The rheological properties of liquid polybutadiene rubber/organo-clay nanocomposite gels were investigated by rheological experiments, focusing on the effects of clay exfoliation and orientation−disorientation as well as polymer−clay interaction and temperature. Both irreversible and reversible viscosity transitions were observed in the temperature range from 26 to 136 °C in steady shear experiments on as-prepared and exfoliated samples. These transitions depend strongly on the end groups, molecular weight of the liquid rubber, and the shear field. The irreversible transition is attributed to the exfoliation of the clay, and the reversible transition can be understood as a shear-induced orientation−disorientation transition of the clay sheets. Polymer−clay interaction is confirmed to be a key controlling factor of the orientation−disorientation transition, whereas the shear field plays a critical role to induce such a transition. To our knowledge, this is the first rheological observation of the in-situ exfoliation process and the shear-induced orientation−disorientation transition of layered silicate in polymer/organo-clay nanocomposites. A tentative model was suggested on the basis of the clay exfoliation and orientation−disorientation transition, and the model is used to explain the observed unique rheological behavior.

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The mobility and glass transition temperature (T g ) for polymers under nanoscale confinement differ substantially from the bulk. Whereas many studies have focused on the one-dimensional confinement, it has great significance to extend studies to higher geometries. Here, we systematically investigate the mobility by dipolar-filter sequence in solid-state NMR and T g by DSC for thiolated polystyrene (PS-SH) on gold nanoparticles. The increase in T g and signal suppression in NMR spectra clearly indicate that the surface confinement dominates molecular mobility as well as T g . The molecular weight of PS-SH and nanoparticles size show significant influence on the immobilization and T g . Our results can be fitted with a core−two shell model; the inner shell is under strong constraints while the outer shell with less confinement. This work is essential to better understand the confinement effect and also provides a step toward the ultimate desire to tailor the properties of nanomaterials.

Incorporation of hydroxyapatite (HA) into the matrix of collagen (Col) and chitosan (Chi) by in situ synthesis was introduced to prepare nanocomposites. Structural investigations of the pure Col-Chi mixture validated the influence of Chi on Col assembly, but the molecular interactions between Col and Chi was partially depressed during the intervention of in situ HA synthesis, as revealed by FTIR and DSC analyses. A series of Col-Chi-HA (CCHA) nanocomposites with varying HA content were thereby prepared by a sequential method, involving in situ synthesis in the Col-Chi system, then gelling at 25 degrees C and subsequently washing the resultant elastic gel followed by dehydration consolidation. The structural characteristics and biological properties of the dehydrated CCHA nanocomposites were further evaluated by using XRD, FTIR, TG, and SEM analyses and the osteoblast culture experiment. Formation of a well integrated microstructure of organic fibers (ca. 90 nm in size) and dense matrix including inorganic aggregates (less than 30 nm in size) was found in these nanocomposites. Rat Ros 17/2.8 Osteoblasts proliferated and attached well on the surface of both CCHA nanocomposite and Col-Chi mixture. These results indicated that in situ HA synthesis in the Col-Chi system provided a feasible route for bone grafting nanocomposites.

There is an ongoing intensive debate on the mechanism of gold nanoparticles formation regarding the intermediate precursors prior to the addition of reducing agent. A new detailed view of the widely used Brust–Schiffrin two-phase method to prepare gold nanoparticles is presented here. Precursor species of these reactions have been identified and quantified by NMR, UV–visible, Fourier-transform Raman spectroscopy, etc. We demonstrate that tetraalkylammonium gold complexes ([TOA][AuX2]) and soluble gold thiolate ([TOA][AuSRX] and [TOA][Au(SR)2]) were detectable as the precursors by NMR spectroscopy. Their relative contents depend on the concentration of reactants. Higher concentration of the reactants is favorable for the formation of soluble thiolate. Polymeric gold thiolate [Au(I)SR] n could eventually precipitate from the solution under specific conditions. The clear mechanism presented here is of great significance to tailor the size and properties of the final products.

