Yajiang Huang

Functionalized Graphene Enables Highly Efficient Solar Thermal Steam Generation

The ability to efficiently utilize solar thermal energy to enable liquid-to-vapor phase transition has great technological implications for a wide variety of applications, such as water treatment and chemical fractionation. Here, we demonstrate that functionalizing graphene using hydrophilic groups can greatly enhance the solar thermal steam generation efficiency. Our results show that specially functionalized graphene can improve the overall solar-to-vapor efficiency from 38% to 48% at one sun conditions compared to chemically reduced graphene oxide. Our experiments show that such an improvement is a surface effect mainly attributed to the more hydrophilic feature of functionalized graphene, which influences the water meniscus profile at the vapor-liquid interface due to capillary effect. This will lead to thinner water films close to the three-phase contact line, where the water surface temperature is higher since the resistance of thinner water film is smaller, leading to more efficient evaporation. This strategy of functionalizing graphene to make it more hydrophilic can be potentially integrated with the existing macroscopic heat isolation strategies to further improve the overall solar-to-vapor conversion efficiency.

The intrinsic thermal-oxidative stabilization effect of chemically reduced graphene oxide on polypropylene

The antioxidative effect of chemically reduced graphene oxide (rGO) on the thermal-oxidative stability of polypropylene (PP) was evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). rGO was prepared by reduction of graphene oxide (GO) and characterized by atomic force microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. PP/rGO nanocomposites were then prepared without using a compatibilizer by melt blending. It was found that the thermal-oxidative degradation of PP was retarded noticeably by the rGO. The stabilization mechanism of rGO was discussed in terms of the changes in carbonyl bands and oxygen diffusion. It was proposed that the improved thermal-oxidation stability of PP/rGO nanocomposites can be attributed to the decline in both the concentration of peroxy radicals and oxygen permeability. The acceptor-like electronic property afforded by the long conjugated CC bonds and the barrier effect of rGO were suggested to be responsible for the improved thermal-oxidation stability of PP.

Morphology and rheology of poly(l-lactide)/polystyrene blends filled with silica nanoparticles

The successful development of co-continuous structure from poly(l-lactide) (PLLA) blends by melt mixing with lower PLLA content is highly desired in preparing macroporous biomaterials. However, the low viscosity of PLLA makes it difficult to prepare co-continuous PLLA blends at low PLLA concentration. In this study, hydrophilic silica nanoparticle is adopted to control the morphology of co-continuous polystyrene (PS)/PLLA blends. The influence of nanoparticle concentration on the co-continuity intervals and rheological properties of PS/PLLA blends are examined. The morphological stability of blends against melt annealing is also determined and discussed with a conceptual coarsening model for co-continuous structure. The results demonstrate that the incorporation of silica nanoparticles into PS/PLLA blends can be used to prepare macroporous PLLA structure with controllable pore size at lower PLLA content.

Preparation of alumina-coated graphite for thermally conductive and electrically insulating epoxy composites

Herein, highly thermally conductive and insulating epoxy composites were reported. Firstly uniform alumina-coated graphite flakes were successfully prepared by a two-step coating method of chemical precipitation with the aid of a sodium dodecyl sulfonate (SDS) surfactant using an inorganic precursor (aluminum nitrate) as the starting material. Then the alumina-coated graphite particles were incorporated into the epoxy resin. The thermal conductivity value of epoxy/alumina-coated graphite composite shows a significant increase from 0.22 W mK−1 (neat epoxy) to 0.64 W mK−1 by a factor of approximately 3 at the filler loading of 18.4%. Moreover, due to the presence of the alumina nanolayers coating on the graphite surface, epoxy/alumina-coated graphite composites could retain high electrical volume resistivity of >1010 Ω cm up to high filler contents, which was much higher than that of epoxy/graphite composites (<105 Ω cm) at the same filler loadings. And they still could be regarded as insulators.

Fractionated crystallization and morphology of PP/PS blends in the presence of silica nanoparticles with different surface chemistries

The fractionated crystallization behavior of polypropylene (PP) droplets in its 20/80 blends with polystyrene (PS) in the presence of hydrophilic or hydrophobic fumed silica nanoparticles was studied by using differential scanning calorimetry, scanning electron microscopy, and transmission electron microscopy. It was found that the fractionated crystallization of PP droplets in the PS matrix was promoted by adding a low content of hydrophobic or hydrophilic nanoparticles due to their morphological refinement effect. However, discrepancies in the fractionated crystallization behavior of PP droplets occurred as the nanoparticle content increased. The crystallization became dominated by the heterogeneous nucleation effect of high content of hydrophilic nanoparticles, which possibly migrated into PP droplets during mixing and significantly suppressed their fractionated crystallization. In contrast, the morphological refinement effect still played a dominated role in promoting the fractionated crystallization of PP droplets in PP/PS blends filled with higher content hydrophobic nanoparticles as a result of the efficiently morphological refinement effect.

