Additive-free hydrogelation of graphene oxide by ultrasonication

Graphene oxide hydrogels have been prepared by ultrasonication of precursor aqueous dispersions. The ultrasonication fractures the nanosheets, reducing their dimensions and exposing new sheet edges that do not possess the stabilizing carboxyl functional groups found along the edge of the as-prepared material. Ultrasonication does not affect the overall chemical functionality of the graphene oxide nanosheets, as spectra (carbon-13 nuclear magnetic resonance spectroscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy) of samples before and after ultrasonication are nearly identical. Gelation is induced after only 30 min of ultrasonication to achieve a relatively weak gel with a shear modulus of 0.3 kPa; however, extension of ultrasonic treatment to 120 min yields a more robust hydrogel with a shear modulus of 1.6 kPa. Such enhancement in the gel’s physical properties can be attributed to the lack of stabilizing carboxyl groups on newly generated nanosheet fragments from the interior regions of the original nanosheets. As prepared, these hydrogels exhibit exceptionally low critical gelation concentrations ranging from ～0.050 to ～0.125 mg mL^?1 that can be tuned according to the extent of ultrasonic treatment.     

Polylactic acid/ethylene glycol triblock copolymer as novel crosslinker for epoxidized natural rubber

Polylactic acid/ethylene glycol triblock copolymer (LLA₄₆EG₄₆LLA₄₆) was prepared and used in a crosslink process of epoxidized natural rubber (ENR) by employing a ring‐opening reaction using Sn(Oct)₂ as a catalyst. The OH‐capped copolymer acts as a macromolecular crosslinking agent in the formation of ENR networks, leading to drastic enhancement in tensile properties. Crosslink efficiency and chemical structures of the cured materials are examined by solvent fractionation, swelling experiments, XRD, ¹H‐NMR, and ATR‐FTIR spectroscopy. The efficiency of the curing process is dependent on the ENR/copolymer feed ratios. The degree of property improvement and gas permeability/selectivity behaviors of the cured materials are strongly dependent on the copolymer content and the efficiency of the curing process. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Effect of physical aging on the shape-memory behavior of amorphous networks

This paper presents an experimental and modeling study of the effects of physical aging on the shape-memory performance of (meth)acrylate-based networks composed of tert-butyl acrylate (tBA) crosslinked by various concentrations of poly(ethylene glycol dimethacrylate) (PEGDMA). The experiments measured the unconstrained recovery response of samples stored at 20 °C (T_g ? 36 °C) for zero to 180 days and evaluated the effects of storage on the strain fixity, activation temperature, and initial recovery rate. A thermoviscoelastic model recently developed for amorphous networks near the T_g was applied to study the influence of structural and viscoelastic relaxation and the aging time and temperature on the recovery response. Results showed that the activation temperature and the initial recovery rate increased with the aging time, producing a sharper initial recovery response. The thermoviscoelastic model predicted that the magnitude of these effects depended on the aging temperature. There was an optimum aging temperature that maximized the initial recovery rate. These results suggest that physical aging can be manipulated to accelerate the recovery performance of shape-memory polymer devices.     

An eco-friendly and efficient route of lignin extraction from black liquor and a lignin-based copolyester synthesis

Traditional method for extraction of lignin from black liquor in chemical pulping processes requires a huge amount of toxic inorganic acids like sulfuric acid which are potentially harmful to our environment and life. Furthermore, the traditional method always contains multiple steps, and thus it is time-consuming process. With a purpose to eliminate all these above disadvantages, in this study, we successfully developed an efficient process for the extraction of lignin from black liquor using a non-toxic aluminum potassium sulfate dodecahydrate (AlK(SO₄)₂·12H₂O). The developed process is simple, efficient, and short-time, which obviously have more advantages over the traditional extraction method. Furthermore, the lignin extracted in this study was used to synthesize a copolyester through polyesterification between lignin and sebacoyl chloride. The copolymer possesses a molecular weight of 31,800, corresponding to four to five repeating units of lignin macromonomers. Notably, it showed a good thermal stability up to 200 °C in TGA analysis. It was also possible to shape the copolymer using solvent casting. We believe that this newly developed method of lignin extraction may exploit new applications for eco-friendly sustainable materials in various fields.     

