Chao Cai

Bioinspired Materials: from Low to High Dimensional Structure

The surprising properties of biomaterials are the results of billions of years of evolution. Generally, biomaterials are assembled under mild conditions with very limited supply of constituents available for living organism, and their amazing properties largely result from the sophisticated hierarchical structures. Following the biomimetic principles to prepare manmade materials has drawn great research interests in materials science and engineering. In this review, we summarize the recent progress in fabricating bioinspired materials with the emphasis on mimicking the structure from one to three dimensions. Selected examples are described with a focus on the relationship between the structural characters and the corresponding functions. For one-dimensional materials, spider fibers, polar bear hair, multichannel plant roots and so on have been involved. Natural structure color and color shifting surfaces, and the antifouling, antireflective coatings of biomaterials are chosen as the typical examples of the two-dimensional biomimicking. The outstanding protection performance, and the stimuli responsive and self-healing functions of biomaterials based on the sophisticated hierarchical bulk structures are the emphases of the three-dimensional mimicking. Finally, a summary and outlook are given.

Mussel Inspired Modification of Polypropylene Separators by Catechol/Polyamine for Li-Ion Batteries

Inspired by the remarkable adhesion of mussel, dopamine, a mimicking adhesive molecule, has been widely used for surface modification of various materials ranging from organic to inorganic. However, dopamine and its derivatives are expensive which impede their application in large scale. Herein, we replaced dopamine with low-cost catechol and polyamine (only 8% of the cost of dopamine), which could be polymerized in an alkaline solution and deposited on the surfaces of various materials. By using this cheap and simple modification method, polypropylene (PP) separator could be transformed from hydrophobic to hydrophilic, while the pore structure and mechanical property of the separator remained intact. The uptake of electrolyte increased from 80% to 270% after the hydrophilic modification. Electrochemical studies demonstrated that battery with the modified PP separator had a better Coulombic efficiency (80.9% to 85.3%) during the first cycle at a current density of 0.1 C, while the discharging current density increased to 15 C and the discharge capacity increased by 1.4 times compared to the battery using the bare PP separator. Additionally, the modification allowed excellent stability during manifold cycles. This study provides new insights into utilizing low-cost chemicals to mimic the mussel adhesion and has potential practical application in many fields.

Robust Polypropylene Fabrics Super-Repelling Various Liquids: A Simple, Rapid and Scalable Fabrication Method by Solvent Swelling

A simple, rapid (10 s) and scalable method to fabricate superhydrophobic polypropylene (PP) fabrics is developed by swelling the fabrics in cyclohexane/heptane mixture at 80 °C. The recrystallization of the swollen macromolecules on the fiber surface contributes to the formation of submicron protuberances, which increase the surface roughness dramatically and result in superhydrophobic behavior. The superhydrophobic PP fabrics possess excellent repellency to blood, urine, milk, coffee, and other common liquids, and show good durability and robustness, such as remarkable resistances to water penetration, abrasion, acidic/alkaline solution, and boiling water. The excellent comprehensive performance of the superhydrophobic PP fabrics indicates their potential applications as oil/water separation materials, protective garments, diaper pads, or other medical and health supplies. This simple, fast and low cost method operating at a relatively low temperature is superior to other reported techniques for fabricating superhydrophobic PP materials as far as large scale manufacturing is considered. Moreover, the proposed method is applicable for preparing superhydrophobic PP films and sheets as well.

Facile preparation of hollow amino-functionalized organosilica microspheres by a template-free method

A facile method was developed to fabricate hollow amino-functionalized organosilica microspheres based on the hydrolysis and condensation of 3-aminopropyltriethoxysilane (APTES) and tetraethoxysilane (TEOS) in an aqueous system without an additional template and catalyst. The hollow hybrid organosilica microspheres obtained have been characterized by scanning electron microscopy, transmission electron microscopy, and N2 adsorption–desorption measurements. The results of Fourier transform infrared spectroscopy, solid-state NMR spectroscopy, X-ray photoelectron spectroscopy and thermogravimetric analysis displayed the composition of the synthesized hollow microspheres and the presence of amino groups on the surface. A self-templated and self-catalyzed mechanism for the formation of the hollow microspheres is proposed: the protonated APTES acts as both a catalyst for the reaction and a stabilizer for the hydrophobic precursor droplets, while the droplets themselves act as soft templates, and the consumption of the precursors leads to the formation of a hollow structure. The morphology and the size distribution of the organosilica microspheres can be controlled by tuning the composition of the precursors, and the stirring speed, as well as the reaction temperature. The large cavity with an open hole on the shell and the reactive amino groups on the surface of the hybrid organosilica microspheres promise many potential applications. Examples of employing the microspheres as adsorbents and phase change microcapsules have been presented.

Fabrication of oriented hBN scaffolds for thermal interface materials

Thermal interface materials are widely used in thermal management, and usually require a high thermal conductivity, low coefficient of thermal expansion (CTE) and adequate softness. Herein, hBN/PDMS composites are fabricated by the infiltration of a PDMS prepolymer in the hBN scaffolds followed by a thermal curing process. The scaffolds are prepared by an ice templating method with hBN microplatelets, leading to a good alignment of hBN platelets along the z direction in the PDMS matrix. This unique structure results in a high thermal conductivity, which is about 3 times higher than that of the composites fabricated by a casting method, and the thermal conductivity is as high as 1.4 W m−1 K−1 along the z direction at ∼20 wt% of hBN microplatelets. The composites also possess low CTEs which are <100 ppm K−1 along the z direction and maintain an adequate softness.

