Self-Healing of a Cross-Linked Polymer with Dynamic Covalent Linkages at Mild Temperature and Evaluation at Macroscopic and Molecular Levels

A lot of self-healing materials using dynamic bonding systems have been reported, while the focus is mainly on the macroscopic self-healing behavior such as visually recognizable healing. Because the healing originates from microscopic chemical reactions of the dynamic bonds, evaluation of the reactions in the materials is necessary for elucidation of the healing mechanisms and development of the healing ability. Herein, we demonstrated self-healing of a cross-linked polymer with diarylbibenzofuranone (DABBF)-based dynamic covalent linkages at mild temperature and investigated the healing behavior from both macroscopic and microscopic viewpoints. The macroscopic behavior was inspected by mechanical tests, and the linkage reaction (equilibrium) was evaluated by electron paramagnetic resonance measurements. These assessments revealed that the healing is strongly dependent on temperature, which is attributable to synergism between changes in the chain mobility and in the equilibrium of the incorporated linkages. These findings would be applicable to other dynamic bonding systems

Multicolor Mechanochromic Polymer Blends That Can Discriminate between Stretching and Grinding

Mechanochromic polymers, which react to mechanical force by changing color, are expected to find applications in smart materials such as damage sensors. Although numerous types of mechanochromic polymers have been reported so far, developing mechanochromic polymers that can recognize different mechanical stimuli remains a formidable challenge. Materials that not only change their color in response to a mechanical stimulus but also detect its nature should be of great importance for practical applications. In this paper, we report our preliminary findings on multicolor mechanochromic polymer blends that can discriminate between two different mechanical stimuli, i.e., stretching and grinding, by simply blending two mechanochromic polymers with different architectures. The rational design and blending of two mechanochromic polymers with radical-type mechanochromophores embedded separately in positions adjacent to soft or hard domains made it possible to achieve multicolor mechanochromism in response to different stimuli. Electron paramagnetic resonance and solid-state UV–vis measurements supported the mechanism proposed for this discrimination

Thermally Healable and Reprocessable Bis(hindered amino)disulfide-Cross-Linked Polymethacrylate Networks

A facile approach to polymethacrylate networks that contain thermally exchangeable bis­(2,2,6,6-tetramethylpiperidin-1-yl)­disulfide (BiTEMPS) cross-linkers is reported, and the easily inducible healability and reprocessability of the obtained networks are discussed. The free radical polymerization of BiTEMPS cross-linkers and hexyl methacrylate (HMA) monomers afforded insoluble and colorless networks of poly­(hexyl methacrylate) (PHMA) films, whose structures were characterized after de-cross-linking via thermal BiTEMPS exchange reactions with added low-molecular-weight BiTEMPS. Swelling experiments and stress-relaxation measurements at elevated temperatures revealed the contribution of BiTEMPS as a polymer chain exchanger both in the gels and in the bulk. The BiTEMPS-cross-linked PHMA networks showed damage healability and repeatable reprocessability in the bulk by simple hot pressing at 120 °C under mild pressure (∼70 kPa). These results should grant facile access to various vinyl polymer networks with on-demand malleability

Polymer–Inorganic Composites with Dynamic Covalent Mechanochromophore

Polymer–inorganic composites with diaryl­bibenzo­furanone (DABBF) moieties, dynamic covalent mechanochromophores, were prepared, and their mechanochromic behavior was systematically investigated. The central C–C bonds in DABBF moieties can be cleaved by mechanical force to form the corresponding stable blue radicals, which can be quantitatively evaluated by electron paramagnetic resonance (EPR) spectroscopy. One controversial issue but attractive property in the DABBF system is the equilibrium between the activated and deactivated states. Although the deactivation process decreases the sensitivity of some equilibrium mechanophores, the equilibrium has rarely been considered when establishing molecular and/or material design of these systems. Herein, a rational macromolecular design to suppress the deactivation of activated dynamic mechanophores and improve sensitivity by limiting their molecular motion is proposed. Polymer–inorganic composite materials with rigid networks prepared from DABBF alkoxysilane derivatives exhibited significant activation of the incorporated DABBF linkages by grinding, with sensitivities more than 50 times as high as that of DABBF monomers. The increased sensitivity is due to the effective transmission of mechanical force to the DABBF moieties in the network structures and suppression of the recombination of the generated radicals by the rigid frameworks. Furthermore, when the rigid frameworks were incorporated into elastomers as inorganic hard domains, the DABBF mechanophores at the interface between the organic and inorganic domains were preferentially activated by elongation

