SYNOPSISThe properties of poly( 1,4-phenylene terephthalamide) (PPTA), Kevlar, and a poly(benzobisoxazole) (PBO) in solution are investigated. Molecular characterization using light scattering and viscometry on dilute solutions shows PBO and PPTA to have a rodlike conformation. Both are prone to interchain aggregation with the tendency for association apparently increasing with the ionic strength of the solvent. Rheological experiments are reported on disordered and ordered concentrated solutions of Kevlar and PBO. The dependence of the viscosity and recoverable compliance on shear rate is discussed and compared with that of flexible-chain polymers. The dependence of the viscosity and recoverable compliance of PBO solutions on temperature and concentration is discussed. PBO PPTAIn this report we discuss some of these properties, including light scattering and viscometry with dilute solutions, and rheological characterization of moderately concentrated solutions. It is shown that appreciable association obtains in many solutions, even at low polymer concentration. Data (apparently) free of association in dilute solution are discussed in terms of a wormlike chain model to assess the rodlike character of PPTA and PBO. The rheological propertieS to be considered include the dependence of the viscosity 9, and the steady-state recoverable compliance R, on the shear rate K . Here, R, is equal to the recovery -y~, after

A rotational cone‐and‐plate rheometer incorporating a drag‐cup torque transducer and a frictionless, wire suspension has been designed and constructed. The instrument design provides for a controlled atmosphere for the sample, including anhydrous conditions necessary for studies on solutions of polymers in strong acids. The rheometer can be used to determine the shear deformation in response to an applied stress, including creep, recovery, and the shear stress in response to an applied steady shear. Both transient and steady‐state measurements are possible. Illustrative examples of the instrument performance are given.

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Transient and steady‐state rheological data are reported for several anionic polystyrene solutions in tritolylphosphate (1. 6 < cM/ρMc < 7). Here c is the concentration of the solution, M is the molecular weight, ρ the density of the undiluted polymer, and Mc the molecular weight between entanglements as determined from zero‐shear viscosity. The polystyrene used had Mw = 410,000 and Mw/Mn < 1.06. Data are also given for solutions of polyisobutylene and poly(vinyl acetate) with larger Mw/Mn. The results give a critical strain γ′ ∝ c−1 such that linear viscoelastic behavior was obtained in a simple shear deformation with shear less than γ′. A simplified version of the constitutive equation of Bernstein, Kearsley, and Zapas is used with an empirical strain function F (γ) which contains γ′ as a parameter to discuss transient and steady‐state behavior in terms of the distribution of relaxation (or retardation) times determined for linear viscoelastic responce. Features of the dependence of the steady‐state viscosity ηk, recoverable compliance Rk, the first‐normal stress function Nk(1) on shear rate k are discussed in terms of F (γ) and the distribution of relaxation times to conclude that the latter plays a dominant role in the behavior observed in the range of k usually studied. The results predict that the reduced functions ηk/η0, Rk/R0, and Nk(1)/N0(1) should depend on η0R0k, and that the functional form depends markedly on the distribution of relaxation times, at least in the range η0R0k < 102. Comparison with the mechanistic model of Doi and Edwards shows a similar F (γ) but substantial differences in the reduced functions caused by a very narrow distribution of relaxation times in the model.