Application of Self-Consistent Field Theory to Model the Temperature Response of Tethered Poly(N-isopropylacrylamide) Chains

Aqueous solutions of poly(N-isopropylacrylamide) (PNIPAM) exhibit lower critical solution temperature (LCST) behavior near 32C. This phenomenon can be exploited to control the release of proteins or cells from PNIPAM modified surfaces with small changes in temperature. Tethered PNIPAM was synthesized using surface-initiated atom transfer radical polymerization from mixed alkanethiolate self-assembled monolayers (SAMs). Both surface plasmon resonance and neutron reflectivity techniques were used to directly measure the polymer brush structure at temperatures above and below the solution LCST. To model the temperature-induced structural changes, we employed self-consistent field (SCF) theory incorporating the chi parameter from the experimental solution phase diagram. We compared the brush structure as predicted by SCF theory to the structure that best fit the neutron reflectivity data that was collected at low and high surface coverage at various PNIPAM molecular weights. At high surface coverage, both SCF theory and experimental data indicate significant changes in the structure between low and high temperatures.