Self-healable and reprocessable networks involving diblock copolymer and hindered urea bonds

Recyclable Dynamic Covalent Networks Derived from Isocyanate Chemistry: The Critical Role of Electronic and Steric Effects in Reversibility

The dynamic covalent networks (DCNs), featuring dynamic covalent bonds (DCBs) formed through isocyanate-involved chemistry, potentially contributes to a circular economy in polyurea and polyurethane industries, due to the inherent recyclability of DCNs. Over the past decade, remarkable progress has been made in the development of isocyanate-derived DCBs (IdDCBs) for the synthesis of recyclable DCNs, aiming to substitute conventional, non-recyclable materials. Herein, the fundamental aspect of the IdDCB-related chemistries reported to date is investigated, and it is found that their reversibility is governed by electronic and steric effects. This discovery encourages us to structure the review into three sections. The first section examines the reversibility of various IdDCBs through the lens of electronic and steric influences. The findings show that the reversibility of some IdDCBs is driven by a single chemical effect, with the examples of steric effect contributing to the dynamic behavior of thiourethanes and hindered ureas, while other cases of reversibility arise from a combination of two or more chemical effects. The knowledge thus established allows to categorize and discuss the technologically relevant DCNs, with particular emphasis on how these chemical effects influence their recyclability. Finally, the review concludes by highlighting several potentially impactful research directions that merit further exploration.

Nanocomposites of Poly(n-Butyl Acrylate) with Fe3O4: Crosslinking with Hindered Urea Bonds, Reprocessing and Related Functional Properties

In this contribution, we reported the synthesis of the nanocomposites of poly(n-butyl acrylate) with Fe3O4 nanoparticles (NPs) via dynamic crosslinking of poly(n-butyl acrylate)-grafted Fe3O4 NPs with hindered urea bonds (HUBs). Towards this end, the surfaces of Fe3O4 NPs were grafted with poly(n-butyl acrylate-ran-2-(3-tert-butyl-3-ethylureido)ethyl acrylate) chains [denoted as Fe3O4-g-P(BA-r-TBEA)] via living radical polymerization. Thereafter, 1,2-bis(tert-butyl)ethylenediamine was used as a crosslinker to afford the organic-inorganic networks with variable contents of Fe3O4 NPs and crosslinking densities. It was found that the fine dispersion of Fe3O4 NPs in the matrix of poly(n-butyl acrylate) was achieved. The nanocomposites exhibited excellent reprocessing properties, attributed to the introduction of HUBs. Owing to the crosslinking, the nanocomposites displayed excellent shape memory properties. Further, the nanocomposites possessed photo- and magnetic-thermal properties, which were inherited from Fe3O4 NPs. These functional properties allow triggering the shape shifting of the nanocomposites in an uncontacted fashion.

Trapping bond exchange phenomenon revealed for off-stoichiometry cross-linking of phase-separated vitrimer-like materials

Vitrimer materials combined with nano-phase separated structures have attracted attention, expanding the tuning range of physical properties, such as flow and creep properties. We recently demonstrated a preparation of vitrimer-like materials with phase-separated nanodomains in which dissociative bond exchange via trans-N-alkylation of quaternized pyridine was operated. In this study, we demonstrate a new finding about the bond exchange mechanism: that is, the trapping bond exchange phenomenon. The component polymer is a poly(acrylate) containing pyridine side groups randomly along the chain, which is cross-linked by diiodo molecules via pyridine-iodo quaternization, where the quaternized pyridines are aggregated to form nano-size domains. When the cross-linking reaction is performed at an off-stoichiometric pyridine : iodo ratio (i.e., an excess of pyridine groups), free pyridine groups are located in the matrix phase. Since the bond exchange in the present system progresses in an inter-domain manner, the dissociated unit bearing pendant iodo is trapped by the free pyridine groups in the matrix, which generates other small aggregates. This trapping phenomenon greatly affects the relaxation and creep properties, which are very different from those found in conventional knowledge about vitrimer physics.

A Self-Healing Thermoset Epoxy Modulated by Dynamic Boronic Ester for Powder Coating

Thermoset powder coatings exhibit distinctive characteristics such as remarkable hardness and exceptional resistance to corrosion. In contrast to conventional paints, powder coatings are environmentally friendly due to the absence of volatile organic compounds (VOCs). However, their irreversible cross-linking structures limit their chain segment mobility, preventing polymers from autonomously repairing cracks. Dynamic cross-linking networks have garnered attention for their remarkable self-healing capabilities, facilitated by rapid internal bond exchange. Herein, we introduce an innovative method for synthesizing thermoset epoxy containing boronic ester moieties which could prolong the life of the powder coating. The epoxy resin system relies on the incorporation of two curing agents: one featuring small-molecule diamines with boronic bonds and the other a modified polyurethane prepolymer. A state of equilibrium in mechanical properties was achieved via precise manipulation of the proportions of these agents, with the epoxy composite exhibiting a fracture stress of 67.95 MPa while maintaining a stable glass transition temperature (Tg) of 51.39 °C. This imparts remarkable self-healing ability to the coating surface, capable of returning to its original state even after undergoing 1000 cycles of rubbing (using 1200-grit abrasive paper). Furthermore, the introduction of carbon nanotube nanoparticles enabled non-contact sequential self-healing. Subsequently, we introduce this method into powder coatings of different materials. Therefore, this work provides a strategy to develop functional interior decoration and ensure its potential for broad-ranging applications, such as aerospace, transportation, and other fields.

Reprocessable and Recyclable Chain-Growth Polymer Networks Based on Dynamic Hindered Urea Bonds

Conventional cross-linked polymers cannot be reprocessed because of the presence of permanent covalent cross-links, preventing reuse and recycling. Covalent adaptable networks (CANs) employ dynamic covalent bonds that undergo dynamic reactions under external stimulus, allowing recyclability of these network materials. Hindered urea chemistry is one of the recently discovered dissociative dynamic chemistries. While hindered urea bonds have traditionally been exploited in the synthesis of step-growth type CANs, the use of hindered urea bonds in the synthesis of chain-growth-type dynamic networks has only been narrowly explored. Here, we present a simple, catalyst-free, fast method to synthesize a hindered-urea-based dynamic cross-linker that can undergo a free radical polymerization with vinyl-type monomers or polymers to form reprocessable CANs. Using this cross-linker, we developed dynamic polymethacrylate networks that can be (re)processed at 80 °C. These dynamic covalent networks exhibit full recovery of cross-link density after multiple recycling steps; they are only the second chain-growth network synthesized directly and exclusively from carbon-carbon double bond monomers to demonstrate such recovery. Unlike other dissociative dynamic polymer networks, polymethacrylate networks that contain dissociative dynamic hindered urea bonds do not flow and maintain their network structure even at high temperature (300 °C). Despite its relatively fast reprocessability, the network showed delayed and extremely slow stress relaxation at the processing temperature. This work offers a simple approach to obtain reprocessable addition-type networks based on hindered urea bonds while revealing the limitations of stress relaxation experiments in relationship to the processability of some dynamic polymer networks.

Dynamic covalent polymers enabled by reversible isocyanate chemistry

Reversible isocyanate chemistry containing urethane, thiourethane, and urea bonds is valuable for designing dynamic covalent polymers to achieve promising applications in recycling, self-healing, shape morphing, 3D printing, and composites.