Flocculation behavior of temperature-sensitive poly(N-isopropylacrylamide) microgels containing polar side chains with OH groups

The flocculation behavior of poly(N-isopropylacrylamide) (pNIPAM) microgels containing polar (OCH2CH2)3OH chains, incorporated by the copolymeric components (triethyleneglycol methacrylate, TREGMA), in aqueous NaCl solution was investigated. Determination of the critical flocculation temperatures (CFTs) and the critical flocculation concentrations (CFCs) of the microgels at 45 °C shows that polar (OCH2CH2)3OH chains have different influence on the flocculation behavior of the microgels at temperatures below and above their volume phase transition temperatures (VPTTs). The flocculation of the microgels becomes more difficult with the increase of (OCH2CH2)3OH chains below the VPTT. In contrast, the microgels flocculate more easily with more (OCH2CH2)3OH chains above the VPTT. Preliminary investigation on the flocculation kinetics of the microgels further shows that (OCH2CH2)3OH chains have different effects on the flocculation rate at temperatures below and above the VPTT. The flocculating rate of the microgels at 25 °C decreases with the increase of (OCH2CH2)3OH chains. While the flocculation rate at 45 °C increases with the increase of (OCH2CH2)3OH chains due to their enrichment on the surface of the microgels as a result of the temperature-induced volume-phase transition, which was verified by variable temperature 1H NMR spectroscopy. The polar (OCH2CH2)3OH chains rich in the surface increase the attractive force between the microgels, promoting the flocculation.

The effect of polar side chains, i.e., (OCH2CH2)3OH, on the CFTs of the microgels in NaCl solutions from 0.02 to 1 M were investigated using turbidity method. The result shows that the CFTs of p(NIPAM-co-TREGMA) microgels increase with the increase of the polar side chains in the microgels, as the existence of the polar side chains strengthens the H-bonding between the microgels and water molecules, restrains the increase of the Hamaker constant of the microgels. As a result, the CFTs of the microgels increase with the increase of the polar side chains.Figure optionsDownload as PowerPoint slide