Dispersion Characteristics of Organoclay in Nanocomposites Based on End-Functionalized Homopolymer and Block Copolymer

Abstract

The dispersion characteristics of organoclay in nanocomposites based on end-functionalized polystyrene (PS) and end-functionalized polystyrene-block-polyisoprene (SI diblock) copolymer were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), and oscillatory shear rheometry. For the study, a low-molecular-weight polystyrene (PSLMW), having molecular weight (M) lower than the viscosity critical molecular weight (Mc), and a high-molecular-weight polystyrene (PSHMW), having M > Mc, were synthesized via anionic polymerization. Each polystyrene (PS) was terminated by carboxylic acid (−COOH) group via carbonation, yielding PSLMW-t-COOH or PSHMW-t-COOH, which were then neutralized, yielding PSLMW-t-COONa or PSHMW-t-COONa. Further, two SI diblock copolymers were synthesized via anionic polymerization. In so doing, the PS block or PI block were end-functionalized via carbonation to yield IS-t-COOH or SI-t-COOH, which were then neutralized to obtain IS-t-COONa or SI-t-COONa. Each of the end-functionalized polymers was mixed in a cosolvent of tetrahydrofuran and water with an organoclay (Southern Clay Products) to prepare nanocomposites. It has been found via XRD and TEM that the polymers terminated by sodium carboxylate (−COONa) group are much more effective in dispersing organoclay aggregates than the polymers terminated by −COOH group. The ionic interaction between the negatively charged carboxylic ion (−COO-) at the polymer chain end and positively charged N+ ion in the surfactant residing at the surface of organoclay is believed to be the driving force that has enhanced the dispersion of organoclay aggregates, giving rise to a very high degree of dispersion. The rheological measurements at elevated temperatures, via oscillatory rheometry, indicate that the storage modulus of the nanocomposites based on end-functionalized polymers is considerably higher than that of neat end-functionalized polymers. This observation is attributed to ionic interactions between the negatively charged anion at polymer chain end and the positively charged ion in the surfactant residing at the surface of organoclay.