Length scale of heterogeneities in glassy propylene carbonate probed by oxygen diffusion

Study of oxygen transport in glassy polymers on a nanometer length scale utilizing the kinetic Monte Carlo simulations

Diffusion-controlled deactivation of excited phenanthrene and oxidation of triplet aryl-nitrene by molecular oxygen were used to determine the energetics of oxygen jump rates in the set of glassy polymers: poly(methyl methacrylate), poly(n-butyl methacrylate), polycarbonate, polystyrene, and polysulfone. To interpret experimental results, a simple model based on the transition state theory of diffusion jump has been used. The kinetic Monte Carlo simulations of phenanthrene deactivation and nitrene oxidation were carried out in a cubic lattice that modeled a polymer matrix. The bonds of the lattice were assigned to be activation barriers for the diffusion jumps of oxygen molecules from one site of the lattice to another. The standard deviation, σ‡, and spatial correlation length, rc, of the free energy of diffusion jump have been determined. It is shown that the spatial correlation of oxygen jump rates on a nanometer scale and the entropic nature of the dynamic heterogeneity are common features of all the studied polymers.

Relation between the activation energy of oxygen diffusion and the instantaneous shear modulus in propylene carbonate near the glass transition temperature

We discuss the transport of small gas molecules in organic glassy matrices using oxygen diffusion in propylene carbonate as an example. The jumps of a penetrant from one interstitial cavity to another require energy to expand the channel between cavities to the size of the penetrant. It has been established that at temperatures below and slightly above the glass transition temperature, the activation energy of oxygen diffusion, E, is related to the instantaneous shear modulus G∞ of propylene carbonate via the equation E = V × G∞, where V is the temperature-independent parameter that characterizes the volume of the channel. Consequently, the E value is the work necessary for elastic deformation of the surrounding matrix to expand the channel available for oxygen diffusion.

Heterogeneity in Single-Molecule Observables in the Study of Supercooled Liquids

Bulk approaches to studying heterogeneous systems obscure important details, as they report average behavior rather than the distribution of behaviors in such environments. Small-molecule and polymeric supercooled liquids, which display heterogeneity in their dynamics without an underlying structural heterogeneity that sets those dynamics, are important constituents of this category of condensed matter systems. A variety of approaches have been devised to unravel ensemble averaging in supercooled liquids. This review focuses on the ultimate subensemble approach, single-molecule measurements, as they have been applied to the study of supercooled liquids. We detail how three key experimental observables (single-molecule probe rotation, translation, and fluorescence lifetime) have been employed to provide detail on dynamic heterogeneity in supercooled liquids. Special attention is given to the potential for, but also the challenges in, discriminating spatial and temporal heterogeneity and detailing the length scales and timescales of heterogeneity in these systems.

Length and time scales of structural heterogeneities in deeply supercooled propylene carbonate

Deactivation of excited phenanthrene by molecular oxygen is utilized to probe the structural heterogeneity of supercooled propylene carbonate. The diffusion rate of oxygen molecules in different regions varies over two orders of magnitude. The size of the regions of different oxygen mobility was determined to be 1.5 nm. Values from 0.2 to 30 s have been obtained for the lifetime of these regions over a temperature range from T(g)-1 to T(g)+4 K (T(g)=158 K). The heterogeneity lifetime is in close agreement with the α-relaxation time determined by dielectric spectroscopy. The obtained results argue in favor of the statement that the heterogeneous cooperative dynamics of host molecules (so-called dynamical heterogeneity) is of structural origin.

Kinetics of azobenzene nitrene oxidation by molecular oxygen in glassy propylene carbonate

The kinetics of azobenzene nitrene oxidation by molecular oxygen dissolved in a matrix was studied in glassy propylene carbonate. The reaction was carried out in excess oxygen within its concentration range 0.008 to 0.048 M. The oxidation kinetics, controlled by oxygen diffusion, is not described by the exponential function. A specific reaction rate decreases in the course of the process. The higher the oxygen concentration in the matrix, the sharper is the decrease in the reaction rate. It is shown that at all concentrations, the oxidation kinetics is described in the framework of the model of heterogeneous matrix with a characteristic size of heterogeneities of 1.5 nm and a dispersion of the activation energy of oxygen molecule jump of 4 kJ/mol. The reaction radius is about 0.5 nm.