Lifetime analysis of positronium annihilating in nanometer voids is used to study the thermal expansion behavior of thin, Si-supported polystyrene films near the glass transition temperature T g . A reduction in void volume expansion is correlated with a reduction in the apparent T g as film thickness decreases. Our results can be fitted using a three-layer model incorporating a 50 Å constrained layer at the Si interface and a 20 Å surface region with reduced T g .[S0031-9007(97)02458-7]

Thermal conductivity is an important property for polymers, as it often affects product reliability (for example, electronics packaging), functionality (for example, thermal interface materials) and/or manufacturing cost. However, polymer thermal conductivities primarily fall within a relatively narrow range (0.1-0.5 W m(-1) K(-1)) and are largely unexplored. Here, we show that a blend of two polymers with high miscibility and appropriately chosen linker structure can yield a dense and homogeneously distributed thermal network. A sharp increase in cross-plane thermal conductivity is observed under these conditions, reaching over 1.5 W m(-1) K(-1) in typical spin-cast polymer blend films of nanoscale thickness, which is approximately an order of magnitude larger than that of other amorphous polymers.

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.

Depth-profiled positronium lifetime spectroscopy is used to probe the pore characteristics ͑size, distribution, and interconnectivity͒ in porous, low-dielectric silica films. The technique is sensitive to the entire void volume, both interconnected and isolated, even if the film is buried beneath a metal or oxide layer. Our extension of a simple quantum mechanical model of Ps annihilation in a pore adequately accounts for the temperature and pore size dependence of the Ps lifetime for pore sizes in the range from 0.1 nm to 600 nm. It is applicable to any porous media. ͓S0163-1829͑99͒51932-2͔ Submicron thin films of porous silica and organosilicates are vigorously being developed as low-dielectric, interlayer insulators for use in future high-speed microelectronic devices. 1 Voids are introduced into the film to produce porosity and hence to lower the dielectric constant. Pores must be plentiful to lower the dielectric constant of solid silica from 4 to less than 2, yet they must be small relative to the device element size which is expected to approach 100 nm in the next decade. Important pore characteristics such as average size, size distribution, and degree of interconnectedness are difficult to probe with standard techniques ͑such as gas absorption͒ because of the submicron film thickness, the presence of a thick Si substrate and, in some cases, by the lack of pore interconnectivity ͑i.e., inaccessibility to gas absorption͒. A less standard technique, positronium annihilation lifetime spectroscopy ͑PALS͒, is well known as a bulk probe of subnanometer voids in polymers and insulators and has recently been extended to probe very thin polymer films using keV beams of positrons. 2 The technique looks promising for probing porous films since it is readily applicable to films less than 0.1 m thick, does not rely on any pore interconnectivity/accessibility, and is expected to be sensitive to pore sizes in the 0.3 nm to 100 nm range.In this paper we will explore the capability of PALS to probe the pores in two different types of porous silica films that are spin-cast on Si substrates. The first is a 0.5-m-thick silica-organic composite in which the organic component is removed by thermal decomposition to create pores after the silica component is fully cured and crosslinked. The second is a 0.9-m-thick film, formed using a sol-gel ͑aerogel/ xerogel͒ technique. We determined the film porosities using Rutherford backscattering spectroscopy to be 52% and 77%, respectively. Details on the methodology of depth-profiled PALS has been presented elsewhere. 2 Briefly, a focussed beam of several keV positrons forms positronium ͑Ps, the electron-positron bound state͒ throughout the film thickness. The binding energy of Ps ͑6.8 eV in vacuum͒ is reduced in the solid dielectric and thus Ps tends to localize in the pores.The natural ͑vacuum͒ lifetime of Ps ͑142 ns͒ is reduced by annihilation with molecular electrons during collisions with the pore surface and thus pore size information can be deduced from measuring this lifetime, ͑...

An existing model that relates the annihilation lifetime of positronium trapped in subnanometer pores to the average size of the pores is extended to account for positronium in any size pore and at any temperature. This extension enables the use of positronium annihilation lifetime spectroscopy in characterizing nanoporous and mesoporous materials, in particular thin insulating films where the introduction of porosity is crucial to achieving a low dielectric constant, K. Detailed results of the model calculations are presented along with extensive experimental results to systematically check the lifetime vs pore size calibration in a variety of low-K materials over a wide range of pore sizes.

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.

There are several compounds for which there exists a disconnect between porosity as predicted by crystallography and porosity measured by gas sorption analysis. In this paper, the Zn-based analogue of Cu 3 (btc) 2 (HKUST-1), Zn 3 (btc) 2 (Zn-HKUST-1; btc = 1,3,5-benzenetricarboxylate) is investigated. Conventional analysis of Zn-HKUST-1 by powder X-ray diffraction and gas sorption indicates retention of crystalline structure but negligible nitrogen uptake at 77 K. By using positron annihilation lifetime spectroscopy, a densified surface layer preventing the entry of even small molecular species into the crystal framework is revealed. The material is shown to have inherent surface instability after solvent removal, rendering it impermeable to molecular guests irrespective of handling and processing methods. This previously unobserved surface instability may provide insight into the failure of other microporous coordination polymers to exhibit significant porosity despite crystal structures indicative of regular, interconnected, microporous networks.

Polymer-based membranes play a key role in several industrially important gas separation technologies, e.g., removing CO 2 from natural gas, with enormous economic and environmental impact. Here, we develop a novel hybrid membrane construct comprised entirely of nanoparticles grafted with polymers. These membranes are shown to have broadly tunable separation performance through variations in graft density and chain length. Computer simulations show that the optimal NP packing forces the grafted polymer layer to distort, yielding regions of measurably lower polymer density. Multiple experimental probes confirm that these materials have the predicted increase in "polymer free volume", which explains their improved separation performance. These polymer-grafted NP materials thus represent a new template for rationally designing membranes with desirable separation abilities coupled with improved aging characteristics in the glassy state and enhanced mechanical behavior.