Phase separation behavior of poly(methyl methacrylate-co-dimethylaminoethyl methacrylate)/methyl silsesquioxane hybrid nanocomposites studied by dansyl fluorescence

Abstract

(Graph Presented) Ultralow dielectric constant (k) materials (k < 2.0) are generally prepared by the sacrificial macromolecular porogen route. Dansyl steady-state and time-resolved fluorescence have been used to study the phase separation behavior of methyl silsesquioxane (MSSQ) and poly(methyl methacrylate-co-dimethylaminoethyl methacrylate) [P(MMA-co-DMAEMA)] hybrid nanocomposites, which are promising candidates for spin-on, ultralow dielectric constant applications. Resins MSSQ-HI and MSSQ-LO, which differ mainly in the concentration of -SiOH (silanol) present in the uncured samples, are used to study the effect of endgroup functionality on the phase separation behavior of the hybrid nanocomposites. Fluorescence results reveal that, for MSSQ-LO resin, a single emission band located in the range of 428-456 nm is observed for P(MMA-co-DMAEMA) loading levels up to 20 wt %. With further increase in polymer loading, a second emission band located at higher emission energy ranging from 408 to 422 nm appears, and the amount of this higher energy band increases with polymer loading. By contrast, for MSSQ-HI, the composition-dependent high-energy band does not appear until polymer loading levels exceed 50 wt %. We infer information about local environment from the emission band position with the low-energy band associated with dansyl surrounded by MSSQ, and the composition-dependent high-energy band associated with dansyl surrounded by P(MMA-co-DMAEMA). Thus, our fluorescence results suggest that both increasing the polymer loading level and lowering the amount of silanol endgroup in the uncured MSSQ promote phase separation in the hybrid nanocomposites. This is evidenced by the increase in the proportion of the high-energy band, which we interpret as indicating that the probe molecules avoid the phase boundary region and seek a polymer-rich environment. This migration results in the formation of larger polymer domains and ultimately translates into larger size pores upon polymer burnout. Our interpretation is consistent with previous pyrene fluorescence, small angle neutron scattering, transmission electron microscopy, and small-angle X-ray scattering results. © 2005 American Chemical Society.