Thermoresponsive Behavior of Polypeptoid Nanostructures Investigated with Heated Atomic Force Microscopy: Implications toward the Development of Smart Coatings for Surface-Based Sensors

The reversible phase transitions of surface tethered thermoresponsive polymers were studied at the nanoscale using in situ atomic force microscopy (AFM) by heating and cooling a test platform of polymer nanostructures. A clear aqueous solution of a random copolymer of poly[(N-ethyl glycine)(32)-ran-(N-butyl glycine)(17)], abbreviated as P(NEG(32)-r-NBG(17)), becomes turbid upon heating and then reverses to become clear upon cooling. The clarity of the solution in response to temperature can be attributed to reversible phase transitions of the polymer. We have designed AFM experiments to evaluate the phase transitions of nanopatterned P(NEG(32)-r-NBG(17)) polymer brushes bound to a substrate by immersing samples in water within a liquid cell mounted on a heated sample stage. As the temperature was increased, the nanostructures shrink in size to form collapsed patterns. When cooled, the polymer strands stretch out to form taller patterns. The morphology differences of copolymer nanopatterns were tracked in situ with AFM topographs to directly investigate the thermoresponsive behavior of P(NEG(32)-r-NBG(17)). The phase transitions of thermoresponsive polymers bound to surfaces can affect the hydrophobic characteristics of surface coatings, which in turn can be used to tune the propensity of the layer to adsorb or repel proteins or other biomacromolecules. Smart coatings of P(NEG(32)-r-NBG(17)) in which properties change in response to environmental stimuli (e.g., temperature) offer promising new functionalities for designing surface-based sensors.