Polymer properties, such as their mechanical strength, barrier properties, and dielectric response, can be dramatically improved by the addition of nanoparticles. This improvement is thought to be because the surface area per unit mass of particles increases with decreasing particle size, R, as 1/R. This favorable effect has to be reconciled with the expectation that at small enough R the nanoparticles must behave akin to a solvent and cause a deterioration of properties. How does this transition in behavior from large solutes to the solvent limit occur? We conjecture that for small enough particles the layer of polymer affected by the particles (“bound” polymer layer) must be much smaller than that for large particles: the favorable effect of increasing particle surface area can thus be overcome and lead to the small solvent limit with unfavorable mechanical properties, for example. To substantiate this picture requires that we measure and compare the “bound polymer layer” formed on nanoparticles with those near large particles with equivalent chemistry. We have implemented a novel strategy to obtain uniform nanoparticle dispersion in polymers, a problem for many previous works. Then, by combining theory and a suite of experimental techniques, including differential scanning calorimetry and positron annihilation lifetime spectroscopy, we show that the immobilized poly(2-vinylpyridine) layer near 15 nm diameter silica particles (∼1 nm) is considerably thinner than that at flat silica surfaces (∼4 to 5 nm), which is the limit of an infinitely large particle. We have also determined that the changes in the polymer’s glass-transition temperature due to the presence of this strongly interacting surface are very small in both well-dispersed nanocomposites and thin films (<100 nm). Similarly, the polymer’s fragility, as determined by dielectric spectroscopy, is also found to be little affected in the nanocomposites relative to the pure polymer. While a systematic study of the dependence of the bound polymer layer thickness on particle size remains an outstanding challenge, this first study provides conclusive evidence for the hypothesis that the bound polymer layer can be significantly smaller around nanoparticles than at chemically similar flat surfaces.

Key Words porous films, pore size, low dielectric constant, positronium ■ Abstract Beam-based positron annihilation spectroscopy (PAS) is a powerful porosimetry technique with broad applicability in the characterization of nanoporous thin films, especially insulators. Pore sizes and distributions in the 0.3-30 nm range are nondestructively determined with only the implantation of low-energy positrons from a table-top beam. Depth-profiling with PAS has proven to be an ideal way to measure the interconnection length of pores, search for depth-dependent inhomogeneities or damage in the pore structure, and explore porosity hidden beneath dense layers or diffusion barriers. The capability of PAS is rapidly maturing as new intense positron beams around the globe spawn more accessible PAS facilities. After a short primer on the physics of positrons in insulators, the various probe techniques of PAS are briefly summarized, followed by a more detailed discussion of the wide range of nanoporous film parameters that PAS can characterize. 51and sensitivity to processing-induced changes. A unique strength associated with beam-based PAS is the capabilities to depth-profile by controlling the positron implantation energy and to resolve laterally by finely focusing the beam on a small spot on the target. The capability to nondestructively detect depth-dependent pore structural characteristics even when the pores are buried under barrier layers will be an increasingly attractive capability as nanoporous films and composites become more complex.

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The long-standing discrepancy [G. S. Adkins, R. N. Fell, and J. Sapirstein, Ann. Phys. (N.Y.) 295, 136 (2002)]] between the theoretical calculations of the orthopositronium (o-Ps) annihilation decay rate (lambda(T)=1/lifetime) and some of the experimental measurements has been resolved. A focused beam of positrons incident on a special nanoporous silica film produces near-thermal energy o-Ps in vacuum that is slow enough to be virtually free of perturbing interactions. The fitted decay rate requires only a 500 ppm correction for nonthermal o-Ps effects. The new value of lambda(T)=7.0404(10)(8) micros(-1) is in excellent agreement with theory.

Over the past decade, nanostructured materials constructed from metal ions/clusters linked by organic groups were demonstrated to have remarkably high porosity and specific surface areas higher than the best activated carbons. [1][2][3][4][5] These microporous coordination polymers (MCPs), are self-assembled, periodic, porous structures that have redefined what is possible with adsorption. [6][7][8][9] Hydrogen storage and CO 2 sequestration are two of the most intensively studied areas with record-setting capacities achieved for several MCPs. [10][11][12] Despite these performance advantages, MCPs suffer from an incomplete understanding of the fundamental mechanisms of adsorption; furthermore problems with structural integrity and poor atmospheric/ temperature stability are compounded by difficulties in characterizing structural damage. Although X-ray diffraction and gas-adsorption techniques have facilitated the development of MCPs, these methods give a structurally averaged picture of the pore structure and are therefore ill-suited to study defects and other nonperiodic phenomena of critical importance for future applications. In particular, their value for monitoring pore structure evolution under conditions directly relevant for sorption applications is limited. Positronium annihilation lifetime spectroscopy (PALS) is an in situ pore-/void-volume characterization technique, [13] in which the shortening of the annihilation lifetime of Ps (Ps ¼ positronium, the hydrogenlike bound state of an electron with its antiparticle, the positron) due to collisions with the pore walls is correlated with the pore size. Ps readily forms by electron capture when positrons are injected into insulators. Moreover, it is energetically favorable for Ps to localize inside voids providing a natural probe from within. The Ps lifetime is correlated with pore size [14][15][16][17] and the relative intensity of this component is related to the porosity of the material. In this first application of PALS to MCPs we demonstrate its unique ability to uncover new phenomena in these materials with nanoscale porosity.MOF-5 [18] (MOF ¼ metal-organic framework) is one of the earliest examples of an exceptionally high surface area MCP and the most thoroughly investigated representative of this class. Therefore, it is an ideal benchmark to test the suitability of PALS and presents an extremely challenging system, in which to reveal new phenomena. The material is an open cubic structure that consists of face-sharing cubic cages that extend in all three dimensions (see Fig. S1 in the Supporting Information). A high-quality sample ($1 cm 3 ) of MOF-5 crystals ($0.5-mm cubic crystals) with a Brunauer-Emmett-Teller theory (BET) surface area of 3500 m 2 g À1 was prepared for the PALS experiments. Ps, formed via electron capture by a positron in this MCP framework, is energetically driven into the pores. Fitting of the PALS time spectrum (a typical spectrum is Fig. S2 in the Supporting Information) reveals, surprisingly in the light of the supposedly u...

