Microwave and millimeterwave obscurants based on typical millimeterwave materials do not environmentally degrade with respect to their physical and electrical properties. The use of conductive polymer coated films and fibers with inherent environmental instabilities are being investigated as possible alternatives, Achieving the optimum electrical performance is highly dependent upon the polymer/dopant combination, deposition conditions, morphology, and volume of the coating that is used. Several conductive polymer coated fiber systems were investigated via thermal analysis. The effects of processing conditions on performance and stability were studied.

Control of the internal morphology of wet-spun fibers from a fluorinated polyimide has been achieved by varying the rate of polymer coagulation through adjustments in nonsolvent/ solvent miscibility and precipitation strength of the coagulation bath. Filament internal morphologies ranged from very porous or sponge-like to fully solid. Intermediate structures included fibers containing a spongy core with a nonporous skin, sponge-like fibers containing large voids, and a relatively solid material containing randomly spaced small voids. The cross-sectional shape of the fiber is dependent upon the coagulation process as well as the volume contraction of the initial extrudate. Drawn fibers (3X) retained the original as-spun cross-sectional shape and also lost porosity. Mechanical properties of poly(6FDA-4BDAF) fibers have an inverse relationship to filament porosity. Maximum modulus and break strength for drawn fibers is approximately 6 GPa and 200 MPa, respectively. As-spun mechanical properties were dependent upon the processing conditions and have moduli between 0.4-3.0 Cpa and break strengths of 10-160 MPa. A dielectric constant of 2.50 for nonporous films was measured over a frequency range between 1.0 MHz to 1.8 GHz, showing little dispersion. (C) 1997 John Wiley & Sons, Inc.

Microwave and millimeterwave obscurants based on typical millimeterwave materials do not environmentally degrade with respect to their physical and electrical properties. The use of conductive polymer-coated films and fibers with inherent environmental instabilities are being investigated as possible alternatives. Achieving the optimum electrical performance is highly dependent upon the polymer/dopant combination, deposition conditions, morphology and volume of the coating that is used. The electrical performance can be assessed by complex permittivity analysis and this was performed on samples of compacted conductive polymer powders. Several conductive polymer-coated fiber systems were also investigated via thermal analysis. The effects of processing conditions on performance and stability were studied.

Organo-soluble rigid-rod and segmented rigid-rod polyimides and their copolyimides exhibit isotropic solutions in hot m-cresol, but form gels upon cooling. A lyotropic liquid crystal phase is observed below the gel/sol transition. Mechanical gel formation is caused by liquid-liquid phase separation, while the liquid crystal phase may be formed through a nucleation process after gelation. High performance fibers can be spun from the hot isotropic solutions using a dry-jet wet spinning method. After the fibers are drawn at high temperatures, they display tensile strength higher than 3.2 GPa and an initial modulus higher than 130 GPa. In particular, the fibers retain relatively high mechanical properties at elevated temperatures. Solution casted films exhibit very low thermal expansion coefficients and dielectric constants. Their structure, morphology and property relationships will also be discussed.

OF THE DISCLOSURE A wet-spinning fiber process which controls the microstructure of the wet-spun fiber by varying the non-solvent/solvent miscibility and precipitation strength.