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IRTG / Soft Matter Science
Freiburger Materialforschungszentrum
Stefan-Meier-Str. 21
79104 Freiburg, Germany

Contact: Jana Husse

+49 761 203 678 34
softmattergraduate[at]uni-freiburg.de


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You are here: Home Events Prof. Greg McKenna "Dynamics and Mechanics of Glass-forming Materials at the Nanoscale: Bubbles, Dewetting and Indentation"

Prof. Greg McKenna "Dynamics and Mechanics of Glass-forming Materials at the Nanoscale: Bubbles, Dewetting and Indentation"

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Department of Chemical Engineering, Texas Tech University, Lubbock, Texas, USA

What
  • Seminar
When Jun 10, 2015
from 10:15 AM to 11:00 AM
Where Seminarraum B, FMF, Stefan-Meier-Str. 21, Freiburg
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Though it has been nearly 25 years since the first reports that confinement of glass-forming materials to nanometer geometries can lead to reductions of the glass transition temperature Tg, there is still a lack of full understanding of the phenomenon/phenomena that occur.  In the work to be presented we will briefly describe some of the issues related to the lack of agreement in the community concerning the behavior of materials at the nanoscale, with particular emphasis on ultrathin polymer films. This will be followed by a description of our own work to use mechanical and rheological methods to investigate nanometer dimensioned materials.  In the first instance we will look at biaxial creep experiments on films in the so-called free standing state in which a membrane inflation (nano bubble tests) is used to probe the rheological and mechanical responses of films as thin as 3 nm. We show that the results are consistent with there being a significant reduction in the glass transition temperature of these films and that the observed reduction is related to the chemical structure of the material.  Furthermore, we observe a surprising “rubbery” stiffening behavior once the material has crept out of the glassy dispersion regime. We find that we are able to relate this behavior to the macroscopic dynamics through either the shape of the segmental relaxation or through the dynamic fragility index m.  We then examine the dewetting of ultrathin films from a liquid surface following the approach from Bodiguel and Fretigny. By extending their results for polystyrene to 4 nm thickness, we also find that the viscoelastic response is much more rapid than expected from macroscopic measurements and consistent with large reductions in the Tg of the samples.  We also demonstrate a new temperature-step method of making dewetting measurements that allow one to determine the Tg vs. film thickness using films of a single initial thickness. Also, because much of the discussion surrounding the reductions observed in the glass temperature of polymers are related to surface properties, we examine polymer surfaces using a novel particle embedment method and show some new results for stacked ultrathin films that are inconsistent with reports of strong Tg gradients in the nano-confined materials. Finally, we show results from nanoindentation measurements on an organic crystal where very large depth dependent properties are observed. These are correlated with the high pressures beneath the indentation tip.

 

invited by Dr. Anne Rubin

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