Epoxy networks reinforced with polyhedral oligomeric silsesquioxanes:: Structure and segmental dynamics as studied by solid-state NMR

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 H-1-H-1 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 H-1-H-1 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 degrees flips dominating to aromatic rings in the POSSPh crystallites are accompanied by the wobbling of the flip axes with an average fluctuation angle ca. 25 degrees. Similarly, cyclopentyl substituents in the POSSCp crystallites undergo to ca. 35 degrees rotational-diffusion motion.