Microstructure and rheology of polyhedral oligosilsesquioxane (POSS) polymeric nanocomposites

Date of Completion

January 2005

Keywords

Chemistry, Polymer|Engineering, Chemical

Degree

Ph.D.

Abstract

This dissertation aims at the research and development of the new hybrid polymeric nano-composites incorporated by Polyhedral Oligosilsesquioxane(POSS) moieties. We seek a deeper understanding of the dependence of the polymer chain dynamics and segmental relaxation behavior on molecular architecture and overall further insight into POSS-polymer structure and properties. ^ Accordingly, this dissertation can be classified into two categories. In the first part, we studied polystyrene(PS)-based polymeric nano-composites incorporating POSS macromers through free radical co-polymerization. Microstructural characterizations show that POSS moieties can be dispersed in the PS matrix nearly at the molecular level. Due to their nano-scale dimension comparable to constituent polymer coils, the presence of POSS moieties significantly modifies the chain topology and plays a dilution role with respect polymer chain entanglement density. With increasing POSS loading level, the rubbery plateau modulus decreased, the associated microscopic "packing length" of polymer chain increasing, accordingly. The temperature sensitivity of the terminal relaxation also increased with the increasing POSS incorporation. Additionally, the corresponding physical properties also strongly depend on R-group, which surrounds silicon-oxygen (Si8O12) cubic cage with T8 spatial symmetric character. The R-group sensitivity is attributed to the interaction between POSS moiety and PS matrix as a function of R-group, following the sequence of interaction strength: CyPOSS > CpPOSS > iBuPOSS. ^ In the second part, we synthesized PEG-segmented multi-block hybrid thermoplastic polyurethane incorporating isobutyl-functionalized POSS unites with systematically varying PEG molecular weight and POSS content. The hydrophobic hard segments (POSS) can form crystalline structures driven by micro-phase separation, itself due to thermodynamic incompatibility. The equilibrium swelling ratio and mechanical properties of the resulting hybrid hydrogels bear a close relationship with PEG loading and POSS crystalline content. Meanwhile, the influence of shear flow on POSS aggregation and crystallization in the hybrid polyurethane stands in contrast to those for polyolefins crystallizing under the influence of shear where flow generally enhances the rate of crystallization. We attribute this qualitative difference to a difference in crystallization mechanism. ^

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