Adsorption of Neutral-Highly Charged Diblock Copolymers in Selective Solvents
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ADSORPTION OF NEUTRAL-HIGHLY CHARGED DIBLOCK COPOLYMERS IN SELECTIVE SOLVENTS N. DAN-BRANDON AND M. TIRRELL Department of Chemical Engineering and Material Science, University of Minnesota, Minneapolis, MN 55455
ABSTRACT
We apply a scaling model to study adsorption of neutral-strongly charged diblock copolymer from concentrated salt solutions, where the range of electrostatic interactions is smaller than that of excluded volume correlations. Adsorbed layer structure is determined by an equilibrium between the solvated block stretching energy, surface-adsorbed block interactions, and the solution chemical potential. We find that adsorption from micellar solutions results in a lower surface density than adsorption from solutions below the CMC. In both cases the surface density increases, and the layer thickness decreases, with salt concentration.
INTRODUCTION
Diblock copolymers in a highly selective solvent aggregate into adsorbed layers on nonrepulsive surfaces. The incompatible 'anchor' block forms a dense film on the surface, to which the soluble block is tethered (see Figure 1). Layer characteristics are determined by a balance between chain stretching energies and the interactions between the anchor block and the surface. Layer structure was shown to be controlled by the solution chemical potential as well, for systems of neutral copolymers in a non-polar, selective solvent [1]. Electrostatic interactions strongly influence the configurations of charged chains in polar solvents. In dilute salt concentration and high charge density along the backbone, electrostatic correlations dominate, resulting in rigid rod-like behavior. Increasing solution salt concentration reduces the range of electrostatic interactions, and the chain regains some flexibility.
Mat. Res. Soc. Symp. Proc. Vol. 248. ©1992 Materials Research Society
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In this study we examine equilibrium properties of adsorbed layers of neutral-strongly charged diblock copolymer, in aqueous salt solutions. We investigate solutions where the electrostatic correlation length is shorter than the range of excluded volume interactions, so that the charged block configurations depend on both.
The insoluble block forms a ated block forms a tethered brush
SCALING MODEL OF HIGHLY CHARGED CHAINS IN SALT SOLUTIONS
We modify the scaling model of polymer brushes [2,3] to apply to charged chains in aqueous salt solutions. The range of excluded volume interactions is defined by a correlation length, or 'blob' size, ý, a function of the monomer concentration:
•
0 is the local,
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3 a r-1/4
(1)
dimensionless, monomer concentration, and a is a segment size. In aqueous
salt solutions, the conformations of a charged chain are affected by electrostatics, as well as excluded volume interactions. The electric persistence length, b, is a measure for the range of electrostatic interactions: chain segments smaller than b behave as rigid rods [4]. We discuss the case where b is smaller than ý, so that the polymer can be envisioned as a chain of rodlike segments, each
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