Gregory B. McKenna

Gregory B. McKenna

Texas Tech University

Chemical Engineer
Fellow, Elected 2020

Prof. McKenna has established an international reputation for his experimental research, which has focused to a large degree on the use and development of rheological methods to interrogate the physics of polymers and glass-forming systems. In particular Professor McKenna is widely recognized for the development and use of novel experimental methods for the investigation of the physics of complex fluids and polymers. During his tenure (1976–1999) at the National Bureau of Standards and then the National Institute of Standards and Technology, McKenna built an experimental program based on both classical and innovative measurement techniques that significantly furthered the fundamental understanding of the physics and mechanics of polymers. He continued and expanded this cutting-edge research after joining the Department of Chemical Engineering at Texas Tech University (August 1999).

His contributions in the rheology of polymers and soft matter can be broken into four areas:

Dynamics of polymers and other glass-forming materials

His work has had a major impact on our understanding of the temperature dependence of the equilibrium dynamics of glass-forming systems deep into the glassy state. The purpose was to test whether super-Arrhenius behavior continues below the nominal Tg. Results from thermoviscoelastic studies of ancient amber ultra-stable glasses showed that the dynamics at the notional "ideal" glass transition do not diverge, as would be expected from the VFT- or WLFtypes of extrapolation. The results were validated using nano-rheological measurements on microgram quantities of a vapor-deposited amorphous PTFE. This body of work challenges theories of an ideal glass transition.

Dynamics in confinement and nanorheology

After the discovery that small molecule glass-forming liquids exhibit reduced calorimetric glass transitions upon confinement in nanoporous matrixes, McKenna and co-workers developed innovative measurement methods to investigate the confinement physics in ultrathin polymer films. Their innovative approach was to shrink the classical membrane inflation experiment so that an AFM could be used to measure, without recourse to contact-mechanics methods, the absolute creep compliance of 1- to 5-μm-diameter polymer membranes having thicknesses as low as 3 nm.

Nonlinear viscoelasticity and rejuvenation of polymer glasses

Through creative measurements and construction of a unique torsional dilatometer, it was demonstrated that, although molecular mobility increases due to the application of large mechanical deformations, the underlying glassy state of an amorphous polymer is decoupled from deformation—contrary to the mechanical rejuvenation hypothesis.

Molecular rheology and rheological characterization of polymer heterogeneity

Demonstrated that the viscosity and the viscoelastic properties of cyclic polystyrene macromolecules that even small amounts of linear chain contamination can lead to dramatic increases in viscosity of the rings. This specific result, interpreted as being due to the threading of the rings by the linear contaminant, has guided the study of the rheology of ring-like molecules in the subsequent decades since the discovery was made.

Based on the documents submitted by Montgomery T. Shaw.