Dielectric and viscoelastic relaxation in dilute solutions of some non-Gaussian chains

Abstract

Dielectric and viscoelastic measurements have been made on dilute solutions of poly(butylisocyanate). The measurements confirm that the molecules are predominantly rod-like in hydrodynamic behaviour. The end-over-end rotation times obtained from both sets of observations have been analyzed by three methods. The monomer projection length along the major axis cannot be evaluated independently of the value chosen for the length-to-thickness ratio, but it is probably less than 1.1 Å and greater than 0.6 Å. Application of the Kratky-Porod model to molecules for which the chain length is less than the persistence length must be treated with caution. For poly(butylisocyanate) the persistence length obtained from dilute solutions is 230 monomer units, although extrapolation of a wider concentration range to infinite dilution yields a value of 400 monomer units. Concentration has a marked effect on chain stiffness parameters (increasing apparent cylinder radii or decreasing persistence lengths) which is explained in terms of side-by-side dipole association.

The dielectric measurements of Phillips ⁴ made on a variety of samples of poly(N-vinyl carbazole) have been analyzed using equations suitable for rod-like or stiff-coil polymers. For molecular weights above 50,000 the molecules are coil-like with a Kratky-Porod persistence length of 250 monomer units. Shorter chains are rod-like in which the projection length of each monomer unit along the rod is roughly ⅔ the monomer length. This suggests that the short chains (8-20 monomer units) adopt a semicircular are outline. Both polymers show increasing departures (in both the dielectric and the viscoelastic parameters) from rod-like behaviour as the temperature is raised. The change is continuous through room temperature so that there is no reason for assigning a perfect structure (helical or otherwise) to solutions at these temperatures. The experimental data do not, generally, allow sensible treatment using number average molecular weights, so that weight (or perhaps even higher) averages must be used for heterodisperse samples.