Dispersion and rheology of polypropylene/organoclay nanocomposites: effect of cation exchange capacity and number of alkyl tails

Nanocomposites of montmorillonite organoclays and polypropylene (PP) were prepared via direct melt intercalation using maleic anhydride functionalized polypropylene (PP-g-MA) as a compatibilizer. Two montmorillonite clays (MMT) with different cation exchange capacities (CEC) were exchanged with alkyl ammonium ions, in which one or two octadecyl chains are attached to the nitrogen atom. The role of alkyl chain numbers and CEC value on the dispersion of clay and rheology of PP nanocomposites under shear and extensional flow was evaluated by X-ray diffraction, scanning electron microscopy, and rheologic techniques. It was found that the low-CEC organoclay with one alkyl chain could only form a conventional composite. However, the low-CEC organoclay with two alkyl chains or high-CEC organoclay with one alkyl chain can disperse finely in the matrix. Nanocomposites containing these two organoclays showed typical shear rheologic properties of intercalated nanocomposites, but only the former showed a mild strain-hardening behavior in uniaxial extensional flow. When using an intercalant with two tails, the high-CEC clay would lead the organoclay to form mixed structures which further resulted in an inferior dispersion quality. It was proposed that the dispersion quality and rheologic properties of nanocomposites were related to the arrangement of modifier molecules in the clay galleries, which was determined by the CEC of clay and the structure of alkyl ammonium ions.