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.

In this work, the effect of both the expandable graphite (EG) and ammonium polyphosphate modified with 3-(methylacryloxyl) propyltrimethoxy silane (M-APP) on the flame retardancy and mechanical properties of the wood-polypropylene composites (WPC) were studied. Cone calorimetry results indicated that both EG and M-APP could effectively improve the flame retardancy of WPC, while the retardancy of EG was better than that of M-APP. When the flame-retardant loading was 25 wt.%, the limiting oxygen index (LOI) values of M-APP-filled WPC (M-APP/WPC) and EG-filled WPC (EG/WPC) were 30.7% and 37.9%, respectively. According to the LOI test, the optimal ratio of M-APP to EG in WPC was 1:1 by weight, at which the LOI value of WPC was 39.3%. Thermogravimetric analysis (TG) results indicated that the addition of M-APP and EG to WPC could lead to an increase of char residue. Under the same conditions, the char residue of composite filled with the mixture of EG and M-APP (at a ratio of 1:1) was greater than that of composites filled individually at the same temperature. Both the tensile strength and flexural strength decreased at a certain extent due to incorporation of EG into WPC, but with the addition of the M-APP, the mechanical properties of these composite samples increased. At the ratio of 1:1 (M-APP to EG), the mechanical properties of the composite were not obviously decreased, and the flame retardancy was higher than the M-APP-filled WPC composites.

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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.

Layer-by-layer (LbL) assembled films have been exploited for surface-mediated drug delivery. The drugs loaded in the films were usually released via diffusion or the degradation of one of the film components. Here we demonstrate that drug release can also be achieved by exploiting the dynamic nature of hydrogen-bonded LbL films. The films were fabricated from tannic acid (TA), a model polyphenolic drug, and poly(vinyl pyrrolidone) (PVPON). The driving force for the film buildup is the hydrogen bonding between the two components, which was confirmed by Fourier transform infrared (FTIR) spectra. The film growth is linear, and the growth rate of the film decreases with increasing assembly temperature. Because of the reversible/dynamic nature of hydrogen bonding, when soaked in aqueous solutions, the PVPON/TA films disassemble gradually and thus release TA to the media. The release rate of TA increases with increasing pH and temperature but decreases with increasing ionic strength. Scanning electron microscopy (SEM) studies on the surface morphology of the film during TA release reveal that the film surface becomes smoother and then rougher again because of the dewetting of the film. The released TA can scavenge ABTS +• cation radicals, indicating it retains its antioxidant activity, a major biological activity of polyphenols.

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