Change in chain stiffness in viscometric and ultracentrifugal fields: Cellulose diacetate in N, N-dimethylacetamide dilute solution

The molecular characteristics and the chain stiffness were investigated for fractionated samples of cellulose diacetate (CDA, degree of substitution DS = 2.40) in N, N-dimethylacetamide (DMAc) through the partial specific volume, the viscometric, the sedimentation velocity, and the sedimentation equilibrium measurements at 30 °C. It was found that CDA dispersed molecularly in DMAc under the external field such as in the viscometric and the sedimentation experiments. The molecular weight dependence of the intrinsic viscosity [η] and the sedimentation coefficient at infinite dilution s0 of the single CDA molecule were expressed by the relation [η] = 1.10×10−2Mw0.85 (cm3 g−1) and s0=2.25×10−14Mw0.20 (s), which exhibit the stiff, or semiflexible chain nature. The semiflexible chain parameters were evaluated by the Yamakawa–Fujii (YF) theory of the unperturbed wormlike cylinder model, first via a combination of the viscosity and the partial specific volume data (method A) and second via a combination of the sedimentation and the partial specific volume data (method B). Method A gave the chain parameters that q = 8.0 nm, ML = 523 nm−1, and d = 0.89 nm, whereas method B gave q = 48 nm, ML = 560 nm−1, and d = 0.93 nm. Here q is the persistence length, ML is the molecular weight per unit contour length, and d is the chain diameter of the wormlike cylinder model. Methods A and B deduce, independent of the method, the definite ML and d values, which are very consistent with the ordinary reported values. However, q estimated by the two methods differs by about six times the other. This fact suggests that the CDA molecule in the ultracentrifugal field has a conformation different from that in the viscometric shear field: The CDA chain may be highly stiff in the ultracentrifuge because of the situation that the adjacent glucose residues are stuck in a rigid conformation by the double stapled hydrogen bonds between the intramolecular hydroxyls and oxygens.