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]

Polyhedral octahydridosilsesquioxanes, [HSiO1.5]8 (1) and [(HSiMe2O)SiO1.5]8 (3) were hydrosilylatively copolymerized with stoichiometric amounts of the octavinylsilsesquioxanes, [vinylSiO1.5]8 (2) and [(vinylSiMe2O)SiO1.5]8 (4) in toluene using platinum divinyltetramethyldisiloxane, “Pt(dvs)”, as catalyst. The degree of condensation of the resultant four copolymers ranges from 43% to 81% depending on intercube chain lengths, as determined by solid state 13C and 29Si MAS NMR analyses, using cross-polarization (CP) techniques. The presence of residual functional groups was confirmed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Polymer porosities were measured using nitrogen sorption, positron annihilation lifetime spectroscopy (PALS), and small angle X-ray scattering (SAXS) methods. The combination of these three techniques allows a relatively complete description of the pore sizes and pore size distributions in these materials. The pores in the cube interiors are ∼0.3 nm in diameter, while those between the cubes range from 1 to 50 nm in diameter (for polymer 3 + 4). Nitrogen sorption analyses give specific surface areas (SSAs) of 380 to 530 m2/g with “observable” pore volumes of 0.19−0.25 mL/g.

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Epoxy resins absorb significant quantities of moisture, typically 1 to 7% by weight for various formulations, which can greatly compromise their physical properties. It is known that polarity of the epoxy is a significant factor in determining the ultimate moisture uptake. However, the contribution from molecular topology still remains vague. In this work, the effects of molecular topology are elucidated by synthesizing novel epoxies where the polarity is maintained constant but the topology is systematically altered. The molecular topology is quantified in part via Positron Annihilation Lifetime Spectroscopy (PALS) in terms of the nanometer‐sized voids, or nanovoids, that are also commensurate with typical interchain distances. The nanovoids are separated into their absolute zero and thermally fluctuating fractions by performing PALS measurements over a wide range of temperatures. A strong correlation is observed between the absolute zero hole volume fraction and the ultimate moisture uptake. Although the correlation is clear, the absolute zero hole volume fraction alone is not sufficient to predict the ultimate moisture uptake, and network polarity must also be considered. It is surmised that the role of the nanovoids is to open the epoxy matrix and alleviate steric hindrances that may prevent a water molecule from associating with a polar group. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3035–3048, 1998

Positron annihilation lifetime spectroscopy (PALS) measurements, in the temperature range 110−480 K, are given for three linear, amorphous polymers. Based on these measurements, a method is proposed for evaluating the hole volume in amorphous thermoplastics. Our studies show that hole volume is composed of static and dynamic components. We demonstrate that the dynamic component, which is a result of the thermal vibrations of the molecular chains, is strongly correlated to thermodynamic volume/density fluctuations. The static hole volume is interpreted as “frozen-in” fluctuations, which are manifested as nanometer-sized flaws in the packing of molecular chains. The results from PALS measurements reported in our work are in very good agreement with results from small-angle X-ray scattering measurements.