Changyu Tang

Resolving the dilemma of gaining conductivity but losing environmental friendliness in producing polystyrene/graphene composites via optimizing the matrix-filler structure

In this work, we report a new approach to resolve a practical problem in producing conductive polystyrene/reduced-graphene-oxide (PS/RGO) composites, i.e. the dilemma that raising the conductivity detrimentally compromises the environmental friendliness of the process because the prevalent chemistry designs emphasize raising conductivity in RGO production by employing strong and toxic chemicals or energy-consuming heat treatments. These designs then rely on organic solvents, chemical functionalization, and stabilizers to overcome the difficulty in dispersing and incorporating RGO into the polymer matrix. In our new design, we emphasize that a compact percolated three-dimensional microcellular RGO network with long-range order is the most effective use of RGO for high composite conductivity. With this guide, GO and non-polar PS mono-dispersed microspheres are co-dispersed in water, with GO stably adsorbing on the microspheres. PS/GO microspheres are then reduced with a nontoxic reducing agent such as vitamin C. PS/RGO microspheres are subsequently hot-pressed into a composite with the required compact percolated three-dimensional microcellular RGO structure. Experimental data collected from the resultant composites validate the design and indeed show low percolation threshold (0.08 vol%), high conductivity (20.5 S m−1), and good compliance with the principles of green chemistry.

Enhanced wetting properties of a polypropylene separator for a lithium-ion battery by hyperthermal hydrogen induced cross-linking of poly(ethylene oxide)

Enhancing the electrolyte wetting of separators by surface modification is very critical to prepare high-performance lithium-ion batteries. Herein, we present a new approach named hyperthermal hydrogen induced cross-linking (HHIC) technology to increase the electrolyte-affinity of polypropylene (PP) separators by covalently cross-linking a thin layer of poly(ethylene oxide) (PEO) on surface-inert PP separators. With the HHIC treatment, the polar functionalities of PEO (e.g. –OH, C–O–C) can be preserved through selective cleavage of C–H bonds and subsequent cross-linking of resulting carbon radicals generated on PEO and PP chains. As proved by solvent rinsing tests, contact angle measurements and Fourier transform infrared spectroscopy, a PEO coating was found firmly fixed on the separator surface, which results in significantly improved wetting with the electrolyte. Electrochemical measurements on subsequent lithium-ion batteries with the modified separator by HHIC treatment exhibit a lower internal resistance but higher capacity retention when compared to the pristine separator. HHIC treatment is concluded to be a highly efficient and environmental-friendly approach for separator surface modification without need for other chemical additives (e.g. chemical cross-linkers, initiators, and catalysts) and can preserve the desired macroscopic material properties of separators such as pore structures and mechanical strength.

3D printing of a mechanically durable superhydrophobic porous membrane for oil–water separation

Although superhydrophobic porous membranes are considered to be very promising candidates for oil–water separation, their fabrication methods often involve complicated treatments to build a coating with micro/nano-features on a porous mesh (called “coating on a mesh structure”), which can lead to weak mechanical stability of the superhydrophobic surfaces and the formation of inhomogeneous membrane pores. Herein, we report a facile and environmentally friendly 3D printing approach to fabricate superhydrophobic membranes with an ordered porous structure for oil–water separation using hydrophobic nanosilica-filled polydimethylsiloxane (PDMS) ink. The addition of nanosilica can improve the mechanical strength of the ink and thus ensures the formation of desired topographical structures without the risk of collapsing during 3D printing. Through adjusting the geometrical parameters, a superhydrophobic PDMS membrane was obtained, which mainly depended on the roughness at the sub-millimeter scale. More importantly, the 3D printing approach described herein integrated the superhydrophobic surface into the porous framework and resulted in a mechanically durable superhydrophobic membrane, which successfully avoids the weak interface adhesion issue that arises from the traditional “coating on a mesh structure.” Moreover, the pore size of the printed membrane could be easily adjusted via a computer program to optimize both the liquid flux and separation efficiency of the membranes. The maximum oil–water separation efficiency (∼99.6%) could be achieved for the printed porous membrane with the pore size of 0.37 mm, which also exhibited a high flux of ∼23 700 L m−2 h−1.

Highly Stretchable Superhydrophobic Composite Coating Based on Self-Adaptive Deformation of Hierarchical Structures

With the rapid development of stretchable electronics, functional textiles, and flexible sensors, water-proof protection materials are required to be built on various highly flexible substrates. However, maintaining the antiwetting of superhydrophobic surface under stretching is still a big challenge since the hierarchical structures at hybridized micro-nanoscales are easily damaged following large deformation of the substrates. This study reports a highly stretchable and mechanically stable superhydrophobic surface prepared by a facile spray coating of carbon black/polybutadiene elastomeric composite on a rubber substrate followed by thermal curing. The resulting composite coating can maintain its superhydrophobic property (water contact angle ≈170° and sliding angle <4°) at an extremely large stretching strain of up to 1000% and can withstand 1000 stretching-releasing cycles without losing its superhydrophobic property. Furthermore, the experimental observation and modeling analysis reveal that the stable superhydrophobic properties of the composite coating are attributed to the unique self-adaptive deformation ability of 3D hierarchical roughness of the composite coating, which delays the Cassie-Wenzel transition of surface wetting. In addition, it is first observed that the damaged coating can automatically recover its superhydrophobicity via a simple stretching treatment without incorporating additional hydrophobic materials.

