Preparation and characterization of N-isopropylacrylamide/acrylic acid copolymer core-shell microgel particles

A new method has been developed to prepare smart copolymer microgels that consist of well defined temperature sensitive cores and pH sensitive shells. The microgels were obtained from N-isopropylacrylamide (NIPAAm) and acrylic acid (AAc), containing different mole ratios of AAc. Transmission electron micrographs of the microgels show that the colloidal copolymers are nearly monodisperse spheres (core-shell structures). The lower critical solution temperatures (LCSTs) (or phase separation temperatures) of the aqueous microgel solutions were measured by cloud-point method. At slight acidic conditions, the LCST decreased with increase in AAc content, which suggests that the hydrophobic group of NIPAAm has a greater influence on the LCST than the polar COOH group at those conditions. An increase of pH value leads to a significant increase in LCST due to the formation of a more hydrophilic copolymer. The LCST were studied as a function of copolymer composition over the pH range from 4.0 to 6.5. Because the pK(a) of the polymers can be tuned to fall close to neutral pH, these polymer compositions can be dispersed to have phase transitions triggered near physiological pH or at slight acidic pH values that fall within acidic gradients found in biology. Because of their stimuli-responsive behavior, these nanoscale materials are excellent candidates for biotechnology and biomedical applications where small changes in pH or temperature are of great consequence.