ABSTRACT
I. INTRODUCTION Thus far, we have examined the flow behavior of flexible-chain polymers. lf the polymers contain para-linked aromatic rings. such as those found in aromatic polyamides. polyesters. and polyazomethines. rotation about the backbone is inhibited, and this leads 10 a stiff. rigid. extended chain structure; chain rigidity typically results in a value of the Mark-Houwink exponent ""a" that is greater than 0.8 (sec Eq. 4.8). When such rodlike molecules. especially those having a large length-to-diameter or aspect ratio, are dissolved in a solvent, the resulting isotropic solution becomes quite viscous at low polymer concentrations. This happens because the polymer molecules in solution form an entangled matlike structure that refuses to accommodate additional molecules without forcing some of the dissolved molecules to bend. lf chain flexibility is prohibited, a further increase in polymer concentration can occur only through the development of an anisotropic or ordered phase that involves parallelization of the polymer chains. Such a phase can possess long-range order similar to crystalline solids. and it is known as a mesophasc or a polymeric liquid crystal. This ordering is accompanied by a sharp reduction in the solution viscosity. This is shown in Fig. 8.1 for a 50/50 copolymer of n-hexyl and n-propylisocyanate of 41,000 molecular weight dissolved in toluene at room temperature [ 1]. Fig. 8.1 clearly illustrates that it is easier for rods to slide past each other when they are oriented parallel to each other; in practical terms this implies ease of processing. Polymers that form a liquid crystalline phase in solution are known as lyotropic. Three different physical structures are found to occur with rodlike molecules; these are shown in Fig.
