Reports: DNI749778-DNI7: Investigating the Micro-Mechanical Properties of Hydrogel Glasses Using Confocal-Rheology

Daniel L. Blair, PhD , Georgetown University

Over the past year we made significant progress this past year with our two specific aims. As outlined in our proposal, our first goal was the production and quantification of N-Isopropylacrylamide (NIPA) particles of sufficient size and stability to be used with confocal microscopy and confocal-rheology. These microgel particles have a very unique feature that sets them apart from conventional colloidal dispersions. With a very small variation of temperature, close to room temperature, the particles undergo a three fold radial variation; leading to a ten fold volumetric change. At the same time, the particles are stable to aggregation through this transition making them ideal for our work. In this report, I will outline the steps in thesynthesis and characterization of these particles that will allow us to proceed in the coming year to accomplishing our second specific aim.

Particle Synthesis and Characterization

Over the past year, Dr. Elizabeth Knowlton, the postdoctoral fellow who's primary support is from this funding has been able to reproducibly synthesize mono-disperse NIPA particles (microgels) with equilibrium sizes that range from sub- to multiple- microns in diameter. Synthesizing microgel particles is not unique, and there are many groups that can produced colloids with similar methods. What makes our work new and exciting, and will help lead to the completion of our second aim, is our ability to produce large, stable, and thermally responsive particles with a covalently crosslinked
fluorophore moiety bound within the particles. The stability and size of these particles is provided in part through the inclusion of a co-monomer backbone. In addition, we include a small amount of co-non-solvent (DMSO) to the reaction vessel which in turn drives the reaction to produce particles at a much larger size.  This work is currently being considered for publication.

We are currently performing a full characterization of these particles with dynamic and static light scattering, rheology, viscometry, and confocal microscopy. These methods allow us to understand the efficacy of our new synthesis recipes. Utilizing a Wyatt technologies Dawn Helios light scattering device located within my laboratory we are able to determine both the stability to aggregation and the thermal response.


To further quantify the mechanical properties of these systems as we change the volume fraction through a variation of the temperature, we perform rheological tests. Primarily we are interested in the material response to an oscillatory shear stress. We observe some intriguing results that will be further enhanced by extending this work into the second aim. We observe that even at very low effective volume fractions the mechanical properties always maintain an elastic component. The lack of a liquid state is intriguing and we have been visualizing this system in our custom confocal rheomter set up. This will enable us to quantify the microscopic properties of these particles before during and after a shear stress is applied.
Signatures of the bulk material properties may be reflected by the microscopic configuration of the particles or their dynamics.

Osmotic Compression

To help us understand our intriguing rheological behavior we have begun a study to quantify the bulk modulus of single particles. One of the most interesting aspects of microgels is that they are largely composed of solvent (water). This implies that if the solvent isn't bound to the polymer chains that under sufficient stress, the particles should either deform or compress. Quantifying the compressibility of each particle should lead to a better understanding of the high volume fraction phases and transition between those phases.To test this, we perform light scattering on low volume fractionsuspensions of microgels with a variable concentration on non-adsorbing polymer chains (PEG) in solution. We find that there is indeed an interplay between the osmotic stress on each particle, mediated by the peg coils, and the size of each particle. The implications of these data are dramatic. We expect that at sufficiently high volume fraction, the particles will impart an osmotic stress on each other through thermal motion. In turn, the osmotic compressibility of the particles should vary as a function of the radius of each particle. We find that as the particle volume fraction is raised through a change in temperature, the mechanical response reflects this interplay between packing and swelling.

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