Traditional covalent organic frameworks (COFs) are prepared via polymerization based on small molecular monomers. However, the employment of polymers as building blocks to construct COFs has not been reported yet. Herein, we create a new concept of polymer covalent organic frameworks (polyCOFs) formed by linear polymers as structural building blocks, which inherit the merits from both COFs and linear polymers. PolyCOFs represent a new category of porous COF materials that demonstrate good crystallinity and high stability. More importantly, benefiting from the flexibility and processability of a linear polymer, polyCOFs can spontaneously form defect-free, flexible, and freestanding membranes that exhibit excellent mechanical properties and undergo reversible mechanical transformation upon exposure to various organic vapors. For the first time, we demonstrated that polyCOF membranes can be used as artificial muscles to perform various complicated motions (e.g., lifting objects, doing “sit-ups”) triggered by vapors. This study bridges the gap between one-dimensional amorphous linear polymers and crystalline polymer frameworks and paves a new avenue to prepare stimuli-responsive actuators using porous COF materials.

Abstract-The snap-through and pull-in instabilities of the micromachined arch-shaped beams under an electrostatic loading are studied both theoretically and experimentally. The pull-in instability that results in a system collision with an electrode substrate may lead to a system failure and, thus, limits the system maximum displacement. The beam/plate structure with a flat initial configuration under an electrostatic loading can only experience the pull-in instability. With the different arch configurations, the structure may experience either only the pull-in instability or the snap-through and pull-in instabilities together. As shown in our computation and experiment, those arch-shaped beams with the snap-through instability have the larger maximum displacement compared with the arch-shaped beams with only the pull-in stability and those with the flat initial configuration. The snapthrough occurs by exerting a fixed load, and the structure experiences a discontinuous displacement jump without consuming power. Furthermore, after the snap-through jump, the structures are demonstrated to have the capacity to withstand further electrostatic loading without pull-in. Those properties of consuming no power and increasing the structure deflection range without pull-in is very useful in microelectromechanical systems design, which can offer better sensitivity and tuning range.

In an era of environmentally friendly development, methods of the green synthesis of zinc oxide nanoparticles (ZnO NPs) from plant extracts have become a focus of research attention because of the benefits of environmental sustainability, simplicity, and low price. The present review introduces a green mechanism for the synthesis of ZnO NPs using the extracts of plants, exploring factors that influence the morphology of ZnO NPs and their antibacterial properties, and the mechanisms of antibacterial action. The results indicate that the factors that influence morphology include the intrinsic crystallographic morphological properties and conditions of the preparation of ZnO NPs. In terms of preparation conditions, the influence of plant extract concentration, precursor concentration, reaction time, and calcination temperature on NP morphology is related to the species of plants used, with precursor concentration the most significant factor affecting the morphology of ZnO NPs. A pH of 12 appears be the most appropriate alkalinity for the synthesis of ZnO NPs from plant extracts. In addition, the synthesized ZnO NPs display excellent antibacterial properties, the mechanism of which involves photocatalysis, reactive oxygen species, and interactions between ZnO NPs and bacterial surfaces. Factors influencing the antibacterial properties are the type of bacteria and the concentration and morphology of ZnO NPs. Finally, the methods of preparation of antibacterial textiles using synthetic ZnO NPs are discussed in relation to the preparation of antibacterial fibers, fabric, and composite textiles. Here, the future trend of such antibacterial textiles is considered, providing the direction for further research of antibacterial textiles.

Influence of temperature on morphology, structure and crystallinity of Poly (vinylidene fluoride) (PVDF) nanofibers was investigated in this study. The Wehilmy technique and viscosity testing apparatus were used to evaluate the surface tension and viscosity of electrospun solutions at various ambient temperatures. Surface morphologies and diameters of nanofibers were examined by Field-emission Scanning Electron Microscopy (FE-SEM) and atomic force microscopy (AFM). It was found that the surface morphologies were obviously affected by ambient temperature. This dependence was attributed to the change of the properties of Poly (vinylidence fluoride) solutions with temperature. The thermal properties and crystal structures of the PVDF nanofibers electrospun at different temperatures were also studied by differential scanning calorimetry (DSC), and Xray diffraction (XRD). The results revealed that the crystallinity and thermal properties were improved by increasing the ambient temperature during electrospinning.

No abstract