摘要：

Viscosity, sedimentation and light scattering measurements were carried out on six fractions of cellulose trinitrate (CTN) in ethyl acetate solution to obtain weight average molecular weights and configurational parameters. In addition, osmotic pressure measurements made on these same fractions in acetone or butanone solution provided information for the evaluation of molecular weight heterogeneity. The molecular weight range of these fractions was Mw = 40,000 to 575,000 with heterogeneities corresponding to Mw/Mn = 1.2 to 2.2. Radii of gyration calculated from light scattering measurements confirm the observations of previous investigators indicating an unusual stiffness of the CTN chain. The persistence length required by the Porod-Kratky chain model corresponds to about 23 structural units; the segment for the Kuhn equivalent chain model is twice as great. The following empirical relations were obtained for the intrinsic viscosity [η] and the sedimentation constant S0: [η] = 2.50 × 10-5 Mw1.01 and S0 = 0.304 × 10-15 Mw0.29. The exponents in these equations differ significantly from those usually found for flexible polymers. Also, [η]M/(s2)3/2 and f0/η0(s2)1/2 differ from their asymptotic values Φ′ and P′, respectively, where s2 is the mean square radius of gyration and f0 is the frictional coefficient. It is shown that excluded volume and thermodynamic effects do not contribute significantly to this behavior. A small contribution to the exponents in the foregoing empirical equations arises from the chain stiffness. The latter manifests itself by a departure of the chain configuration from random flight behavior and a concomitant decrease in s02/M with decreasing M, which can be accounted for by the Porod-Kratky chain model. While the remaining portions of these exponents could be accounted for by hydrodynamic effects considered in the Kirkwood-Riseman or Debye-Bueche theories, the parameters thereby deduced lead to erroneous asymptotic values for [η]M/(s2)3/2 and f0/η0(s2)1/2. In view of the failure of these theories to apply it is concluded that hydrodynamic permeation is not an important factor. Alternatively it is suggested that, because of chain stiffness, the configuration of CTN in ethyl acetate exhibits severe deviations from random flight statistics with a related deviation from spherical symmetry over the molecular weight range investigated. Presumably, if similar investigations could be carried out in a higher molecular weight range these anomalies would disappear, and the behavior of CTN at high molecular weight would be similar to that observed for more flexible polymers at much lower molecular weight. Finally, on the basis of the results obtained for the unusually stiff chains of CTN and also for flexible polymers, it appears unlikely that hydrodynamic permeation is ever of appreciable significance in the viscosity and sedimentation behavior of dilute polymer solutions.

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