Synthesis of single-crystal SnO2 nanowires for NOx gas sensors application

Single-crystal SnO₂ nanowires (NWs) were successfully synthesized and characterized as sensing materials for long-term NO_x stability detection in environmental monitoring. Reproducible and selective growths of the SnO₂ NWs on a patterned, 5 nm-thick gold catalyst coated on a SiO₂/Si wafer as substrate were conducted by evaporating SnO powder source at 960 °C in a mixture of argon/oxygen ambient gas (Ar: 50 sccm/O₂: 0.5 sccm). The as-obtained products were characterized by field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Raman scattering, and photoluminescence (PL). The SEM and HRTEM images revealed that the products are single-crystal SnO₂ NWs with diameter and length ranges of 70 nm–150 nm and 10 μm–100 μm, respectively. The three observed Raman peaks at 476, 633, and 774 cm⁻¹ indicated the typical rutile phase, which is in agreement with the XRD results. The NWs showed stable PL with an emission peak centered at around 620 nm at room-temperature, indicating the existence of oxygen vacancies in the NW samples. The electrical properties of synthesized SnO₂ NWs sensor were also investigated and it exhibited a negative temperature coefficient of resistance in the measured range (300–525 K). The calculated activation energy E_c of SnO₂ NWs was 0.186 eV. Moreover, the SnO₂ NW sensors exhibited good response to NO_x gas. The response of the sensors to 5 ppm NO_x reached 105% at an operating temperature of 200 °C.     

Downlink Performance Analysis of MIMO Relaying Networks

In recent years, there has been an upsurge of interest in relay-augmented infrastructure-based networks. On the other hand, multiple-input-multiple-output (MIMO) wireless system becomes a hot topic for it can achieve significant increases in overall system performance. This paper gives a detailed analysis of downlink performance of MIMO relaying networks, in which different channel models and classic MIMO cellular system model with fixed relay are considered. Our results in single cell scenario show that when meeting effective relay system efficiency of 100%, the required relay SNR gain of 2-antenna system can be reduced to less than half of that with only one transceiver antenna. We also observe that MIMO is more tolerant to interference compared with the one-transceiver antenna system. After that, multi-hop architecture is researched and statistics figures out the tolerable number of hops which is supported in wireless communication with a view to real application. In addition, multi-cell scenario is also studied. Statistics results show that there is more 8 dB average spectral efficiency gain in a two-hop fixed relay 4 × 4 MIMO system than that in a SISO system. These results present significant indications in deployment and optimization of relay-based MIMO networks.     

Investigation of Generalized SIFs of cracks in 3D piezoelectric media under various crack-face conditions

This paper investigates the influence of crack geometry, crack-face and loading conditions, and the permittivity of a medium inside the crack gap on intensity factors of planar and non-planar cracks in linear piezoelectric media. A weakly singular boundary integral equation method together with the near-front approximation is adopted to accurately determine the intensity factors. Obtained results indicate that the non-flat crack surface, the electric field, and the permittivity of a medium inside the crack gap play a crucial role on the behavior of intensity factors. The mode-I stress intensity factors (K_{\mathrm{I}}) for two representative non-planar cracks under different crack-face conditions are found significantly different and they possess both upper and lower bounds. In addition, K_{\mathrm{I}} for impermeable and semi-permeable non-planar cracks treated depends strongly on the electric field whereas those of impermeable, permeable, and semi-permeable penny-shaped cracks are identical and independent of the electric field. The stress/electric intensity factors predicted by permeable and energetically consistent models are, respectively, independent of and dependent on the electric field for the penny-shaped crack and the two representative non-planar cracks. Also, the permittivity of a medium inside the crack gap strongly affects the intensity factors for all crack configurations considered except for K_{\mathrm{I}} of the semi-permeable penny-shaped crack.     

Simulating secondary organic aerosol activation by condensation of multiple organics on seed particles

The conditions under which semivolatile organic gases condense were studied in a Teflon particle chamber by scanning mobility particle sizing (SMPS) of the resultant particles. Benzaldehyde, maleic and citraconic anhydrides, n-decane, trans-cinnamaldehyde, and citral were introduced in various combinations into a particle chamber containing either particle-free nitrogen or nitrogen with dry seed particles made out of sodium chloride, D-tartartic acid, ammonium sulfate, or 1,10-decanediol. No organic gas was allowed to reach its saturation point relative to the vapor pressure of its pure liquid in any experiment. In the absence of seed particles, organic aerosol particles formed by ternary nucleation when the sum of the individual organic saturation levels reached a threshold between 1.17 and 1.86. With seed particles present, particle sizes began to increase when the sum of organic saturation levels reached 1.0. This size increase corresponds to the establishment and activation of ternary organic layers on the "clean" seed particles, as predicted by absorption partitioning theory. The observed increases in particle volume depended on initial seed particle volume, indicating that either gas diffusion rates or chemical reactions were controlling the rate of uptake.