Internal Structure and Charge Compensation of Polyelectrolyte Multilayers Department of Chemical & Biological Engineering Colorado State University David (Qiang) Wang

PE are important materials Can be soluble in water Can be adsorbed onto charged surfaces PE are difficult to study PE are charged polymers Both long-range (Coulomb) and short-range (excluded volume) interactions present in the system Decher, Science, 277, 1232 (1997) PE Layer-by-Layer (LbL) Assembly Polyelectrolytes (PE)

“Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites” Decher, Science, 277, 1232 (1997) Black curve: Concentration profile of each layer. Blue (Red) dots: Total concentration profile of anionic (cationic) groups from all layers. Green dots: Concentration profile of a labeling group applied to every fourth layer.

Model System for PE Adsorption 0x P = A,1, l  A,b c s,bA  b  0 solvent molecule (S) cation (+) anion (  ) Monovalent, 1D system Ions from salt  counterions from PE and substrate Ions have no volume and short-rang interactions Polymer segments have the same density  0 as solvent molecules All polymer segments have the same statistical segment length a No short-range interactions between polymers Parameters in the model:  SF substrate charge density; v P charge valency of PE; p P degree of ionization of PE (Smeared or Annealed);  PS Flory-Huggins parameter for solvent quality;  A,b bulk polymer concentration; c s,bA bulk salt concentration;  80dielectric constant. Quantities to be solved:  (x)electrostatic potential (in units of k B T/e);  A (x)polymer segmental density.

x w (1) Layer Profiles – Symmetric, Smeared PE  SF  0.1 (2.61mC/m 2 ), v 1  v 2 , p 1  p 2  0.5,  1S  2S  1, c s,b1  c s,b2  0.05 (0.667M),  A,b  7.5×10  4 (10mM) (with a  0.5nm and  0  a  3 )

 SF  0.1 (2.61mC/m 2 ), v 1  v 2 , p 1  p 2  0.5,  1S  2S  1, c s,b1  c s,b2  0.05 (0.667M),  A,b  7.5×10  4 (10mM) (with a  0.5nm and  0  a  3 ) Layer Profiles – Symmetric, Smeared PE

 SF  0.1 (2.61mC/m 2 ), v 1  v 2 , p 1  p 2  0.5,  1S  2S  1, c s,b1  c s,b2  0.05 (0.667M),  A,b  7.5×10  4 (10mM) (with a  0.5nm and  0  a  3 ) Layer Profiles – Symmetric, Smeared PE

Three-Zone Structure – Symmetric, Smeared PE  SF  0.1 (2.61mC/m 2 ), v 1  v 2 , p 1  p 2  0.5,  1S  2S  1, c s,b1  c s,b2  0.05 (0.667M),  A,b  7.5×10  4 (10mM) (with a  0.5nm and  0  a  3 )

Charge Compensation – Smeared PE  SF  0.1 (2.61mC/m 2 ), v 1  v 2 ,  A,b  7.5×10  4 (10mM), c s,b1  c s,b2  0.05 (0.667M),  1S  2S  1 (with a  0.5nm and  0  a  3 )

Charge Compensation – Asymmetric, Smeared PE  SF  0.1 (2.61mC/m 2 ), v 1  v 2 ,  A,b  7.5×10  4 (10mM), p 1  p 2  0.5 (with a  0.5nm and  0  a  3 )

Charge Density Profiles – Asymmetric, Smeared PE  SF  0.1 (2.61mC/m 2 ), v 1  v 2 , p 1  p 2  0.5,  1S  1,  2S  0.6, c s,b1  c s,b2  0.05 (0.667M),  A,b  7.5×10  4 (10mM) (with a  0.5nm and  0  a  3 )  1S  2S  1

Annealed vs. Smeared PE – 1 st Layer  SF  0.1 (2.61mC/m 2 ), v 1 , p 1  0.5,  1S  1, c s,b1  0.05 (0.667M),  A,b  7.5×10  4 (10mM) (with a  0.5nm and  0  a  3 )

Charge Fractions in Multilayer – Symmetric, Annealed PE  SF  0.1 (2.61mC/m 2 ), v 1  v 2 , p 1  p 2  0.5,  1S  2S  1, c s,b1  c s,b2  0.05 (0.667M),  A,b  7.5×10  4 (10mM) (with a  0.5nm and  0  a  3 ) Each deposition changes the charges carried by the PE in a few previously deposited layers, of which the density profiles are fixed in our modeling. Thus,  (i) : charges carried by PE adsorbed in the i th deposition.  (i) : amount of PE adsorbed in the i th deposition.

 SF  0.1 (2.61mC/m 2 ), v 1  v 2 , p 1  p 2  0.5,  1S  2S  1, c s,b1  c s,b2  0.05 (0.667M),  A,b  7.5×10  4 (10mM) (with a  0.5nm and  0  a  3 ) Annealed vs. Smeared PE – Polymer Density in Zone II

 SF  0.1 (2.61mC/m 2 ), v 1  v 2 , p 1  p 2  0.5,  1S  2S  0.5, c s,b1  c s,b2  0.05 (0.667M),  1,b  2,b  7.5×10  4 (10mM) (with a  0.5nm and  0  a  3 ) Non-Equilibrium & Solvent Effects – Symmetric, Smeared PE Multilayer does not form in  or good solvent.

We have used a self-consistent field theory to model the layer- by-layer assembly process of flexible polyelectrolytes (PE) on flat surfaces as a series of kinetically trapped states. Our modeling, particularly for asymmetric PE having different charge fractions, bulk salt concentrations, or solvent qualities, reveals the internal structure and charge compensation of PE multilayers. We have also compared multilayers formed by strongly and weakly dissociating PE. Our results qualitatively agree with most experimental findings. Summary Q. Wang, Soft Matter, in press