Probing Biomembranes with Positrons

Nanostructural aspects, free volume and phase constitution of rapidly solidified Nd–Fe–B

Modification of microstructure of the surface and the bulk in ion-irradiated membrane studied using positron annihilation spectroscopy

The free volume of metallic glasses has a significant effect on atomic relaxation processes, although a detailed understanding of the nature and distribution of free volume sites is currently lacking. Positron annihilation spectroscopy was employed to study free volume in a Zr–Ti–Ni–Cu–Be bulk metallic glass following plastic straining and cathodic charging with atomic hydrogen. Multiple techniques were used to show that strained samples had more open volume, while moderate hydrogen charging resulted in a free volume decrease. It was also shown that the free volume is associated with zirconium and titanium at the expense of nickel, copper, and beryllium. Plastic straining led to a slight chemical reordering.

It is well known that structural relaxation in a bulk metallic glass affects some properties such as viscosity, electrical conductivity, and ductility. The free volume in glassy alloy, which is open volume retained by rapidly solidification from liquid state, has a significant roll for those properties. In order to discuss the nature of free volume in a Zr50Cu40Al10 bulk metallic glass, positron lifetime measurements have been performed for this metallic glass before and after annealing, and all positron lifetime spectra have been decomposed into multi components as a size distribution function by use of CONTIN-PALS II program. The positron lifetime distribution for this metallic glass has a broad spectrum comparing to crystal metal including a single vacancy, and its average lifetime corresponds to the free volume size. This width of positron lifetime (free volume size) distribution decreases by the annealing at 673 K. This change of the positron lifetime distribution can be attributed to a free volume relaxation.

In order to predict and enhance the material properties of blends, it is important to understand the nature and the underlying mechanism of blending at the molecular level. As such miscibility and phase-separation phenomena of polymer blends have received significant attention in polymer applications. In this chapter, the connection between free-volume and blend miscibility is explored. In recent years, Positron Annihilation Spectroscopy (PAS) has emerged as a unique and potent probing tool for characterizing free-volume properties of polymers and polymer based materials. In the investigations spanning two decades both discrete lifetime components (average free volume size) and the distribution of free volume hole sizes have been analysed. It has been found that connecting free volume to polymer miscibility directly is inadequate and hence the hydrodynamic interactions approach for the description of composition dependent miscibility level seems to be versatile. Considerable amount of data in literature on binary polymer blends in has been revisited and interpreted in this work. We show that, it is more effective and especially for partially miscible blends in which some of the inferences were in vague. Therefore, the present chapter establishes hydrodynamic interaction approach as a versatile method developed based on the same free volume data.

In the quest to elucidate the solid‐state structures of polymers, insight into the amorphous phase is particularly elusive. Although the permeability of small molecules is often measured as an important performance property, numerous researchers have found that a deeper analysis of the transport characteristics provides insight into polymer morphology, especially if used in combination with more usual characterization techniques. The transport of small gas molecules senses the permeable amorphous structure and probes the nature of the free volume. In recent years, our interest in the gas barrier of polyesters has resulted in an unusual opportunity to investigate the nature of the free volume in the polymer glassy state. This effort has been aided by access to aromatic polyesters with designed variations in their chemical structure. This review focuses on oxygen transport, supplemented with other methods of physical analysis, as a probe of the excess‐hole free volume. The review addresses the profound effects of orientation and crystallization on the free volume of the glassy state. The discussion also presents a simple odel for the gas permeability of the isotropic glass based on lattice concepts and tests more sophisticated models for the gas permeability of semicrystalline polymers. The final section addresses other opportunities for fruitful applications of oxygen transport as a solid‐state structure probe. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1047–1063, 2005

Positron annihilation spectroscopy (PAS) is a novel method that provides molecular-level information about complex macromolecular structure in a manner different from, but complementary to, conventional physical and chemical methodology. This paper presents a perspective of PAS in polymeric systems covering 12 aspects: historical, spacial, spherical quantum model, anisotropic structure, voids, positronium chemistry, time, positron annihilation lifetime spectroscopy and data analysis, variable monoenergetic slow positron beam techniques and depth profiling, elemental analysis, multidimensional instrumentation advances in PAS, and free volume and free-volume theories.

The macroscopic Bi precipitation process in highly heterogeneous Sn65.4Bi34.6 eutectic alloy is investigated on an atomic scale by positron annihilation spectroscopy. High concentration of vacancy-sized free volumes are located at interfaces among short-range ordered phases. Along with Bi precipitation, the chemical surroundings around the free volumes are changed from Bi- to Sn-rich environment and the concentration of free volume increases. The results demonstrate that free volume-intermediated atomic diffusion at the interfaces is responsible for macroscopic Bi precipitation in Sn65.4Bi34.6.

