Xingru Yan

Electrically conductive thermoplastic elastomer nanocomposites at ultralow graphene loading levels for strain sensor applications

An electrically conductive ultralow percolation threshold of 0.1 wt% graphene was observed in the thermoplastic polyurethane (TPU) nanocomposites. The homogeneously dispersed graphene effectively enhanced the mechanical properties of TPU significantly at a low graphene loading of 0.2 wt%. These nanocomposites were subjected to cyclic loading to investigate the influences of graphene loading, strain amplitude and strain rate on the strain sensing performances. The two dimensional graphene and the flexible TPU matrix were found to endow these nanocomposites with a wide range of strain sensitivity (gauge factor ranging from 0.78 for TPU with 0.6 wt% graphene at the strain rate of 0.1 min−1 to 17.7 for TPU with 0.2 wt% graphene at the strain rate of 0.3 min−1) and good sensing stability for different strain patterns. In addition, these nanocomposites demonstrated good recoverability and reproducibility after stabilization by cyclic loading. An analytical model based on tunneling theory was used to simulate the resistance response to strain under different strain rates. The change in the number of conductive pathways and tunneling distance under strain was responsible for the observed resistance-strain behaviors. This study provides guidelines for the fabrication of graphene based polymer strain sensors.

Advanced micro/nanocapsules for self-healing smart anticorrosion coatings

Smart self-healing coatings for corrosion protection of metallic substrates (steel, magnesium, and aluminium, and their alloys) have attracted tremendous interest due to their capability to prevent crack propagation in the protective coatings by releasing active agents from micro/nanocapsules, that is, micro/nano particles consisting of a coating layer or a shell (micro/nanocontainers) and core material (solids, droplets of liquids or gases), in a controllable manner. This paper aims to give a concise review on the most recent advances in preparing micro/nanocapsules based on different types of micro/nanocontainers, i.e., organic polymer coatings, inorganic clays, mesoporous silica nanoparticles, polyelectrolyte multilayers, etc. for smart coatings with self-healing properties. The state-of-the-art design and preparation of micro/nanocapsules are highlighted with detailed examples.

An overview of multifunctional epoxy nanocomposites

Epoxy is a crucial engineered thermosetting polymer with wide industrial applications in adhesive, electronics, aerospace and marine systems. In this review, basic knowledge of epoxy resins and the challenge for the preparation of epoxy nanocomposites are summarized. The state-of-art multifunctional epoxy nanocomposites with magnetic, electrically conductive, thermally conductive, and flame retardant properties of the past few years are critically reviewed with detailed examples. The applications of epoxy nanocomposites in aerospace, automotives, anti-corrosive coatings, and high voltage fields are briefly summarized. This knowledge will have great impact on the field and will facilitate researchers in seeking new functions and applications of epoxy resins in the future.

Cellulose derived magnetic mesoporous carbon nanocomposites with enhanced hexavalent chromium removal

Magnetic carbon–iron nanoadsorbents fabricated by carbonizing cellulose and reducing Fe3O4 nanoparticles or Fe(NO3)3 (the products are denoted as MC–O and MC–N, respectively) have demonstrated great Cr(VI) removal. MC–N with a higher proportion of zero-valence iron (ZVI) and bigger specific surface area exhibited better resistance to oxygen and acid than MC–O due to its smaller pore size. The Cr(VI) removal was highly pH-dependent. For example, 4.0 mg L−1 Cr(VI) neutral solution was completely purified by 2.5 g L−1 MC–O and MC–N within 10 min. 1000 mg L−1 Cr(VI) solution at pH 1.0 was completely removed by both nanoadsorbents in 10 min. The MC–O nanoadsorbents had a higher removal percentage (98.1%) than MC–N (93.5%) at pH 7.0, while MC–N had a removal capacity of 327.5 mg g−1, much higher than 293.8 mg g−1 of MC–O at pH 1.0. A chemical adsorption was revealed from the pseudo-second-order kinetic study. Monolayer adsorption of Cr(VI) was revealed by a better fitting of the Langmuir model isotherm, rather than multilayer adsorption for the Freundlich model. These nanoadsorbents could be easily separated from solution by using a permanent magnet after being treated with Cr(VI). Finally, the Cr(VI) removal mechanisms were proposed considering the Cr(VI) reduction and precipitation of Cr(III).

