Linear dynamic viscoelastic properties of functionalized block copolymer/organoclay nanocomposites

Linear dynamic viscoelastic properties of functionalized block copolymer/organoclay nanocomposites were investigated. The organoclay employed (Cloisite 30B, Southern Clay Products) is natural clay modified with methyl, tallow, bis(2-hydroxyethyl), quaternary ammonium chloride (MT2EtOH). Nanocomposites were prepared by solution blending Cloisite 30B with a polystyrene-block-hydroxylated polyisoprene (SIOH diblock) copolymer having hexagonally packed cylindrical microdomains of polystyrene (PS). For comparison, nanocomposites were also prepared by solution blending Cloisite 30B with a polystyrene-block-polyisoprene (SI diblock) copolymer having lamellar microdomain structure. It was found from the linear dynamic viscoelastic measurements that the dynamic storage modulus (G') and complex viscosity (\eta*\) of 95/5 SIOH/Cloisite 30B nanocomposite increased as the temperature was increased from 170 to 240 degreesC, while the G' and \eta*\ of 95/5 SI/Cloisite 30B nanocomposite decreased as the temperature was increased from 90 to 125 degreesC, where 95/5 refers to the weight percent of the constituent components. In situ Fourier transform infrared spectroscopy indicated that hydrogen bonding persisted in the 95/5 SIOH/Cloisite 30B nanocomposite at temperatures as high as 240 degreesC, the highest experimental temperature employed, while no measurable hydrogen bonding existed in the 95/5 SI/Cloisite 30B nanocomposite at temperatures ranging from 30 to 240 degreesC. Thus, it is concluded that the rather unusual temperature dependence of G' and \eta*\ for the 95/5 SIOH/Cloisite 30B nanocomposite is attributable to an enhanced dispersion, and thus larger surface areas available, of the silicate layers of clay aggregates as the temperature is increased. It is speculated that the enhanced dispersion of the silicate layers of organoclay aggregates occurs due to the compatibilization, via hydrogen bonding, between the hydroxyl groups in SIOH and the polar groups in MT2EtOH on the surfaces of organoclay aggregates. It was found from both the dynamic frequency sweep and isochronal dynamic temperature sweep experiments that the order-disorder transition temperature (T-ODT) of the 95/5 SIOH/Cloisite 30B nanocomposite exceeded 240 degreesC, the highest experimental temperature employed, while the T-ODT of the neat SIOH diblock copolymer was 198 degreesC and the T-ODT of the 95/5 SI/Cloisite 30B nanocomposites was 115 degreesC, the same as that of the neat SI diblock copolymer. The above observation, once again, suggests that strong attractive interactions exist, via hydrogen bonding, between the hydroxyl groups in SIOH and the polar functional groups in MT2EtOH of the chemically modified clay. Studies using X-ray diffraction and transmission electron microscopy support the conclusions drawn above.