Temperature and Salt Responsivity of Anionic, Cationic and Amphoteric Nanogels Based on N-Isopropylacrylamide, 2-Acrylamido-2-Methyl-1-Propanesulfonic Acid Sodium Salt and (3-Acrylamidopropyl) Trimethylammonium Chloride

Abstract

Three different nanogels possessing anionic, cationic and amphoteric character were synthesized via conventional redox initiated free radical copolymerization of N-isopropylacrylamide (NIPAM), 2-acrylamido-2- methyl-1-propanesulfonic acid sodium salt (AMPS) and (3-acrylamidopropyl) trimethylammonium chloride (APTAC). The negatively charged [NIPAM]:[AMPS] = 90:10 mol.%, positively charged [NIPAM]:[APTAC] = = 90:10 mol.%, and charge-balanced amphoteric nanogels [NIPAM]:[APTAC]:[AMPS] = 90:5:5 mol.% abbreviated as NIPAM90-AMPS10, NIPAM90-APTAC10, and NIPAM90-APTAC5-AMPS5, respectively, were characterized by FTIR spectroscopy, TGA, UV-Vis spectroscopy and DLS measurements. The temperature and salt responsive properties of nanogels in aqueous and aqueous-salt solutions were studied in the temperature range of 25–60 °C and ionic strength (μ) of 0.001–1.0 M NaCl. Anionic NIPAM90-AMPS10 and cationic NIPAM90-APTAC10 nanogels, exhibit a pronounced polyelectrolyte effect in aqueous-salt solution due to screening of the negative or positive charges by low-molecular-weight salt. Whereas the charge-balanced amphoteric nanogel NIPAM90-APTAC5-AMPS5 exhibits an antipolyelectrolyte effect due to the screening of electrostatic attraction between opposite charges by low-molecular-weight salt. The difference between the temperature-dependent behaviors of anionic, cationic and amphoteric nanogels is explained by shrinking (polyelectrolyte effect) and expanding (antipolyelectrolyte effect) of macromolecular chains in aqueous-salt solutions.