Polymer–Inorganic Composites with Dynamic Covalent Mechanochromophore

Polymer–inorganic composites with diaryl­bibenzo­furanone (DABBF) moieties, dynamic covalent mechanochromophores, were prepared, and their mechanochromic behavior was systematically investigated. The central C–C bonds in DABBF moieties can be cleaved by mechanical force to form the corresponding stable blue radicals, which can be quantitatively evaluated by electron paramagnetic resonance (EPR) spectroscopy. One controversial issue but attractive property in the DABBF system is the equilibrium between the activated and deactivated states. Although the deactivation process decreases the sensitivity of some equilibrium mechanophores, the equilibrium has rarely been considered when establishing molecular and/or material design of these systems. Herein, a rational macromolecular design to suppress the deactivation of activated dynamic mechanophores and improve sensitivity by limiting their molecular motion is proposed. Polymer–inorganic composite materials with rigid networks prepared from DABBF alkoxysilane derivatives exhibited significant activation of the incorporated DABBF linkages by grinding, with sensitivities more than 50 times as high as that of DABBF monomers. The increased sensitivity is due to the effective transmission of mechanical force to the DABBF moieties in the network structures and suppression of the recombination of the generated radicals by the rigid frameworks. Furthermore, when the rigid frameworks were incorporated into elastomers as inorganic hard domains, the DABBF mechanophores at the interface between the organic and inorganic domains were preferentially activated by elongation

Polymer–Inorganic Composites with Dynamic Covalent Mechanochromophore

Polymer–inorganic composites with diaryl­bibenzo­furanone (DABBF) moieties, dynamic covalent mechanochromophores, were prepared, and their mechanochromic behavior was systematically investigated. The central C–C bonds in DABBF moieties can be cleaved by mechanical force to form the corresponding stable blue radicals, which can be quantitatively evaluated by electron paramagnetic resonance (EPR) spectroscopy. One controversial issue but attractive property in the DABBF system is the equilibrium between the activated and deactivated states. Although the deactivation process decreases the sensitivity of some equilibrium mechanophores, the equilibrium has rarely been considered when establishing molecular and/or material design of these systems. Herein, a rational macromolecular design to suppress the deactivation of activated dynamic mechanophores and improve sensitivity by limiting their molecular motion is proposed. Polymer–inorganic composite materials with rigid networks prepared from DABBF alkoxysilane derivatives exhibited significant activation of the incorporated DABBF linkages by grinding, with sensitivities more than 50 times as high as that of DABBF monomers. The increased sensitivity is due to the effective transmission of mechanical force to the DABBF moieties in the network structures and suppression of the recombination of the generated radicals by the rigid frameworks. Furthermore, when the rigid frameworks were incorporated into elastomers as inorganic hard domains, the DABBF mechanophores at the interface between the organic and inorganic domains were preferentially activated by elongation

Self-healing and shape-memory polymers based on cellulose acetate matrix

The creation of self-healing polymers with superior strength and stretchability from biodegradable materials is attracting increasing attention. In this study, we synthesized new biomass-derived cellulose acetate (CA) derivatives by ring-opening graft polymerization of δ-valerolactone followed by the introduction of ureidopyrimidinone (Upy) groups in the polymer side chains. Due to the semicrystalline aliphatic characteristics of the side chain poly(δ-valerolactone) (PVL) and quadruple hydrogen bonds formed by the Upy groups, the stretchability of the resulting polymers was significantly enhanced. Moreover, the shape memory ability and self-healing property (58.3% of self-healing efficiency) were successfully imparted to the polymer. This study demonstrates the great significance of using biomass sources to create self-healing polymers. This paper describes the first successful demonstration of self-healing polymers with superior strength and stretchability from a biodegradable material, cellulose acetate (CA). We initially introduced the ureidopyrimidinone (Upy) groups in the side chains of CA. However, the resulting polymer was not soluble and processable. In order to solve this issue, a new strategy based on the ring-opening graft polymerization of δ-valerolactone followed by the introduction of ureidopyrimidinone (Upy) groups was adopted. Due to the semicrystalline aliphatic characteristics of the side chain poly(δ-valerolactone) (PVL), the resulting polymers were soluble and processable. In addition, the quadruple hydrogen bonds formed by the Upy groups enhanced the stretchability of the resulting polymers. Moreover, the shape memory ability and self-healing property were successfully achieved due to the presence of PVL and Upy. The developed new strategy can be applied to a variety of polymers including biomass-based polymers and materials.</p

