Marlene D. Houtz

Synthesis and thermal properties of thermosetting bis‐benzocyclobutene–terminated arylene ether monomers

A series of new bis-benzocyclobutene-endcapped arylene ether monomers was prepared and characterized. Whereas 2,6-bis(4-benzocyclobutenyloxy)benzonitrile (BCB-EBN) could be prepared in good yield using the standard procedure (K2CO3/NMP/toluene/Dean–Stark trap/120°C), other bis(benzocyclobutene) (BCB)-terminated monomers containing ether-benzophenone (BCB-EK), ether-phenylsulfone (BCB-ES), and ether-6F-benzoxazole (BCB-EBO) moieties were invariably contaminated by mono-endcapped products under similar reaction conditions. This can be attributed to a much greater activating effect of the nitrile group on the ortho-fluorides in the aromatic nucleophilic displacement reaction than the carbonyl, sulfonyl, and benzoxazolyl groups. However, the latter monomers could be synthesized (70–80%) from 4-trimethylsiloxybenzocyclobutene and respective aromatic fluorides in the presence of CsF at 140°C. Similar curing behaviors under N2 (DSC: extrapolated onset and peak temperatures at 227–230° and 260–262°C, respectively) characterized all four monomers. BCB-EK, BCB-ES, and BCB-EBN showed melting transitions at 108, 119, and 146°C, in that order. As BCB-EBO contained more rigid benzoxazole segments, it only exhibited a glass transition (Tg) at 85°C prior to curing exotherm, after it had been previously heated to 125°C. The following Tgs were observed for the cured materials: BCB-EK (201°C), BCB-EBN (224°C), BCB-ES (264°C), and BCB-EBO (282°C). The relative thermal stability according to TGA (He) results is: BCB-ES < BCB-EBN < BCB-EK < BCB-EBO. Finally, the results from thermal analysis, infrared spectroscopic, and variable temperature microscopic studies indicated that the nitrile group plays an important role in the cure chemistry, thermal, and microstructural properties of BCB-EBN.

Novel Ternary Molecular Composites Prepared by a Sol–Gel Process and Their Conversion into Microcellular Foams

Abstract A carboxylated polysulfone (C‐PSF) was reacted with 3‐(glycidoxypropyl)trimethoxysilane (GPS) in a sol–gel reaction to form hybrid networks. Specifically, GPS was grafted to the C‐PSF chains at their epoxy ends by ring‐opening reactions and then the methoxy silane groups at their other ends were crosslinked through the usual hydrolysis and condensation reactions. These C‐PSF/GPS cross‐linked binary networks are of interest in their own right, but this type of network was also used in the incorporation of polybenzimidazole (PBI) rigid‐rod polymer to form a C‐PSF/PBI/GPS ternary composite. In this case, the network structures obtained by carrying out the same reaction in the presence of PBI chains successfully suppressed their undesirable segregation within the composite. The transparencies of the preparative solutions and the dried films indicated that the dispersion of PBI was maintained in these environments, presumably at the nano‐ or molecular level. The ternary composites were characterized using Fourier‐transform infrared (FT‐IR) spectroscopy, scanning electron microscopy (SEM), thermal mechanical analysis (TMA), and thermogravimetric analysis (TGA). Some of these composites were converted to closed‐cell microcellular foams, which were then characterized in a preliminary way with regard to their morphologies.