David A. Hoagland

Research Interests

Electron Microscopy of Solvated Polymeric Materials, Polyelectrolytes, Polymer Solutions and Gels, Interfacial Nanoparticles, Single Molecule Visualization, Polymers in Ionic Liquids

Current Research

"Soft materials" range from solutions and gels to suspensions, emulsions, and pastes. Projects in the Hoagland group examine polymer structure and dynamics in these materials using a range of experimental methods. The typical goal is to understand the behaviors of individual polymer molecules, i.e., their average conformation, where and how they move, or how rapidly they deform/recover when challenged with an external force.

Current projects focus specifically on (i) polyelectrolytes (highly charged polymers), (ii) polymers in ionic liquids (salts that melt near or below room temperature), (iii) imaging of the dynamics of confined polymers, and (iv) carbon nanotube membranes. Some highlights: Due to ionic liquid nonvolatility, soft materials wet by these liquids can be imaged in the high vacuum environment of the electron microscope, affording opportunities for the in situ and real time tracking of nanoscale structure and dynamics. Such imaging is impossible with ordinary liquids. Frames extracted from image sequences for two ionic liquid systems are given (F1,F2). The (F1) SEM image shows crystallized monolayers on the surface of an ionic liquid containing nanoparticles; crystal melting into the liquid was observed as temperature rises. The (F2) TEM image illustrates the network of a poly(oxyethylene) gel; gelation here is driven by polymer crystallization.

Three frames of a fluorescence microscopy image sequence (F3) revealing the conformations of a flexible polymer (large DNA) moving through a chromatography column (beads not visible). Driven by solvent flow (toward upper left), the molecule initially “entangles” around a contact point between two beads, and then through a "pulley-like" motion about this constraint, extends in the flow direction before final release.

Ionic liquids have attractive properties as room temperature media for protein storage and shipping. The dynamic light scattering correlation function (F4) was obtained for the protein lysozyme in the neat room temperature ionic liquid [EMIM][EtSO4]. The form of the decay reveals that the protein molecularly dissolves in the neat salt, retaining a size comparable to that in aqueous buffer. Circular dichroism and infrared spectroscopy reveal the extent to which the native conformation is retained; upon subsequent transfer to buffer, lysozyme’s native activity is recovered.