Formation of nanostructured epoxy networks containing polyhedral oligomeric silsesquioxane (POSS) blocks

Abstract

Nanostructured epoxy networks, based on DGEBA and poly(oxypropylene)diamine (Jeffamine D), containing nano-sized inorganic blocks, polyhedral oligomeric silsesquioxanes (POSS), were investigated. The POSS were incorporated in the network as crosslinks or as pendant units by using octa- or monoepoxy-POSS monomers, respectively, as well as diepoxides with pendant POSS. The authors focused on investigating the relationship between the network formation process and the final product properties. The reactivity of the epoxy-functional POSS monomers, the hybrid systems' time of gelation, the gel fractions and the phase structure of the networks were determined using 1H or 13C NMR spectroscopy, chemorheology experiments, sol–gel analysis and transmission electron microscopy (TEM). All the POSS epoxides tested show a reduced reactivity if compared to their respective model compounds due to sterical crowding in the neighborhood of their functional groups and due to reduced epoxy group mobility. The incorporation of pendant POSS into networks of the type DGEBA–Jeffamine D–monoepoxy-POSS hence took place only in the late reaction stage. Together with the high tendency of these POSS to aggregation, the kinetics favors the formation of small nano-phase-separated POSS domains, which act as physical crosslinks due to their covalent bonds to the organic matrix. At POSS loadings higher than 70%, topological constraint by POSS leads to a strongly reduced elastic chain mobility, thus additionally strongly reinforcing the networks. The network build-up and gelation of the octaepoxy-POSS–Jeffamine D system were slow compared to the reference DGEBA–Jeffamine D network due to a low octaepoxy-POSS reactivity and due to its strong tendency to cyclization reactions with primary amines. The topology of the amino groups is shown to be very important. In contrast to monoepoxy-POSS, the octaepoxy-POSS becomes dispersed as oligomeric junctions (purely chemical crosslinks) of the network in the cured product. The octaepoxide's reinforcing effect is small and is given only by its high functionality and not by its inorganic nature. The functionality effect is reduced by the mentioned cyclizations.