Material Behavior at the Nanoscale

The behavior of polymers at the nanometer scale remains an intense area of investigation, but most studies have been limited to the case of the substrate supported thin films even though studies suggest much larger effects occur in the freely standing state. We have been examining both conditions through a range of experimental methods from nanobubble inflation, nanoindentation and dewetting of thing films from a liquid substrate. One especially intriguing work is related to the relationship between rubbery stiffening and the dynamic fragility suggested by the coupling model and and we are testing the prediction that extremely fragile systems should show extreme stiffening and strong systems should show a weak stiffening. Branched polymers have been shown to exhibit different nanoscale behavior from linear counterparts upon confinement on a supporting layer and we use the TTU bubble inflation method to examine the effects of branching and unentangled polymer chain length on the viscoelastic properties of freely standing ultrathin films. We have also been investigating the dewetting response of extremely thin films of star-branched polystyrene to compare with the bubble inflation films. Furthermore, there is currently no work available that characterizes the yield and modulus behavior as a function of film thickness and temperature for the freely standing state, and we make measurements to provide such novel data to the field. In addition, we examine the impact of nano-indentor geometry and surface detection capabilities on the apparent modulus of soft materials.

Video of Ultrathin Film Project

 

Some Publications

1. Z. Qian, J. Risan, B. Stadnick and G. B. McKenna*, "Apparent Depth Dependent Modulus and Hardness of Polymers by Nanoindentation: Investigation of Surface Detection Error and Pressure Effects," Journal of Polymer Science. Part B. Polymer Physics, 56, 414-428 (2018).

2. H. Yoon and G.B. McKenna*, ""Rubbery Stiffening" and Rupture Behavior of Freely Standing Nanometric Thin PIB Films," Macromolecules, 50, 9821-9830 (2017).

3. O. Brazil, J.P. de Silva, M. Chowdhury, H. Yoon, G.B. McKenna, W.C. Oliver, J. Kilpatrick, J.B. Pethica and G.L.W. Cross*, "In situ measurement of bulk modulus and yield response of glassy thin films via confined layer compression," Journal of Materials Research, 35, 644-653 (2020).

4. P. Chapuis*, P. C. Montgomery, F. Anstotz, A. Leong-Hoï, C. Gauthier, J. Baschnagel, G. Reiter, G. B. McKenna, and A. Rubin, "A novel interferometric method for the study of the viscoelastic properties of ultra-thin polymer films determined from nanobubble inflation," Rev. Sci. Instr., 88, 093901 (2017).

5. J. Wang and G.B. McKenna*, "Viscoelastic Properties of Ultrathin Polycarbonate Films by Liquid Dewetting," J. Polymer Science. Part. B. Polymer Physics, 53, 1559-1566 (2015).

 

Funding

• National Science Foundation. Division of Materials Research (DMR), Polymers Program.
• John R. Bradford Endowment at Texas Tech University.