Electrically Conductive Polypropylene Nanocomposites with Negative Permittivity at Low Carbon Nanotube Loading Levels

Polypropylene (PP)/carbon nanotubes (CNTs) nanocomposites were prepared by coating CNTs on the surface of gelated/swollen soft PP pellets. The electrical conductivity (σ) studies revealed a percolation threshold of only 0.3 wt %, and the electrical conductivity mechanism followed a 3-d variable range hopping (VRH) behavior. At lower processing temperature, the CNTs formed the network structure more easily, resulting in a higher σ. The fraction of γ-phase PP increased with increasing the pressing temperature. The CNTs at lower loading (0.1 wt %) served as nucleating sites and promoted the crystallization of PP. The CNTs favored the disentanglement of polymer chains and thus caused an even lower melt viscosity of nanocomposites than that of pure PP. The calculated optical band gap of CNTs was observed to increase with increasing the processing temperature, i.e., 1.55 eV for nanocomposites prepared at 120 °C and 1.70 eV prepared at 160 and 180 °C. Both the Drude model and interband transition phenomenon have been used for theoretical analysis of the real permittivity of the nanocomposites.

Polymer nanocomposites for energy storage, energy saving, and anticorrosion

Polymer nanocomposites exhibit unique physicochemical properties that cannot be obtained with individual components acting alone. Polymer nanocomposites have attracted significant research interests due to their promising potential for versatile applications ranging from environmental remediation, energy storage, electromagnetic (EM) absorption, sensing and actuation, transportation and safety, defense systems, information industry, to novel catalysts, etc. In particular, polymer nanocomposites have attracted intensive research interest for solving both energy and environmental issues. This review paper mainly focuses on the most recent advances in polymer nanocomposites for energy storage (i.e., electrochemical capacitors and batteries), energy saving (i.e., electrochromic devices and carbon dioxide capture), and anticorrosion (conductive and non-conductive polymer nanocomposite anticorrosive coatings) applications.

Organic vapor sensing behaviors of conductive thermoplastic polyurethane–graphene nanocomposites

Conductive thermoplastic polyurethane (TPU) nanocomposites filled with graphene were fabricated and tested for organic vapor sensing. The observed finely dispersed graphene in the TPU matrix benefited from the formation of efficient conductive paths and the generation of stable electrical signals. Organic vapor sensing behaviors of the conductive polymer composites (CPCs) were evaluated using four kinds of organic vapors possessing different polarities (p), including cyclohexane (p = 0.1), tetrachloromethane (CCl4, p = 1.6), ethylacetate (p = 4.3) and acetone (p = 5.4). Unlike conventional CPCs that only respond to certain specific groups of organic vapors, the current CPCs showed a novel negative vapor coefficient (NVC) effect for all tested vapors. This observed NVC was due to both the inherent microphase segregation structure of TPU containing soft and hard segments and the wrinkled structure of graphene. In successive immersion-drying runs (IDRs) at 30 °C, fast response, good reversibility and reproducibility were observed for the non- and low- polar vapors (cyclohexane and CCl4), but residual resistance was observed for polar organic vapors (ethylacetate and acetone) after their desorption. The temperature dependent vapor sensing behaviors indicated that the vapor sensing responsivity increased with increasing the temperature due to higher absorption activation energy at higher temperature. This study provides guidelines for the fabrication of organic vapor sensors using CPCs possessing fast response, good discrimination ability and reproducibility.

