Role of Confinement, Molecular Connectivity, and Flexibility on Entropic Driven Surface Segregation of Polymer-Colloid Mixtures
The relative surface affinity between polymers and colloids are leveraged in many applications like filtration, adhesion, bio-sensing, etc. The surface affinity is governed by both enthalpic (relative interactions between 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 the species 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 dramatically change the entropy of the system and thus enhance or decrease the surface affinities of the constituent species. In this article, we use coarse-grained metropolis Monte Carlo simulations to delineate the role of these factors on entropic surface segregation in binary mixture of polymers and colloids. At low number densities, excluded volume effects are negligible and we do not observe any entropically driven surface segregation. Therefore our system of interest is binary polymer-colloids mixtures at moderate to high number densities where excluded volume effects are predominant. Our results indicate that for flexible polymer chains, the surface is always enriched with colloids compared to polymers and this effect is enhanced for longer polymer chains. The configurational entropy of flexible polymers is significantly reduced near the surface and therefore they prefer to stay in bulk (away from the surface). However, this behavior can be completely reversed by introducing a large degree of confinement and making the chains relatively rigid (less flexible). We observe that for longer polymer chains with an equilibrium bond angle θ=π, both confinement and chain stiffness enhance surface segregation of polymers relative to colloids. However, the segregation behavior within confinement is dependent on the polymer chain length. Our results show that polymer segregation of long stiff chains in slit pore geometry is driven by nematization near the surface while looping of polymers is observed under large degree of confinement. The surface segregation of polymers is dramatically decreased for chains with equilibrium bond angles θ0=π/3, π/2, 2π/3 and 5π/6 independent of chain length and flexibility due to excluded volume effects as this leads to inefficient packing near the surface.