Low-cost mussel inspired poly(catechol/polyamine) coating with superior anti-corrosion capability on copper.

A low-cost mussel inspired approach was developed to produce anti-corrosion coating on copper substrate. Catechol (CA) and polyamine (PA) were spontaneously polymerized to form adhesive coating of poly(cetechol/polyamine) (P(CA/PA)) onto copper surface and then P(CA/PA) was grafted by 1-dodecanethiol. The SEM, contact angle, XPS, FTIR and TG results demonstrated the formation of uniform, compact and thermal stable coatings through multiple interactions and chemically grafting. Electrochemical tests indicated of Cu-P(CA/PA)-SH possessed a highest corrosion potential of -81mV, a lowest corrosion current density of 0.15μA/cm(2), and a highest coating resistance of 57.19kΩcm(2), and also exhibit great long-term stability whether in solution immersion or salt spray tests. The remarkable anti-corrosion capability of Cu-P(CA/PA)-SH could be ascribed to the synergistic effect of the hydrophobicity, good stability, and strong wet adhesion of the mussel-inspired coating. This study provides an effective and cheap way for material protection and may give inspiration in the fields of material, biology and medicine relating to surface and interface engineering.

Ultra Water Repellent Polypropylene Surfaces with Tunable Water Adhesion

Polypropylene (PP), including isotactic PP (i-PP) and atactic PP (a-PP) with distinct tacticity, is one of the most widely used general plastics. Herein, ultra water repellent PP coatings with tunable adhesion to water were prepared via a simple casting method. The pure i-PP coating shows a hierarchical morphology with micro/nanobinary structures, exhibiting a water contact angle (CA) larger than 150° and a sliding angle less than 5° (for 5 μL water droplet). In contrast, the pure a-PP coating has a less rough morphology with a water contact angle of about 130°, and the water droplets stick on the coating at any tilted angles. For the composite i-PP/a-PP coatings, however, ultra water repellency with CA > 150° but water adhesion tailorable from slippery to sticky can be realized, depending on the contents of a-PP and i-PP. The different wetting behaviors are due to the various microstructures of the composite coatings resulting from the distinct crystallization ability of a-PP and i-PP. Furthermore, the existence of a-PP in the composite coatings enhances the mechanical properties compared to the i-PP coating. The proposed method is feasible to modify various substrates and potential applications in no-loss liquid transportation, slippery surfaces, and patterned superhydrophobic surfaces are demonstrated.

Robust anti-reflective silica nanocoatings: abrasion resistance enhanced via capillary condensation of APTES

This paper demonstrates a facile and effective improvement of abrasion resistance of silica nanoparticle (NP) based anti-reflective coatings via capillary condensation of 3-aminopropyl triethoxysilane (APTES). The quartz crystal microbalance (QCM) is used to test the abrasion resistance property. The versatility of this developed method is further illustrated by the successful application to the poor heat-resistant polymer substrates.

Fabrication of oriented wrinkles on polydopamine/polystyrene bilayer films

Wrinkles exist widely in nature and our life. In this paper, wrinkles on polydopamine (PDA)/polystyrene (PS) bilayer films were formed by thermal annealing due to the different thermal coefficients of expansion of each layer. The factors that influenced the dimensions of wrinkles were studied. We found that oriented wrinkles could be formed if the bilayer films were patterned with micro-grooves, and the degree of the orientation depended on the thickness of the PDA and the dimensions of the grooves. Combined with the strong adhesion, biocompatibility and reactivity of PDA, the oriented wrinkles on PDA/PS patterned bilayers may find potential application in diffraction gratings, optical sensors and microfluidic devices.

Precise preparation of highly monodisperse ZrO2@SiO2 core–shell nanoparticles with adjustable refractive indices

This paper describes a straightforward method to precisely prepare monodisperse ZrO2@SiO2 core–shell nanoparticles (CSNs) by sol–gel polymerization of TEOS in the presence of ZrO2 cores without additional capping agents. We studied the morphology and composition of CSNs by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD). It has been found that the citric acid groups adsorbed onto the ZrO2 surface during the preparation of ZrO2 as well as the slow hydrolysis and condensation of TEOS in isopropanol are crucial for the selective growth of silica on the surface of ZrO2 cores. The thickness of the silica shell can be facilely controlled from about 4 to 30 nm by varying the concentration of TEOS, with the refractive indices of the prepared CSNs tuned accordingly from about 1.2 to 1.9. The coated silica shell, which can be well dispersed in solvents such as methanol and ethanol, can improve the dispersibility of pure ZrO2 nanoparticles which can only be dispersed in water. Moreover, the formation of the silica shell improved the chemical reactivity of pure ZrO2 nanoparticles. After modifying with methyltriethoxysilane (MTES), the CSNs can be well dispersed in various organic solvents, such as dimethylbenzene, chloroform and isoamyl alcohol, which may find applications in optically transparent resins with controllable refractive index or organic silicon packaging materials with high refractive index for LED and so on. Additionally, other silane coupling agents can also be applied to modify the surface chemical properties of CSNs, which may provide more potential applications.