Shear induced fluctuations and fluctuation-length transition in the quasiflexible polymer in a solution of a hydrogen-bond enhancing solvent

Abstract

Dynamics of cellulose diacetate (a typical polysaccharide) in N,N-dimethylacetamide (a hydrogen-bond enhancing solvent) at 27 °C was investigated in a shear field of velocity gradient γ=0–5.3 s−1 through dynamic light scattering under circular Couette flow. The polymer mass concentration was set around the crossover of dilute and semidilute regimes. Growth of extra-large length-scale fluctuations and their transitions with shear were detected, which were originated by the long-ranged interaction, or the hydrogen bond, operating on the OH and CO groups in the glucose residues of cellulose acetates via high electronegative solvent. Two modes of fluctuations with fast and slow decay rates Γf and Γs were induced and each Γ value changed stepwise and linearly with γ, respectively. The decay rate distribution, which was estimated from the electric-field time correlation function expressed in the KWW-type of stretched exponential decay form g(1)(t)=A exp[−(t/τ)β] with β the index of decay rate distributions, was changed from 1/2 (one-sided stable distribution) to 3/2 (Holtzmark distribution) around γ=0.8–1 s−1 for the fast mode, while β=2 (Gaussian, or the sharpest distribution) over the full range of 0.40<γ<5.3 s−1 for the slow mode. The usual Cauchy distribution (β=1) at γ=0 (the quiescent state) was destroyed above a threshold γ*=0.033 s−1. At γ*<γ<0.2 s−1, fluctuations of various length-scale modes (0<β<1/2) were created but finally converged to two modes of fixed β(=1/2, 3/2, 2) at γ>0.4 s−1. In addition, oscillatory feature was conspicuous in g(1)(t) at every γ of 0.40<γ<5.3 s−1 and periodic fluctuations became remarkable in the scattered intensity IVv at γ>3.7 s−1, which may come from the competition between shear flow distortion and the Brownian diffusion of polymer chain in solution and can be grasped qualitatively by the initial stage of spinodal decomposition under the control of long-ranged attractive interactions.