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A method for the measurement of Hildebrand solubility parameter has been developed. This method was based on the measurement of the sessile drop contact angle and contact base width during its evaporation. Experimental results showed that this method is accurate and insensitive to the measurement conditions. The major advantages of this method are that it is time efficient and requires a small amount of liquid (microliter) for the measurement. [ABSTRACT FROM AUTHOR]

We used molecular dynamics simulationsand experiments to studythe rheology of polymer–clay nanocomposite hydrogels. The moleculardynamics simulations studied the formation of physical cross-linkednetworks as a function of the clay concentration. Simulations showedthat while the local structure changed from isolated polymer–clayclusters to a percolating network with increasing clay concentrationthe networks were only able to sustain stress at concentrations ofroughly 1.5 times the percolation transition. Experiments using poly(N-isopropylacrylamide) (PNIPA)–clay nanocompositesat different clay concentrations were compared with simulation results.Experiments showed a transition from viscous to gel like behaviorat a clay concentration of 15.24g/L, in good agreement with simulations.The modulus, G′ and G″,prior to yield was observed to increase exponentially with clay concentrationand G′ at yield could be fitted to the modifiedMooney’s equation. [ABSTRACT FROM AUTHOR]

Abstract: We have shown that the addition of nanoclays is an effective means for enhancing the flame retardant properties of polymer blends. Polymer blends are difficult to render flame retardant even with the addition of flame retardant agents due to dispersion and phase segregation during the heating process. We show that the addition of 5% functionalized Cloisite 20A clays in combination with 15% decabromodiphenyl ether and 4% antimony trioxide to a polystyrene/poly(methyl methacrylate) blend can render the compound flame resistant within the UL-94-V0 standard. Using a variety of micro-characterization methods, we show that the clays are concentrated at the interfaces between the polymers in this blend and completely suppress phase segregation. The flame retardant (FR) is absorbed onto the clay surfaces, and the exfoliation of the clays also distributes the FR agent uniformly within the matrix. TGA of the nanocomposite indicates that prior to the addition of clay, the dissociation times of the individual components varied by more than 20°C, which complicated the gas-phase kinetics. Addition of the clays causes all the components to have a single dissociation temperature, which enhanced the efficacy of the FR formula in the gas phase. Cone calorimetry also indicated that the clays decreased the heat release rate (HRR) and the mass loss rate (MLR), due to the formation of a robust char. In contrast, minimal charring occurred in blends containing just the FR. SEM examination of the chars showed that the clay platelets were curved and in some cases tightly folded into nanotube-like structures. These features were only apparent in blends, indicating that they might be associated with thermal gradients across the polymer phase interface. SEM and SAXS examinations of the nanocomposites after partial exposure to the flame indicated that the clays aggregated into ribbon-like structures, approximately microns in length, after the surfactant thermally decomposed. Thermal modeling indicated that these ribbons might partially explain the synergy due to better distribution of the heat and improve the mechanical properties of the melt at high temperatures, in a manner similar to the one reported for carbon nanotubes. [Copyright &y& Elsevier]

Abstract: The high concentration triblock copolymer poly(ethylene oxide)99–poly(propylene oxide)69–poly(ethylene oxide)99 (Pluronic F127) aqueous solutions with the addition of different components commonly used in physiologically relevant applications were characterized by rheological measurements, differential scanning calorimetry (DSC) and small angle X-ray/neutron scattering. The sol–gel transition temperature, as well as the storage modulus of the F127 solution depend both on the concentration of polymer and of clay. Above the gel transition, the storage modulus of the solutions increased with clay concentration. Yield strain is independent of polymer and clay concentrations. Two different kinds of inorganic salts, sodium chloride (NaCl) and calcium chloride (CaCl2) were added into the polymer and polymer–clay solutions. The sol–gel transition temperature decreased noticeably, but the storage modulus decreased only a small amount with increasing concentration of inorganic salts. Addition of salts to polymer–clay solutions resulted in precipitation of the clays which decreased the modulus. No effect on the mechanical properties was observed with the addition of common serum proteins. However, addition of 0.5–10% glucose decreased the transition temperature between 3° and 7°, without significantly affecting the modulus. The depression of the transition temperature by glucose was similar to that found with salts and indicated that the mechanism, namely competition for water, may be similar. [Copyright &y& Elsevier]

