The term "monomer unit" will always be used in the present invention to mean "rigid-rod monomer unit." In the instances where a flexible or non-rigid-rod monomer unit is discussed, it will be indicated as a "non-rigid monomer unit." Most non-rigid monomer units cannot attain a conformation in which the bonds to the polymer chain are parallel, for example, the 1,3-phenylene group or the 4,4'-diphenylether group. However, some non-rigid monomer units will admit a conformation in which the bonds to the polymer chain are parallel, such as the phenylene amide type non-rigid monomer units of KEVLAR (TM DuPont) (polyamide of 1,4-phenylenediamine and terepthalic acid). Polymers comprised of such non-rigid monomer units are "pseudo-rigid" due to the possibility of bent or kinked conformations. Rigid-rod polymers are, in general, stiffer than pseudo-rigid polymers.