Effects of triblock copolymer architecture and the degree of functionalization on the organoclay dispersion and rheology of nanocomposites

The effects of triblock copolymer architecture (ABA-type vs ABC-type) and the degree of functionalization on the organoclay dispersion in nanocomposites were investigated using X-ray diffraction (XRD), transmission electron microscopy (TEM), and linear dynamic viscoelasticity. For the study, a lamella-forming polystyrene-block-polyisoprene-block-polystyrene (SIS triblock) copolymer and a homogeneous polyisoprene-block-polystyrene-block-polybutadiene (ISB triblock) copolymer were synthesized via sequential anionic polymerization. Subsequently, the polyisoprene block of the SIS triblock copolymer was hydroxylated yielding an SIOHS triblock copolymer, and the polybutadiene block of the ISB triblock copolymer was hydroxylated yielding an ISBOH triblock copolymer. The degree of hydroxylation in both SIOHS and ISBOH triblock copolymers was varied. Both the unfunctionalized (SIS and ISB) and functionalized (SIOHS and ISBOH) triblock copolymers were used to prepare nanocomposites with two different types of organoclays: one (Cloisite 30B) treated with a surfactant having hydroxyl groups and the other (Cloisite 15A) treated with a surfactant having no hydroxyl groups. It was found from TEM that Cloisite 30B aggregates had a very high degree of dispersion in the ISBOH triblock copolymer, while Cloisite 15A aggregates had a low degree of dispersion in both ISBOH and SIOHS triblock copolymers. A very unusual temperature dependence of linear dynamic viscoelasticity of the ISBOH/Cloisite 30B nanocomposites was observed. An optimum degree of hydroxylation in the ISBOH triblock copolymer existed such that it gave rise to the highest degree of dispersion of Cloisite 30B aggregates. These observations are supported by XRD patterns. In situ Fourier transform infrared spectroscopy confirms the presence of hydrogen bonds in ISBOH/Cloisite 30B nanocomposites and not in ISBOH/Cloisite 15A nanocomposites.