Monte Carlo Simulations of End-Adsorption of Head-to-Tail Reversibly Associated Polymers

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Using Monte Carlo simulations we study reversible end-adsorption of head-to-tail associating polymers on a surface containing adsorption sites. The adsorption energy was considered to be similar to the association energy (10kT) leading to the competition between association in the bulk and adsorption. We found that for all considered volume fractions of polymer in the bulk Φ and all densities of adsorption sites σ the density profile of adsorbed polymer layer follows an exponential dependence (∼exp(−r/ξ), with the decay length ξ being a function of the average chain length in the bulk and independent of σ). The chain length distribution for adsorbed polymer follows a similar exponential dependence as in the bulk, except for an enhancement of short chains in the distribution, especially for large σ. With an increase in σ adsorbed polymers start to overlap leading to a decrease in the chain length to avoid stretching. Up to a certain concentration σcr the fraction of occupied sites on the surface remain nearly constant. At σ ≥ σcr adsorbed oligomers start to overlap leading to a decrease in the fraction of occupied sites. Because of the reversibility of association adsorbed chains are not stretched in the adsorbed layer, as the average radius of gyration of the chains remains equal to that for bulk polymers. At a low density of adsorption sites, the height of the adsorbed polymer layer is defined by the average radius of gyration for adsorbed chains, as expected for the mushroom regime. With an increase in σ, the average height of the polymer layer adsorbed from concentrated solutions increases, while for polymer layers adsorbed from more dilute solutions the height remains practically at the same level or even slightly decreases. The increase of the height for larger Φ is due to the chain orientation along the surface normal. At low σ chains adsorbed from relatively concentrated solutions are preferably oriented along the surface (due to the narrow depletion zone). With an increase in σ, chains become more crowded at the surface and start to orient away from it leading to an increase in the height of the adsorbed layer. Different regimes of the adsorbed chain behavior are summarized in the diagram of states, which can be applied for systems with different adsorption energies or spacer lengths.