Influence of polymer architecture and polymer-wall interaction on the adsorption
of polymers into a slit-pore
Zhong Chen and Fernando A. Escobedo*
School of Chemical Engineering, Cornell University, Ithaca, New York, 14850-5201, USA
Received 25 August 2003; published 17 February 2004
The effects of molecular topology and polymer-surface interaction on the properties of isolated polymer
chains trapped in a slit were investigated using off-lattice Monte Carlo simulations. Various methods were
implemented to allow efficient simulation of molecular structure, confinement force, and free energy for a
chain interacting with such ‘‘sticky’’ surfaces. The simulations were performed in the canonical ensemble, and
the free energy was sampled via virtual slit-separation moves. Six different chain architectures were studied:
linear, star-branched, dendritic, cyclic, two-node i.e., containing two tetrafunctional intramolecular crosslinks,
and six-node molecules. The first three topologies entail increasing degrees of branching, and the last three
topologies entail increasing degrees of intramolecular bonding. The confinement force, monomer density
profile, and conformational properties for all these systems were compared for identical molecular weight N
and analyzed as a function of adsorption strength. The compensation point where the wall attraction counter-
balances the polymer-slit exclusion effects was the focus of our study. It was found that the attractive energy
at the compensation point,
c
, is a weak increasing function of the chain length for excluded-volume chains.
The value of
c
differs significantly for different topologies, and smaller values are associated with better-
adsorbing molecules. Due to their globular shape and numerous chain ends, branched molecules e.g., stars and
dendrimers experience a relatively small entropic penalty for adsorption at low adsorption force and moderate
confinement. However, as the adsorption force increases, the more flexible linear chains reach the compensa-
tion point at a weaker attractive energy because of the ease with which monomers can be packed near the
walls. In moderate to weak confinement, molecules with intramolecular cross-links, such as cyclic, two-node,
and six-node molecules, always adsorb better than the other chains with the same N. Especially at strong
adsorption, two-node and six node molecules are highly localized in the region near the walls. Under strong
confinement conditions, chain rigidity becomes the dominating factor and the more flexible linear chain
adsorbs the best at all adsorption strengths. These results provide useful insights for controlling confinement
and depletion forces of polymers with different molecular architectures in the presence of attractive polymer-
surface interactions.
DOI: 10.1103/PhysRevE.69.021802 PACS numbers: 36.20.-r
I. INTRODUCTION
The conformational and thermodynamic properties of
polymer chains are strongly affected by geometric confine-
ment. This phenomenon is relevant to numerous applications
of polymers, as in chromatographic separations, colloidal
stabilization, thin-film processing, and the preparation of
clay-based nanocomposites.
While rigorous formulations exist for the configurational
properties and partitioning of a Gaussian chain in pores 1,a
rigorous analytical theory has not been worked out for
excluded-volume EV chains a model adequate to describe
good solvent conditions, and only approximate theories are
available, e.g., mean field theory 2 and scaling theory 3.
Scaling theory has had the most general applicability so far,
although it is valid for limiting conditions and can give only
semiquantitative predictions. Most of the theoretical and
simulation work on confined polymers so far has been con-
cerned with linear chains; studies on polymers with more
complicated topology are very limited. It is well known that
polymers with complex internal architecture have quite dif-
ferent static and dynamic properties. The question then arises
as to how the internal architecture affects the properties of
confined polymer chains. Just as in the case of linear poly-
mers, rigorous theoretical solutions are available for an ideal
self-crossing cyclic chain confined in slitlike pores 4 and
scaling results are available for a cyclic EV chain 5. These
theories predict substantial differences in the partition coef-
ficients of linear and cyclic macromolecules under weak ad-
sorption conditions, opening up the possibility for the devel-
opment of a method for separating cyclic and linear
polymers 6. An on-lattice simulation study was carried out
for the adsorption of polymer chains with different molecular
architectures, including linear, star, and cyclic chains, on a
solid surface 7. For weakly adsorbed chains it was shown
that ring polymers are always adsorbed ca. 50% more than
linear and star-branched ones, while the properties of ad-
sorbed linear and star polymers are very similar. Experimen-
tal work in this area has been even more scarce 8.
In a previous paper 9, we investigated the properties of
topologically complex polymers, such as rings, stars, den-
drimers, and hyperbranched polymers, confined in repulsive
slit-pores. It was found that scaling theory for linear chains
well describes all the properties of star molecules examined
and the scaling of the linear dimensions of dendrimers and
hyperbranched polymers. The relative partition coefficient at
the dilution limit was estimated from the data of F vs slit
separation D. It was shown that for very narrow D branched
polymers tend to be depleted in the pore relative to linear
*Corresponding author. Email address: fe13@cornell.edu
PHYSICAL REVIEW E 69, 021802 2004
1063-651X/2004/692/02180210/$22.50 ©2004 The American Physical Society 69 021802-1