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Sam Starcevic, John Hyatt, Xiaobo Hu, L.A. Lyon, Alberto Fernandez-Nieves


My name is Sam Starcevic and I am a Research Assistant in the Soft Condensed Matter Laboratory at Georgia Tech.  I work under a Ph.D. student, John Hyatt, and Professor Alberto Fernandez-Nieves, who heads the lab.  The following highlights the work I have done while working with John.  There are other aspects of this overarching project that are not detailed, as well as other projects that I am not a part of that John works on.  If you wish to learn more about them, please visit John Hyatt’s page on this website.


Phase Behavior of pNIPAM-PEG-AAc Microgels Studied by Viscometry and Oscillatory Rheology


The size of colloidal particles can range from 10 to 1000 nanometers.  Microgels are colloidal particles of crosslinked polymers that are both swollen and suspended by a solvent. Some microgels swell and deswell depending upon ambient properties of the suspension.  Parameters such as temperature and pH are among those that can affect the size of the microgel.  The capacity of a microgel to well within a solvent, then deswell, expelling the solvent, especially at biological conditions, has possible applications in the medical field.  The structure and properties, for example the softness, of the microgels can be radically different at different levels of swollenness.  This allows for characteristics of the particles at different softness, such as the glass transition, to be studied.


This project is of particular interest because of the temperature at which pNIPAM-PEG-AAc microgels undergo a change from a deswollen state to a swollen state.  This occurs around 31-33oC, which is close to body temperature.  This coincidence makes mapping of the behavior of the microgels important base research for medical applications.  pNIPAM has a lower critical solution temperature transition from hydrophilic to hydrophobic.  Additionally, the AAc copolymerized with the pNIPAM further ionizes at higher pH, drawing in counterions to neutralize the charge and swelling the microgel more, with some intraparticle Coulombic repulsion from the ions.  Our interest is in the phase behavior of dense suspensions of these particles, as well as intraparticle structure, specifically that of the crosslinker position, as they deswell.


One critical step to characterize the particles involves viscometry.  To obtain a phase transition curve dependent on temperature and pH, a range of suspensions were prepared with differing microgel weight percentages.  The dynamic viscosity is determined using an Ubbelohde viscometer (see picture below).  The Einstein-Batchelor equation (see below) is used to relate the dynamic viscosity to the volume fraction of the microgels in the suspension.  It is worth noting that the Einstein-Batchelor equation is a model for hard spheres.  However, the microgels are hydrodynamically opaque, which contributes to their likeness to hard spheres and allows the Einstein-Batchelor equation to be used for their analysis.  The kinematic viscosity is measured from the viscometry.  The dynamic viscosity is then obtained from this value, and used to determine the volume fraction of the particles in suspension.  Volume fraction is the proportion of space that a solute occupies within the total volume of the solvent.  This volume fraction corresponds to swollen or deswollen microgels.  Since the Einstein-Batchelor equation is only valid for lower volume fractions (~0.2), this project is focused on establishing a base line phase behavior graph from which higher volume fraction suspension behaviors can be extrapolated.


Where η is the dynamic viscosity of the suspension, ηo is the dynamic viscosity of the solvent, and is the volume fraction.


The Ubbelohde Viscometer is shown lying in front of a water bath setup used to control the temperature of the suspension.

Some of our recent results from Viscometry.  The conditions of the experiment are a pH of 4.5 and a progression of temperature, separated by color in the graph.  Kinematic viscosity is obtained from the viscometry and converted to dynamic viscosity by multiplying by the density of the solvent. This is the value of  in the Einstein Batchelor equation.  This is divided by the viscosity of the solvent (water) to obtain the ratio .  This value is shown above plotted against the concentration of the microgel in the suspension.  A parabolic fit is applied to the data.  The coefficients,  and , correspond to kc and k2c2, where c is the concentration of the microgel in the suspension and k is the coefficient relating concentration and volume fraction.  The aim of this process is to eventually have a graph of k dependent on both pH and temperature for extrapolation and prediction purposes.


The second portion of the project utilizes the viscoelastic properties of the suspensions that can be analyzed with rheology (see picture below).  The phase transitions of hard sphere suspensions is much more studied and understood than that of microgels.  Microgels do not perfectly match the behavior of hard spheres, but bare some resemblance to them, depending on how swollen they are.  As they swell more, multiple microgels can occupy the same space, allowing for higher effective volume fraction to be attained than with hard spheres.  This distinction differentiates the phase transition of hard sphere suspensions and microgel suspensions.  One area of interest to us is the glass transition of the pNIPAM-PEG-AAc microgels.  The rhemoeter will be used to study the storage modulus and loss modulus of suspensions of different microgel weight percentage at various temperatures and pH values.  This information will be used to determine the behavior of the suspensions and the volume fraction range that corresponds to the glass transition.  Additionally, the relaxation time of the suspensions is of interest, which describes the time scale at which the microgels in suspension are able to flow around each other.


The Anton Paar Rheometer


Soft Condensed Matter Laboratory, School of Physics, Georgia Institute of Technology
770 State Street NW, Atlanta, GA, 30332-0430, USA
Phone: 404-385-3667 Fax: 404-894-9958
sstarcevic3 [at] gatech.edu