Viscosity and the prolate ellipsoid model applied to low molecular weight cellulose triacetate fractions

Cellulose triacetate of molecular weight 5,600, obtained by acetolysis, and separated into 15 fractions by preparative permeation chromatography, was analyzed for intrinsic viscosity, number average, and weight average molecular weight. Glucose pentaacetate, cel-lobiose octaacetate, and cellotriose hendecaacetate are also included in the analysis. The axial ratios of a prolate ellipsoid calculated from the viscosity of the low DP CT A fractions, when compared to a molecular model and to the fully extended contour length, confirm the persistence length found by viscosity theory at high molecular weight levels. A long plateau region of slope 0.28 to ca. DP 8, is found in the log-log [n] versus DPn plot. The curve then rises sharply to slope ca. 2.0 at DPn 10 and declines again to slope ca. 0.9 above DP 30. A close correspondence is found with the hydrodynamic predictions of Peterlin [5, 16] for short-chain wormlike or inflexible polymers. Plots of R2/DPn versus DPn for CTA and CA show that coil randomness is apparently not achieved before DP 400. No change in viscosity was detectable for CTA of DP 50, when increasing amounts of nonsolvent isopropyl alcohol were added to 1,2-dichloroethane, short of the precipitation point.