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Soft Materials and Rheology Group Research Most of this research has been supported by the National Science Foundation, the Petroleum Research Fund, and the University of Pittsburgh. Compatibilizers Immiscible polymers are often blended together to realize
properties that cannot be realized by single homopolymers. In such cases,
block copolymers are often added or reactively-generated to stabilize the
two-phase morphology and improve interfacial adhesion. A long-standing interest
of our group has been elucidating the rheological and structural consequences
of compatibilizers. Fig. 1 illustrates one significant result: diblock copolymers added as compatibilizers raise the
viscosity of immiscible blends (Fig. 1). This viscosity increase can be
regarded as an interfacial immobilization effect of the compatibilizer. We are now turning to more complex compatibilizers such as
multifunctional reactive compatibilizers that crosslink the interface (Fig.2)
and form an interfacial skin.
Fig. 1: Viscosity of droplet/matrix blends with compatibilizer
as the volume fraction of drops changes. The compatibilizer free blends have
viscosities that agree well with an emulsion model, A tiny amount of
compatibilizer raises the viscosity significantly. Download full
details from Martin & Velankar, J. Rheol., 51,
669-692, 2007.
Fig.
2: Top: Schematic of a crosslinked interface created by interfacial reaction
of multifunctional polymers. Bottom: Confocal image
of a droplet-matrix blend with crosslinked interfaces
that are fluorescently tagged. Note how some drops are not spherical. Download full details from DeLeo & Velankar, J. Rheology, 52, 1385-1404, 2008. Photonic
crystal sensors When colloidal particles suspended in water are organized in a
well-ordered lattice, they can diffract light as per Bragg’s law and
show iridescence. Such colloidal ordered lattices are called Colloidal
Crystalline Arrays (CCA). Prof. Sanford
Asher at the University of Pittsburgh has developed methods of
infiltrating the interstitial spaces of a CCA with a water-soluble monomer,
and then polymerizing it to obtain a polymerized CCA hydrogel
(PCCA). Since the hydrogel is sensitive to stimuli
such as pH or chemical environment, the corresponding change in optical
properties can be used for sensing applications. We are collaborating with
Prof. Asher in developing and characterizing such photonic crystal sensors. Fig.
3: A PCCA based on polystyrene particles at two different magnifications.
Note that the particles are not in
contact; this is clear in B where some particles have fallen out of the
matrix. Download
full details in Muscatello et al, Macromolecules,
42, 4403-4406, 2009. Hydrogels based on Extracellular Matrix (ECM) Proteins In collaboration with Prof. Stephen Badylak,
McGowan Institute for Regenerative medicine, we are assisting with the
development of hydrogel scaffolds made from ECM
proteins. Prof. Badylak has pioneered the use of
ECM scaffolds for tissue regeneration. Since collagen is a major component of
ECM, hydrogels can be constituted readily. Our
group has conducted rheological characterization (Fig. 4) to quantify the
kinetics of gelation and the differences in the gelation characteristics of ECM derived from various
sources. In collaboration with Prof. Lawrence Block, we are examining
the rheological properties of polymeric excipients
used in pharmaceutical formulations. More specifically, we are interested in whether
a single point specification (e.g. viscosity at a particular concentration or
stress) is a sufficient quality control tool in the pharmaceutical industry
(Fig. 4).
Fig. 4: Left: Gelation of ECM hydrogels based on the urinary baldder
matrix (UBM) examined rheologically. Download
full details in Freytes et al, Biomaterials, 29,
1630-1637, 2008. Right: Cox-Mertz rule applied to aqueous solutions of sodium
alginate. Download full details in Fu et al, AAPS PharmSciTech,
11, 1662-1674, 2010 Drag
reducing polymers in blood flow Prof. Marina Kameneva, McGowan Institute for Regenerative Medicine, has had a long-standing interest in using high
molecular-weight water-soluble polymers as blood additives. These polymers
strongly modify the flow characteristics of red blood cells resulting in
improved oxygenation of tissues. Our collaborative research using
microfluidic devices has established that the mechanism of this improved
tissue oxygenation is the near-elimination of the cell-free layer near the
walls of the blood vessels (Fig. 5). This reduces the “plasma
skimming” effect near bifurcations and increases the number of blood
cells entering the bifurcations.
Fig. 5: Blood flow in a contraction. The flow is directed
upwards and the images show half of the contraction. Top image: without added
particles, the cells stay away from the walls of the channel and the inner
corner o the contraction is cell-free. Bottom: With added PEO, the cells
occupy the entire cross section of the channel. Download full details from Marhefka et al, Biorheology,
46, 281-292, 2009. Questions, Suggestions,
Comments? Send e-mail to velankar@pitt.edu
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Current projects Interfacially-active
particles Natural and
synthetic papillae
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