47016-G7
Modeling Polyelectrolyte Layer-By-Layer Assembly
Our project is to model the polyelectrolyte (PE) layer-by-layer (LbL) assembly process in order to gain better understanding and predictions on the structure and properties of polyelectrolyte multilayer (PEM). In this process, a charged substrate is alternatively exposed to solutions of polyanions and polycations to build-up hundreds of thin adsorbed layers of polyions on the substrate; there is a washing step after each deposition to remove loosely adsorbed chains, in order to avoid their contamination to the solution used in the next deposition.
In this project, we use a numerical self-consistent field theory (SCFT) suitable for monovalent PE systems, which is based on the ground-state dominance approximation and non-linear Poisson equation, to model the sequential LbL assembly process of flexible PE as a series of kinetically trapped states. While SCFT is an equilibrium theory, we modify in each washing step the segmental density profile of adsorbed polymers to mimic the chain re-arrangement and partial desorption, and in subsequent depositions fix the total segmental density profiles of previously deposited polyanions and polycations to mimic the irreversibility of PE adsorption. These introduce significant non-equilibrium effects into our modeling. In fact, we do not find multilayering in the equilibrium adsorption of PE mixtures.
We have studied the internal structure and charge compensation of PEM formed by strongly dissociating polyanions and polycations on flat substrates, and the effects of various parameters affecting the long-range electrostatic interactions (including the substrate charge density, polymer charge fractions, bulk salt concentrations) and short-range interactions (including the solvent qualities and repulsion between polymers) in the system. For each set of parameters, we calculate the segmental density profiles of each deposited layer and the corresponding electrostatic potential profiles without a priori assumptions, from which we then quantify the internal structure and charge compensation of PEM by various quantities, including the total segmental density profiles of deposited polyanions and polycations, the charge density profiles of polyions and small ions, the amount of adsorbed polymers in each deposition, the thickness of each deposited layer and the overall thickness of PEM, the charges carried by polymers in each layer and the overall charges of PEM, etc.
In particular, while all theoretical and computational studies on PEM reported to date are limited to symmetric PE (where the only difference between polyanions and polycations is that they carry opposite charges), our modeling of multilayers formed by asymmetric PE (which have different charge fractions, bulk salt concentrations, or solvent qualities) provides us with more insights into the internal structure and charge compensation of PEM. We have also studied PEM formed by weakly dissociating PE, where the charges carried by polymers (including those already adsorbed in the PEM) change with the local electrostatic environment and can thus be tuned by solution pH, providing another convenient way to control the PEM structure and properties. Our modeling of PE LbL assembly is in good qualitative agreement with most experiments and molecular dynamics simulations. Many of our results can also be quantitatively tested in experiments.
Our results were recently reported in a paper (Q. Wang, “Internal Structure and Charge Compensation of Polyelectrolyte Multilayers: A Numerical Study”, Soft Matter, in press. DOI:10.1039/B809095E). According to the referee: “This very nice manuscript reports the results of a comprehensive, numerical mean-field study of layer-by-layer polyelectrolyte adsorption. Layer-by-layer assembly has quickly become an important tool for a variety of materials chemistry applications, and therefore, a careful study, like this one, of the thermodynamic aspects of this problem provides necessary insight into the many parameters controlling this non-equlibrium process.” This work has also been disseminated by the PI through the following presentations:
· · · We
are currently continuing our project in the following two directions: (1) Examine
the position-dependent dielectric constant and image-charge effects and (2) Investigate
the PEM formation on spherical colloids and the stability of PEM-coated
colloids against aggregation, as originally planned. This project will help us
better understand the formation mechanism and internal structure of PEM, and
further guide experimental design to obtain PEM with desired properties. This
very first research grant awarded to the PI is also crucial for him to
establish his research group and start his academic career. Furthermore, the
postdoctoral researcher (Ms. Yuhua Yin) has also gained
from her participation in this project lots of fundamental understanding on
polymer physics and numerical modeling techniques. Her work along the second
direction above is in progress and will be published later.
