Dynamic Bond Exchange Research Articles

Modeling self-healing behavior of vitrimers using molecular dynamics with dynamic cross-linking capability

Vitrimers are a special class of polymers that undergo dynamic cross-linking under thermal stimuli. Their ability to exchange covalent bonds can be harnessed to mitigate damage in a composite or to achieve recyclable aerospace composites. This work addresses the primary challenge of modeling dynamic cross-linking reactions in vitrimers during thermomechanical loading. Dynamic bond exchange reaction probability change during heating and its effect on dilatometric and mechanical response are simulated for the first time in large scale molecular dynamics simulations. Healing of damage under thermal cycling is computed with mechanical properties predicted before and after self–healing.

Recyclable and Self-Repairable Fluid-Driven Liquid Crystal Elastomer Actuator.

Liquid crystal elastomer (LCE) is a newly emerging soft actuating material that has been extensively explored for building novel soft robots and diverse active devices, thanks to its large actuation stress and strain, high work density, and versatile actuation modes. However, there have also been several widely recognized limitations of LCE-based actuators for practical applications, including slow response and narrow range of operation temperature. Herein, we develop fluid-driven disulfide LCE actuators through facile laminate manufacturing enabled by a dynamic bond exchange reaction. Because of the merits of the active heating/cooling mechanism of the fluidic structure, this newly developed disulfide LCE actuator can generate large cyclic actuation at a frequency around 1 Hz and can operate in a wide range of temperatures. The unique combination of the fluidic structure design and the dynamic covalent bonds in the elastomer has also enabled the full recyclability and self-repairability of the actuator. Using the newly developed actuator as building block, we further constructed soft robotic systems that can realize manipulating and programmable movement. The design principle demonstrated in the current work opens a promising avenue for exploring more novel applications of LCE-based soft actuators.

Stress Relaxation via Covalent Dynamic Bonds in Nanogel-Containing Thiol-Ene Resins.

Functional nanogels are attractive additives for use in polymer composites. In this study, nanogels with internal allyl sulfide moieties throughout their network structure were prepared via a thiol-Michael addition reaction. The excess thiol-functionalized nanogels were less than 60 nm as discrete particles but act as room-temperature liquids in the bulk state. The reactive nanogels can be dispersed in and swollen by a thiol-ene matrix resin, which upon photopolymerization yields dramatically decreased levels of polymerization shrinkage stress. Furthermore, the postcured nanogel-modified polymers effectively relaxed applied stresses as well as enhanced toughness during exposure to a UV light source that activated the addition-fragmentation as a means for dynamic bond exchange. These nanogels provide a generic approach to introduce adaptable network performance that significantly improves a number of key properties of glassy cross-linked polymer.

Siloxane crosslinks with dynamic bond exchange enable shape programming in liquid-crystalline elastomers

Liquid crystalline elastomers (LCE) undergo reversible shape changes in response to stimuli, which enables a wide range of smart applications, in soft robotics, adhesive systems or biomedical medical devices. In this study, we introduce a new dynamic covalent chemistry based on siloxane equilibrium exchange into the LCE to enable processing (director alignment, remolding, and welding). Unlike the traditional siloxane based LCE, which were produced by reaction schemes with irreversible bonds (e.g. hydrosilylation), here we use a much more robust reaction (thiol-acrylate/thiol-ene ‘double-click’ chemistry) to obtain highly uniform dynamically crosslinked networks. Combining the siloxane crosslinker with click chemistry produces exchangeable LCE (xLCE) with tunable properties, low glass transition (−30 °C), controllable nematic to isotropic transition (33 to 70 °C), and a very high vitrification temperature (up to 250 °C). Accordingly, this class of dynamically crosslinked xLCE shows unprecedented thermal stability within the working temperature range (−50 to 140 °C), over many thermal actuation cycles without any creep. Finally, multiple xLCE sharing the same siloxane exchangeable bonds can be welded into single continuous structures to allow for composite materials that sequentially and reversibly undergo multiple phase transformations in different sections of the sample.

