Equilibria of extended‐chain polymers exhibiting crystalline and liquid‐crystalline phases

The phase diagram of poly( p ‐benzamide) (PBA) in N , N ‐dimethylacetamide/LiCl solutions was determined for two PBA samples having weight‐average molecular weights of about 10,000. The various equilibria were studied using analytical, viscometric, and optical microscope measurements. The phase diagram at 25°C, taking as variables the concentration of polymer ( C p ) and LiCl ( C s ), involves several equilibria which can be summarized as follows: solid ↔ isotropic solution when 0.75 ≤ C s ≤ 2 g/dl, solid ↔ anisotropic solution when 2 ≤ C s ≤ 4 g/dl, isotropic solution ↔ anisotropic solution when 2 ≤ C s ≤ 4 g/dl, and C p > 6 g/dl, and dilute isotropic solution ↔ gel when C s > 4 g/dl. In the C p range in which the isotropic and anisotropic phases coexist, enrichment of the high‐molecular‐weight component of the polymer in the anisotropic phase becomes more marked as the volume fraction of the latter phase is decreased. The two PBA samples exhibit noticeable differences in solubility, absolute viscosity, and in their viscosity‐concentration behavior. The location of the maximum in the latter dependence does not necessarily coincide with the first appearance of the anisotropic phase. In the absence of a flow field, anisotropic solutions exhibit an irreversible increase in viscosity. Inclusion of the equilibria involving the crystalline state furnishes insight into some of the common observations for extended‐chain polymers. A diagram illustrates the superposition of the solubility curves for a crystalline polymer and the liquid‐crystal regions. This indicates that, for the high melting crystalline polymers, the crystalline phase should be stable relative to the concentrated anisotropic phase of the wide biphasic region.