Surfaces & Materials Characterization

Bioinspired and soft materials

Complex coacervation is a phenomenon characterized by the association of oppositely charged polyelectrolytes into transient liquid complexes forming micron-scale condensates. This process is the first step in the formation of underwater adhesives by sessile marine organism, and in the complexation of many other synthetic and protein-based polyelectrolytes to contemporary materials. Efforts to understand the physical nature of complex coacervates are important for developing robust adhesives, injectable materials, or novel drug delivery vehicles for biomedical applications, however their internal fluidity necessitates the use of in-situ characterization strategies of their local dynamic properties—capabilities not offered by conventional techniques such as x-ray scattering, microscopy, or bulk rheological measurements. We employ the novel magnetic resonance technique Overhauser DNP together with EPR line shape analysis, to concurrently quantify local molecular and hydration dynamics, with species- and site-specificity. Acquiring molecular-level insight to the internal structure and dynamics of dynamic polymer complexes in water through the in situ characterization of site- and species-specific local polymer and hydration dynamics is a promising general approach that we pursue, but has not been widely utilized for materials characterization.

Members involved:

Nanostructured coacervate hydrogel

Our group is developing new spectroscopic tools to probe the surface and interfacial properties of polymer-based, functional materials, gain structural insight at the nanometer scale and ultimately advance the understanding of structure-property relationship of these synthetic systems that find applications in wet adhesion, drug delivery vehicles to coating materials. One particular pursuit has been the study of coacervate-based hydrogels, that are formed in aqueous solution by simple mixing of two oppositely charged ABA block copolyelectrolytes. This synthesis approach, pioneered by the group of Profs. Hawker and Kramer at UCSB represent a new and versatile approach to the design of bio-inspired gelators. While coacervate-based hydrogels promise high tunability of a range of desirable properties, little is understood about the molecular-level makeup of the nanometer-scale domains. In collaboration with the group of Prof. Kramer, small angle neutron scattering was employed to quantify the effective polymer density and water content of each domain. The Han lab is employing electron paramagnetic resonance (EPR) and Overhauser dynamic nuclear polarization (ODNP) measurements of block-specific spin labels to elucidate domain-specific, local, polymer and water dynamics. This unique combination of techniques reveals that the charged A blocks segregate into spherical domains with a radius of 8 nm, and are dispersed in a continuous matrix of water soluble, PEO B blocks (see figure). The edges of the spherical A block-containing domains are found to be soft and diffuse, and the B block-based matrix to exhibit higher water and polymer dynamics than the A block-based domains. The selective measurement of the local water and polymer dynamics shows a viscous and dense, but fluidic environment in the spherical A block-based domains, thus permitting the designation as a complex coacervate phase. Further, the physical properties of the analogous homopolymers mixed at equal composition to that of the triblock copolyelectrolytes leads to the conclusion that “the whole is greater than the sum of its parts”: nanometer scale complex coacervates only form when the two charged A blocks are covalently linked by a PEO midblock that serves as an intrinsic osmolyte.

Members involved:

Wet adhesion mechanisms

Marine mussels perfectly understand how to adhere to virtually any surface, under water. However, man-made adhesive performance and water are fundamentally in conflict. The presence of stable hydration layers around both the adhesive polymer and surface results in strong repulsive hydration forces that undermine adhesion. Understanding the adaptive mechanisms by which mussels, for example, overcome repulsive hydration forces to adhere to any type of surface underwater can provide the key to inventing a new generation of water-resistant adhesives with wide-ranging applications from biomedical implants to coatings. Using our surface water and dynamics measurement tools, we study what happens to intervening water at the interface when adhesive mussel proteins (and controls) approach a wet solid surface. Results suggest that under force-free conditions, different proteins have differential abilities to locally “dry” a surface in advance of binding interactions. It was expected that Dopa would mediate the drying action as DOPA is usually regarded the most potent functional groups in facilitating wet adhesion, but instead hydrophobic side-chains, not Dopa, are found to be the critical mediators of surface preparation, prior or concurrent to wet adhesion. Our group is exploring direct measurement of interfacial water dynamics during spontaneous (adhesive) interactions between proteins and a solid surface under force-free conditions to understand and single out the “building blocks” that achieve wet adhesion. It is already clear that there are at least two functional units of importance: hydrophobic molecular vanguards that help weaken surface hydration forces to pave the way for wet adhesion and the functional groups responsible for adhesive bond formation, such as DOPA groups or other chelate forming agents. Likely, there are other important factors for successful wet adhesion, such as a mechanism to seal the surface-adhesive interface from debonding.

Members involved:

  • Yasar Akdogan (Collaboration with Prof. Herb Waite, Dr. Wei Wei and others of the Waite lab)

Figure: The ability of mussel foot proteins (Mfp) to spontaneously approach a solid surface by breaking its surface hydration layer (here of polystyrene surfaces) is measured by means of the translational correlation time (?), and was found to mainly scale with the hydrophobicity of the protein, under force-free conditions. Larger ??means that the protein is effectively covering the polystyrene surface, and thus slowing its surface water dynamics.