Abstract: Monolayers of organoclay platelets were formed at the air/water interface using the Langmuir technique and were then investigated either by in situ or lifted onto Si wafers and studied ex situ, using X-ray reflectivity (XR) methods. The XR data showed that the surfactant molecules on the clay platelets formed a dense, self-assembled monolayer where the molecules were tilted at an angle of from the normal to the dry clay surface. The surfactant layers only covered a fraction of the clay platelet surface area, where the fractional surface coverage for the three clays studied (C6A, C15A, and C20A) was found to be 0.90, 0.86, and 0.73, respectively. These values were significantly higher than those estimated from the cation exchange capacity (CEC) values. Rather than being uniformly distributed, the surfactant was clustered in patchy regions, indicating that the surface of the clay platelets had both polar and non-polar segments. This heterogeneity confirmed the hypothesis which was previously invoked to explain the distribution of the clay platelets in melt mixed homopolymer and polymer blend nanocomposites. [Copyright &y& Elsevier]

Abstract: We demonstrated that self-extinguishing polymer nanocomposites, which can pass the stringent UL 94 V0 standard, can be successfully prepared by combining modified organoclays with traditional flame retardant (FR) agents. Using secondary ion mass spectrometry (SIMS) and transmission electron microscopy (TEM), we determined that the addition of modified clays, which can intercalate or exfoliate in the matrix, also improved the dispersion of the FR agents. Dynamic mechanical analysis (DMA) indicated that the clays increased the modulus of the polymer above T g, which prevented dripping during burning. Cone calorimetry test showed that the nanocomposites with both FR and organoclay, had a lower peak heat release rate (PHRR) and average mass loss rate (MLR) than those with only clay or the FR agents. Extended X-ray absorption fine structure (EXAFS) data confirmed that no FR/clay interactions occurred in the solid phase, and that the synergistic effects were due to gas phase reactions. Since this mechanism is not specific, it opens the possibility of formulating self-extinguishing materials from a large class of polymers. [Copyright &y& Elsevier]

Abstract: A three-dimensional (3D) hyaluronic acid (HA) nanofibrous scaffold was successfully fabricated to mimic the architecture of natural extracelluar matrix (ECM) based on electrospinning. Thiolated HA derivative, 3,3′-dithiobis(propanoic dihydrazide)-modified HA (HA-DTPH), was synthesized and electrospun to form 3D nanofibrous scaffolds. In order to facilitate the fiber formation during electrospinning, Poly (ethylene oxide) (PEO) was added into the aqueous solution of HA-DTPH at an optimal weight ratio of 1:1. The electrospun HA-DTPH/PEO blend scaffold was subsequently cross-linked through poly (ethylene glycol)-diacrylate (PEGDA) mediated conjugate addition. PEO was then extracted in DI water to obtain an electrospun HA-DTPH nanofibrous scaffold. NIH 3T3 fibroblasts were seeded on fibronectin-adsorbed HA-DTPH nanofibrous scaffolds for 24h in vitro. Fluorescence microscopy and laser scanning confocal microscopy revealed that the 3T3 fibroblasts attached to the scaffold and spread, demonstrating an extended dendritic morphology within the scaffold, which suggests potential applications of HA-DTPH nanofibrous scaffolds in cell encapsulation and tissue regeneration. [Copyright &y& Elsevier]

Flame-retardant Elvacite acrylic resin/Cloisite 6A nanocomposites were prepared via direct melt intercalation. Transmission electron microscopy micrographs showed that 75% of the clay platelets were completely exfoliated. This high degree of exfoliation resulted in a large improvement in thermal stability and UV absorption properties without sacrificing optical clarity. Cone calorimetry tests clearly showed that the heat release rate was far lower and more gradual in the nanocomposites than in pure resins. Thermal gravimetric analysis measurements showed that the thermal stability of nanocomposites was enhanced by almost 50°C (at 50 wt% loss) when the samples were thermally degraded under nitrogen. These results are consistent with a dramatic increase in the specific heat of the nanocomposites as verified by high precision differential scanning calorimetry measurements. Additionally, Fourier transform infrared spectroscopy results indicated that the introduction of clay did not change the chemical structure of acrylic resins. [ABSTRACT FROM AUTHOR]

Studies of kaolin have revealed an effect characterized by an unusual temperature-induced change of the EPR spectrum of the Fe3+ ion, which is the magnetic probe in kaolin–clay. At low temperature (T=4.2 K) a resonance line with an effective g-value g1=4.13±0.16 is observed. At high temperature (T=288 K) one observes a resonance line with the effective g-value g2=2.15±0.1. The transition from the low- to high-temperature spectrum is gradual and it is accompanied by a redistribution of the absorption intensity. The observed properties of the temperature dependence of the EPR spectrum are characteristic of systems with a multiminimum potential. [Copyright &y& Elsevier]