Some dilute solution properties of highly isotactic polyacrylonitrile in aq. nitric acid

An attempt was made to investigate the dilute solution properties of highly isotactic polyacrylonitrile (PAN) dissolved in aq. nitric acid and to estimate the molecular parameters, such as the unperturbed chain dimension A and the characteristic ratio C∞. For this purpose a highly isotactic polymer with isotactic triad content of 73.4% was synthesized by irradiating γ-ray on acrylonitrile-urea canal complex at -78degC and was subjected to successive precipitation fractionation, using dimethyl sulfoxide as solvent and toluene as precipitant, into 7 fractions. Among them 2 fractions with isotactic triad of 71.0 and 77.9%, respectively were employed for further study. The weight-average molecular weight Mw and the second virial coefficient A2 of these fractions in 70wt% aq. nitric acid soln. were measured by the light scattering method. The 70wt% aq. nitric acid soln. was predicted to be able to dissolve perfectly-isotactic PAN with Mw of 1.5-2.6 x 105 at room temperature. The Mark-Houwink-Sakurada equations in aq. nitric acid solutions differ significantly depending on the stereoregularity of PAN employed, even if highly concentrated acids are used. A 57.5wt% aq. nitric acid soln. is determined, by the dependence of A2 on the nitric acid concentration, to be a Flory's theta solvent for PAN with isotactic triad of 71.0% at 25degC. A of PAN increases rapidly with isotactic triad content, especially in the range higher than 50%. C∞ of PAN is always far larger than the calculated value of non-polar Monte Carlo chain for polypropyrene [P. J. Flory et al., J. Am. Chem. Soc., 88, 639 (1966)] over the whole range higher than 50% isotactic diad. (author)