Biomedical subjects

Dan E Demco

Publications and source records attributed to Dan E Demco.

3 recordsLinked to original sources

PubMed2006

Segmental mobility in short-chain grafted-PDMS by homo- and heteronuclear residual dipolar couplings.

Different homo- ((1)H-(1)H) and heteronuclear ((1)H-(13)C) multiple quantum solid-state NMR experiments were applied to a series of short poly (dimethylsiloxane) (PDMS) chains grafted onto hydrophilic silica. The existence and clear separation of a strongly dipolar coupled region which is attributed to the silica-PDMS interface and a weakly dipolar coupled region attributed to the mobile chain portions outside the interface is evident in all experiments. In a sample series with different average chain lengths the residual dipolar couplings and with that the average order parameters are constant for the interface part and correlate for the mobile part linearly with the average chain length. Temperature dependent studies using (1)H double quantum experiments indicate the different stages of immobilization of the polymer chains. Solid-state NMR techniques therefore can give detailed information about the microscopic structure of this material and especially the effect of chain anchoring. It is also demonstrated that sample rotation influences the mobility of the mobile component and therefore the analysis of such materials should always be done under static conditions.

Journal Article↗

PubMed2002

Particle motion in gas-fluidized granular systems by pulsed-field gradient nuclear magnetic resonance.

Nuclear magnetic resonance k-space and q-space imaging methods were applied for the first time to monitor the time-averaged density variations and the random motion of granular particles in the presence of gas flow of variable velocity in a model gas-fluidized bed reactor. The transitions toward fluidization, bubbling phase, and slugging phase are visible as sudden changes in the density and the root mean-squared displacement. The onset of these transitions is shown to depend on the particle sizes for a given cell geometry. The effective diffusion coefficients are measured and the distributions of particle displacements, or propagators, are presented.

Journal Article↗

PubMedDate not supplied

Molecular mobility in fixed-bed reactors investigated by multiscale NMR techniques.

The complex problem of a fixed-bed reactor consisting of catalytically active particles provides an exceptional opportunity of combining a wide range of NMR methods which have become available over time as tools to probe porous media. This work demonstrates the feasibility of different NMR techniques for the investigation of the intra- and interparticle pore space over length scales from nanometers up to centimeters. Many industrially relevant cracking reactions leave a coke residue on the inner surface of the porous catalyst particles so that the active sites become inaccessible to the reactants. Moreover, the pore space shrinks due to the formation of coke, thereby hindering molecular transport. The presence of the coke residue and its influence on the mobility of adsorbed fluid molecules are probed by 129Xe spectroscopy, NMR cryoporometry, relaxation dispersion measurements, and investigations of the reduced diffusivity in the intraporous space. The voids surrounding the random arrangement of catalyst pellets represent another pore space of much larger dimensions, the properties of which can be more directly investigated by mapping the fluid density and the velocity distribution from velocity-encoded imaging. Propagator representations averaged over large sample volumes are discussed and compared to velocity images obtained in selected axial slices of the reactor.

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