1 research outputs found
Probing the Internal Heterogeneity of Responsive Microgels Adsorbed to an Interface by a Sharp SFM Tip: Comparing Core–Shell and Hollow Microgels
Microgels composed
of thermoresponsive polymer polyÂ(<i>N</i>-isopropylacrylamide)
(PNIPAM) are interfacial active. Their adsorption
leads to deformation, causing conformational changes that have profound
effects on the macroscopic properties of these films. Yet, methods
to quantitatively probe the local density are lacking. We introduced
scanning force microscopy (SFM) to quantitatively probe the internal
structure of microgels physically adsorbed on a solid (SiO<sub>2</sub>)/water interface. Using a sharp SFM tip, we investigated the two
types of microgels: (i) core–shell microgels featuring a hard
silica core and a PNIPAM shell and (ii) hollow microgels obtained
by dissolution of the silica core. Thus, both systems have the same
polymer network as the peripheral structure but a distinctly different
internal structure, that is, a rigid core versus a void. By evaluating
the force–distance curves, the force profile during insertion
of the tip into the polymer network enables to determine a depth-dependent
contact resistance, which closely correlates with the density profiles
determined in solution by small-angle neutron scattering. We found
that the cavity of the swollen hollow microgels is still present when
adsorbed to the solid substrate. Remarkably, while currently used
techniques such as colloidal probe or reflectometry only provide an
average of the <i>z</i>-profile, the methodology introduced
herein actually probes the real three-dimensional density profile,
which is ultimately important to understand the macroscopic behavior
of microgel films. This will bridge the gap between the colloidal
probe experiments that deform the microgel globally and the insertion
in which the disturbance is located near the tip