Synthesis of Highly Efficient Flame Retardant High-Density Polyethylene Nanocomposites with Inorgano-Layered Double Hydroxides As Nanofiller Using Solvent Mixing Method

High-density polyethylene (HDPE) polymer nanocomposites containing Zn2Al-X (X= CO3(2-), NO3(-), Cl(-), SO4(2-)) layered double hydroxide (LDH) nanoparticles with different loadings from 10 to 40 wt % were synthesized using a modified solvent mixing method. Synthesized LDH nanofillers and the corresponding nanocomposites were carefully characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, etc. The thermal stability and flame retardancy behavior were investigated using a thermo gravimetric analyzer and microscale combustion calorimeter. Comparing to neat HDPE, the thermal stability of nanocomposites was significantly enhanced. With the addition of 15 wt % Zn2Al-Cl LDH, the 50% weight loss temperature was increased by 67 °C. After adding LDHs, the flame retardant performance was significantly improved as well. With 40 wt % of LDH loading, the peak heat release rate was reduced by 24%, 41%, 48%, and 54% for HDPE/Zn2Al-Cl, HDPE/Zn2Al-CO3, HDPE/Zn2Al-NO3, and HDPE/Zn2Al-SO4, respectively. We also noticed that different interlayer anions could result in different rheological properties and the influence on storage and loss moduli follows the order of SO4(2-) > NO3(-) > CO3(2-) > Cl(-). Another important finding of this work is that the influence of anions on flame retardancy follows the exact same order on rheological properties.

Interfacially reinforced unsaturated polyester composites by chemically grafting different functional POSS onto carbon fibers

Monofunctional (methacrylolsobutyl) and multifunctional (methacryl) polyhedral oligomeric silsesquioxane (POSS) were successfully grafted on a carbon fiber (CF) surface to enhance the interfacial strength of CF reinforced unsaturated polyester resin (UPR) composites. The silicon containing functional groups were obviously increased on the CFs after the successful grafting of POSS onto the CF surface. Both kinds of POSS were uniformly grafted on the CF surface and the surface roughness of the CFs grafted with methacryl POSS and methacrylolsobutyl POSS was almost the same (131.6 and 129.6 nm) but much higher than that of the as-received CFs (57.9 nm). Dynamic contact angle analysis of the CFs grafted with both POSS showed almost the same surface energy but higher than that of the as-received CFs. After being grafted with methacrylolsobutyl POSS, the interlaminar shear strength of the composites was 62 MPa, increased by 31.9%, however, in terms of methacryl POSS the value was 67 MPa, increased by 42.6% compared with that of the as-received CFs (47 MPa). The interfacial shear strength (IFSS) of the composites with methacryl POSS grafted CFs (93 MPa) was significantly increased by 102.2% compared with the composites with the as-received CFs (46 MPa) and is even higher than that of the composites with methacrylolsobutyl POSS grafted CFs (87 MPa). The impact energy of 1.72 J for the composites with methacryl POSS grafted CFs is higher than that of the composites with as-received CFs (1.00 J) and methacrylolsobutyl POSS grafted CFs (1.43 J).

Polypyrrole doped epoxy resin nanocomposites with enhanced mechanical properties and reduced flammability

For the liquid epoxy nanosuspensions with both fibril and spherical polypyrrole (PPy) nanostructures, a stronger PPy nanofibers/epoxy interaction and more temperature stable behavior with a lower flow activation energy of nanosuspensions with nanofibers (54.34 kJ mol−1) than that with nanospheres (71.15 kJ mol−1) were revealed by rheological studies. As well as the common enhancing mechanism of limiting crack propagation in the polymer matrix, the nanofibers further initiated the shear bands in the epoxy resin to give a higher tensile strength (90.36 MPa) than that of pure epoxy (70.03 MPa) and even that of the epoxy nanocomposites with nanospheres (84.53 MPa). With a larger specific surface area, the nanofibers rather than nanospheres were observed to reduce the flammability of epoxy more efficiently by assisting more char formation of the epoxy resin. The hydroxyl groups formed between the protons of the doped acid in the PPy nanofillers and the epoxy broke the conjugate structure of PPy, leading to a higher bandgap in the nanocomposites (Eg1 = 3.08 eV for 1.0 wt% PPy nanofibers) than that of pure nanofillers (1.8 eV for PPy nanofibers and 1.2 eV for PPy nanospheres). Due to the high aspect ratio, the PPy nanofibers could form the conductive path more easily than the PPy nanospheres to provide a lower percolation threshold value. The real permittivity was observed to increase with increasing the PPy nanofiller loading, and the enhanced permittivity was interpreted by the interfacial polarization.