Investigation of the shear stress effect on montmorillonite platelet aspect ratio by atomic force microscopy

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The effect of shear stress on the aspect ratio of montmorillonite platelets was investigated using atomic force microscopy (AFM). Montmorillonite, Cloisite® Na +, and organically modified montmorillonite, Cloisite® 20A, were examined under different shear stress conditions. The use of ultrasonication as an exfoliation method helped disperse mineral particles but had little impact on platelet size and was therefore used to determine the native aspect ratio of the clay minerals. The aspect ratio of both Cloisite® Na + and Cloisite® 20A platelets had a wide distribution with values ranging from 180 to 2900 and 192 to 2800, respectively. More than 40% of the Cloisite® Na + platelets and 77% of the Cloisite® 20A platelets had aspect ratios higher than 1000. Single delaminated platelets were shown to be broken when minimal shear stress, 2 Pa for example, was applied to dispersions of Cloisite® Na +/deionized water and Cloisite® 20A/toluene. The platelets from these experiments showed a most probable aspect ratio of 200 and 300, respectively. Increasing the magnitude of the shear stress further resulted in a reduction in platelet size, although not to the same degree as the initial application of shear stress. This accurate and unbiased measurement of the aspect ratio of the platelet increases the understanding of the nature of montmorillonite fillers during the extrusion process.