Fast Scanning Calorimetry Studies of Molecular Dynamics in Crystals, Liquids, and Glasses

摘要

There is a dearth of experimental techniques that can probe the dynamic properties of non-equilibrium condensed phases, particularly in materials characterized by very slow kinetics, such as glasses. Although fundamental theories on the glass transition and the properties of glass forming liquids abound, none can be validated because there is insufficient experimental data on the molecular dynamics of these materials at temperatures near the glass transition. In this Dissertation, I demonstrate the utility of a novel technique, Fast Scanning Calorimetry (FSC), to interrogate the kinetic and thermodynamic parameters of non-equilibrium condensed phases. The custom-built, quasi-adiabatic, thin-wire calorimeter can quickly heat micrometer scale crystalline, liquid, or glassy samples, all rapidly prepared by vapor deposition, to obtain high resolution and accurate data for non-equilibrium structural relaxation. Essentially, fast temperature scanning vibrationally perturbs the material far from its initial or equilibrium state and the calorimeter can measure the structural response of the material to these thermal perturbations.;My FSC studies reveal that, in the limit of high heating rates on the order of 106 K/s, the kinetics of any phase transition, e.g. glass devitrification, viscous liquid relaxation, or crystalline melting, simplify greatly to follow a zero-order rate law with an Arrhenius-like temperature dependence. The discovery of this unique kinetic regime, along with the analytical methodology developed in this Dissertation, have opened a new venue for gathering data on the molecular mobility of glassformers at low temperatures. Studies on the melting of superheated molecular crystals illustrate the capabilities of FSC and expose long-held misconceptions on the mechanism of heterogeneous melting. The observed high activation energy barriers to this diffusion-limited process demonstrate that, under conditions of rapid heating, the structure of the material at the crystalline-amorphous interface is surprisingly similar to that of a glass.;Furthermore, FSC studies that compare the devitrification of ordinary, melt-cooled glasses and vapor deposited amorphous phases confirm that both materials can undergo heterogeneous, surface-facilitated relaxation when heated with a sufficiently high rate. The high activation energies for these relaxation processes reflect the kinetic properties of the materials in their initial states at low temperatures, not at the temperatures at which transformation occurs. FSC studies that utilized a set of glass-forming liquid samples with well-defined initial states confirm the impact of initial sample temperature on the transformation rate and provide validation for a proposed model of front-propagated structural relaxation. In fact, the data measured by FSC can be used to calculate the thermodynamic driving force for front propagation and provide quantitative validation of the proposed relaxation mechanism.;Finally, the FSC relaxation rate data with the aforementioned sample set shows an astounding correlation with molecular self-diffusion at low temperatures, demonstrating that the technique can be used to measure diffusivity in condensed phases with very slow dynamics, even below the glass transition. The last study of this Dissertation is a preliminary exploration of the slow molecular kinetics in amorphous vapor-deposited materials with very stable initial structures. The results of these experiments implicate the existence of a crossover in equilibrium mobility parameters at temperatures below the glass transition, an unprecedented finding that may have a significant impact on developing an accurate theoretical framework for the formation and devitrification of glasses.