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.
As-made and calcined forms of large-pore (5.3 nm) mesostructured silica with a wormhole framework structure, denoted MSU-J, have been used to form rubbery epoxy mesocomposites containing 1.0-12% (w/w) silica. The tensile modulus, strength, toughness, and extension-at-break for the mesocomposites formed from as-made and calcined forms of MSU-J silica are systematically reinforced by up to 4.8, 5.7, 1.6, and 8.5 times, respectively, in comparison to the pure epoxy polymer. The composites represent the first examples wherein the reinforcement benefits provided by mesostructured silica particles are comparable to those provided by exfoliated organoclay nanolayers at equivalent loadings. Moreover, the reinforcement benefits are realized without the need for organic modification of the silica surface, and the increases in tensile properties occur with little or no sacrifice in optical transparency or thermal stability. The oxygen permeability of the mesocomposites prepared from as-made MSU-J silica increases dramatically at loadings g5.0% (w/w), whereas the compositions made from the calcined form of the mesostructure show no permeation dependence on silica loading. For instance, the oxygen permeability of the mesocomposites containing 12% (w/w) as-made MSU-J silica is 6-fold higher than that of the silica-free epoxy membrane. Positron annihilation lifetime spectroscopy established the absence of free volume in the mesocomposites, thus precluding the possibility of facile oxygen diffusion through the framework pores of the silica. The increase in oxygen permeability is correlated with the partitioning of curing agent between the as-made mesostructure and the liquid prepolymer, which leads to coronas of permeable polymer with reduced chain cross-linking in the vicinity of the silica particles. Mesocomposites made from calcined forms of the mesostructured silica do not allow for curing agent partitioning, and the oxygen permeability is not significantly influenced by the silica loading.
High concentration and stable few-layer graphene dispersions prepared by the exfoliation of graphite in different organic solvents
Relaxation and chain dynamics of polymer nanostructures after release of spatial confinement were studied using line gratings as small as 33 nm on polystyrene surface fabricated by nanoimprint lithography. The temperature for "slumping";rapid line height relaxation;decreased as the line width diminished for all molecular masses (MWs), but a simple explanation based on enhanced surface mobility fails to explain the results. When MW was low and the structure was large, the line height monitored with an AFM reduced as surface tension overcame the polymer viscosity. Interesting and complex behaviors were observed when the radius of gyration (R g ) of the polymer molecules was not small compared with the dimension of the nanostructure. Careful examination of the surface viscosity shows that confinement of polymer chains into space comparable to or even smaller than its R g appears to enhance relaxation, which is the major factor for the surprisingly low temperature at which nanostructures of high MW slumps.
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