Dispersion and rheology of polypropylene/organoclay nanocomposites: effect of cation exchange capacity and number of alkyl tails

Nanocomposites of montmorillonite organoclays and polypropylene (PP) were prepared via direct melt intercalation using maleic anhydride functionalized polypropylene (PP-g-MA) as a compatibilizer. Two montmorillonite clays (MMT) with different cation exchange capacities (CEC) were exchanged with alkyl ammonium ions, in which one or two octadecyl chains are attached to the nitrogen atom. The role of alkyl chain numbers and CEC value on the dispersion of clay and rheology of PP nanocomposites under shear and extensional flow was evaluated by X-ray diffraction, scanning electron microscopy, and rheologic techniques. It was found that the low-CEC organoclay with one alkyl chain could only form a conventional composite. However, the low-CEC organoclay with two alkyl chains or high-CEC organoclay with one alkyl chain can disperse finely in the matrix. Nanocomposites containing these two organoclays showed typical shear rheologic properties of intercalated nanocomposites, but only the former showed a mild strain-hardening behavior in uniaxial extensional flow. When using an intercalant with two tails, the high-CEC clay would lead the organoclay to form mixed structures which further resulted in an inferior dispersion quality. It was proposed that the dispersion quality and rheologic properties of nanocomposites were related to the arrangement of modifier molecules in the clay galleries, which was determined by the CEC of clay and the structure of alkyl ammonium ions.

Unusual hierarchical structures of micro-injection molded isotactic polypropylene in presence of an in situ microfibrillar network and a β-nucleating agent

The microstructural and mechanical properties of isotactic polypropylene (iPP), in situ PET microfibrils, and β-nucleating agent blends obtained from micro-injection molding were investigated via polarized light microscopy, differential scanning calorimetry, scanning electron microscopy, and two-dimensional wide-angle X-ray diffraction. The results indicate that addition of PET microfibrils markedly increases crystallization temperatures, and increases the thickness of the final oriented layer. Introduction of PET microfibrils to β-nucleation agent-nucleated iPP samples leads to formation of oriented β-crystals epiphytic on the surface of PET fibers in the inner region; this feature improves adhesion between the fiber and the matrix and simultaneously improves the strength and toughness of the final PP/0.5/15 microparts (e.g., the tensile strength increased by 12 MPa and the elongation at break increased by 1.2%) compared with those of iPP microparts. Taken together, the results of this study introduce an alternative approach to optimize the properties of MIM parts.

Effect of air plasma treatment on the mechanical properties of polyphenylene sulfide/glass fiber cloth composites

In this research study, air plasma-treated polyphenylene sulfide film was used to prepare polyphenylene sulfide/glass fiber cloth composites for improving interfacial adhesion and mechanical properties. The effect of air plasma treatment on the properties of polyphenylene sulfide films was investigated by X-ray photoelectron spectroscopy and water contact angle analysis. According to the X-ray photoelectron spectroscopy analysis, it was found that plasma treatment led to the emergence of oxygen-containing polar groups on the film surface. The intensity of sulfide oxides (S = O and O = S = O) on the surface of polyphenylene sulfide films increased to 79.03% after it was modified by air plasma. Water droplet contact angle analysis indicated that plasma treatment significantly increased the hydrophilicity of the polyphenylene sulfide film. The contact angle was 38 degrees for plasma-treated polyphenylene sulfide film, compared to 78.5 degrees for untreated polyphenylene sulfide film. Tensile strength and notched impact strength of plasma-treated polyphenylene sulfide/glass fiber cloth (50/50) composite increased by 11% (from 248.49 MPa to 275.70 MPa) and 18% (from 52.26 KJ/m2 to 61.43 KJ/m2), respectively, compared with the raw specimen, which was consistent with the morphology analysis result. Meanwhile, the mechanical properties of the composites were improved with the increased discharge power density and treatment time.

Morphological hysteresis in immiscible PIB/PDMS blends filled with fumed silica nanoparticles

The morphological hysteresis behavior of immiscible polymer blend reflects the dependence of their steady-state morphology on the shear protocol applied. In this work, the influences of hydrophobic and hydrophilic fumed silica nanoparticles on the morphology hysteresis behavior of immiscible polyisobutylene (PIB)/polydimethylsiloxane (PDMS) (10/90) blends under simple shear flow were investigated by using optical shear technique. Compared with particle-free blend, the morphology hysteresis zone of filled blends was found to be expanded by the addition of hydrophobic or hydrophilic fumed silica nanoparticles. It was found that the expansion of the morphology hysteresis zone in hydrophobic nanoparticle-filled blend stemmed from the suppression of droplet coalescence. However, the expansion in the morphological hysteresis zone for hydrophilic nanoparticle-filled blend, which was less noticeable, might originate from the more difficult breakup of PIB droplets upon the addition of nanoparticles.

Pickering emulsions stabilized by shape-controlled silica microrods

Silica microrods with aspect ratios (AR) varying from 1 to 16 but similar surface chemical characteristics are synthesized and their potential in preparing stable oil-in-water Pickering emulsions is explored. The stability of hexadecane/water emulsions is found to strongly depend on the AR and concentration of particles. Emulsions stabilized with these silica microrods are quiescently stable for quite a long period of time (over months), while emulsions with spherical particles of similar diameters destabilize after only dozens of hours. The superior stabilization efficiency of microrods with larger ARs is attributed to their higher steric hindrance, interface adsorption energy and capillary forces.