Facile Preparation of Polyimine Vitrimers with Enhanced Creep Resistance and Thermal and Mechanical Properties via Metal Coordination

Vitrimers undergoing dynamic bond exchange enable reprocessing and recycle of thermosets. However, vitrimers are susceptible to creep, leading to their poor dimensional stability, which limits thei...

Accelerating dynamic exchange and self-healing using mechanical forces in crosslinked polymers

Surprisingly, a few seconds–minutes of compression at room temperature can increase the rate of dynamic bond exchange as measured by better self-healing, even for thermoresponsive dynamic bonds which do not exchange under ambient conditions.

Impact of Dynamic Bond Concentration on the Viscoelastic and Mechanical Properties of Dynamic Poly(alkylurea‐co‐urethane) Networks

AbstractPolymer networks containing dynamic covalent bonds do not exhibit traditional thermoset material properties. Such dynamic covalent networks have the ability to undergo stress relaxation processes associated with dynamic covalent bond exchange, imparting these materials with adaptive/responsive properties. Reported herein is an investigation on the effect that changing the amount of dynamic hindered alkylurea bonds has on the viscoelastic behavior of a series of poly(alkylurea‐co‐urethane) networks prepared by reacting a trifunctional isocyanate crosslinker with varying ratios of an N‐isopropyl amine endcapped poly(propylene glycol) and a poly(propylene glycol). Films that contain >50% dynamic alkylurea bonds (wrt. alkylurea + urethane bonds) exhibit facile reprocessability, while those films with <50% dynamic alkylurea bonds exhibit poor reprocessability under these same conditions. Analysis of the temperature‐dependent shear rheometry and uniaxial stress relaxation measurements demonstrates that the primary stress relaxation mode in these materials is linked to the dynamic bond exchange process. Interestingly, these films exhibit an increasingly rich viscoelastic spectrum with increasing fraction of non‐dynamic urethane bonds. In addition to the primary relaxation process an order‐of‐magnitude slower relaxation emerges, which is identified as being related to the relaxation of larger, permanently crosslinked polymeric clusters in an otherwise dynamic matrix.

3D Printable and Reconfigurable Liquid Crystal Elastomers with Light-Induced Shape Memory via Dynamic Bond Exchange.

3D printable and reconfigurable liquid crystal elastomers (LCEs) that reversibly shape-morph when cycled above and below their nematic-to-isotropic transition temperature (TNI ) are created, whose actuated shape can be locked-in via high-temperature UV exposure. By synthesizing LCE-based inks with light-triggerable dynamic bonds, printing can be harnessed to locally program their director alignment and UV light can be used to enable controlled network reconfiguration without requiring an imposed mechanical field. Using this integrated approach, 3D LCEs are constructed in both monolithic and heterogenous layouts that exhibit complex shape changes, and whose transformed shapes could be locked-in on demand.

Vitrimerization: A Novel Concept to Reprocess and Recycle Thermoset Waste via Dynamic Chemistry.

A new approach for reprocessing of existing thermoset waste is presented. This work demonstrates that unrecyclable thermoset materials can be reprocessed using the concept of associative dynamic bonding, vitrimers. The developed recycling methodology relies on swelling the thermoset network into a solution of a catalyst, which enables transesterification reactions allowing dynamic bond exchange between ester and hydroxyl groups within the thermoset network. Thermal and mechanical properties for recycled polyurethane and epoxy networks are studied and a strategy to maintain the properties of recycled materials is discussed. The developed methodology promises recycling and even upcycling and reprocessing of previously thought intractable materials. Moreover, processability of vitrimerized thermosets with common thermoplastic manufacturing methods opens up the possibility of tuning recycled networks by adding nanoparticles. This flexibility keeps the application window of recycled thermosets very broad.

