Sandwich structures are important innovative multifunctional structures with the advantages of low density and high performance. Creative design for sandwich structures is a design process based on sandwich core structure evolution mechanisms, material design method, and panel (including core structure and facing sheets) performance prediction model. The review outlines recent research efforts on creative design for sandwich structures with different core constructions such as corrugated core, honeycomb core, foam core, truss core, and folded cores. The topics discussed in this review article include aspects of sandwich core structure design, material design, and mechanical properties, and panel performance and damage. In addition, examples of engineering applications of sandwich structures are discussed. Further research directions and potential applications are summarized.
4D printing deforms a 2D foldable structure to another shape evolve over time by using heterogeneous material. The deformation of the 2D foldable structure is stimulated by actuators that are fabricated by shape memory materials or bilayer structures. Therefore, the deformation programming method of actuators is a critical technology in 4D printing. This paper proposes a method for programming the deformation of a temperature-driven bilayer structure actuator in 4D printing. The thermo-mechanical mechanism of the bilayer structure actuator is analyzed and three kinds of deformation behavior are modeled. Then a constitutive model with five main deformation programming parameters including the line width, the print height, the print temperature, the filled form, and the stimulation temperature is fitted by the orthogonal experiment and response surface method. The permanent deformation of the bilayer structure actuator results from the programmed parameters and time evolution of the 3D printed structure upon heating. A typical temperature shape memory material polylactic acid is used as a case study to illustrate the methodology and a desired programmed deformation is achieved.
Developing hydrogels with new structures and extraordinary performances is fundamental and mission-critical for the advancements of gel materials. Here, we report a class of tough supramolecular hydrogels facilely prepared by polymerizing methacrylic acid precursor solution in the presence of hexadecyltrimethylammonium chloride micelles. After swelling the as-prepared hydrogels in water, strong polyelectrolyte/surfactant complexes (PESCs) are formed between the weakly charged polymer chains and oppositely charged surfactants, serving as the physical cross-links of the gel matrix. The equilibrated hydrogels are transparent with a water content of 50–85 wt % and possess excellent mechanical properties, with a tensile breaking stress of 0.1–5 MPa, a breaking strain of 600–1200%, and Young’s modulus of 1–70 MPa. Typical yielding is observed at a small tensile strain of ∼10%, followed by forced elastic deformation of the hydrogels, which are in a glassy state due to the reduced segmental mobility of the matrix highly cross-linked by PESCs. The plastic-like mechanical properties of hydrogels could be well tuned by pH, temperature, and ionic strength that influence the ionization of polymer chains and the strength of PESCs. These dynamic behaviors render the hydrogels with self-recovery and shape-memory properties. Furthermore, the nanosized hydrophobic pockets within the hydrogels afford the capacity of loading functional hydrophobic molecules with promising applications as fluorescent materials and drug delivery systems. The design principle and strategy should be extended to other systems, including biomacromolecules and lipids, toward broad applications in biomedical and engineering fields.
In this study, fluorescent molecularly imprinted polymers (FMIPs), which were for the selective recognition and fluorescence detection of λ-cyhalothrin (LC), were synthesized via fluorescein 5(6)-isothiocyanate (FITC) and 3-aminopropyltriethoxysilane (APTS)/SiO2 particles. The SiO2@FITC-APTS@MIPs were characterized by Fourier transform infrared (FT-IR), UV-vis spectrophotometer (UV-vis), fluorescence spectrophotometer, thermogravimetric analysis (TGA), confocal laser scanning microscope (CLSM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The as-synthesized SiO2@FITC-APTS@MIPs with an imprinted polymer film (thickness was about 100 nm) was demonstrated to be spherically shaped and had good monodispersity, high fluorescence intensity, and good selective recognition. Using fluorescence quenching as the detection tool, the largest fluorescence quenching efficiency (F0/F - 1) of SiO2@FITC-APTS@MIPs is close to 2.5 when the concentration of the LC is 1.0 μM L(-1). In addition, a linear relationship (F0/F - 1= 0.0162C + 0.0272) could be obtained covering a wide concentration range of 0-60 nM L(-1) with a correlation coefficient of 0.9968 described by the Stern-Volmer equation. Moreover, the limit of detection (LOD) of the SiO2@FITC-APTS@MIPs was 9.17 nM L(-1). The experiment results of practical detection revealed that the SiO2@FITC-APTS@MIPs as an attractive recognition element was satisfactory for the determination of LC in Chinese spirits. Therefore, this study demonstrated the potential of SiO2@FITC-APTS@MIPs for the recognition and detection of LC in food.
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