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

Particle Synthesis and Characterization
Over the past year, Dr. Elizabeth Knowlton, the postdoctoral fellowwho's primary support is from this funding has been able toreproducibly synthesize mono-disperse NIPA particles (microgels) withequilibrium sizes that range from sub- to multiple- microns indiameter.  Synthesizing microgel particles is not unique, and thereare many groups that can produced colloids with similar methods.  Whatmakes our work new and exciting, and will help lead to the completionof our second aim, is our ability to produce large, stable, andthermally responsive particles with a covalently crosslinkedfluorophore moiety bound within the particles.  The stability and sizeof these particles is provided in part through the inclusion of aco-monomer backbone.  In addition, we include a small amount ofco-non-solvent (DMSO) to the reaction vessel which in turn drives thereaction to produce particles at a much larger size.
We are currently performing a full characterization of these particleswith dynamic and static light scattering, rheology, viscometry, andconfocal microscopy.  These methods allow us to understand theefficacy of our new synthesis recipes.  Utilizing a Wyatt technologiesDawn Helios light scattering device located within my laboratory weare able to determine both the stability to aggregation and thethermal response.

Rheology
To further quantify the mechanical properties of these systems as wechange the volume fraction through a variation of the temperature, weperform rheological tests.  Primarily we are interested in thematerial response to an oscillatory shear stress.  We observe someintriguing results that will be further enhanced by extending thiswork into the second aim.  We observe that even at very low effectivevolume fractions the mechanical properties always maintain an elasticcomponent.  The lack of a liquid state is intriguing and we are nearlyready to start visualizing this system in our custom confocal rheomterset up.  This will enable us to quantify the microscopic properties ofthese particles before during and after a shear stress is applied.Signatures of the bulk material properties may be reflected by themicroscopic configuration of the particles or their dynamics.

Osmotic Compression
To help us understand our intriguing rheological behavior we havebegun a study to quantify the bulk modulus of single particles.  Oneof the most interesting aspects of microgels is that they are largelycomposed of solvent (water).  This implies that if the solvent isn'tbound to the polymer chains that under sufficient stress, theparticles should either deform or compress.  Quantifying thecompressibility of each particle should lead to a better understanding ofthe 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 onnon-adsorbing polymer chains (PEG) in solution.  We find that there isindeed an interplay between the osmotic stress on each particle,mediated by the peg coils, and the size of each particle.  Theimplications of these data are dramatic.  We expect that atsufficiently high volume fraction, the particles will impart anosmotic stress on each other through thermal motion.  In turn, theosmotic compressibility of the particles should vary as a function ofthe radius of each particle.  We find that as the particle volumefraction is raised through a change in temperature, the mechanicalresponse reflects this interplay between packing and swelling.

Next Steps: Confocal-Rheology
In the coming year we will focus on the final steps of our project.We will utilize our three dimensional laser scanning confocalmicroscope made by Lecia GmBH that is coupled with an customized AntonPaar MCR-301 rheometer.  This combined system will allow us to deeplyinvestigate the structural and dynamic response of these unique materials.

Impact:
The work thus far has been very exciting for our group.  Thedevelopment an Undergraduate student (Leah Nestico) has been quite goodand my Post-Doctoral fellow Dr. Knowlton has performed very well inthe lab and as a mentor.  This project has two women scientists involved.