When 2:00 PM - 4:00 PM Aug 26, 2015
Where NCRC, Bldg. 10, Room ACR1
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The Elastic Mechanical Properties of Supported Thin Polymer Films


Peter C. Chung
Thesis Defense

Peter F. Green, advisor

 

An understanding of the mechanical properties of supported thin polymer films is of crucial importance for the stability and performance of various polymeric thin film applications. Despite the technological and scientific importance, the mechanical response of supported thin polymer films is not well understood. Nanoindentation studies on thin polymer films with thicknesses on the order of few hundreds of nanometers supported by stiff substrates showed that the elastic modulus increases with decreasing film thickness due to the effects associated with the underlying stiff substrates. Specifically, when film thickness is less than a threshold film thickness, the indentation-induced stress field propagates throughout the entire film and strongly interacts with the underlying hard substrate, leading to the enhanced modulus. This is the so- called “substrate effect”. In this dissertation, nanoindentation measurements were performed on three classes of polymeric films in order to understand the factors that determine the overall mechanical response of supported thin polymer films: (1) linear-chain polymers, (2) miscible polymer/polymer blends, and (3) star-shaped polymers.

 

In general, a substrate effect is observed for sufficiently thin polymer films supported by stiff substrates. Prior and simulations and experiments on a limited number of systems suggested that the results could be rationalized in terms of the moduli of the polymer and substrate and the Poisson’s ratio of the polymer. After a more extensive study, we showed this not to be true and that the local vibrational force constant (i.e. local chain stiffness), measured by incoherent neutron scattering, of individual polymers provides a satisfactory rationalization of the overall elastic mechanical response of linear-chain polymers. We performed nanoindentation experiments of miscible polymer/polymer blends and showed how the compositional dependence of the mechanical response could be described in terms of the average vibrational force constant of the components. In the case of star shaped polymers, however, the effects of molecular structure on the packing of molecules possessing a large number of arms f > 32, had to be considered.

 

The study of these different systems provided a reasonable comprehensive insight into the nanoscale mechanical response of polymer films.