Equilibrium Phase Behavior and Mass Transport in Neutral and Oppositely Charged Polymer Assemblies

Polyelectrolyte Compelxation;Mass Transport;Thermodynamics;Modeling;Layer-by-Layer Assembly;Chemical Engineering;Engineering;Chemical Engineering

Salehi, AliKotov, Nicholas ;

University of Michigan

关键词: Polyelectrolyte Compelxation;    Mass Transport;    Thermodynamics;    Modeling;    Layer-by-Layer Assembly;    Chemical Engineering;    Engineering;    Chemical Engineering;   

Others  :  https://deepblue.lib.umich.edu/bitstream/handle/2027.42/143922/salehi_1.pdf?sequence=1&isAllowed=y

瑞士|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Polyelectrolyte (PE) complexation (PEC) occurs upon mixing solutions of oppositely charged ‎polyelectrolytes. This electrostatic self-assembly paradigm is also extended to layer-by-layer ‎‎(LbL) assembled polyelectrolyte multilayers (PEM). Despite the broad applications of both PEC ‎and PEM, bulk phase behavior of PEC and mass transport controlling the structure and film ‎growth rate of PEMs and their connection is poorly understood. In this doctoral work, we ‎present a combined experimental and theoretical investigation of PEC and PEM LbL assembly. ‎We first observe that polymer-specific interactions have a profound effect on both PEC and LbL ‎growth rate while salinity has a non-monotonic and a rather universal effect on LbL growth rate ‎of fully ionized polyelectrolytes when normalized by the critical salinity required to suppress ‎PEC. We next develop a free energy model of PEC by incorporating counterion association-‎dissociation, cross-chain ion pairing (IP) and protonation, treating each as a reversible reaction ‎using laws of mass action. The importance of each reaction is controlled by a corresponding ‎chemistry-dependent standard free energy input parameter that could be measured via ‎experimentation or molecular simulations. In monophasic systems, the thermodynamic model can ‎qualitatively explain the shifts in acidity and basicity observed in potentiometric titration of weak ‎PEs in the presence of salt and oppositely charged PEs in accordance with Le Châtelier’s ‎principle. We demonstrate how a competition between counterion condensation and IP can ‎explain the complex coacervation of strongly charged PEs. Binodal diagrams predicted in our ‎model are most sensitive to IP strength both for weak and strong PEs. We compare binodal ‎diagrams predicted by our model against experimental data, and find a plausible parameter set ‎that leads to agreement between them. Finally, we develop a transport modeling framework for ‎LbL assembly by variational minimization of the Rayleighian of a mixture of oppositely charged ‎PEs, simple salt and water with respect to species velocities yielding species flux laws that equate ‎the net mutual friction between components with the diffusional driving force on each species. ‎The latter includes gradients in the conventional mixing chemical potential, electrostatic potential ‎and mechanical stress (only for PEs). We also develop a constitutive equation for mixtures of PEs ‎that accounts for solvent imbibition and IP. The result is a modification of the upper-convected ‎Maxwell model. Our LbL transport model captures PE adsorption and film swelling in the ‎equilibrium limit. A dynamic coupling of elastic stress and diffusion is applied in a different ‎context to an electroneutral system involving drug release from polymer tablets, capturing ‎Fickian, anomalous and case II modes of drug transport that arise naturally from the model. In ‎addition to LbL, the transport framework proposed in this work can be applied to any system of ‎charged and neutral components.‎