Gregory B. McKenna was born in Pittsburgh, PA in 1949. His interest in science was cultivated from a young age through exposure to youth programs at the Buhl Planetarium on
Pittsburgh’s North Side as well as through the dinosaur collection at the Carnegie Museum in Oakland. His academic training came from the US Air Force Academy (B.S.
Engineering Mechanics in 1970), Massachusetts Institute of Technology (M.S. Composite Materials in 1971), and the University of Utah (PhD. Material Science and Engineering
in 1976). McKenna worked as a Test Evaluation-Engineer at the Hill Air Force Base in Ogden, Utah from 1971 to 1975. He took a position as a Post-Doctoral Researcher in
the Polymers Division at the National Bureau of Standards (NBS) from 1976 to 1977 then served as a Physical Scientist in the Division until 1992. He became the Structure
and Mechanics Group Leader in the Polymers Division in 1992, just a few years after NBS changed its name to the National Institute of Standards and Technology (NIST).
After seven years, McKenna left NIST and moved to Texas Tech University to serve as Professor and J.R. Bradford Endowed Chair in Engineering. Additionally, he served as
the Chair of the Department of Chemical Engineering from 2001 to 2004 and was named the Paul Whitfield Horn Professor in 2004.
Greg’s contributions to rheology have been made in four major areas: (1) nanorheology and surface rheological methods, (2) nonlinear viscoelasticity and rejuvenation of
polymer glasses, (3) molecular rheology and rheological characterization of polymer heterogeneity, and (4) mechanics and thermodynamics of cross-linked rubbers.
In the area of nanorheological measurements, McKenna’s miniaturization of the classical membrane inflation experiment involved the application of an AFM instrument to measure
the creep compliance of 1-5 micron diameter polymer membranes with thicknesses as small as 27 nm. The measurements were the first absolute determinations of creep compliance
of ultrathin free-standing films providing direct information about the glass transition and chain dynamics of polymers in confinement. In his glass-rejuvenation work,
McKenna applied a novel torsional dilatometer to study the volume relaxation of glasses, demonstrating that the rate of relaxation was unaffected by the magnitude of a large
mechanical deformation – contrary to the prevailing rejuvenation hypothesis. With his co-workers they showed how these features of the rejuvenation phenomenon could be
explained with a nonlinear constitutive model without invoking a change in thermodynamic state. McKenna’s contributions to molecular rheology were collaborations with A.J.
Kovacs and D.J. Plazek in which the viscosity and viscoelastic properties of cyclic polystyrene molecules were measured as a function of molecular weight and the concentration
of linear chains in blends of rings and chains. Their work provided the first observations that even small amounts of linear chain contamination can lead to dramatic increase
in the viscosity of rings. A novel rheological technique, Mechanical Spectroscopic Hole Burning (MSHB) was used to study the dynamic heterogeneity of materials. When applied
to small molecule glass formers, they showed that these systems exhibit the same type of dynamic mechanical behavior as polymer glass-formers; they all exhibit strong deviations
from classical VFT or WLF temperature dependence, and the viscosity does not diverge as Tg is approached. Finally, in the area of rubber elasticity, McKenna
showed that the fundamental assumption of the Flory-Rehner hypothesis of separability of the mixing and elastic contributions to the free energy function is essentially correct.
This work relied upon the use of careful torque and normal force measurements to obtain the strain energy function for rubber. McKenna’s work to understand the thermodynamics
and mechanism of crosslinked networks has had substantial impact on current research in this area because of the merging of continuum concepts to fully explore the consequences
of the Flory-Rehner molecular approach to rubber elasticity.
McKenna is a fellow of the American Physical Society, Society of Plastics Engineers, Society of Engineering Science, North American Thermal Analysis Society, and American Association
for the Advancement of Science. He has been awarded the Mercator Fellowship, Barnie E. Rushing Award for Distinguished Research, Michelin Chair for Visiting Professors at ESPCI in
Paris, Society of Plastics Engineers International Award, and the North American Thermal Analysis Society (NATAS) Award among others. Additionally, he has served the scientific
community in roles such as Member of the Governing Board of the American Institute of Physics (1995-1999), President of The Society of Rheology (2015-2017), and as a member of the
Editorial Board for the Journal of Rheology (1990-present).
Sources
McKenna, Gregory B..
Biographies of Astronaut and Cosmonaut Candidates. Space Facts (accessed Aug 22, 2019).
Gregory B. McKenna, Ph.D.
Chemical Engineering, Texas Tech Univeristy (accessed Aug 22, 2019).
Note: This biography is an adaptation of the following article previously published by The Society of Rheology.
Greg McKenna is the 2009 Bingham Medalist.
Rheology Bulletin 2009, 78(2), 4.