Effect of different monomer precursors with identical functionality on the properties of the polymer network

The association between thermo-mechanical properties in polymers and functionality of monomer precursors is frequently exploited in the materials science. However, it is not known if there are more variables beyond monomer functionality that have a similar link. Here, by using simulations to generate spatial networks from chemically different monomers with identical functionality we show that such networks have universal graph-theoretical properties as well as a near-universal elastic modulus. The vitrification temperature was found to be universal only up to a certain network density, as measured by the bond conversion. The latter observation is explained by the fact that monomer’s tendency to coil enhances formation of topological holes, which, when accumulated, amount to a percolating cell complex restricting network’s mobility. This higher-order percolation occurs late after gelation and is shown to coincide with the onset of brittleness, as indicated by a sudden increase in the glass transition temperature.

On the effect of the flexibility parameter on thermody­namic properties of acrylonitrile and methylvinylpyridine copolymers

We used thermal analysis to investigate the effects of temperature (20--300 °C) on the behaviour of acrylonitrile and methylvinylpyridine copolymers and determine the temperature ranges in which segmental mobility develops in them, as well as their glass transition temperatures. Data obtained via differential thermal analysis allowed us to compute activation energies for vitrification processes in copolymers of the series specified and to plot vitrification temperature as a function of copolymer composition. We used the method of increments to compute flexibility parameters for acrylonitrile and 2-methyl-5-vinylpyridine and analyse the effects of flexibility parameters on the manifestation of relaxation processes. The data obtained may be used to amend manufacturing parameters when developing chemisorptive filter materials based on the specified copolymer series.

Increasing vitrification temperature improves the cryo-electron microscopy reconstruction

At the initial stage of the cryo-electron microcopy (cryo-EM) samples irradiated by electrons, the cryo-EM samples suffer from a rapid “burst” phase (first 3~4 e−/Å2) of beam induced motion (BIM) which is too fast to be corrected by existing motion correction software, and lowers the quality of the initial frames. Therefore, these least radiation damaged, but ruined frames are commonly excluded or down-weighted during data processing, which reduces the undamaged signals in the reconstruction and decreases the reconstruction resolution by weakening the averaging power. Here, we show that increasing the freezing temperature of cryo-EM samples suppresses the BIM in this phase. The quality of initial frames is partially recovered after BIM correction and is better than that of subsequent frames in certain frames. Incorporating these initial frames into the reconstruction increases the resolution, at an equivalent of ~60% extra data. Moreover, these frames are least radiation damaged, thus preserves the high quality cryo-EM density of radiation sensitive residues. Such density is usually damaged or very weak in the canonical 3D reconstruction. In addition, we found that a different kind of radiation damage neglected previously occurs in the per-frame reconstruction after the exposure of 2.5 e−/Å2. Such radiation damage distorts the density of atoms. The deformation can be avoided by only including the frames from the first 2.5 e−/Å2 into the reconstruction. Overall, the high temperature freezing not only provides extra undamaged signal to the reconstruction, but also increases the resolution of the reconstruction.

Estimation of Firing Temperature and Compositional Variability of Archaeological Pottery by Differential Scanning Calorimetry

ABSTRACTDigital Scanning Calorimetry (DSC), a thermal characterization technique, can be used to rapidly obtain a rough upper estimate of the firing temperature of archaeological pottery as well as an indication of its composition. The technique involves heating a small sample (10–20 mg) of ground ceramic above the vitrification temperature, cooling and reheating. The curves of the two heating cycles are then compared. The validity of the technique was evaluated by a blind test in which 35 tiles fired at different temperatures were analyzed without knowing their firing point, and by analysis of archaeological pottery samples assumed to be local or imported based upon stylistic criteria.