Positron annihilation lifetime spectroscopy, which is applied to measure distributions of hole volume sizes in polymers, uses a model to relate the positronium (Ps) lifetime to hole volume dimensions. It is assumed in this model that the Ps remains in one site until annihilation. Recently, the validity of this last assumption has been questioned by Yu et al.

The thermally induced cross-linking of nadic-end-capped polyimides has been studied by a variety of spectroscopic techniques. Two nadic-end-capped polyimides were subjected to 371 °C heat treatment for 100 h in order to study cross-linking resulting from elevated temperature. The glass transition temperature of the treated samples increased to greater than 400 °C, indicating that both polyimides underwent extensive chemical changes during heat treatment. 13C spin counts were measured before and after heat treatment by 13C cross-polarization magic angle spinning nuclear magnetic resonance. Electron spin resonance measurements showed an increase in the concentration of stable free radicals in the treated samples. Dynamic nuclear polarization and electron spin resonance results show that two species of free radicals are present in the treated samples; the first is located on the polymer backbone and the second on the cross-linking network that arises from the reaction of the nadic end-capping groups. On the basis of these results, new cross-linking structures of nadic-end-capped polyimides before and after heat treatment at 371 °C are proposed.

Rotational-echo double-resonance (REDOR) 13C NMR spectra (with 19F dephasing) have been obtained of 6-fluorotryptophan complexed by a polymeric amphiphilic nanosphere consisting of a polystyrene core covalently attached to a poly(acrylic acid)-polyacrylamide shell. The REDOR spectra show that aromatic carbons from the polystyrene core and oxygenated carbons in the poly(acrylic acid)-polyacrylamide shell are both proximate to the 19F of 6-fluorotryptophan. Molecular modeling restrained by distances inferred from the REDOR spectra suggests that all of the 6-fluorotryptophans are in the shell but within 10 A of the core-shell interface.

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