.2% were obtained without gel formation or extensive broadening of the dispersity. When the same conditions were applied in the copolymerization of 4CPA with lauryl acrylate (LA), methyl acrylate (MA), and isobornyl acrylate, 4CPA conversions of the vinyl group between 63 and 95% were reached. The additional functionality of 4CPA in copolymers was demonstrated by model studies with 4-oxocyclopent-2-en-1-yl acetate (1), which readily dimerized under UV light via [2 + 2] photocyclodimerization. First-principles quantum mechanical simulations supported the experimental observations made in NMR. Based on the calculated energetics and chemical shifts, a mixture of head-to-head and head-to-tail dimers of (1) were identified. Using the dimerization mechanism, solvent-cast LA and MA copolymers containing 30 mol % 4CPA were cross-linked under UV light to obtain thin films. The cross-linked films were characterized by dynamic scanning calorimetry, dynamic mechanical analysis, IR, and swelling experiments. This is the first case where 4CPA is described as a monomer for functional biobased polymers that can undergo additional UV curing via photodimerization.

Additive Manufacturing for Self-Healing Soft Robots.

The field of self-healing soft robots was initiated a few years ago. A healing ability can be integrated in soft robots by manufacturing their soft membranes out of synthetic self-healing polymers, more specifically elastomeric Diels-Alder (DA) networks. As such they can recover completely from macroscopic damage, including scratches, cuts, and ruptures. Before this research, these robots were manufactured using a technique named "shaping-through-folding-and-self-healing." This technique requires extensive manual labor, is relatively slow, and does not allow for complex shapes. In this article, an additive manufacturing methodology, fused filament fabrication, is developed for the thermoreversible DA polymers, and the approach is validated on a soft robotic gripper. The reversibility of their network permits manufacturing these flexible self-healing polymers through reactive printing into the complex shapes required in soft robotics. The degree of freedom in the design of soft robotics that this new manufacturing technique offers is illustrated through the construction of adaptive DHAS gripper fingers, based on the design by FESTO. Being constructed out of self-healing soft flexible polymer, the fingers can recover entirely from large cuts, tears, and punctures. This is highlighted through various damage-heal cycles.

Aromatic sulfonation with sulfur trioxide: mechanism and kinetic model.

Electrophilic aromatic sulfonation of benzene with sulfur trioxide is studied with ab initio molecular dynamics simulations in gas phase, and in explicit noncomplexing (CCl3F) and complexing (CH3NO2) solvent models. We investigate different possible reaction pathways, the number of SO3 molecules participating in the reaction, and the influence of the solvent. Our simulations confirm the existence of a low-energy concerted pathway with formation of a cyclic transition state with two SO3 molecules. Based on the simulation results, we propose a sequence of elementary reaction steps and a kinetic model compatible with experimental data. Furthermore, a new alternative reaction pathway is proposed in complexing solvent, involving two SO3 and one CH3NO2.

Towards multifunctional cellulosic fabric: UV photo-reduction and in-situ synthesis of silver nanoparticles into cellulose fabrics.

Herein, the highly multifunctional cotton fabric surfaces were designed with excellent coloration, UV-protection function, and antimicrobial activity. These multifunctional functions were developed by in-situ synthesis of silver nanoparticles (Ag NPs) into the cotton fabric surface using a simple green one-pot "UV-reduction" method. Cotton fabrics were pretreated with non-anionic detergent, immersed into alcoholic silver nitrate solution (concentration ranging from 100 to 500ppm), squeezed to remove excess solution and then exposed to UV-irradiation (range 320-400nm) for 1h. The influence UV-irradiation on the thermal, chemical, optical and biological properties of the cotton fabric surface was discussed in details. The UV-irradiation promotes reducing of Ag+ ions and the cotton fabrics act as seed medium for Ag NPs formation by "heterogeneous nucleation". Increasing Ag+ concentration (from 100 to 500ppm) results in Ag NPs of particle size (distribution) of 50-100nm. Interestingly, the Ag NPs exhibited different localized surface Plasmon resonance properties causing a coloration of the cotton fabrics with different color shades ranging from bright to dark brown with excellent color fastness properties. The treated cotton fabrics also show high protecting functions against UV-transmission (reduction of 65%) and Escherichia coli growth (99%). The side-effects of the UV-reduction process are further investigated.

