Session R4: Ionic, Dipolar and H-bonding Polymers

Solvent-induced changes in the structure and rheology of polyelectrolyte solutions.

By integrating microfluidics and particle tracking microrheology, we have constructed a dialysis cell for microrheology, which provides unique opportunities for studying the dynamics of microstructural changes induced by changes in solvent composition. Such experiments are virtually impossible with mechanical rheometers. The concept and design of the microdialysis cell will be discussed, and data will be presented on the structural and rheological response of polyelectrolyte solutions to changes in ionic strength. Sulphonated polystyrene is a water-soluble polymer and its molecular conformation in solution strongly depends on ionic strength of the solution. It will be shown that quantitative measurements of transient solution viscosity during solvent exchange can be performed with the new dialysis cell. Experiments were also performed on amphiphilic block copolypeptide (BCP) hydrogels, which self-assemble into fibrillar structures due to a subtle balance between attractive and repulsive intermolecular forces. Electrostatic repulsion between the hydrophilic L-lysine blocks plays a key role. Therefore, changes in ionic strength have a significant effect on the self-assembled local structure and mechanical properties of the BCP gels, as was previously observed in rheometer experiments. Microrheology in the dialysis cell provided a much more complete picture, revealing the occurrence of microscopic phase separation upon the addition of salt. For example, in a K160L40 lysine-leucine block copolypeptide, the motion of tracer particles in the hydrogel is homogeneous in DI water. After the addition of salt, microrheology reveals the co-existence of populations of freely moving and immobilized particles. The changes in local microstructure were found to be reversible when the ionic strength of the solution was lowered again. Data will be presented on the dynamics of the morphological and rheological changes of various block copolypeptide hydrogels.   

Theoretical Modeling of Hydrogen Bonded and Metal-Ligand Associating Polymers

Applying analytical modeling in combination with Monte Carlo simulations we have studied the association behavior and properties of two types of supramolecular polymers employing 1) hydrogen bonded and 2) metal-ligand associative motifs. In the first case association between the hydrogen bonded arrays results in numerous donor-acceptor interactions between the complementary end groups of linear oligomers leading primarily to formation of linear chains or rings. Similar architectures of self-assembled polymers can also be obtained by reversible 1:2 complex formation between metal ions (such as Zn(+2), Cd(+2), Co (+2), etc.) and ligands of end-functionalized oligomers. In this case the association is strongly influenced by metal- ligand ratio. We analyze the chain-ring equilibrium and study the influence of the strength and type of association, rigidity of the complex and solution composition on the degree of association and the average molecular weight of the supramolecular polymers. For metal ions (such as La(+3), Nd(+3), Eu(+3), etc) capable of 1:3 complex formation with the ligands, self-assembly results in reversible polymers of more complex architecture, in particular a reversible network (in the percolation limit). Since the coordination sites of the metal possess unequal reactivity (having different energies of association with the first, second and third ligand) and due to the cooperativity of binding, network formation is influenced by different factors such as metal/ligand ratio, oligomer length and concentration. Predictions of an analytical model based on the equilibrium among different associating species and classical percolation theory compare favorably with simulation results for gel fraction and average molecular weight. Simulation results and theoretical predictions will be compared with available experimental data.   

Soultion assembly of charged block copolymers and block copolypeptides

By considering peptidic or charged, synthetic polymers in the materials self-assembly design process, one can take advantage of inherently biomolecular attributes; intramolecular folding events, secondary structure stabilized by hydrogen bonding, and electrostatic interactions; in addition to more traditional self-assembling molecular attributes such as amphiphilicty, to define hierarchical material structure and consequent properties. The solution assembly behavior of two charged block copolymers will be discussed. First, diblock copolypeptides consisting of a hydrophilic, charged block and a hydrophobic block were designed to self-assemble due to their amphiphilic nature. The defined helical secondary structure of the leucine hydrophobic block forces these molecules to form a membraneous local nanostructure. However, diverse materials, ranging from rigid hydrogel, vesicle suspension, or hexagonal single crystal, can be formed depending on assembly pathway. Second, synthetic triblock copolymers with charged corona blocks can be assembled in dilute solution with multivalent organic counterions to produce complex micelle structures such as toroids and discs. Nanostructure can be tuned with different concentrations or molecular volumes of organic counterion. Transmission electron microscopy, small-angle neutron scattering, multiphoton confocal microscopy, dynamic light scattering, and atomic force microscopy results will be discussed.   

Water Solubility of Polymers with Salt: the Hofmeister Series

We have designed temperature gradient microfluidic devices that allow high throughput, low sample volume assays to be performed on the folding of thermoresponsive polymers and proteins. These macromolecular systems are insoluble at high temperatures, but become hydrated and unfold as the temperature is decreased in a process analogous to the cold denaturation of proteins. Our assays enable highly precise measurements to be made rapidly of the physical behavior of the polymers. The device is specifically used to obtain data on poly (N-isopropylacrylamide) and alpha-elastin at multiple concentrations in the presence of a variety of ions. The results indicate that the folding process follows the Hofmeister series. This series, which dates back to 1888, is a rank ordering of anions and cations based upon their ability to salt-out or salt-in proteins. It had been historically believed that ions affect macromolecule solubility indirectly through their interactions with bulk water. This idea has been largely disproved by a variety of characterization techniques over the last decade. A new theory to explain the mechanism of the Hofmeister effect, however, still needs to be developed. Microfluidic assays in combination with vibrational sum frequency spectroscopy allowed us to develop a model based solely on the direct interaction of the ions with a macromolecule and its first hydration shell. In fact, the protein folding properties can be related to a few simple factors: an ion's hydration entropy, its effect on the surface tension of an aqueous interface, and its ability to interact directly with binding sights on a protein.   

Polyelectrolyte effects in polymers for lithography

The transformation of a solid-like film into a solution upon exposure to a miscible solvent is a complex process involving sluggish kinetic pathways associated with the slow transport of the liquid into the film and the evolution of the thermodynamic driving forces during the course of the dissolution process. In complex materials such as polymers, this process occurs in stages from the transformation of the glassy or crystalline film into a swollen state, followed at longer times by the final dissolution of the film. Dissolving polyelectrolyte films exhibit additional complexities in their dissolution dynamics over uncharged polymer films. Interfacial charge density, the dielectric constant of the medium, ionic strength and valence influence the phase behavior of charged polymers thus affecting their dissolution behavior. The dissolution mechanism can be tailored for different applications, for instance the microelectronics industry utilizes the selective dissolution of one component enabling lithographic pattern formation. We present neutron reflectivity and quartz crystal microbalance results to address polyelectrolyte effects in thin films such as the counterion distribution, quasi-equilibrium swelling and kinetics. V.M. Prabhu, R.L. Jones, E.K. Lin, W-L Wu. ``Polyelectrolyte effects in model photoresist developer solutions.'' J. Vac. Sci. and Tech.B, 21, 1403 (2003). V.M. Prabhu, B.D. Vogt, W-L. Wu, J. Douglas, E. Lin, S. Satija, D. Goldfarb, and H. Ito. ``Direct measurement of the counterion distribution within swollen polyelectrolyte films'' Langmuir Letter, 21, 6647 (2005).