Abstract
We compute desorption rates for isolated polymers adsorbed to a solid wall with a rare event sampling technique called multilevel splitting, also known as forward flux sampling. We interpret computed rates with theories based on the conjecture that the product of the desorption time and diffusivity divided by squared radius of gyration scales with exp(h/Rg) where h is the equilibrium ratio of adsorbed surface concentration of polymer to bulk concentration of polymer . As the polymer–wall interaction energy is increased, the slope of vs. nearly approaches unity, as expected for strongly-adsorbing chains, where N is the degree of polymerization and is the height-averaged monomer–wall interaction energy for a strongly adsorbed chain. However, we also find that this scaling law is only accurate when adsorption strength per monomer exceeds a threshold value on the order of 0.3–0.5 kBT for a freely jointed chain without or with excluded volume effects. Below the critical value, we observe that becomes nearly constant with N, so that , with . This suggests a crossover from “strong” detachment-controlled to a “weak” diffusion-controlled desorption rate as VMF/kBT drops below some threshold. These results may partially explain experimental data, that in some cases show “strong” exponential dependence of desorption time on chain length, while in others a “weak” power-law dependence is found. However, in the “strong” adsorption case, our results suggest much longer desorption times than those measured, while the reverse is true in the weak adsorption limit. We discuss possible reasons for these discrepancies.
Highlights
To the best of our knowledge, the rate of desorption of an isolated chain from an interface has not been predicted using detailed theory or simulation, and even experiments measuring desorption rates are rare [1,2,3]
Published works that are the most similar to what we propose here include that of Wang, Rajagopalan, and Mattice [8], who performed Monte Carlo (MC) simulations on a cubic lattice of the desorption of an adsorbed film of multiple polymers, from a surface
We never observe in our simulations the dimensionless desorption time decrease with increasing N, for weak adsorption, the dimensionless desorption time does become insensitive to chain length
Summary
To the best of our knowledge, the rate of desorption of an isolated chain from an interface has not been predicted using detailed theory or simulation, and even experiments measuring desorption rates are rare [1,2,3] In their textbook, Fleer et al noted that in most cases, even in the absence of a kinetic barrier to desorption, polymer adsorbed at levels up to a significant fraction of saturation of the surface yields an equilibrium concentration in solution too small to drive sufficiently fast diffusion to be measurable [4]. The adsorption energy per monomer may be very feeble, εPW
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