Ponente: Gregory Mckenna
Procedencia: Unis. Tecnologica de Texas


Though it has been nearly 25 years since the first reports[1,2] 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 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 is followed by a description of our own work to use mechanical and rheological methods to investigate nanometer dimensioned materials. In the first instance we look at biaxial creep experiments on films in the so-called free standing state in which a membrane inflation (nano bubble tests)[3] 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 [4]. We also examine the dewetting of ultrathin films from a liquid surface and 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[5].

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