Vitrimers make up a class of polymeric materials combining the advantages of thermosets and thermoplastics, because they can be reprocessed while being at the same time permanently cross-linked. However, a long heating duration or an elevated temperature is necessary for most vitrimers to relax the stress from deformation and exhibit malleability. Herein, a disulfide-containing carboxylic acid is applied as a curing agent to synthesize epoxy vitrimers with simultaneous disulfide metathesis and carboxylate transesterification. The insoluble networks exhibit rapid stress relaxation and have relaxation times ranging from 1.5 s (200 °C) to 5500 s (60 °C), while the temperature of malleability is as low as 65 °C. Moreover, this vitrimer can be efficiently reprocessed at 100 °C in 1 h with full recovery of mechanical strength for at least four cycles. Additionally, such a material is simply synthesized from commercially available chemicals and may have potential applications in the electronics industry where a high temperature is not allowed.

Into and out of the blue: The highly ordered structure of a PNIPAM microgel colloidal crystal (MCC) is stabilized by photopolymerization of its surface-bound vinyl groups. The resulting polymerized MCCs can respond reversibly and quickly to external stimuli, including temperature and ionic strength of the surrounding media, allowing the color and band gap to be finely tuned in the whole visible range.

No abstract

Vitrimer materials that obscure the line between ″thermoset″ and ″thermoplastic″ have already attracted a great deal of interest since they exhibit dynamic properties of stress relaxation or reprocessability without losing their permanently cross-linked networks. While much research has been aimed at developing new exchangeable dynamic bonds to broaden the scope, there is a dearth of effort in exploring the key factors that influence the dynamic properties of vitrimer materials. These explorations are not only useful in the exact design and synthesis of vitrimer materials but also quite important in their theory establishment and computer simulation. However, only a few ways including catalyst control have been confirmed to be the key factors in controlling the dynamic properties of classical vitrimer materials like the polyester-based epoxy vitrimer. Here, we proposed that the density of exchangeable ester bonds (υ) in networks also has a crucial role in adjusting the dynamic properties of epoxy vitrimers. Four polyester-based epoxy vitrimers were synthesized from the tricarboxylic acid curing agent and different epoxy monomers, resulting in various υ’s. These epoxy vitrimers were named ″E202-vitrimer″, ″E380-vitrimer″, ″E500-vitrimer″, and ″E640-vitrimer″ according to the different molecular weights of the epoxy monomer. It was revealed that the dynamic properties of these vitrimer materials varied with the υ value. From the E640-vitrimer to E202-vitrimer, the stress relaxation behavior sped up with the rise in υ and the characterized relaxation times (τ’s) obviously decreased. Meanwhile, the activation energy (E a) and the Arrhenius prefactor (τ0), which are two important intrinsic parameters in evaluating the dynamic properties of vitrimers, exhibited heavy dependence on υ, while linear models were applied to describe their relationships. Moreover, the τ at various temperatures could be also predicted from a linear model that relied on the υ in polyester vitrimers. These results indicated that υ has a crucial role in controlling the dynamic properties of polyester-based epoxy vitrimers and the value of υ could be applied to calculate the τ, E a, and τ0. Besides, the influence of υ on the dynamic properties of polyester-based epoxy vitrimers and the resulted models might be workable in other vitrimer materials, which can be utilized to design desired vitrimer materials for various applications.

Stimuli-responsive hydrogels whose swelling degree changes in response to certain external stimulus are usually constructed by the introduction of a functional group. Here we show that they can also be constructed using dynamic bonds as crosslinks because the equilibrium of their formation and breakage can be shifted by external stimuli. As an example, hydrogel films were fabricated from partially oxidized dextran (PO-Dex) and chitosan (Chi) using the layer-by-layer assembly technique. The driving force for the film buildup is the in situ formation of Schiff base bonds between the aldehyde groups on PO-Dex and amino groups on Chi. The swelling of the film was studied using the Fabry-Perot fringes on the reflection spectra. Like ordinary hydrogels, the PO-Dex/Chi hydrogel films swell in water. Their swelling degree decreases with increasing oxidation degree of PO-Dex. The films swell to a larger degree when the pH is lowered. The pH-sensitivity was attributed to amino groups on chitosan and also the dynamic Schiff base linkages, because pH change shifts the equilibrium of the Schiff base reaction in the films, resulting in a change in the crosslink density and therefore a change in swelling degree. When the transient linkages were fixed by NaBH 4 reduction, the pH-sensitivity of the films was significantly reduced. The films were found to be sensitive to other external stimuli, including temperature, L-lysine and pyridoxal.These stimuli-responsivities were also attributed to the dynamic Schiff base linkages in the film.

