Controllable inter-action between cations and thermally sensitive poly(N-vinylcaprolactam-co-sodium acrylate) microgels in water

The microgels made of poly(N-vinylcaprolactam) (PVCL) and a few percent of sodium acrylate (SA) were used as a model system to study the cation induce d aggregation of colloid particles. Using such microgels make the aggregati on controllable and reversible because PVCL can gradually change from hydro philic (soluble) to hydrophobic (insoluble) in water when the temperature i s increased in the range 25-35 degreesC. The aggregation induced by differe nt kinds of cations and at different temperatures was investigated by using a combination of static and dynamic laser light scattering (LLS). At tempe ratures lower than similar to 30 degreesC, the microgel shrinks with increa sing the cation concentration due to the increase in the ionic strength and the intra-microgel complexation. The extent of the shrinking induced by di fferent cations follows the order of Hg2+ much greater than Cu2+ > Ca2+ > N a+. At temperatures higher than similar to 32 degreesC, Ca2+ and Cu2+ can i nduce inter-microgel aggregation. The Ca2+ induced aggregation is essential ly reversible in the heating-and-cooling cycle, but there exists a large hy steresis in the Cu2+ induced one. As expected, monovalent Na+ was not able to induce the inter-microgel aggregation The complexation between Hg2+ and carboxylic groups is so strong that the intra-microgel complexation becomes so dominant that Hg2+ was also not able to induce the inter-microgel aggre gation. (C) 2001 Elsevier Science Ltd. All rights reserved.