Solid state 2H-NMR studies of segmental dynamics in polymer blends

Recent two-dimensional (2D) 2H-NMR studies on nearly ideal mixtures are reviewed. The use of selective deuterium labeling allows the unambiguous observation of the individual segmental dynamics of each component in the blend. 2D exchange spectra provide both the mean motional rates and the motional distributions, which was necessary to discriminate among the disparate explanations of blend behavior. The components exhibit very different mean mobilities and broad mobility distributions near the glass transition. The broad macroscopic glass transition in the blends is attributed to both these two kinds of dynamic heterogeneities. The individual mean motional rates are used to define distinct glass transition temperatures, T(g,i) for each species, i. The separation between the T(g,i),s of the two species increases with the content of the high-T(g) component. The widths of their individual motional distributions also increase with the content of the high-T(g) component. These two effects combine to produce the increase in both the anomalously broad glass transition and the thermorheological complexity of these blends with fraction of the high-T(g) component.     

Single polymer dynamics in an elongational flow

The stretching of individual polymers in a spatially homogeneous velocity gradient was observed through use of fluorescently labeled DNA molecules. The probability distribution of molecular extension was determined as a function of time and strain rate. Although some molecules reached steady state, the average extension did not, even after a approximately 300-fold distortion of the underlying fluid element. At the highest strain rates, distinct conformational shapes with differing dynamics were observed. There was considerable variation in the onset of stretching, and chains with a dumbbell shape stretched more rapidly than folded ones. As the strain rate was increased, chains did not deform with the fluid element. The steady-state extension can be described by a model consisting of two beads connected by a spring representing the entropic elasticity of a worm-like chain, but the average dynamics cannot.     

Slow cellular dynamics in MDCK and R5 cells monitored by time-lapse atomic force microscopy

We have examined dynamic events that occur on a time scale of minutes in an epithelial monolayer of Madine-Darby Canine Kidney (MDCK) cells and in ras-transformed MDCK cells by atomic force microscopy (AFM). Cells were imaged under physiological conditions, and time-lapse movies representing approximately 60 s real time per frame were assembled. In normal MDCK cells, two types of protrusions in the apical plasma membrane exhibit dynamic behavior. First, smooth bulges formed transiently over the time scale of minutes to tens of minutes. Second, spike-like protrusions appear initially as bulges, extend well above the apical surface and, finally, seem to detach. R5, an oncogenic transformant derived from MDCK cells, grows very flat on glass. During AFM imaging, these cells sometimes round up and detach from the substrate. In light microscopic observations of parallel preparations, cells rarely detach, suggesting that this is an active response of these cells to irritation by the AFM tip. R5 cells often extend processes that are supported by actin stress fibers. During imaging with the AFM, these processes withdraw at a rate of 1-5 microns/min, similar to that observed by light microscopy. During the withdrawal, movement of the stress fibers can be clearly seen. In the flat periphery of these cells, the transport of intracellular particles along cytoskeletal elements was seen. In addition, we have observed two types of wave-like movements through the cell, which appear to be an organized rearrangement of cytoplasm. One type of wave moves radially out from center of the cell while the other moves circularly along the cell periphery.     

The intergranular segregation of boron in Ni3Al: Equilibrium segregation and segregation kinetics

The grain boundary B content of high-purity Ni-24 at.% Al alloys containing 0.048, 0.144, 0.240 and 0.480 at.% B (100, 300, 500, 1000 ppm mass) has been determined for samples aged from 1323 to 873 K for sufficient times to attain equilibrium. The B content was derived from Auger electron spectra of the intergranular fracture facets. Many facets were exposed during fracture at ≈ 300 K, and additional facets were formed upon fracturing following hydrogen charging after heat treatment. For each alloy sample, about 25 facets were analyzed. The grain boundary B contents were in the range of 0.5–2.5 at.%. The grain boundary B content increased with decreasing temperature and with increasing bulk B content in the alloys. The energy of binding of a B atom to the grain boundary was calculated using McLean's segregation theory and assuming a unique binding energy for each alloy. The values were in the range of 0.15–0.45 eV/atom, and increased with increasing temperature and with decreasing bulk B content. These results have been rationalized in terms of a spectrum of binding energies for a given alloy. However, when the entropy of adsorption was taken into account, an enthalpy of adsorption of B to the grain boundary of 0.13 eV/atom was obtained, independent of temperatire and bulk B content. This is interpreted to mean that the spectrum of binding energies is quite restricted. The grain boundary B content of these alloys has also been measured as a function of annealing time at 773, 873, 973 and 1173 K. The diffusion coefficient of B in Ni₃Al at 773 K is about 5 × 10⁻²¹ m²/s, and the equilibrium grain boundary B content is attained at about 3000 s. The diffusion coefficient at 973 K is between 10⁻¹⁶ and 10⁻¹⁷ m²/s. The activation energy for diffusion of B in Ni₃Al is between 200,000 and 300,000 J/mol.