Functional group distributions in carboxylic acid containing poly(N-isopropylacrylamide) microgels

Control of the functional group distribution is of fundamental importance in the design of functional polymer particles, particularly in biological applications. Surface-functionalized particles are useful for bioconjugation and medical diagnostics, while internally functionalized particles may have applications in drug delivery. We have prepared a series of temperature-sensitive poly(N-isopropylacrylamide) (PNIPAM)based microgels containing carboxylic acid functional groups via copolymerization with methacrylic acid and acrylamide, which was selectively hydrolyzed under optimized conditions to generate the carboxylic acid functionality. The resulting microgels were analyzed using conductometric and potentiometric titration, dynamic light scattering, and electrophoresis. Acrylamide-containing microgels hydrolyzed below the volume phase transition temperature (VPTT) show broad particle size versus temperature profiles, relatively low electrophoretic mobilities at basic pH, and time-dependent base titration profiles, suggesting the presence of internal functional groups whose titration is diffusion-controlled. Methacrylic acid containing microgels show sharper particle size versus temperature profiles, higher electrophoretic mobilities at basic pH, and time-independent base titration profiles, suggesting the presence of a "core-shell" structure with primarily surface functionalization. Similar results were obtained when acrylamide-containing microgels were hydrolyzed at temperatures above the VPTT. Thus, through selection of comonomer and hydrolysis conditions, we have developed strategies to control and characterize the number and distribution of carboxylic acid functional groups in PNIPAM-based microgels.