Received September 25, 2007

Revised Manuscript Received November 12, 2007

Abstract:

The epoxy networks based on poly(propylene oxide) chains cross-linked by diglycidyl ether of Bisphenol A and reinforced by polyhedral oligomeric silsesquioxanes (POSS) provide a typical example of polymer nanocomposites with hierarchical architecture. In addition to characterizing the epoxy-POSS composites, this contribution demonstrates valuable applications of solid-state NMR spectroscopy. The size of domains in the nanocomposites was determined by high-speed MAS ¹H-¹H spin-diffusion experiments, offering an alternative to the established methods like SAXS, EM, or AFM. While the latter might fail under certain circumstances (low contrast, or small domains), the ¹H-¹H spin-diffusion measurement yielded the size of unbroken primary domains, making also possible their distinction from "aggregates of primary domains", the size of the latter being measured by EM or SAXS. Depending on the type of the investigated network the size of the POSS aggregates arising in the nanocomposites was determined to be ca. 1-20 nm. Investigations of molecular dynamics (various "domain-selective" relaxation and recoupling solid-state NMR experiments were applied) yielded information making possible the assignment of the contribution of molecular segments to thermomechanical properties like glass transition temperature and storage shear modulus, and to predict the products' ability to absorb mechanical energy. Remarkable motional heterogeneities were found not only in the amorphous phase, where mobile polymer segments of the "free" domains coexist with the immobilized chains of the "constrained" ones, but also in the crystallites of POSS building blocks, where the amplitudes of segmental reorientations occurring in the midkilohertz frequency region remain relatively large: two-site 180 flips dominating to aromatic rings in the POSS_Ph crystallites are accompanied by the wobbling of the flip axes with an average fluctuation angle ca. 25. Similarly, cyclopentyl substituents in the POSS_Cp crystallites undergo to ca. 35 rotational-diffusion motion.

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