The definition of a molecular composite is a blend of a rigid rod poly
mer and a flexible coil polymer that is miscible at the molecular leve
l. This concept has been tested using systems in which the chain flexi
bilities differ as widely as possible as judged by the difference in g
lass transition temperatures (Delta Tg). The biggest variation (Delta
T similar to 360 degrees C) was obtained by mixing poly benzimidazole
with copolymers of poly(vinyl acetate-ran-vinyl alcohol). It was obser
ved that the blends were distinctly two phase when the hydroxyl conten
t was less than 50 mol %. Above this value clear blends were obtained
with finely dispersed phases although it is doubtful if mixing at the
molecular level takes place. Miscible blends could be obtained from co
mbinations of the sodium salt of poly(phenylene terephthalamide) with
poly(4-vinylpyridine) and mixtures of poly(phenyl imino-1,4-phenyleneo
xyterephthalate) with poly(styrene-stat-hydroxylstyrene) where coulomb
ic interactions and hydrogen bonding respectively promoted the miscibi
lity.