Complexation Phenomena in Polyelectrolyte Microgels
Abstract
Polyelectrolyte (PE) microgels represent an important yet understudied materials class able to responsively interact with a changing aqueous environment. Like uncrosslinked polyelectrolyte homopolymers, PE microgels bring an intricate array of complexation phenomena associated with the compensation of their electrostatic charge by counterions. They do so, however, with the additional constraints introduced by a crosslinked polymer mesh and the dynamic balance between swelling and deswelling processes that come with it. The situation is further complicated when multivalent counter macro-ions are involved since these bring additional entropic effects on the microgel network as well as richer enthalpic interactions including various types of secondary bonding. This project is designed to experimentally study complexation between model PE microgels and model counter macro-ions with the overarching objective of establishing design rules able to guide the development of new microgel-based materials systems with responsive and functional properties. Most central to this effort is understanding how to control the strength of microgel/micro-ion complexation. While conditions of pH and ionic strength can usually be identified where such a complex is thermodynamically stable, these conditions are often divorced from those relevant to practical applications with pH values near 7 and with elevated ionic strength, conditions which can significantly interfere with the complexation process. This project will probe properties of both the microgel and the macro-ion to determine which among these are most critical for enhanced complexation strength. Systematic variations in the microgel anionic groups using carboxylates, sulfates, and phosphates, introduced during microgel synthesis, will explore a wide range of possible microgel chemistries. Relative complexation strength will be assessed by determining the threshold combinations of pH and ionic strength needed to destabilize microgel complexation with a specific amine-rich cationic macro-ion (colistin) using in situ measurements of microgel diameter in a customized microfluidic liquid-chromatography system. Similarly, in addition to colistin, oligopeptides will be used as model macro-ions with systematic changes to total charge, charge distribution, hydrophobicity, and aromaticity that take advantage of the ability of modern peptide-synthesis methods to make single-residue substitutions. Notably, these will include histidine substitutions that bring the ability to turn on/off positive charge with small pH variations. This base of new fundamental knowledge will be exploited to develop responsively functional materials derived from microgel heterostructures. This aspect of the project combines concepts of advanced electron-beam patterning with multiple steps of directed hierarchical self-assembly to create site-specific microgel multilayers with controllable size in all three dimensions and comprising microgel elements that are differentially responsive both to macro-ion loading and to prevailing environmental triggers. These structures can ultimately bring a range of new behaviors including differentially responsive macro-ion release and highly anisotropic shape changes driven by environmental cues.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Aug 31, 2020
- Source ID
- W911NF2010277
Entities
People
- Matthew Libera
Organizations
- Army Contracting Command
- Stevens Institute of Technology
- United States Army