The interaction of water with the silica surface is explored using *H NMR techniques. The silica is examined at various levels of hydration/dehydration and temperature, using line narrowing by magic-angle spinning, with or without multiple-pulse averaging ofdipolar interactions. Water physisorbed on the silica surface exhibits a freezing point depression of roughly 40 K. Molecular reorientation of physisorbed water at rates in the tens of kilohertz range is observed through interference with multiple-pulse line narrowing. These results are correlated with variabletemperature NMR spectra of pure water. Isotropic

No abstract

4027in more interaction between layers or strings and thus increase the excess viscosity in larger systems.As yet we have not attempted to vary particle number near 500 due to the extensive computer time involved. Xue and Grest18 have reported no change in the string ordered structure in a system similar to ours when the number of particles was changed from 500 to 504. However, their chief concern was whether the cubic simulation box used with 500 particles was responsible for the ordered structure, and they tested this by using a rectangular box of appropriate dimensions with the increase to 504 particles necessary to maintain constant volume fraction. It is likely that the adoption of the rectangular cell nullifies the type of geometric constraints we have discussed by permitting the four added particles to fit 'smoothly" into the packing pattern of the cubic 500-particle system. From our standpoint, a more interesting test would be 504 particles in a slightly larger cubic box. Perhaps an even more interesting test would be 505 particles, which would require either one string of eight, or a completely reordered structure to allow for another string, since 504 particles would undoubtedly be structured as our 500-particle system with exactly seven particles in each string.Finally, what role might hydrodynamic interactions play here? Bossis and Brady" found that layering occurred in strongly interacting systems but was not induced by hydrodynamic interactions alone12 or in more weakly interacting systems" at any shear rate. Thus we might infer that, in systems in which some type of ordering is favored, hydrodynamic interactions may not play a major role in determining system behavior. ConclusionWe have shown that in simulations of sheared colloidal suspensions the number dependence of the results intimately coupled to the geometry of the ordered phases found and to the size of the periodic box holding the particles. Clearly, one has to be very careful in interpreting numerical results from such systems. Although the observed tendency to order is probably real, the type of ordering achieved may not become independent of the system size until the long-range correlations can safely die out within the simulation box. We also note that the strength and range of the repulsive potential must play a major role in determining which type of ordered structure is realized for a given N. As noted earlier,26 significant differences exist between these aqueous simulations with a relatively long range repulsive potential and the nonaqueous simulationsZo with a fairly short range repulsive potential, when the same shear rate and number of particles are used. Clearly in the aqueous suspension, the longer range of the repulsive potential makes the suspension appear more concentrated. This may indicate that even more interesting structures lie ahead at higher volume fractions.Acknowledgment. It is with great pleasure that we dedicate this paper to Marshall Fixman, our good friend, former mentor, and all too infrequent companion on hi...

No abstract

The structures of poly(allyl alcohol) were characterized by one-and two-dimensional gradient-enhanced heteronuclear multiple quantum coherence (gHMQC) and gradient-enhanced heteronuclear multiple-bond connectivity (gHMBC) NMR spectroscopy. Main-chain structures and chain-end structures were identified from the spectra. The polymerization of allyl alcohol can be described by degradative chain transfer to allyl alcohol monomers, which results in distinctive chain ends as well as low molecular weight polymers. After the allylic hydrogen abstraction by initiator radicals, allyl radicals produce chain ends with vinyl groups and aldehyde groups. The chain-end structures were identified by the correlation peaks in gHMQC and gHMBC spectra.