A nest of structures in dynamics of cellulose diacetate in N,N-dimethylacetamide in quiescent solution state studied by dynamic light scattering

Abstract

In the quiescent state, dynamic light scattering measurements were performed for a polysaccharide, cellulose diacetate (CDA, the degree of substitution 2.40), in a liquid-crystalline promoting solvent N, N-dimethylacetamide (DMAc) in the range from dilute to crossover concentrations. Three modes of motions were detected simultaneously over all concentrations measured. They are in a nest of structures, which are caused by long-range interactions acting between OH groups of glucose units in a highly dielectric-constant solvent DMAc. Denoting these modes as Mode I, II, and III from fast to slow motions, the decay rates Γ of each mode showed the squared scattering-vector dependence at lower angles, i.e., the diffusion nature. In dilute solution, fast Mode I is the translational diffusion of a single CDA chain and others represent the dissipation of locally associated CDA clouds which grow by the interactions specified above. Mode II is related to a coarse cloud formed in a limited time but spread over a wide space A, while Mode III is related to a dense cloud created with a very small space B inside >A. The tentative increase in CDA concentrations in the spaces A and B relaxes to the level of bulk concentration with a large decay rate ΓII for Mode II and a small ΓIII for Mode III. With the increase of c, the number of A and the bulk concentration increase, and the decay rate ΓIII for dissipating B becomes smaller than that in dilute solution. Above the crossover concentration c*, however, the space A becomes close each other and almost all the CDA are packed in A. Then fast Mode I represents the relaxation of concentration fluctuations, or cooperative diffusion, with the length scale as small as a single CDA chain, ξI. Mode II represents another cooperative diffusion, which is originated by the relaxation of concentration fluctuations effective only in A of the correlation length ξII. Mode III is related to the time–space fluctuation in the distribution of A, which emerges as another cooperative diffusion of much larger correlation length ξIII. Mode II was contaminated by a self-diffusionlike (SF) motion, though both were separated into two branches in more concentrated regions. The SF mode is assigned to a reptation motion of CDA, which can be described by the constrained translational motion of a stiff chain in the tube with interacting wall. The dynamical feature of these modes of motions are influenced strongly by strength and nature of long-range interactions between unsubstituted OH groups in glucose residues, or the hydrogen bonds in solvents of high electronegativity.