Particle size-induced transition between surface segregation and bulk aggregation in a thin film of athermal...

Surface segregation and bulk aggregation in a thin film of athermal polymer-nanoparticle blends have been investigated by dissipative particle dynamics simulations. The thin film is confined between two athermal walls and the shape of the nanoparticles is spherical or cubic. Both phases are driven purely by the entropic effect, i.e., depletion attraction, which depends significantly on the nanoparticle size. At a specified particle volume fraction, surface segregation dominates for small nanoparticles but bulk aggregation emerges for large ones. The transition between the two phases is a result of the competition between particle-wall and particle-particle depletion attractions. The dominance of the former leads to surface segregation while the control of the latter results in bulk aggregation. Since nanocubes possess more contact areas and thus exhibit stronger depletion attractions than nanospheres do, the crossover from surface segregation to bulk aggregation occurs at smaller particle size for nanocubes.