Solid-state cpmas 13C-NMR studies of the reaction of an epoxy resin with masked isocyanates

It has been shown that incorporation of masked isocyanates in the MY720/DDS epoxy significantly reduces the equilibrium moisture absorption by blocking of residual functional groups ( oxirane group) by the isocyanates released in the deblocking reaction of the masked isocyanates. We have now used high resolution ¹³C-CP/MAS NMR to follow the reactions in the solid state and to identify intermediates and by-products. The deblocking reaction of the masked isocyanates also releases the corresponding alcohol, part of which may evaporate during the curing reaction. The resolution in the solid-state spectra is good enough to identify all the reactants and the intermediates involved in the curing reaction. Difference spectra are used to emphasize changes between systems that differ in treatment or composition.     

Explicit solutions to the reverse order law (AB)+ = B-mrA-lr

Shinozaki and Sibuya have shown that the Moore-Penrose inverse (AB)⁺ can always be expressed as B^-A^- for generalized inverses A^- and B^- of matrices A and B, respectively. In this paper, explicit solutions B^-_mr and A^-_lr to (AB)⁺ = B^-_mrA^-_lr are given. A class of solutions is obtained which is related to an equation of Greville, and expressions for the general solutions are presented.     

Hypergeometrics and the cost structure of quadtrees

Several characteristic parameters of randomly grown quadtrees of any dimension are analyzed. Additive parameters have expectations whose generating functions are expressible in terms of generalized hypergeometric functions. A complex asymptotic process based on singularity analysis and integral representations akin to Mellin transforms leads to explicit values for various structure constants related to path length, retrieval costs, and storage occupation.     

Delayed fluorescence ODMR as a probe of triplet-triplet fusion probability

A kinetic scheme is presented to rationalize recent observations of delayed fluorescence ODMR in isotopically mixed naphthalene crystals, in which the sign of the signal changes from negative to positive as the critical concentration for triplet-triplet transport among the perprotonated traps is approached. The effects of chopped optical excitation, prolonged microwave pumping, temperature and light intensity are considered. The observation of positive DF-ODMR in the high concentration case is used to obtain a lower limit for the ratio of the annihilation constantk ₂ to the pair dissociation constantk _?1.     

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