Fibers consisting of a rigid rod polymer and thermoset resin matrices were prepared. Poly(benzo‐[1,2‐d : 5,4‐d′]bisoxazole‐2,6‐diyl)‐1,4‐phenylene} (PBO) in polyphosphoric acid (PPA) was blended with isophthaloyl bis‐4‐benzocyclobutene (1) or 2,6‐bis‐4‐benzocyclobutene benzo[1,2‐d: 5,4‐d′]bisoxazole (2), and fibers were spun from these dopes. As‐spun fibers that did not show phase segregation between the two components as examined with an optical microscope, were soluble in methanesulfonic acid (MSA). After heat treat‐ment, the fibers swelled but did not dissolve in MSA. A fiber cross section of heat‐treated PBO‐1 fiber showed well‐dispersed benzocyclobutene polymer domains of 200–500 Å by transmission electron microscopy (TEM). Films cast from MSA solutions of PBO and 2 were homogeneous, and TEM of heat‐treated fiber showed only one phase. A molecular composite fiber was made. Some of these fibers showed 20–30% improvement in compressive strength over unmodified PBO fiber. © 1995 John Wiley & Sons, Inc.

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Films consisting of a rigid-rod polymer and thermoset resin matrixes were prepared. Poly{(benzo [1,2-d : 5,4-d]bis(oxazole-2,6-diyl))-1,4-phenylene} (PBO) in polyphosphoric acid (PPA) was blended with 2,6-bis(4-benzocyclobutene) benzo[1,2-d : 5,4-d]bis(oxazole) ( 1), and films were extruded from these solutions. The coagulated films were soluble in methanesulfonic acid (MSA). After heat treatment at 300ЊC, the films became insoluble in MSA. Crosslinked films were homogeneous and did not show phase segregation between the two components. These were composite films at the molecular level. Transmission electron microscopy (TEM) showed enhanced interlayer integrity and reduced microfibril separation for the molecular composite films as compared to normal PBO film. These films had significantly better torsion and tension delamination resistance. The incorporation of a second component did not sacrifice the tensile properties of PBO film. Thermal stability of these composite films was only slightly lower than that of normal PBO film.

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