Self-healing soft pneumatic robots.

Inspired by the compliance found in many organisms, soft robots are made almost entirely out of flexible, soft material, making them suitable for applications in uncertain, dynamic task environments, including safe human-robot interactions. Their intrinsic compliance absorbs shocks and protects them against mechanical impacts. However, the soft materials used for their construction are highly susceptible to damage, such as cuts and perforations caused by sharp objects present in the uncontrolled and unpredictable environments they operate in. In this research, we propose to construct soft robotics entirely out of self-healing elastomers. On the basis of healing capacities found in nature, these polymers are given the ability to heal microscopic and macroscopic damage. Diels-Alder polymers, being thermoreversible covalent networks, were used to develop three applications of self-healing soft pneumatic actuators (a soft gripper, a soft hand, and artificial muscles). Soft pneumatic actuators commonly experience perforations and leaks due to excessive pressures or wear during operation. All three prototypes were designed using finite element modeling and mechanically characterized. The manufacturing method of the actuators exploits the self-healing behavior of the materials, which can be recycled. Realistic macroscopic damage could be healed entirely using a mild heat treatment. At the location of the scar, no weak spots were created, and the full performance of the actuators was nearly completely recovered after healing.

Seed-Mediated Hot-Injection Synthesis of Tiny Ag Nanocrystals on Nanoscale Solid Supports and Reaction Mechanism.

Controlling the size and shape of noble Ag nanocrystals (NCs) is of great interest because of their unique size- and shape-dependent properties, especially below 20 nm, and because of interesting applications in drug delivery, sensing, and catalysis. However, the high surface energy and tendency of these tiny NCs to aggregate deteriorates their unique properties and limits their applications. To avoid the aggregation of Ag NCs and improve their performance, we report a seed-mediated hot injection approach to synthesize highly dispersed tiny Ag NCs on a nanosized solid CaCO3 support. This simple, low-cost, and effective chemical approach allows for synthesizing highly uniform Ag NCs (∼10 nm) on the surface of presynthesized CaCO3 single NCs (∼52 nm) without any aggregation of the Ag NCs. Viscose fibers were coated with the Ag@CaCO3 composite nanoparticles (NPs) produced, as well as with ∼126 nm Ag NPs for reference. The Ag@CaCO3 composite NPs show excellent UV protection and antibacterial activity against Escherichia coli. In addition, they give a satin sheen gold to a dark gold color to the viscose fibers, while the Ag NPs (∼126 nm) result in a silver color. The proposed synthesis approach is highly versatile and applicable for many other noble metals, like Au or Pt.

Monitoring the morphology development of polymer-monolithic stationary phases by thermal analysis.

Thermal analysis and SEM were employed to gain insights in the different stages of morphology development and the thermal properties of polymer-monolithic stationary phases. The studied system was a thermally initiated free-radical copolymerization reaction at 70°C of styrene and divinylbenzene in the presence of tetrahydrofuran and 1-decanol. The key events in the early stages of morphology development are initiation, chain growth, branching, and cyclization, leading to microgel particles. Interparticle reactions through pendant vinyl groups lead to the formation of microgel clusters. The rapid increase in molecular weight and cross-link density of the microgel clusters causes a reaction-induced phase separation, and the formation of a macroscopic network of interconnected globules was observed (macrogelation) at around 45 min. After 3 h or 65% conversion, a space-filling macroporous monolithic network was observed. Afterwards, mainly growth of existing globules takes place, reducing the macropore size. The porogen ratio affects the timing of the reaction-induced phase separation, strongly influencing the morphology of the polymer material. The use of a mixture of divinylbenzene isomers yielded a monolithic material that is less cross-linked at the surface compared to the central part of the polymer backbone due to copolymerization-composition drift. The less cross-linked outer layer starts devitrifying at 100°C.

