Dr. Greg McKenna
Research Interests
Small Molecule Interactions with Glassy Polymers
Mathematical models of structural recovery and physical aging are able to describe the behavior of glassy polymers subjected to temperature-jump (T-jump) conditions. In addition, when polymers are exposed to small molecule plasticizers it is known that the glass transition temperature is depressed. Therefore, one can anticipate that a rapid change in plasticizer content would result in changes in material response similar to those which occur following a T-jump. While some preliminary experiments indicate this to be the case, viz., both structural recovery and physical aging are observed to occur, quantitative analysis of the kinetics associated with these events has not yet been done. Furthermore, the possibility exists that, because plasticizer content changes can lead to larger volume changes than are normally found in T-jump experiments, structural recovery and physical aging will not occur in the same fashion as occurs after T-jumps. To address these issues, we are performing novel humidity-jump and CO2 pressure-jump experiments to investigate the kinetics of structural recovery and physical aging in polymers. This project is funded by the National Science Foundation.
Torsion and Normal Force Measurements
By making an analogy between our understanding of the nonlinear elastic response of rubbery materials 50 years ago, polymer melt and solution nonlinear rheology 25 years ago and the current state of knowledge about the nonlinear viscoelastic response of glassy polymers today we can make important progress in our understanding of the mechanical response of polymeric glasses when their chemical structure is changed. In this study we examine the effects of chemical structure on the glassy state normal force response in torsional deformations. Torque and normal force measurements are being made and then analyzed using equations similar to those of finite elasticity theory to extract the nonlinear material parameters for materials having different chemical structures, including side group bulkiness in thermoplastics and the network architecture in thermosets. In addition, chain rigidity effects are being investigated. This project is funded by the Petroleum Research Fund of the American Chemical Society.
Nanorheology and Nanomechanics
Novel experiments are being performed using the atomic force microscope in order to investigate the influence of material dimensions on the rheological and mechanical response of polymeric glass forming materials. Thin films are examined by examining the temperature dependence of the viscosity and viscoelastic properties as a function of film thickness.
Melt and Solution Rheometry
The nonlinear rheology of polymers has been extensively studied as a function of molecular architecture and temperature. In particular, shear rate dependence of the viscosity and first normal stress difference have been extensively examined as have the so-called damping function in time-strain separable Wagner or Doi-Edwards type of single integral constitutive model. At the same time, relatively few systematic evaluations of polymer concentration on the same material parameters have been performed. Here we examine the impact of polymer concentration on the viscoelastic response of the polystyrene/ortho-terphenyl system. We probe the response primarily using torsional measurements and simultaneously recording the torque and the normal force. Following the application of the Penn and Kearsley Scaling Law for elastic cylinders to viscoelasticity media used by McKenna and Zapas, we obtain the time dependent strain potential function derivatives. This approach can also be formally arrived at from the K-BKZ single integral model. Results will be reported for concentrations of 25, 50 and 75% polystyrene at 35 °C. Data are analyzed within the frameworks of both damping function and the time-dependent strain potential function approaches.
Residual Stresses in Composite Materials
Residual Stresses in composites can lea to early failure in finished parts. We are investigating the impact of hydrostatic residual stresses induced during thermoset cure on the behavior of composites in a study funded by the Air Force Office of Scientific Research. The work involves modeling of the cure process and how process parameters affect residual stress development. In addition, experiments to validate the models are being developed.