Abstract
The rheopectic behavior of sacran aqueous solutions, a natural giant molecular polysaccharide with a molecular weight of 1.6 × 107 g/mol, was investigated. When a low shear was applied to 1.0 wt.% sacran solution, the shear viscosity increased from 7.2 to 34 Pa·s. The increment in the viscosity was enhanced as the shear rate decreased. The shear viscosity was independent of the time at a shear rate of 0.8 s−1; simultaneously, thixotropic behavior was observed at shear rates higher than 1.0 s−1. A crossover was observed at 0.15 wt.% for the concentration dependence of both the viscosity increase and zeta potential, which was the vicinity of the helix transition concentration or gelation concentration. It was clear that the molecular mechanism for the rheopexy was different at lower and higher regions of the crossover concentration.
Highlights
Sacran is a supergiant cyanobacterial polysaccharide extracted from the jelly extracellular matrix of a river plant named Aphanothece sacrum [1], which has been mass cultivated in Japan for a long time
Due to the electric charges on the chain, sacran changes its conformation depending on the sacran concentration, e.g., helix transition concentration, where the sacran chain change from random coil to double helix occurs at 0.09 wt.% and gelation concentration, where sacran showed a transition from liquid to weak gel at 0.25 wt.%
Many reports describe the phenomenon of negative thixotropy or rheopexy, which is the increase of viscosity with time under a fixed-shear rate
Summary
Sacran is a supergiant cyanobacterial polysaccharide extracted from the jelly extracellular matrix of a river plant named Aphanothece sacrum [1], which has been mass cultivated in Japan for a long time. Similar high swelling behaviors were found for cross-linked sacran hydrogels, e.g., 6100 mL/g for pure water and 530 mL/g for 0.9 wt.% for saline solution [2]. Many reports describe the phenomenon of negative thixotropy or rheopexy, which is the increase of viscosity with time under a fixed-shear rate. The aqueous solution of polymers containing inorganic particles demonstrates the negative thixotropy depending on the rest time between the pre-shear and viscosity measurements [23] or the value of pH [24,25]. A synovial fluid and bovine serum albumin (BSA) solution demonstrates rheopectic behavior at shear rates below 10−1 s−1 due to protein aggregation [29]. The influence of the macroscopic LC domain on the rheopectic behavior is discussed
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