Dynamics of amylopectin in semidilute aqueous solution

The dynamic behaviors of potato amylopectin and waxy corn amylopectin in semidilute solution were investigated by laser light scattering and viscometer. For potato amylopectin with relatively smaller molecular weight, only pure diffusion motion of amylopectin was found by LLS in dilute regime. When the concentration was above the critical overlapping concentration (C^∗), three relaxation modes were found. The line-width of fast mode (Γ_f) had a q² dependence, where q is the scattering vector, and the correlative length (〈ξ_h〉) could be scaled to concentration (C) as 〈ξ_h〉 ∼ C^−0.79±0.1 when C > 2%. This mode was attributed to the cooperative relaxation motion of the “blobs” in the transient network. The line-width of slow relaxation mode (Γ_s) could be scaled with q as Γs∼qαs, α_s varying from 2.0 to 2.66 as the concentration increased. The relaxation time of slow relaxation mode (τ_s) had a C^1.8±0.1 dependence. This mode was originated from the association of the amylopectin. The medium mode was found when C > 4%. The line-width of medium relaxation mode (Γ_m) could be scaled to q as Γm∼qαm, α_m varying from 2.7 to 2.5 with the increasing concentration. The relaxation time of medium relaxation mode (τ_m) had C^0.7±0.1 dependence. The relative intensity contribution of the medium relaxation mode decreased with a rise in the concentration. This mode was attributed to the thermally agitated density fluctuation in semidilute solution induced by heterogeneities of the transient network. For waxy corn amylopectin with relatively huge molecular weight (∼10⁸ g/mol), only the internal motion of the single amylopectin molecule was found in dilute regime when qR_g ≥ 2, where R_g is the gyration radius of amylopectin. It was also found that there were three relaxation modes in semidilute solution of waxy corn amylopectin. The fast relaxation mode was found to be caused first by the internal motion of the single amylopectin molecule, and then, with the increasing concentration, by the cooperative motion of the transient network. The medium and slow relaxations for waxy corn amylopectin have the same physical origin as those for potato amylopectin. However, the C dependence and the q dependence of the medium and slow relaxation times for waxy corn amylopectin were different from those for potato amylopectin. This was attributed to the strong dynamic coupling effect in semidilute solution of the waxy corn amylopectin. The concentration dependence of the viscosity of amylopectin in semidilute solution indicated that the topological entanglement of amylopectin was weak due to the highly branching.