Quantitative analysis of polymer mixtures in solution by pulsed field-gradient spin echo NMR spectroscopy.

Pulsed Field-Gradient Spin Echo (PGSE) NMR, which associates to a spectral dimension the measure of diffusion coefficients, is a convenient technique for mixture analysis. Unfortunately, because of relaxation, the quantification of mixtures by PGSE NMR is far from straightforward for mixtures with strong spectral overlap. Antalek (J. Am. Chem. Soc. 128 (2006) 8402-8403) proposed a quantification strategy based on DECRA analysis and extrapolation to zero of the diffusion delay. More recently, Barrère et al. (J. Magn. Reson. 216 (2012) 201-208) presented a new strategy based also on DECRA and on the renormalization of the intensities using estimates of the T1 and T2 relaxation times. Here we report an alternative quantification approach in which the fractions are obtained by analyzing the PGSE attenuation profile with a general Stejskal-Tanner equation that explicitly includes the relaxation effects. The required values of T1 and T2 relaxation times are either independently measured with conventional sequences or determined, along with the fractions and the diffusion coefficients, from the simultaneous analysis of up to 6 PGSE data sets recorded with different diffusion delays. This method yields errors lower than 3% for the fractions, even for complete spectral overlap, as demonstrated on model binary and ternary mixtures of polystyrene in the case of a convection compensating double stimulated echo (DSTE) sequence.

Phase behavior of PCBM blends with different conjugated polymers.

In this work the phase behavior of [6,6]-phenyl C(61)-butyric acid methyl ester (PCBM) blends with different poly(phenylene vinylene) (PPV) samples is investigated by means of standard and modulated temperature differential scanning calorimetry (DSC and MTDSC) and rapid heat-cool calorimetry (RHC). The PPV conjugated polymers include poly(2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylene vinylene) (MDMO-PPV), High T(g)-PPV which is a copolymer, and poly((2-methoxy-5-phenethoxy)-1,4-phenylene vinylene) (MPE-PPV). Comparisons of these PPV:PCBM blends with regioregular poly(3-hexyl thiophene) (P3HT):PCBM blends are made to see the different component miscibilities among different blends. The occurrence of liquid-liquid phase separation in the molten state of MDMO-PPV:PCBM and High T(g)-PPV:PCBM blends is indicated by the coexistence of double glass transitions for blends with a PCBM weight fraction of around 80 wt%. This is in contrast to the P3HT:PCBM blends where no phase separation is observed. Due to its high cooling rate (about 2000 K min(-1)), RHC proves to be a useful tool to investigate the phase separation in PPV:PCBM blends through the glass transition of these crystallizable blends. P3HT is found to have much higher thermal stability than the PPV samples.

Dynamics of the crystal to plastic crystal transition in the hydrogen bonded N-isopropylpropionamide.

N-Isopropylpropionamide (NiPPA), which can self-associate via hydrogen bonds, was found to undergo a solid-solid transition as identified by DSC and X-ray diffraction. Below the melting temperature of 51 °C NIPPA adopts a plastic crystalline state with a tetragonal unit cell until it transforms into an ordered crystal with a monoclinic structure at temperatures ≤10 °C. Dielectric spectroscopy was used to characterize the dynamics of the system, determining the activation parameters for the plastic to crystalline phase transition. The activation enthalpy is relatively high, as expected for a system that involves hydrogen bonds. However, most of the activation energy as the plastic phase assumes a more crystalline state is due to the activation entropy, suggesting that the increased cooperativity observed in the relaxation processes is due to a steric locking of the molecules.