Cleaving C–H bonds with hyperthermal H2: facile chemistry to cross-link organic molecules under low chemical- and energy-loads

A facile method for cross-linking organic molecules has been developed by computational modeling, instrumentation design, and experimental research. Briefly, organic molecules are hit by H2 with controllable kinetic energy in our novel apparatus where a high flux of hyperthermal H2 is generated. When a C–H bond of the organic molecule is hit by H2 at about 20 eV, efficient kinematic energy-transfer in the H2→H collision facilitates the C–H dissociation with nearly 100% reaction probability. When H2 hits other atoms which are by nature much heavier than H2, mass disparity bars effective energy transfer and this both blocks undesirable bond dissociation and reduces unnecessary energy wastage. The recombination of the carbon radicals generated by the C–H cleavage efficiently completes the production of C–C cross-links at room temperature with no additional energy/chemicals requirements. In addition to these green chemistry merits, this new method is better than other cross-linking techniques which rely on prerequisite reactions to add cross-linkers to the organic molecules or additional reactants and additives. These promising features are validated by several cross-linking trials which demonstrate desirable mechanical, electrical, chemical, and biochemical changes while inducing no undesirable damage of chemical functionalities in the original molecules.

“Zero-transfer” production of large-scale, flexible nanostructured film at water surface for surface enhancement Raman spectroscopy

Reduced complexity in production of large-scale, flexible surface enhancement Raman spectroscopy (SERS) active substrate has been implemented at the water surface, when polydimethylsiloxane (PDMS) solution having lower density than the water meets with self-assembled polystyrene (PS) nanoparticles (NPs). Through tuning its flowability, the PDMS solution could effectively wet PS NPs, wherein the PS NPs can be embedded in the PDMS during the curing process. This technical innovation saves ill-posed transfer steps as present in traditional operations which may cause damaged nanostructures, and it could be beneficial for preparing a large scale, high quality, and flexible SERS active substrate. Field tests demonstrated that the Raman signal enhancement factor could reach up to the order of ∼107 with decent repeatability less than 10%.

Surface enhanced Raman scattering fiber optic sensor as an ion selective optrode: the example of Cd2+ detection

Here in this work, we report the fabrication of a (metal) ion selective surface enhanced Raman scattering (SERS) optrode, the counterpart of an ion selective electrode, for the detection of metal ions in solution. Following our previous work, a layer-by-layer self-assembly strategy was used to fabricate the SERS optrode, followed by modification with an ion chelating reagent, 4-(4-phenylmethanethiol)-2,2′:6′,2′′-terpyridine (PMTTP). The SERS spectrum change after binding with metal ions was used to identify and detect metal ions in solution. Cd2+ in aqueous solution was chosen as a sample analyte. Similar to standard pH measurement, through simple single point known standard solution calibration, a quick (semi-)quantitative analysis of Cd2+ was realized.

Cross-Linking Poly(lactic acid) Film Surface by Neutral Hyperthermal Hydrogen Molecule Bombardment.

Constructing a dense cross-linking layer on a polymer film surface is a good way to improve the water resistance of poly(lactic acid) (PLA). However, conventional plasma treatments have failed to achieve the aim as a result of the unavoidable surface damage arising from the charged species caused by the uncontrolled high energy coming from colliding ions and electrons. In this work, we report a modified plasma method called hyperthermal hydrogen-induced cross-linking (HHIC) technology to construct a dense cross-linking layer on PLA film surfaces. This method produces energy-controlled neutral hyperthermal hydrogen, which selectively cleaves C-H bonds by molecule collision from the PLA film without breaking other bonds (e.g., C-C bonds in the polymer backbone), and results in subsequent cross-linking of the carbon radicals generated from the organic molecules. The formation of a dense cross-linking layer can serve as a barrier layer to significantly improve both the hydrophobicity and water vapor barrier property of the PLA film. Because of the advantage of selective cleavage of C-H bonds by HHIC treatment, the original physical properties (e.g., mechanical strength and light transmittance) of the PLA films are well-preserved.

Azobenzene dendronized carbon nanoparticles: the effect of light antenna

Fluorescent carbon nanoparticles are grafted with azobenzene dendrons, giving unique dendronized carbon nanoparticles that exhibit an increased quantum yield by as much as ∼74% at a low concentration. Such an improvement is attributed to the strong light harvesting capability of grafted azobenzene dendrons that act as “light antenna” to collect photons and the excitons generated then traverse to adjacent central carbon nanoparticles.

Patternable Poly(chloro-p-xylylene) Film with Tunable Surface Wettability Prepared by Temperature and Humidity Treatment on a Polydimethylsiloxane/Silica Coating

Poly(chloro-p-xylylene) (PPXC) film has a water contact angle (WCA) of only about 84°. It is necessary to improve its hydrophobicity to prevent liquid water droplets from corroding or electrically shorting metallic circuits of semiconductor devices, sensors, microelectronics, and so on. Herein, we reported a facile approach to improve its surface hydrophobicity by varying surface pattern structures under different temperature and relative humidity (RH) conditions on a thermal curable polydimethylsiloxane (PDMS) and hydrophobic silica (SiO2) nanoparticle coating. Three distinct large-scale surface patterns were obtained mainly depending on the contents of SiO2 nanoparticles. The regularity of patterns was mainly controlled by the temperature and RH conditions. By changing the pattern structures, the surface wettability of PPXC film could be improved and its WCA was increased from 84° to 168°, displaying a superhydrophobic state. Meanwhile, it could be observed that water droplets on PPXC film with superhydrophobicity were transited from a “Wenzel” state to a “Cassie” state. The PPXC film with different surface patterns of 200 μm × 200 μm and the improved surface hydrophobicity showed wide application potentials in self-cleaning, electronic engineering, micro-contact printing, cell biology, and tissue engineering.