Abstract
Rheopectic behavior of the aqueous polymethacrylic acid (PMA) solution was investigated with a Zimm-Crothers type viscometer. Measurements were performed for the solutions of PMA ranging in molecular weight from 6×105 to 16×105, at concentration of 0.05—5.0 wt% and shear rate of 0.5—7.0 s-1. The solution viscosities of high molecular weight PMA (Mv > 10×105) increase linearly with the elapse of shearing time at early stage, reach a maximum and then fall off. Ratio of the maximum viscosity to the initial one, ηmax/ηinit, decreases with decreasing the concentration of polymer and increasing the rate of shear, indicating that rheopexy is attributed to a competition between growth and disruption of the molecular clusters in the shear field. The influences of four additives (HCl, NaOH, NaCl, and urea) on rheopexy were examined; it was found that, regardless of the additive species, there is a correlation between rheopexy and the initial rate of shear. These results suggest that a particular balance of hydrophilic and hydrophobic bonding abilities of PMA molecule is essential for the occurrence of rheopexy in the solution.
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