Sharp Transition of End-Grafted PNIPAM Studied by Interfacial Force Microscopy

Poly N-isopropylacrylamide (PNIPAM) experiences a transition from a swollen hydrophilic state to a collapsed hydrophobic state at the lower critical solution temperature (LCST) in solution. Based on the transition behavior, the PNIPAM has been end-grafted to a surface for future applications of the polymer, such as in microfluidics, drug delivery, and tissue engineering. Recent studies have shown that the end-grafted PNIPAM does not collapse above the LCST at low grafting density and at low molecular weight. On the other hand, a sharp transition is observed with intermediate grafting density and intermediate molecular weight. To explore the transition of PNIPAM in the crossover region between low and intermediate molecular configuration, we investigated the PNIPAM end-grafted to an oxidized silicon surface at low molecular weight and intermediate grafting density. This was achieved using interfacial force microscopy, and was observed as solution temperature varies. We measured the force as a function of distance between a PNIPAM polymer brush and a tip coated with a hydrophobic molecule of octadecyltrichlorosilane. Repulsive forces were observed to exponentially decrease with the distance, corresponding to a swollen, hydrated state at temperatures below the LCST. Above the LCST, adhesive forces were present in the force-distance curves, thus consistent with a collapsed, hydrophobic state. The results suggest that, in the crossover region, the end-grafted PNIPAM experiences a sharp transition as the temperature increases.