Highly pH and Temperature Responsive Microgels Functionalized with Vinylacetic Acid

Temperature-responsive microgels based on poly(N-isopropylacrylamide) (PNIPAM) and functionalized with vinylacetic acid (VAA) are observed to exhibit a host of novel swelling responses compared with equally functionalized microgels prepared using the conventional acrylic acid (AA) and methacrylic acid (MAA) comonomers. VAA−NIPAM microgels are ionized over a narrow pH range and show functional group pKa values which are independent of the degree of ionization. Ionization induces a much larger swelling response in VAA−NIPAM microgels than in the conventional microgels; upon ionization at physiological temperature, VAA−NIPAM swells 3 times more than either AA−NIPAM or MAA−NIPAM. VAA−NIPAM microgels also display sharp, PNIPAM-like thermal deswelling profiles when protonated but, upon ionization, undergo no volume phase transition up to at least 70 °C. The highly responsive and tunable ionization and swelling profiles observed for VAA−NIPAM are consistent with the tendency of VAA to behave as a chain transfer agent, resulting in the incorporation of a large number of well-separated VAA units on highly mobile chain ends at or near the microgel surface. VAA−NIPAM microgels may thus be ideal for use in biomolecule separation, medical diagnostics, and biodelivery applications in which sharp responses to multiple environmental stimuli are required.