Role of confinement, molecular connectivity, and flexibility on entropic driven surface segregation of polymer-colloid mixtures

Abstract

Relative surface affinity between polymers and colloids is leveraged in many applications like filtration,adhesion, bio-sensing,etc.The surface affinity is governed by both enthalpic (relative interactionsbetween the species and surface) and entropic (excluded volume) effects. Neglecting enthalpic effects,i.e.for purely athermal systems, entropy is the only driving force that controls the surface affinity of thespecies in a binary or multi-component mixture. Many intensive (relative size of colloids, chain length,equilibrium bond angle, chain flexibility) and extensive (confinement, temperature) factors can dramaticallychange the entropy of the system and thus enhance or decrease the surface affinities of the constituentspecies. In this article, we use coarse grained metropolis Monte Carlo simulations to delineate the role ofthese factors in entropic surface segregation in a binary mixture of polymers and colloids. At low numberdensities, excluded volume effects are negligible and we do not observe any entropic driven surfacesegregation. Therefore our system of interest is binary polymer�colloid mixtures at moderate to highnumber densities where excluded volume effects are predominant. Our results indicate that for flexiblepolymer chains, the surface is always enriched with colloids compared to polymers and this effect isenhanced for longer polymer chains. The configurational entropy of the flexible polymers is significantlyreduced near the surface and therefore they prefer to stay in the bulk (away from the surface). Howeverthis behavior can be completely reversed by introducing a large degree of confinement and making thechains relatively rigid (less flexible). Our results show that polymer segregation of long stiff chains in slitpore geometry is driven by nematization near the surface while looping of polymers is observed under alarge degree of confinement. We observe that for longer polymer chains with an equilibrium bond angle(y=p), both confinement and chain stiffness enhance the surface segregation of polymers relative tocolloids. However, the segregation behavior within a confinement is dependent on the polymer chainlength. The surface segregation of polymers is dramatically decreased for chains with equilibriumbond anglesy0�p3;p2;2p3and5p6independent of chain length and flexibility due to excluded volumeeffects and inefficient packing near the surface.