In Situ Monitoring of “Smart” Polyacrylamide Copolymer Phase Transition at the Solid-Liquid Interface
Lei Wan1, Harender S. Bisht2, David Oupicky2, and Guangzhao Mao1. (1) Chemical Engineering and Materials Science, Wayne State University, 5050 Anthony Wayne Drive, Detroit, MI 48202, (2) Pharmaceutical Sciences, Wayne State University, 5050 Anthony Wayne Drive, Detroit, MI 48202
The coil-to-globule and globule-to-coil transitions of N-isopropylacrylamide (NIPA)-vinliidazol (VI)-polyethylene glycol (PEG) terpolymer are monitored in situ by dynamic light scattering (DLS) and atomic force microscopy (AFM). The NIPA terpolymer is positively charged at pH < 6 and adsorbs on negatively charged mica. The adsorption saturation concentration was determined by AFM to be 1×10^(-3) mg/mL for the polymer below its lower critical solution temperature (LCST) and 5×10^(-4) mg/mL above its LCST. The solution temperature was varied between 23°C and 60°C by an AFM thermal controller during AFM imaging in the Fluid Tapping Mode. From 23°C to 42°C, the adsorbed NIPA copolymer layer is molecularly smooth corresponding to the close-packed structure of the flexible coils. Particles with average size of 2.7 nm in height and 35 nm in width appear in a narrow temperature range of 42 to 43°C and in less than 5 minutes. The number of particles increases with increasing temperature from 42°C to 60°C. The particles correspond to the collapsed and associated state of the copolymer globule. The film morphological changes are reversible during the cooling cycle and subsequent heating and cooling cycles. The copolymer association behavior at the solid-liquid interface parallels that in solution as studied by DLS.