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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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Record number 429590
Title Reconciling lattice and continuum models for polymers at interfaces
Author(s) Fleer, G.J.; Skvortsov, A.M.
Source Journal of Chemical Physics 136 (2012)13. - ISSN 0021-9606
DOI http://dx.doi.org/10.1063/1.3693515
Department(s) Physical Chemistry and Colloid Science
VLAG
Publication type Refereed Article in a scientific journal
Publication year 2012
Keyword(s) segmental adsorption energies - scheutjens-fleer theory - analytical approximation - depletion thickness - dilute-solution - surface - macromolecules - silica
Abstract It is well known that lattice and continuum descriptions for polymers at interfaces are, in principle, equivalent. In order to compare the two models quantitatively, one needs a relation between the inverse extrapolation length c as used in continuum theories and the lattice adsorption parameter ¿¿(s) (defined with respect to the critical point). So far, this has been done only for ideal chains with zero segment volume in extremely dilute solutions. The relation ¿¿(s)(c) is obtained by matching the boundary conditions in the two models. For depletion (positive c and ¿¿(s)) the result is very simple: ¿¿(s) = ln(1 + c/5). For adsorption (negative c and ¿¿(s)) the ideal-chain treatment leads to an unrealistic divergence for strong adsorption: c decreases without bounds and the train volume fraction exceeds unity. This due to the fact that for ideal chains the volume filling cannot be accounted for. We extend the treatment to real chains with finite segment volume at finite concentrations, for both good and theta solvents. For depletion the volume filling is not important and the ideal-chain result ¿¿(s) = ln(1 + c/5) is generally valid also for non-ideal chains, at any concentration, chain length, or solvency. Depletion profiles can be accurately described in terms of two length scales: ¿ = tanh(2)[(z + p)/d], where the depletion thickness (distal length) d is a known function of chain length and polymer concentration, and the proximal length p is a known function of c (or ¿¿(s)) and d. For strong repulsion p = 1/c (then the proximal length equals the extrapolation length), for weaker repulsion p depends also on chain length and polymer concentration (then p is smaller than 1/c). In very dilute solutions we find quantitative agreement with previous analytical results for ideal chains, for any chain length, down to oligomers. In more concentrated solutions there is excellent agreement with numerical self-consistent depletion profiles, for both weak and strong repulsion, for any chain length, and for any solvency. For adsorption the volume filling dominates. As a result c now reaches a lower limit c ˜ -0.5 (depending slightly on solvency). This limit follows immediately from the condition of a fully occupied train layer. Comparison with numerical SCF calculations corroborates that our analytical result is a good approximation. We suggest some simple methods to determine the interaction parameter (either c or ¿¿(s)) from experiments. The relation ¿¿(s)(c) provides a quantitative connection between continuum and lattice theories, and enables the use of analytical continuum results to describe the adsorption (and stretching) of lattice chains of any chain length. For example, a fully analytical treatment of mechanical desorption of a polymer chain (including the temperature dependence and the phase transitions) is now feasible.
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