The glass transition behavior of polystyrene (PS) nanotubes confined in cylindrical alumina nanopores was studied as a function of pore diameter (d) and polymer tube thickness (δ). Both the calorimetric glass transition temperature and the microstructure measured by a nonradiative energy transfer method indicated that the polymer nanotube, or concave polymer thin film, exhibited significant differences in vitrification behavior compared to the planar one. A closer interchain proximity and an increased T_{g} were observed for polymer nanotubes with respect to the bulk polymer. T_{g} for polymer nanotubes was primarily dependent on the curvature radius d of the template, while it was less dependent on the thickness δ of the PS tube wall in the range of 11-23 nm. For small nanotubes (d=55nm), the T_{g} increased as high as 18 °C above the bulk value. This vitrified property reverted back to the bulk value when the substrate was chemically removed, which indicated the crucial importance of the interfacial effect imposed by the hard wall with a concave geometry.

Understanding interfacial water behavior is essential to improving our understanding of the surface chemistry and interfacial properties of nanomaterials. Here using 1H solid-state nuclear magnetic resonance (1H SSNMR), we successfully monitored ligand exchange reaction between oleylamine (OLA) and adsorbed water on titanium dioxide nanoparticles (TiO2 NPs). Three different types of interfacial waters with different reactivities were distinguished. The mobility of the adsorbed water molecules was characterized by dipolar filtered 1H SSNMR. Our experimental results demonstrate that the adsorbed water can be categorized into three different layers: (i) rigid water species with restricted mobility closest to the surface of TiO2 NPs, (ii) less mobile water species weakly confined on TiO2 NPs, and (iii) water molecules with high mobility. Water in the third layer could be replaced by OLA, while water in the first and second layers remained intact. The finding that the interfacial water with the highest mobility has the strongest reactivity has guiding significance for tailoring the hydrophilic and hydrophobic properties of TiO2 NPs.

Poly(vinyl chloride) (PVC) samples for the conformation and structural relaxation studies were prepared by freeze-extracting the solutions in various solvents and with varying concentrations. The partially disentangled PVC samples could be obtained by freeze-extracting very dilute PVC/ tetrahydrofuran (THF) and PVC/dimethyl phthalate (DMP) solution (0.05 wt %), while the PVC sample remains in an entangled state after freeze-extracting the concentrated solutions in THF and in DMP. Surprisingly, the extent of interpenetration of PVC chain decreases, even in concentrated solution in dioctyl phthalate (DOP) (5 wt %), which is contrary to accepted theories. DSC and infrared spectroscopic studies showed that a higher ordered conformation could be achieved for the disentangled specimens recovered from concentrated solution in DOP than for the entangled samples prepared from THF and DMP. DSC heating curves showed strong and sharp endothermal peaks for the disentangled polymer after sub-T g annealing. The structural relaxation behavior of the disentangled PVC freeze-extracted from DOP solutions is more predominant, which is in good agreement with the modeling predictions. IntroductionIt is well-known 1 that atactic poly(vinyl chloride) (PVC) is a low crystalline polymer. Commercial PVC contains less than 10% crystalline segments. But this relatively low level of order has a surprisingly significant effect on both its processing and properties. As early as 1926, Flumiani 2 reported that the dispersion state of PVC in the solution could be influenced by temperature, the various heating period, the aging of the solution, the solvent itself, and even the method of preparation of different concentrations of the solution. Despite numerous studies 2-6 dealing with the gelation and aging process of PVC solutions, the mechanism of formation and the structure of PVC physical gel have still not been fully elucidated. It is generally accepted that the cross-links in these gels are tiny crystallites, although according to Guenet et al.,7 hydrogen bonding also plays an important role. Effects of polymerization temperature, 8 heat treatment, 9,10 and processing temperature 11 on crystalline of PVC have been investigated. However, until now, little has been done on the conformation and structural relaxation of PVC affected by solvent because of the complexity of the solvents applied.In our previous work, we prepared partially disentangled PVC samples by the freeze-extraction method from dilute PVC/THF solutions and found that the conformation and relaxation behavior of PVC depended strongly on the entanglements in polymers. 12 We also found that the molar volume of solvents plays an important role in the entanglement state of polymer in solution. 13-15 Therefore, it is necessary for us to explore the effect of solvent on the conformation of PVC. In this work, we selected a normal plasticizer DOP, which has large molar volume, as well as DMP and THF, as the solvents for PVC. Some attempts [16][17][18] have been made to study the extent of...