The formation and subsequent thermalization of positronium ͑Ps͒ produced at a few eV in gases are investigated using time-resolved Doppler-broadening measurements of the annihilation photons. A static magnetic field quenches the Ps enabling Doppler energy measurements from 25 to 70 ns after the Ps is formed. Varying the gas density permits a significant range of the thermalization process to be observed. Seven different gases are studied, He, Ne, Ar, H 2 , N 2 , isobutane, and neopentane. A classical elastic scattering model fits all the gas data reasonably well. For each gas, an elastic scattering cross section and an average Ps formation energy are determined from the classical model fit. When comparisons can be made, these cross sections are often significantly smaller than most quantum-mechanical-theory predictions and most previous experimental results obtained using the angular correlation technique. Various systematic tests have been applied to the apparatus and the analysis, reinforcing the discrepancy with previous works.

Positronium (Ps) is shown to exist in a delocalized state in self-assembled metalorganic crystals that have large 1.3-1.5 nm cell sizes. Belonging to a class of materials with record high accessible specific surface areas, these highly porous crystals are the first to allow direct probing with simple annihilation lifetime techniques of the transport properties of long-lived triplet Ps in what is hypothesized to be a Bloch state. Delocalized Ps has unprecedented (high) Ps mobility driven primarily by weak phonon scattering with unusual and profound consequences on how Ps probes the lattice.

We report precision measurements of the excited state lifetime of the 5p 2 P 1/2 and 5p 2 P 3/2 levels of a single trapped Cd + ion. The ion is excited with picosecond laser pulses from a mode-locked laser and the distribution of arrival times of spontaneously emitted photons is recorded. The resulting lifetimes are 3.148 ± 0.011 ns and 2.647 ± 0.010 ns for 2 P 1/2 and 2 P 3/2 respectively. With a total uncertainty of under 0.4%, these are among the most precise measurements of any atomic state lifetimes to date. Here we report excited state lifetime measurements using a time-correlated single photon counting technique on a single atom. This method, designed especially to eliminate common systematic errors, involves selective excitation of a single trapped ion to a particular excited state (lifetime of order nanoseconds) by an ultrafast laser pulse (duration of order picoseconds). Arrival of the spontaneously-emitted photon from the ion is correlated in time with the excitation pulse, and the excited state lifetime is extracted from the distribution of time delays from many such events.By performing the experiment on a single trapped ion, we are able to eliminate prevalent systematic errors, such as: pulse pileup that causes multiple photons to be collected within the time resolution of the detector, radiation trapping or the absorption and re-emission of radiation by neighboring atoms, atoms disappearing from view before decaying, and subradiance or superradiance arising from coherent interactions with nearby atoms. By using ultrafast laser pulses, we eliminate potential effects from applied light during the measurement interval.With this setup, at most one photon can be emitted following an excitation pulse. While this feature is instrumental in eliminating the above systematic errors, it would appear that this signal would require large integration times for reasonable statistical uncertainties. However, with a lifetime of only a few nanoseconds, millions FIG. 1: The experimental apparatus. (a) A picosecond mode locked Ti:Saph laser is tuned to four times the resonant wavelength for either the 5p 2 P 1/2 or the 5p 2 P 3/2 level of Cd + . Each pulse is then frequency-quadrupled through non-linear crystals, filtered from the fundamental and second harmonic, and directed to the ion. An amplified cw diode laser is also frequency quadrupled and tuned just red of the 2 P 3/2 transition for Doppler cooling of the ion within the trap. Acoustooptic modulators (AOM) are used to switch on and off the lasers as described in the text. Photons emitted from the ion are collected by an f /2.1 imaging lens and directed toward a photon-counting photo multiplier tube (PMT). The output of the PMT provides the start pulse for the time to digital converter (TDC), whereas the stop pulse is provided by the reference clock of the mode-locked laser. of such excitations can be performed each second, thus potentially allowing sufficient data for a statistical error of under 0.1% to be collected in a matter of minutes [6].A diagram of th...