November 29-December 4, 2015 | Boston
Meeting Chairs: T. John Balk, Ram Devanathan, George G. Malliaras, Larry A. Nagahara, Luisa Torsi
Determination of the mechanical response of polymeric materials with dimensions less than 100 nm is a continuing challenge. Here we describe a novel membrane (“nano-bubble”) inflation method we have developed for the purpose of making measurements of the creep response of ultrathin polymer films and show two major findings. The first is that the material dynamics as measured by the creep response of the membranes depends dramatically on film thickness. For example, in polystyrene films, the dynamics is accelerated so much that the glass transition temperatureTg of a 11 nm thick film is reduced by approximately 50 K relative to the macroscopic Tg . Furthermore, we have discovered that the nominal rubbery plateau in ultrathin films is stiffened by upwards of two orders of magnitude relative to the macroscopic state and the rate of stiffening (stiffening index S) correlates with the shape of the segmental relaxation in accordance with a recent model proposed by Ngai, Prevosto and Grassia. We have elaborated this finding further and observe a strong correlation with the fragility index m that is related to glass formation according to the Angell categorization  of super cooled liquids. These results will be discussed in terms of current understanding of the impact of nanoconfinement on the glass transition behavior of polymers. In addition to being able to characterize the creep response of the ultrathin polymer films, we have also succeeded in adapting the bubble inflation method to make measurements on a graphene/polymer nano-sandwich structure and show that the method can be used to not only extract the stiffness of the graphene inner layer of the composite but that the method can be used to extract the interfacial shear strength of the polymer-graphene couple . P.A. O&’Connell, S.A. Hutcheson and G.B. McKenna, Journal of Polymer Science: Part B: Polymer Physics, 46, 1952-1965 (2008). K.L. Ngai, D. Prevosto and L. Grassia, Journal of Polymer Science: Part B: Polymer Physics, 51, 214-224 (2013). C.A. Angell, Journal of Non-Crystalline Solids, 73, 1-17 (1985). X. Li, J. Warzywoda and G.B. McKenna, Polymer, 55, 4976-4982 (2014).
The measurement of thin layer properties is important for both fundamental and technological reasons. Measurement of stress versus strain mechanics in one of the fundamental tenets upon which materials engineering is based. Direct measurements of elastic moduli and plasticity in nanometer thick polymeric glass layers is non-trivial, and one must often turn to indirect probes or small scale mechanical measurements that require complex interpretation. We introduce a novel strain-gradient free nanoscale compression test for thin soft layers on high stiffness substrates, which permits direct extraction of stress versus strain mechanics of the layer in elastic and plastic states from a single loading curve. Confined layer compression testing is built upon the compression of a precisely aligned flat probe into the layer forming a confined, stationary volume of highly compressed material in a well-defined, invariant geometry. Due to natural confinement from the surrounding material, we show that a state of uniaxial strain is created beneath the probe under small axial strains. Under a uniaxial strain stress state lateral strains within the film are zero throughout the entire volume under the probe. By this methodology we are able to directly probe via load-displacement measurement the stress-strain relationship in uniaxial flows under both reversible elastic and irreversible plastic conditions. Such nanoscale uniaxial strain testing is in some sense the complementary antithesis of nanopillar compression testing that has become so valuable for mechanical testing of size effects in hard materials. However, nanoscale uniaxial strain testing is suitable for in situ testing of both soft materials and continuous coatings on solid supporting substrates, in both elastic and plastic states. Confined compression measurements via i