No abstract

The mechanics of nanocomposites is critical in the design of nanomaterials with desirable properties. In this paper, the mechanics of polymer−clay nanocomposites is studied using a designed polymer and solution nanocomposite synthesis. A copolymer latex, with function groups that strongly interact with the surface of the clay nanoplatelet and glass-transition temperature lower than room temperature, was synthesized. Uniformly dispersed nanocomposites were then generated using water as the intercalation agent through the solution process. The chain mobility in the nanocomposites is greatly reduced as studied by dynamic mechanical thermal analysis (DMTA) and dielectric thermal analysis (DETA). The modulus of the composite increases significantly. The modulus enhancement strongly relates to the volume of the added clay as well as the volume of the constrained polymer. This modulus enhancement follows a power law with the content of the clay and is modeled well by Mooney's equation for this soft-polymer-based nanocomposite. Modeling suggests that the nanocomposite modulus enhancement is determined by the high aspect ratio of the intercalated clay and the strong interfacial strength, in the form of the Einstein coefficient, K, when the modulus of the matrix phase is much lower than that of the clay, i.e., E f/E m > 100. This study also indicates that the structure of clay nanocomposites with strong interfacial interactions is analogous to that of semicrystalline polymers. In the case of polymer−clay nanocomposites, the intercalated clay phase serves as an unmeltable crystalline phase that results in improvement in mechanical and thermal properties.

No abstract

In-situ polymerization of silicon alkoxide was used to prepare composites of silicon dioxide and poly(vinyl acetate) (PVAc). The local environment of the PVAc chains was probed using FTIR, dielectric, and dynamic mechanical spectroscopy. Dielectric relaxation master curves were prepared, and the dielectric data were fit to the empirical Kohlrausch-Williams-Watts function (KWW). The results showed that 0 decreased with increasing sol-gel concentration, indicating that there is a broadening in the distribution of relaxation times for PVAc chains. The results suggest that there exists an interfacial region in the composites where the mobility of the PVAc is reduced by the interactions with the silicate network. In addition, at high concentrations of tetraethoxysilane (TEOS) it was not possible to shift the data because of a breakdown of time-temperature superposition. Further, at these concentrations a plateau region above Tt was observed by dynamic mechanical analysis. The results show that the silicate network inhibits the relaxation of at least a portion of the PVAc chains, and it is believed that the plateau is a manifestation of the PVAc chains that are entrapped in a continuous silicate network and of the nature of the network itself. It was also shown that the distribution of relaxation times narrows as the cure temperature increases. In addition, as the composite is cured, the amount of hydrogen bonding between the PVAc and the silicate network decreases. This is presumably due to further condensation of the silicate and the proportional decrease in the amount of available silanol groups.