From shape and color memory PCL network to access high security anti-counterfeit material

Shape memory polymers have been widely applied in actuators and space deployable materials due to the shape-change feature under external stimuli. Actually, it will show greater potential in smart materials if the shape memory technique creatively combined with other functions. Herein, a shape and color memory material was prepared by incorporating a photochromic molecule spiropyran into the shape memory poly(ε-caprolactone) network. Integrating shape memory effect with color variance in this network, we firstly mimic a blooming rose with color change as it warms through specific assembly and procedures. Assisted with dynamic bond exchange of urethane and ester linkages, surface patterning, and molding technology, a series of application models can execute SME accompanied with color change were demonstrated, including high security information storage material, anti-counterfeit label and encryption label.

Dynamics and reaction kinetics of coarse-grained bulk vitrimers: a molecular dynamics study.

Vitrimers with dynamic covalent bonds make thermosetting materials plastic, recyclable and self-repairing, and have broad application prospects. However, due to the complex composition of vitrimers and the dynamic bond exchange reactions (BERs), the mechanism behind their unique dynamic behavior is not fully understood. We used the hybrid molecular dynamics-Monte Carlo (MD-MC) algorithm to establish a molecular dynamics model that can accurately reflect BERs, and reveal the intrinsic mechanism of the dynamic behavior of the vitrimer system. The simulation results show that BERs change the diffusion mode of the vitrimer's constituent molecules, which in turn affects the BER and other relaxation dynamics. This provides a theoretical basis and a specific method for the rational design of the rheological properties of vitrimers.

Digital coding of mechanical stress in a dynamic covalent shape memory polymer network

Controlling stresses in materials presents many unusual opportunities for their engineering applications. The potential for current approaches is severely limited by the intrinsic tie between the stress and the geometric shape. Here, we report a material concept that allows stress management in a highly efficient digital manner while decoupling the stress and the geometric shape. This is realized in a dynamic covalent shape memory polymer network, for which the elastic shape memory sets the baseline stress level and maintains the geometric shape while the plasticity enabled by the dynamic bond exchange allows stress tuning. With a digital gray scale photothermal mechanism, any arbitrarily defined stress distribution can be created in a free-standing polymer film. The naturally invisible stresses can be further visualized as mechanical colors under polarized light, revealing its potential for encoding hidden information. Our approach expands the technological potential in many areas for which stresses are relevant.

A readily programmable, fully reversible shape-switching material.

Liquid crystalline (LC) elastomers (LCEs) enable large-scale reversible shape changes in polymeric materials; however, they require intensive, irreversible programming approaches in order to facilitate controllable actuation. We have implemented photoinduced dynamic covalent chemistry (DCC) that chemically anneals the LCE toward an applied equilibrium only when and where the light-activated DCC is on. By using light as the stimulus that enables programming, the dynamic bond exchange is orthogonal to LC phase behavior, enabling the LCE to be annealed in any LC phase or in the isotropic phase with various manifestations of this capability explored here. In a photopolymerizable LCE network, we report the synthesis, characterization, and exploitation of readily shape-programmable DCC-functional LCEs to create predictable, complex, and fully reversible shape changes, thus enabling the literal square peg to fit into a round hole.

Dynamics of Vitrimers: Defects as a Highway to Stress Relaxation.

We propose a coarse-grained model to investigate stress relaxation in star-polymer networks induced by dynamic bond-exchange processes. We show how the swapping mechanism, once activated, allows the network to reconfigure, exploring distinct topological configurations, all of them characterized by complete extent of reaction. Our results reveal the important role played by topological defects in mediating the exchange reaction and speeding up stress relaxation. The model provides a representation of the dynamics in vitrimers, a new class of polymers characterized by bond-swap mechanisms which preserve the total number of bonds, as well as in other bond-exchange materials.

Maximizing the symbiosis of static and dynamic bonds in self-healing boronic ester networks

Networks that contain boronic ester crosslinks undergo dynamic bond exchange that enables self-healing behavior and reprocessing.