Microgel in a Pore: Intraparticle Segregation or Snail-like Behavior Caused by Collapse and Swelling

We demonstrate that
a microgel can enter and exit a narrow cylindrical
pore under external stimuli leading to collapse and swelling of the
microgel. Attractive interactions between the microgel and the pore
surface stimulate the microgel entering upon its collapse. The entering
is driven by a gain in the surface energy: the area of the microgel–pore
contacts is maximized within the pore. Swelling of the microgel within
the pore of a finite size is thermodynamically favorable if the pore
thickness exceeds a certain threshold value. Otherwise, the swelling
leads to the microgel exit. The physical reason for this is a gain
in the elastic free energy of the subchains which are less stretched
outside the pore. We systematically study swelling and collapse of
the microgel within the pore. Both longitudinal size and radial concentration
profiles are calculated for different strength of interactions of
the beads with each other and the pore surface. We predict an intramicrogel
“phase” coexistence leading to the formation of a dense
adsorbed layer near the pore surface and highly swollen central part
of the microgel. Furthermore, the permeation of nanoparticles, whose
size is smaller than the mesh size of the microgels, was simulated
under different swelling and adsorption degrees. It is demonstrated
that the microgel can slow down and completely stop the permeation
of nanoparticles through the pore.