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Surface forces in solutions containing rigid polymers: Approaching the rod limit

Forsman, Jan LU and Woodward, CE (2006) In Macromolecules 39(3). p.1269-1278
Abstract
In a recent paper,(1) we investigated how the interaction between two adsorbing surfaces in a polymer solution responds to changes of the intrinsic stiffness of the polymer chains. We demonstrated that as the chain rigidity is increased, starting from the fully flexible limit, a free energy barrier develops. In this work, we shall continue our studies of semiflexible polymers but focus on the other extreme, where the stiffness is increased to the extent that the chains become rodlike. In this limit, the free energy barrier disappears and is replaced by a long-ranged attraction. A weak barrier at separations corresponding to the length of a fully stretched chain is, however, present. The approach to rodlike behavior is very slow at low... (More)
In a recent paper,(1) we investigated how the interaction between two adsorbing surfaces in a polymer solution responds to changes of the intrinsic stiffness of the polymer chains. We demonstrated that as the chain rigidity is increased, starting from the fully flexible limit, a free energy barrier develops. In this work, we shall continue our studies of semiflexible polymers but focus on the other extreme, where the stiffness is increased to the extent that the chains become rodlike. In this limit, the free energy barrier disappears and is replaced by a long-ranged attraction. A weak barrier at separations corresponding to the length of a fully stretched chain is, however, present. The approach to rodlike behavior is very slow at low polymer concentrations; i.e., rods will under these conditions behave in a manner which is qualitatively different from that of even extremely rigid semiflexible chains. At higher concentration, a slightly different picture emerges, since saturation effects will prevent an exceptionally strong adsorption of rods. Semiflexible polymers will then display reasonably "rodlike" behavior at a more moderate chain rigidity. Still, our results suggest that models of semiflexible chains as rods or cylinders should be treated with considerable caution, at least in terms of adsorption or surface force properties. Our results furthermore show that the results for explicit solvent models, adopted in our previous work, are in this case qualitatively reproduced by simpler models, where the solvent enters implicitly. The agreement across a range of different model systems, of which some are solved exactly, lends support to the qualitative conclusions drawn from our studies on these systems. Finally, we compare structural density functional predictions with corresponding simulation results. The predictions are accurate, except in the case of extremely stiff chains and rodlike chains. These display considerable ordering at the surfaces, a phenomenon that cannot be captured by the present density functional. This leads to a significant underestimation of the primary adsorption peak. However, comparisons between surface forces in rod solutions, as obtained from grand canonical simulations and density functional calculations, demonstrate that the ordering transitions have little influence on the surface interactions. This is because the surface interactions are governed by the free chains, which are not strongly adsorbed to any surface. We also show that the small quantitative effect that surface ordering does have on the surface forces, in principle, can be handled in an effective manner by a simple increase of the surface potential strength. (Less)
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author
and
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Macromolecules
volume
39
issue
3
pages
1269 - 1278
publisher
The American Chemical Society (ACS)
external identifiers
  • wos:000235200500047
  • scopus:32544447431
ISSN
0024-9297
DOI
10.1021/ma052472+
language
English
LU publication?
yes
additional info
The information about affiliations in this record was updated in December 2015. The record was previously connected to the following departments: Theoretical Chemistry (S) (011001039)
id
01648385-428e-4e8d-b418-8fc40c7b3094 (old id 417698)
date added to LUP
2016-04-01 11:39:42
date last changed
2020-08-05 01:47:01
@article{01648385-428e-4e8d-b418-8fc40c7b3094,
  abstract     = {In a recent paper,(1) we investigated how the interaction between two adsorbing surfaces in a polymer solution responds to changes of the intrinsic stiffness of the polymer chains. We demonstrated that as the chain rigidity is increased, starting from the fully flexible limit, a free energy barrier develops. In this work, we shall continue our studies of semiflexible polymers but focus on the other extreme, where the stiffness is increased to the extent that the chains become rodlike. In this limit, the free energy barrier disappears and is replaced by a long-ranged attraction. A weak barrier at separations corresponding to the length of a fully stretched chain is, however, present. The approach to rodlike behavior is very slow at low polymer concentrations; i.e., rods will under these conditions behave in a manner which is qualitatively different from that of even extremely rigid semiflexible chains. At higher concentration, a slightly different picture emerges, since saturation effects will prevent an exceptionally strong adsorption of rods. Semiflexible polymers will then display reasonably "rodlike" behavior at a more moderate chain rigidity. Still, our results suggest that models of semiflexible chains as rods or cylinders should be treated with considerable caution, at least in terms of adsorption or surface force properties. Our results furthermore show that the results for explicit solvent models, adopted in our previous work, are in this case qualitatively reproduced by simpler models, where the solvent enters implicitly. The agreement across a range of different model systems, of which some are solved exactly, lends support to the qualitative conclusions drawn from our studies on these systems. Finally, we compare structural density functional predictions with corresponding simulation results. The predictions are accurate, except in the case of extremely stiff chains and rodlike chains. These display considerable ordering at the surfaces, a phenomenon that cannot be captured by the present density functional. This leads to a significant underestimation of the primary adsorption peak. However, comparisons between surface forces in rod solutions, as obtained from grand canonical simulations and density functional calculations, demonstrate that the ordering transitions have little influence on the surface interactions. This is because the surface interactions are governed by the free chains, which are not strongly adsorbed to any surface. We also show that the small quantitative effect that surface ordering does have on the surface forces, in principle, can be handled in an effective manner by a simple increase of the surface potential strength.},
  author       = {Forsman, Jan and Woodward, CE},
  issn         = {0024-9297},
  language     = {eng},
  number       = {3},
  pages        = {1269--1278},
  publisher    = {The American Chemical Society (ACS)},
  series       = {Macromolecules},
  title        = {Surface forces in solutions containing rigid polymers: Approaching the rod limit},
  url          = {http://dx.doi.org/10.1021/ma052472+},
  doi          = {10.1021/ma052472+},
  volume       = {39},
  year         = {2006},
}