AQUEOUS POLYMER ASSEMBLY
11 PARTICIPATING FACULTY: BHATIA, EMRICK, HOAGLAND, HSU, KOKKOLI, MUTHUKUMAR, PENELLE, ROBERTS, SANTORE, STREY, TEW (WITH 9 GRADUATE STUDENTS, 1 POST-DOCTORAL FELLOW)
Unlike conventional organic polymers, the assembly of water-soluble polymers into ordered microstructures involves both hydrophobic and electrostatic forces. For polymers dissolved in aqueous media, the ability to control placement of hydrophilic, hydrophobic, and charged sequences presents a myriad of self-assembly strategies that can capitalize on the great specificity and long range nature of intermolecular forces in water. Further, the competition between these forces can be adjusted by adding appropriate amounts of cosolute (e.g. salts, surfactants, other polymers) and cosolvent, or by applying external forces (e.g. osmotic pressure, surface forces, electric fields). The assembly of polymer structure in aqueous solution is common in nature but has not been widely used as yet for the preparation of synthetic materials. This IRG will examine how to tune the interactions of water-soluble polymers and cosolutes, illustrated in the top half of the figure on the right, so as to induce their assembly into useful microstructures, such as those illustrated in the bottom half of the same figure. In this context, the term “assembly” refers to the spontaneous or field-driven rearrangement of dissolved and disordered molecular building blocks, including polymers, into ordered microstructures. The type and degree of order in the final microstructures reflects the thermodynamic interactions that occur in solution among the building blocks and possibly the energetic landscape that must be traversed to produce these microstructures. For example, if a solid material is desired, extra processing steps may be needed to solidify or “lock in” the fluid-like structure initially formed first in solution.
The microstructures proposed in this IRG are largely inspired by biology,94 reflecting lessons learned from natural processes such as formation of the extracellular matrix, cell recognition, and packaging of DNA into nucleosomes and chromosomes. Other aspects of the research are influenced by the polymer community’s long experience with “polysoaps”95 and similar long chain water-soluble molecules that incorporate surfactant-like features to enhance molecular association. The preponderance of proposed research involves synthetic materials or synthetically modified biomaterials. Although solution properties and “soft” associations have long been examined for these substances, our goal reaches beyond prior investigations to the preparation of highly structured, robust materials from aqueous environments rather than weakly structured or soft materials. This goal will require the design and synthesis of monodisperse (or nearly so) building blocks, an understanding of transport and assembly kinetics, theoretical prediction and experimental measurement of macromolecular interactions, and the characterization of structure across a broad range of length scales.
SCOPE
The IRG’s primary objectives are to:
- form novel, microstructured polymeric materials in water by the intra- and inter-molecular association of complementary chain sequences and architectures
- measure the contributions of electrostatic, hydrophobic, surface, and hydrodynamic forces to aqueous supramolecular assembly
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- develop a predictive understanding of aqueous assembly
- promulgate protocols for fabricating new nanostructured materials that have value as membranes, aqueous sensors, controlled release devices, and responsive/compatible biomaterials
To meet these objectives, several fundamental materials-related issues must be addressed and understood, including (i) impact of hydrophobic, electrostatic, and hydrogen bonding interactions in terms of strength and range, (ii) manipulation of macromolecular entropy and enthalpy by judicious choice of chemical sequence and chain length, (iii) entropy change of neutralizing counterions, (iv) mechanism and extent of Manning condensation, (v) structural rearrangement of water and concomitant alteration of its dielectric properties, (vi) compatibility (or incompatibility) of polymer configuration with the shape/size of micelles or other complexing species, (vii) influence of nearby surfaces, (viii) hindered macromolecular diffusion and attenuated solvent flow in a nanoporous assembly, and (ix) selection of favored kinetic pathways within an ordered microstructure. All of these issues will be examined in IRG-III research.
For the benefits of clarity and focus, the organization of proposed research is as follows. First, we describe organization of microstructure in bulk aqueous solution via three broad strategies that include complexation of polyelectrolytes with oppositely charged surfactants (forming Polyelectrolyte-Surfactant Complexes or PSCs), complexation of polyelectrolytes with oppositely charged polyelectrolytes containing neutral blocks or grafts (forming Polyelectrolyte-Polyelectrolyte Complexes or PPCs), and assembly of giant, tube-like “bottlebrushes” with hydrophilic (or hydrophobic) interiors insulated within a hydrophobic (or hydrophilic) coronas (defining polymer assemblies or PAs). Next, we discuss research projects exploring how the assembly of PSCs, PPCs, and PAs are affected on aqueous surfaces that express an affinity for one polymer component through charge or hydrophobicity. Finally, we present proposals for investigating processing and properties of the developed microstructures. In particular, we will examine diffusion within highly ordered nanoporous arrays, the ability of shear to select and modify morphology, and the use of the porous microstructures for encapusulation of living cells.
