Polycarbonate-silsesquioxane and polycarbonate-siloxane nanocomposites: synthesis, characterization, and application in the fabrication of porous inorganic films
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Three types of poly(norbornane carbonate) or PNC oligomers were synthesized and characterized via spectroscopic methods and elemental analyses to validate their chemical structures. Using the results from proton nuclear magnetic resonance (1H NMR) experiments, the degree of polymerization and size of each PNC chain was estimated via end-group analysis. All three types of PNC structures were both thermally-labile and acidolytically-labile, allowing them to be used as sacrificial materials in both direct-write and thermally-processed template systems. Thermogravimetric analysis (TGA) data was used to determine the kinetic parameters for the thermolytic decomposition reactions and evolved-gas analysis via mass spectrometry (TGA-MS) was used to determine the mechanisms for thermolytic degradation. PNC oligomers were freely-mixed with hydrogen silsesquioxane (HSQ) to form solutions that were spin-coated to form templated films. Transmission electron microscopy (TEM) showed that the free-mixing of PNCs with HSQ resulted in the agglomeration of the porogen molecules during the spincoating step. This phase-segregation produced domain sizes much larger than those of the individual chains, and during decomposition large pores were produced. To combat the phase segregation, hydrosilylation reactions were used to covalently bond vinyl end-capped PNC chains to silane-functionalized siloxane and silsesquioxane molecules. These matrix-like materials served as compatibilizers in order to improve the phase-compatibility of the sacrificial polymers in HSQ films. NMR and GPC analyses showed that the solids recovered from the hydrosilylation reactions were binary mixtures of hybrid nanocomposite molecules and residual ungrafted PNC chains. TEM imaging showed that the domains in these nanocomposite films had bimodal size distributions due to the presence of two components in the mixtures. The hybrid molecules produced pores ranging in size from about 6-13 nm as a result of improvements in the phase-compatibility of the grafted oligomers. However, the residual ungrafted oligomers in the blends produced larger domains measuring 30-40 nm. It is believed that separation difficulties can be avoided if the vinyl termination reaction conditions can be adjusted to ensure 100% conversion of all the terminal hydroxyl groups to vinyl groups. Doing so would allow all PNC chains to be grafted during hydrosilylation reaction; thus, avoiding the recovery of free PNC oligomers.