Self-assembled hybrid materials based on ionomers of styrene/ethylene-co-butylene/styrene (SEBS): Evaluation of morphology and rheo-mechanical behavior
Organic-inorganic hybrid materials were fabricated via an in-situ sol-gel reaction of RX -Si-(OR)4-X within a partially sulfonated styrene/ethylene-co-butylene/styrene (sSEBS) tri-block copolymer (BCP). It was hypothesized that the micro-phase separated sulfonated polystyrene domains provided a morphological template for the synthesis of the silicate network. The nanostructured hybrids were created from co-miscible solutions containing the sol-gel precursors and sSEBS and were, therefore, considered self-assembled. Sulfonation was necessary, though only in small amounts, to prevent macro-phase separation of the organic and inorganic phases. The progressive effect of sulfonation, including degree and counterion type, and hybrid formation on morphology and rheo-mechanical properties was investigated. Atomic force microscopy and small angle x-ray scattering indicated a shift in BCP morphology from hexagonally packed cylinders to lamellar after sulfonation. This was explained on the basis of an increase in the Flory-Huggins interaction parameter, χ, between the two blocks. Silicate incorporation did not further influence morphology, supporting the idea of a templating effect. Dynamic mechanical and creep properties were affected by sulfonation and hybrid formation indicating an influence of each on both short and long-range relaxations. Creep compliance with increasing temperature was evaluated and master curves were generated from time-temperature superposition of this data. Superposition was possible over limited temperature ranges. Creep and recovery behavior could be described accurately by a relatively simple Burger's mode modified to account for the non-linear time dependent behavior of these complex systems. An order-order transition (OOT) was observed in the parent BCP using DMA. The OOT shifted to higher temperatures after sulfonation and hybrid formation. The OOT could also be predicted from creep compliance vs. temperature data. No order-disorder transition (ODT) was observed up to 300°C in the dynamic mechanical data, but a transition possibly attributed to an ODT was observed for some samples in the creep compliance data. Initial work was undertaken in an attempt to covalently bind the organic and inorganic phases in order to further improve the rheo-mechanical properties, especially at elevated temperatures or when subjected to large stresses.