Synthesis of Vinylic Macromolecular Rotaxane Cross-Linkers Endowing Network Polymers with Toughness

Macromolecular rotaxane cross-linkers having two radically polymerizable vinyl groups (RCs) were first synthesized and used to prepare network polymers. A crown ether/<i>sec-</i>ammonium-type pseudorotaxane initiator having an OH terminal-containing axle and a crown ether wheel with a vinyl group was subjected to the living ring-opening polymerization of δ-valerolactone followed by end-capping with a bulky isocyanate to yield a polyester axle-tethering macromolecular [2]­rotaxane cross-linker (RC). Rotaxane cross-linked polymers (RCPs) were prepared by the radical polymerization of <i>n</i>-butyl acrylate in the presence of RCs (0.25, 0.50 mol %). The properties of the RCPs and covalently cross-linked polymers (CCPs) were characterized mainly by mechanical properties. Both fracture stress and strain values of RCPs were much higher than those of CCPs, probably owing to the increased network homogeneity by the rotaxane cross-link. The hybrid-type RCPs obtained from a mixture of RC and covalently connected cross-linker (CC) showed poorer mechanical properties similar to that of CCPs, indicating the importance of RCs in increasing the toughness of the network polymers

Triggered Structural Control of Dynamic Covalent Aromatic Polyamides: Effects of Thermal Reorganization Behavior in Solution and Solid States

Thermally rearrangeable aromatic polyamides (TEMPO-PA) and random copolyamides (TEMPO-PA-COOH) incorporating alkoxyamine moieties in the main chain were synthesized, and the effects of thermal reorganization behavior on their solution and solid-state structures were investigated. The hydrodynamic radius in solution decreased as the solution temperature increased because of the dissociation of the alkoxyamine unit. Additionally, the dry density of the thin films decreased as the fabrication temperature increased because of the suppression of polymer aggregation caused by the thermally induced radical crossover reaction. In addition, at the film surface of the random copolyamide containing hydrophobic TEMPO and hydrophilic 3,5-diaminobenzoic acid (DABA) units, the hydrophilicity decreased as the fabrication temperature increased. This is because hydrophobic TEMPO and hydrophilic DABA units tend to be discretely aggregated near the film surface to minimize the surface energy and suppress the hydrogen bonding via a radical crossover reaction during the thin-film fabrication process. The present study clearly shows that both the solution structure and the solid-state molecular aggregation structure of the dynamic covalent polymers can be easily controlled by a thermal trigger, and it provides a new method for controlling the higher-order structure of polymer solutions and solids

Network Reorganization of Dynamic Covalent Polymer Gels with Exchangeable Diarylbibenzofuranone at Ambient Temperature

Reversible bonds and interactions have been utilized to build stimuli-responsive and reorganizable polymer networks that show recyclability, plasticity, and self-healing. In addition, reorganization of polymer gels at ambient temperature, such as room or body temperature, is expected to lead to several biomedical applications. Although these stimuli-responsive properties originate from the reorganization of the polymer networks, not such microscopic structural changes but instead only macroscopic properties have been the focus of previous work. In the present work, the reorganization of gel networks with diarylbibenzofuranone (DABBF)-based dynamic covalent linkages in response to the ambient temperature was systematically investigated from the perspective of both macroscopic and microscopic changes. The gels continued to swell in suitable solvents above room temperature but attained equilibrium swelling in nonsolvents or below room temperature because of the equilibrium of DABBF linkages, as supported by electron paramagnetic resonance measurements. Small-angle X-ray scattering measurements revealed the mesh sizes of the gels to be expanded and the network structures reorganized under control at ambient temperature