| STUDY |
An example is poly(styrene)-poly(acrylic acid) (PS-PAA). Authors has shown that PS-PAA polymers organize into specific structures depending on the relative lengths of the two blocks. In water, they form colloidal objects consisting of a dense, hard, glassy core surrounded by a charged brush. The core, made up of PS units, can be cylindrical or lamellar. PAA units make up the brush. The size of the object and the charge, height, and density of the brush vary with diblock length, pH, and salt concentration. "This is a way to produce colloids of controlled shapes and sizes that can be dispersed in water," she says. In water, PS-PAA forms gels. Authors has been investigating the mechanical properties of the gels and how those are influenced by changes in the chemistry of the diblocks. For example, he considered a PS-PAA in which PAA is chemically modified to incorporate hydrophobic stickers. The micelles thus consist of a glassy PS core surrounded by a charged hydrophilic corona punctuated by hydrophobic segments. The segments, author thought, would act like a glue, sticking individual micelles together to form a network that is more rigid than that formed by regular PS-PAA. What he found surprised him: "The first action of the glue is to soften the gel and make it flow," he says. "But as you add more and more glue, you end up with a very rigid structure." Author explains what's happening by comparing it with a traffic jam: "If you put more and more cars on a highway, eventually it will be so crowded that the system is rigid; no car can move. Sticking some cars together will create space for other cars to move. But if all of the cars are glued, no car can move." With or without stickers, PS-PAA forms gels. The difference, author says, is that without stickers the gel formed is in equilibrium with the liquid, whereas with stickers the gel formed is more permanent. PS-PAA gels form at a concentration of 2% in water, whereas other gel-forming materials of comparable molecular weight require up to 20 times higher concentrations, says author. And because the stickers increase gel strength by up to 10 times, the concentration required to get a desired gel strength can go down to about 1%. These gels, she points out, can be used in oil-drilling operations to suspend sand. |
| UPDATE | 08.03 |
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| LITERATURE REF. | This data is not available for free |
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