Covalent adaptable networks (CANs) relying on dynamic cross-links have been developed to make the cross-linked polymeric materials and composites degradable. However, due to the reversibility of dynamic bonds, the CANs and composites suffer accidental degradation and failure upon a certain stimulus (moisture, acid/base, reductant/oxidant, etc.) in the application environment. Herein, inspired by parallel circuits, interlocked covalent adaptable networks (ICANs) were prepared by one-pot reactions from epoxy monomers and two curing agents that contained different dynamic bonds of aromatic disulfide and aromatic imine bonds, resulting in dual dynamic parallel cross-links in homogeneous epoxy networks. The ICANs exhibited outstanding mechanical properties and improved stability, relying on the topological interlocking structure. The ICANs could be unlocked and became degradable only when two stimuli were both applied to completely break the cross-links of disulfide bonds and imine bonds. When applying ICANs as a matrix to form carbon fiber-reinforced polymer (CFRP) composites, the resulted CFRP inherited the interlocking properties from the ICANs, exhibiting improved stability and nondestructive recyclability. Maintaining the degradable properties, the interlocking structure of networks provided a facile way to optimize the stability of CANs and their composites.

Polymers with regulated alternating structures are attractive in practical applications, particularly for main-chain fluoropolymers. We for the first time enabled controlled fluoropolymer synthesis with alternating sequence regulation using a novel fluorinated xanthate agent via a light-driven process, which achieved on-demand copolymerization of chlorotrifluoroethylene and vinyl esters/amides under both batch and flow conditions at ambient pressure. This method creates a facile access to fluoropolymers with a broad fraction range of alternating units, low dispersities and high chain-end fidelity. Moreover, a two-step photo-flow platform was established to streamline the in-situ chainextension toward unprecedented block copolymers continuously from fluoroethylene. Influences of structural control were illustrated with thermal and surface properties. We anticipate that this work will promote advanced material engineering with customized fluoropolymers.

To meet the demand of sustainable development, epoxy vitrimers based on the dynamic transesterification reaction (DTER) have received considerable attention recently due to their reprocessability and repairability. However, they suffer from low mechanical strength and rely heavily on external catalysts to ensure their curing and repair. Herein, we report a facile design of a novel ZnAl-LDH-catalyzed epoxy vitrimer nanocomposite via introducing ZnAl-layered double metal hydroxide (ZnAl-LDH) nanosheets. Our results show that ZnAl-LDH can be well dispersed in the epoxy vitrimer. Notably, ZnAl-LDH has multifunctionality, which can simultaneously catalyze the curing reaction and enhance the mechanical strength and repairable efficiency of the resultant vitrimer. For instance, the peak curing temperature of epoxy vitrimer with 2 wt % ZnAl-LDH is 8 °C lower than that of an epoxy vitrimer under the same loading between Zn2+ of Zn­(OAc)2, demonstrating a strong catalytic action. The tensile strength and Young’s modulus of ZnAl-LDH/epoxy resin (ER) increase from 18 and 156 MPa to 42 and 307 MPa, respectively, due to the reinforcing effect of ZnAl-LDH and the increased cross-linking density. The repairable efficiency of ZnAl-LDH/ER can reach 95% after repair at 200 °C for 1 h, which is mainly due to the abundant catalytic sites and large contact areas of the ZnAl-LDH lamella. Hence, this work offers an innovative and scalable strategy for creating epoxy vitrimers combining exceptional mechanical strength and high repairable efficiency, which holds great promise for many practical applications in the industry.