Dynamic light scattering study of structural changes of cellulose diacetate in solution under Couette flow

摘要

Dynamic light scattering (DLS) measurements were conducted under a simple shear flow, i.e., the Couette flow field, to cellulose diacetate (CDA; a typical polysaccharide) in semidilute N,N-dimethylacetamide (DMAc) solution. The shear rate gamma was varied From 0 to 5.25 s(-1). At quiescence, CDA showed three dynamic modes, which correspond to two kinds of associations and a single chain. In the weakly sheared system (0.402 s(-1) less than or equal to gamma less than or equal to 5.25 s(-1)), the three dynamic modes collapsed, and alternatively two completely different modes were observed. The fast mode experienced a transition around gamma = 0.81 s(-1), the decay rate distribution changing from a broader bimodal to a narrower unimodal shape with increasing shear. The slow mode had a unimodal decay rate distribution, but the length scale of fluctuations was larger by 3 orders of magnitude compared to those observed at gamma = 0 s(-1); that is, the length distribution was extremely uniform irrespective of gamma, and the length xi(s) was proportional to gamma(-1). This xi(s) proportional to gamma(-1) relation was interpreted by Taylor's treatment of the behavior of viscous drops. Thus, the repetition of the burst and the coalescence growth of the associations was suggested for the slow mode. The xi(s) proportional to gamma(-1) relation implied that CDA was molecularly disperse in DMAc under the viscometric field in a capillary viscometer at gamma = 920 s(-1). [References: 47]