Viscoelastic and glass transition behavior of organic and inorganic materials at the nano and micro scale
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The slow dynamics of glass forming systems in the deep glassy state as well as the glass transition and dynamic behavior at the nanoscale are the two major current challenges in the field of glass transition. The dynamics of glass forming materials have generally been described by super-Arrhenius behaviors, often expressed in terms of the Vogel-Fulcher-Tammann (VFT) equation. The VFT equation well describes dynamic behavior of experimental and theoretical results of glass forming systems in temperatures above the Tg. However, there is a lack of understanding of the deep glassy regime because it requires geological time scales to obtain dynamic data at equilibrium condition. In the case of the glass transition at the nanoscale, it has been reported from several studies that the Tg and related physical properties can change with decreasing size of the materials. Considerable efforts have been made to understand Tg changes at the nanoscale, but the exact mechanism is still unclear. The goal of the current dissertation is to investigate these two major challenges by applying various experimental techniques. First, to investigate the substrate and free surface effects on glass transition and dynamic behavior of the nanometric thin films, I performed particle embedment experiments on multi-layer films where 13 and 20 nm PS films were supported on PS, PMMA, and P2VP films with different thickness ranging from 13 to 350 nm. I found that the dynamics of top layer PS films were faster than the macroscopic below Tg, but slower above Tg. In addition, the glass transition and dynamics of top layer PS films were less sensitive to under-layer substrates than that of pseudo-thermodynamic measurements. Next, I investigated the glass transition and dynamic behavior of freestanding nanometric selenium films by applying the TTU bubble inflation method. The dynamics of thin selenium films followed Arrhenius behavior, which was different from the macroscopic VFT behavior reported in several studies. Furthermore, the Tg was decreased by less than 3 K when the film thickness decreased to 60 nm. In order to study the slow dynamics of glass forming systems in the deep glassy regime, I developed a new ultra-stable polymeric glass. Amorphous Teflon was used to create ultra-stable polymeric glasses by using physical vapor deposition technique. The ultra-stable polymeric glass produced in the current study showed a Tf reduction of 57 K, a reduction higher than the stable glasses produced from low-molecular-weight glass formers as well as 20 million years aged amber. I then used this material to test the super-Arrhenius behavior in the deep glassy state by performing TTU bubble inflation method. The creep experiments were performed by following Struik’s protocol in the upper bound regime where the temperature is higher than Tf but lower than Tg. In the upper bound regime, we see the de-aging behavior in which the relaxation time decreases with increasing aging time. I found that the dynamic behavior in the deep glassy state strongly deviate from the macroscopic VFT behavior.