Plasticization of a novel polysulfone based mixed matrix membrane with high-performance CO[formula omitted] separation studied by positron annihilation

Fluorescent molecular rotors embedded in polymer matrices can probe the local changes in the dynamics of the polymer matrix (such as the glass transition of polymers near interfaces/surfaces) that are not accessible from macroscopic measurements. Yet, there is little consensus as to how the fluorescence data should be analyzed, what property the fluorescence intensity actually represents, and what are appropriate/optimal rotors for glass transition measurements. By experimentation with a model fluorescent rotor, farnesyl-(2-carboxy-2-cyanovinyl)-julolidine (FCVJ), and also re-analysis of data available in the literature for other types of molecular rotors, we investigated the correlation between the relaxation processes of photo-excited rotors and free volume changes in matrix polymers. The temperature dependences of the fluorescence intensities from FCVJ-doped polystyrene (PS), poly(vinyl acetate) (PVAc), and poly(isobutyl methacrylate) (PiBMA) materials across their glass transition temperatures (Tg) were successfully modeled using a free volume model which is based on the original theory of Loutfy that relates the fluorescence of a rotor probe to the free volume of the matrix, and the revised definition of the total free volume quantity proposed by Lipson and White (the Locally Correlated Lattice model-based free volume) that includes both the vibrational free volume and the excess free volume. Atomistic molecular dynamics simulation supports that the timescale of the vibrational motion of PS monomers is comparable to the intrinsic timescale of a rotor’s internal rotation known in the literature (on the order of picoseconds), and thus the rotation of a rotor is controlled by the friction from the hard-core volume (not by the friction from the vibrational volume) of the monomers. Measurements on FCVJ-loaded polystyrene–poly(ethylene glycol) (PS–PEG) micelles in water suggest that the PS core domain of the PS–PEG micelle has a broad distribution of glass transition temperatures; a quantitative analysis based on the free volume model enabled to estimate the actual range of Tg. These results allow us to conclude that fluorescent molecular rotors with a tendency toward rapid non-radiative (rotational) relaxation and bulky rotating subgroups (such as FCVJ) exhibit a strong coupling between the fluorescence of the rotor and the free volume of the matrix and are thus useful probes for Tg measurements.

Characterization of dynamics in complex lyophilized formulations: II. Analysis of density variations in terms of glass dynamics and comparisons with global mobility, fast dynamics, and Positron Annihilation Lifetime Spectroscopy (PALS)

Our work proves that positron annihilation spectroscopy is an excellent tool to follow the structural changes in chemical species. We present four examples to support the above statement. The sizes of defects in electrodeposited chromium layers were studied and estimated on the basis of positron lifetime spectra decomposed into two components. Vacancies, di-vacancies and vacancy-clusters could be identified in the electrodeposites. The changes of the size distribution of the free volume units in poly(methylmetacrylate) on the dependence of molecular weight and dispersity were described by the correlation between the lifetime ofortho-Ps and the free volume units in polymers. It was found that the free volume is affected by both the molecular weight and dispersity. The effect of dispersity was explained by the sample preparation technique, namely by the application of high pressure. The ortho-para conversion ofortho-Ps was used to follow the partial spin-crossover in [Fe(1-ethyl-1H-tetrazole)6](BF4)2. The spin-crossover temperature was identified to be 105 K. A conformal structural transformation was found in [Zn(1-propyl-1H-tetrazole)6](BF4)2 between 170 and 90 K by positronium lifetime measurement and the role of (BF4)2− anion, in this transformation, was proved by19F NMR spectroscopy.

The effects of hydrogen on viscoelastic relaxation in Zr–Ti–Ni–Cu–Be bulk metallic glasses: implications for hydrogen embrittlement

Free volume evolution in polyoxymethylene studied by positron annihilation spectroscopy

The application of plasma polymerized silicon-based coatings on plastic substrates is an effective way to adjust the permeability of the substrate. However, the permeation mechanisms are yet not fully understood. Here, the activation energy of permeation can offer valuable insights. In order to understand how the activation energy of permeation depends on the coating structure, five silicon-based coatings with varying oxygen content were analyzed, which led to property modifications ranging from silicon-oxidic to silicon-organic. Positron annihilation spectroscopy was employed to characterize the free volume and quartz crystal microbalance measurements were used to determine the density of the coating. These results were compared to water vapor permeation measurements with a temperature variation in the range of 15°C to 50°C. As expected, the silicon-organic coatings do not significantly impact the permeation rates, while the silicon-oxidic coatings do exhibit a barrier effect. The density of the coatings increases for the more silicon-oxidic coatings. A coating with an unusually high oxygen to precursor ratio forms the exception in both permeation and density. The free volume appears to increase for the more silicon-organic coatings. The pore wall chemistry is also affected, hinting at a structural transition from silicon-organic to silicon-oxidic. With this approach, we aim for an in-depth understanding of the chemical structure of silicon-based thin film coatings and its influence on gas permeation through those coatings.