Dynamics in Stimuli-Responsive Poly(N-isopropylacrylamide) Hydrogel Layers As Revealed by Fluorescence Correlation Spectroscopy
Abstract
We employ fl uorescence correlation spectros- copy (FCS) to study the translational mobility of molecular tracers in stimuli-responsive grafted poly( N -isopropylacryla- mide)(PNiPAAm)hydrogels,undervariablesolvency conditions. Tracer MINUS SIGN matrix interactions were tuned by selecting three di ff erent molecular tracers.
In contrast to a non- interacting tracer (Alexa 647), the mobility of a weakly (Alexa 488) and a strongly interacting (Rhodamine 6G) tracer deviates from a simple single Fickian di ff usion. In addition to pure crowding e ff ects, the mobility of both Alexa488 and Rhodamine 6G is in fl uenced by tracer MINUS SIGN polymer interactions.
We interpret the observed trends in tracer mobility in terms of the interplay between Coulombic repulsions and short-range attractions. Although tracer dynamics and hydrogel swelling ratio are interdependent properties, their relation turns out to be nontrivial and does not allow predictions of tracer dynamics on the basis of polymer structural information.
Hence, a universal scaling behavior is not possible, due to tracer MINUS SIGN polymer interactions.