Structure-property relationships from universal signatures of plasticity in disordered solids.

When deformed beyond their elastic limits, crystalline solids flow plastically via particle rearrangements localized around structural defects. Disordered solids also flow, but without obvious structural defects. We link structure to plasticity in disordered solids via a microscopic structural quantity, "softness," designed by machine learning to be maximally predictive of rearrangements. Experimental results and computations enabled us to measure the spatial correlations and strain response of softness, as well as two measures of plasticity: the size of rearrangements and the yield strain. All four quantities maintained remarkable commonality in their values for disordered packings of objects ranging from atoms to grains, spanning seven orders of magnitude in diameter and 13 orders of magnitude in elastic modulus. These commonalities link the spatial correlations and strain response of softness to rearrangement size and yield strain, respectively.

Simultaneous monitoring of electroformation of phospholipid vesicles by quartz crystal microbalance and optical microscopy.

The electroformation of giant vesicles from 1,2-Dimyristoyl-sn-Glycero-3-Phosphocholine (DMPC) was monitored using quartz crystal microbalance with dissipation monitoring (QCM-D) and optical microscopy, simultaneously using a novel sample cell design. A gold-coated QCM crystal was used as one of the electrodes and an Indium-tin-oxide (ITO)-coated glass slide was used as the second electrode for electroformation. Increases in the frequency and decreases in the dissipation were observed immediately upon voltage application between the two electrodes, indicating the loss of lipid from the QCM surface. Concurrently, we observed vesicles on the QCM electrode surface by differential interference contrast (DIC)-optical microscopy. The lipid-coated substrates were measured with AFM at various stages in the electroformation, and a significant change in the morphology of the lipid film was observed. Ellipsometry was used to find the average thickness of lipid film. The QCM data were fitted to a viscoelastic model to determine the viscoelastic properties and time dependence of the film thickness. All methods used to determine film thickness give values in reasonable quantitative agreement. Differences between the methods are consistent with what one might expect due to what is actually measured in the individual techniques. The comparison between mass loss and observed vesicles suggest that the vesicles formed are first localized to the substrate and then slowly released into the solution. By comparing the mass lost from the lipid film, to the total surface area of lipid vesicles observed, it is apparent that only a relatively small fraction of the lipid goes into the production of unilamellar vesicles with sizes detectable with optical microscopy.

Substrate and chain size dependence of near surface dynamics of glassy polymers.

We use nanohole relaxation to study the surface relaxation of films of glassy isotactic poly (methyl methacrylate) (i-PMMA) films. These measurements allow us to obtain the time dependent relaxation function at a number of different sample temperatures for the first 2-3 nm of the free surface in a system often used as a model system for the effect of the substrate on thin film dynamics. The surface is observed to relax at temperatures up to 42 K below the bulk Tg value, even on systems where the thin film Tg is known to be greater than the bulk value. We are able to determine the range over which the substrate directly affects the free surface relaxation, and determine a surprisingly large (Mw independent) limiting thickness of approximately 180 nm where the free surface relaxation is not affected by the substrate. For thick films (h>200 nm) we find an unexpected linear Mw dependence of the near surface relaxation time.

Measuring the surface dynamics of glassy polymers.

The motion of polymer chain segments cooled below the glass transition temperature slows markedly; with sufficient cooling, segmental motion becomes completely arrested. There is debate as to whether the chain segments near the free surface, or in thin films, are affected in the same way as the bulk material. By partially embedding and then removing gold nanospheres, we produced a high surface coverage of well-defined nanodeformations on a polystyrene surface; to probe the surface dynamics, we measured the time-dependent relaxation of these surface deformations as a function of temperature from 277 to 369 kelvin. Surface relaxation was observed at all temperatures, providing strong direct evidence for enhanced surface mobility relative to the bulk. The deviation from bulk alpha relaxation became more pronounced as the temperature was decreased below the bulk glass transition temperature. The temperature dependence of the relaxation time was much weaker than that of the bulk alpha relaxation of polystyrene, and the process exhibited no discernible temperature dependence between 277 and 307 kelvin.

Qualitative discrepancy between different measures of dynamics in thin polymer films.

We have used ellipsometry to measure the initial stages of interface healing in bilayer polystyrene films. We also used ellipsometry to measure the glass transition temperature T(g) of the same or identically prepared samples. The results indicate that as the film thickness is decreased, the time constant for the interface healing process increases, while at the same time the measured glass transition temperature in the same samples decreases as the film thickness is decreased. This qualitative difference in the behavior indicates that it is not always possible to make inferences about one probe of polymer dynamics from measurements of another. We propose a reason for this discrepancy based on a previously discussed origin for reduction in the T(g) value of thin films.