The processing of a new series of liquid crystalline (LC) epoxy networks was evaluated. Above the glass transition temperature, the low crosslink density networks could be mechanically aligned. The mechanically oriented networks readily lost orientation upon heating. Highly anisotropic liquid crystalline (LC) epoxy networks were also prepared by aligning the mesophase of the prepolymer during the curing process under the influence of a magnetic field. Orientation parameters (f) of 0.13 to 0.57 were achieved by these processes as determined by x‐ray diffraction analysis. The ability of the magnetically aligned networks to retain their orientation above the glass transition temperature was determined by time‐resolved x‐ray diffraction. The stability of the alignment of these networks was found to depend on crosslink density. The effect of the anisotropy of these networks was investigated by measuring the coefficient of thermal expansion (CTE). In the aligned networks, there was a substantial reduction in CTE parallel to the direction of the applied field compared to the randomly oriented networks. © 1992 John Wiley & Sons, Inc.

This article describes the synthesis and characterization of a new series of liquid crystalline thermosets. Nematic epoxy‐terminated oligoethers based on dihydroxy‐α‐methylstilbene were synthesized for this study. These prepolymers were crosslinked within the nematic mesophase using methylenedianiline. Depending upon the molecular weight and polydispersity of the oligoether, the crosslinking reaction resulted in networks with either a smectic or nematic molecular organization in contrast to the simple nematic phase of the oligoether. The formation of a smectic‐like structure on curing was found to be related to the breadth of the prepolymer molecular weight. In those networks with a low crosslink density a clearing transition could be observed, whereas in the more highly crosslinked networks the molecular organization was frozen in until decomposition. The glass transition temperature of these LC networks rose as the crosslink density was increased, ranging from 35 to 152°C. In agreement with theory, the clearing transition of the networks was found to be dependent on the phase state during curing © 1992 John Wiley & Sons, Inc.

The resistance of AlN powder to oxidation upon heat treatment in air is investigated using thermogravimetric analysis (TGA) and x-ray photoelectron spectroscopy (XPS). The results reveal that oxidation is initiated at temperatures above 700 °C. For temperatures between 750 and 900 °C, an intermediate oxidation state, which involves the formation of oxynitrides, is detected by XPS and only a linear increase in weight with time is observed with TGA. Oxynitride formation precedes the formation of aluminum oxide and the oxynitride layer persists forming an interfacial layer between oxide and nitride. At temperatures above 900 °C XPS indicated the formation of a thick oxide layer while TGA measurements showed a linear followed by a nonlinear increase in weight with time. These results suggest that the initial stage of oxidation is controlled by surface reactions followed by diffusion controlled oxidation. The activation energy for surface reaction and diffusion oxidation are 36 and 56 kcal/mol, respectively.