Structure And Properties Of Layered Silicate / Rubber Nanocomposites

Recently layered silicate/polymer nanocomposites have attracted many researchers' interests. Consequently, it is of great important to well understand their structures and properties in order to obtain information for their applications in industry. Several layered silicate/rubber nanocomposites are prepared by directly co-coagulating the rubber latex and layered silicate aqueous suspension (rubbers including styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR) and natural rubber (NR); layered silicate including montomorillonite(MMT) and rectorite). X-ray Diffraction (XRD) patterns and Transmission Electron Microscopy (TEM) micrographs show that the nanocomposites by this method are a novel "separated" layered silicate/rubber nanocomposites, in which the rubber molecules separate the layered silicate particles into either individual sheet, or just nano-scale silicate sheet aggregates without the intercalation of rubber molecules. Compared with carbon black or silica filled rubber, this kind of layered silicate/rubber namocomposites exhibit excellent mechanical properties, such as high hardness, high stress at a certain strain and high tear strength. The results of DMTA reveal that the nanocomposites represent a lower tanδ peak value within the zone of glass transition temperature (Tg), while a higher tanδvalue at high temperature. The elastic modulus of the nanocomposites is highest compared to the unfilled rubber and carbon black filler rubber. These results are in consistent with the results from Rubber Processing Analysis (RPA). The processing properties of MMT/SBR nanocomposite are investigated by means of Monsanto Rheometer and RPA. At the same time, these results are compared with those of carbon black filled SBR. The results indicate that the rheology of these two composites is similar, however, layered silicate/SBR nanocomposites exhibit stronger filler network and higher viscosity compared to those of carbon black/SBR composites. Moreover, the extruder surface of layered silicate/SBR nanocomposite is more smooth than that of SBR filled by carbon black. All these results reveal that the nanocomposites have good processing properties. The permeabilities of several layered silicate/rubber nanocomposites and carbon black filled rubber are measured. The aspect ratios (area/thickness) of these nanocomposites are obtained by use of the TEM and XRD. Based on reasonable hypothesis, a theoretical mode is established to predict the reduction of permeability in a filled rubber according to the content and the structure size of the filler. Compared with the Nielsen model this model is more suitable to describe the experimental data. The factors of the permeability in filled rubber are discussed according to the theoretical mode. Positron annihilation lifetime spectroscopy (PALS) has been used to study the available free volume hole size(o-Ps lifetime) and concentration (o-Ps intensity) of MMT/SBR nanocomposites and SBR filled with carbon black. The variation of filler type and the filler content have little effect on the o-Ps lifetime, but alter the o-Ps intensity in the filled rubber. With the amount of filler increasing, the o-Ps intensity decreases in carbon black filled SBR, but increases followed with decrease in MMT/SBR nanocompoistes. This can be attributed to their different interface with SBR. Although a good correlation is found between diffusivity (D) and free volume fraction(Fv) based on D=Aexp(-B/Fv) according to free volume theory in carbon black/SBR, a non-linear relationship is observed in MMT/SBR. The reason is that the addition of impermeable fillers (such as MMT) may not significantly modify the available free volume but will cause a significant decrease in the diffusivity due to tortuosity effects. The effect of temperature on the free volume and permeability are also studied. Although the free volume and the permeability of filled SBR increase obviously with temperature increasing, the activation energies calculated according to the Arrhenius' law are almost th...