Neutron reflectometry can capture the rapid collapse and swelling of a polymer brush

Measuring dynamic processes in responsive soft matter systems remains a critical frontier in bridging fundamental studies and applied sciences. In particular, determining structural changes of surface-grafted polymers due to the application of stimuli remains poorly examined due to historical instrument limitations. Herein, we exploit the high flux capabilities of the D17 neutron reflectometer (Institut Laue-Langevin, France) to examine kinetic changes in the internal nanostructure of a poly(N-isopropyl-acrylamide) (PNIPAM) brush with temperature or the presence of a common osmolyte (glucose).

Ellipsometry and neutron reflectometry (NR) was employed to examine changes in brush thickness as a function of temperature in water and aqueous glucose solutions under equilibrium conditions. NR captured the kinetics of brush swelling/collapse triggered by rapid temperature or glucose concentration changes.

Ellipsometry and NR revealed a decrease in the lower critical solution temperature (LCST) of a PNIPAM brush with increasing glucose concentration. Equilibrium NR measurements showed vertical phase separation within the brush when in a near-collapsed state. This stratified structure was expected for the thermally triggered conformation changes, but has not been observed for glucose-triggered collapse/swelling. Excellent statistics were achieved over intervals for the NR kinetic measurements with modelled profiles comparable to longer equilibrium measurements across a wider q range. A small hysteresis was observed in the temperature induced swelling/collapse, while a more pronounced hysteresis was observed in the glucose triggered conformational changes. This hysteresis was equivalent for both swelling and collapse transitions and is attributed to preferential adsorption of the glucose in the brush.