Effect of NBR and organoclay contents on physical gelation of the uncured EVA/NBR/OMMT nanocomposites

The influence of acrylonitrile-butadiene rubber (NBR) and organoclay (OMMT) contents on the physical gelation of the uncured poly(ethylene-co-vinyl acetate) (EVA)/NBR blends and their OMMT-filled nanocomposites were investigated using the Winter-Chambon method. The rheological results revealed that the storage modulus, loss modulus and complex viscosity values increased with the increase in the rubber and OMMT contents in the blends and nanocomposites, respectively. The three parameters for the pristine NBR were lower than those of the nanocomposites having 3 wt% OMMT and beyond. The parameters of gel point (P-g), relaxation exponent (n), gel strength (S-g) and fractal dimension (d(f)) of the network were estimated at the sol-gel transition. The results showed that no gel point could be clearly detected using the Winter-Chambon method for the unfilled blend in the range 20-60 wt% NBR. In contrast, P-g was found to take place at about 3.9 wt% OMMT for the uncured nanocomposite. The n and S-g values were calculated to be about 0.376 and 32,040 +/- 940 Pa.s(n), respectively. Moreover, the estimated d(f) value pointed out that the physical gel formed throughout the uncured nanocomposites behaved as if the excluded volume interactions were almost screened. The values of the surface free energy, work of adhesion, interfacial free energy, spreading coefficient and Girifalco-Good's interaction parameters were also worked out for the blends and nanocomposites, using the contact angle data of water. The results revealed that the hydrophilicity of the materials increased with the increase of OMMT content. [GRAPHICS] .