Influence of Chain Rigidity and Dielectric Constant on the Glass Transition Temperature in Polymerized Ionic Liquids.

Polymerized ionic liquids (PolyILs) are promising candidates for a wide range of technological applications due to their single ion conductivity and good mechanical properties. Tuning the glass transition temperature (Tg) in these materials constitutes a major strategy to improve room temperature conductivity while controlling their mechanical properties. In this work, we show experimental and simulation results demonstrating that in these materials Tg does not follow a universal scaling behavior with the volume of the structural units Vm (including monomer and counterion). Instead, Tg is significantly influenced by the chain flexibility and polymer dielectric constant. We propose a simplified empirical model that includes the electrostatic interactions and chain flexibility to describe Tg in PolyILs. Our model enables design of new functional PolyILs with the desired Tg.

Role of quantum fluctuations in structural dynamics of liquids of light molecules.

A possible role of quantum effects, such as tunneling and zero-point energy, in the structural dynamics of supercooled liquids is studied by dielectric spectroscopy. The presented results demonstrate that the liquids, bulk 3-methyl pentane and confined normal and deuterated water, have low glass transition temperature and unusually low for their class of materials steepness of the temperature dependence of structural relaxation (fragility). Although we do not find any signs of tunneling in the structural relaxation of these liquids, their unusually low fragility can be well described by the influence of the quantum fluctuations. Confined water presents an especially interesting case in comparison to the earlier data on bulk low-density amorphous and vapor deposited water. Confined water exhibits a much weaker isotope effect than bulk water, although the effect is still significant. We show that it can be ascribed to the change of the energy barrier for relaxation due to a decrease in the zero-point energy upon D/H substitution. The observed difference in the behavior of confined and bulk water demonstrates high sensitivity of quantum effects to the barrier heights and structure of water. Moreover, these results demonstrate that extrapolation of confined water properties to the bulk water behavior is questionable.

Why many polymers are so fragile: A new perspective.

Many polymers exhibit much steeper temperature dependence of their structural relaxation time (higher fragility) than liquids of small molecules, and the mechanism of this unusually high fragility in polymers remains a puzzle. To reveal additional hints for understanding the underlying mechanism, we analyzed correlation of many properties of polymers to their fragility on example of model polymer polystyrene with various molecular weights (MWs). We demonstrate that these correlations work for short chains (oligomers), but fail progressively with increase in MW. Our surprising discovery is that the steepness of the temperature dependence (fragility) of the viscosity that is determined by chain relaxation follows the correlations at all molecular weights. These results suggest that the molecular level relaxation still follows the behavior usual for small molecules even in polymers, and its fragility (chain fragility) falls in the range usual for molecular liquids. It is the segmental relaxation that has this unusually high fragility. We speculate that many polymers cannot reach an ergodic state on the time scale of segmental dynamics due to chain connectivity and rigidity. This leads to sharper decrease in accessible configurational entropy upon cooling and results in steeper temperature dependence of segmental relaxation. The proposed scenario provides a new important insight into the specifics of polymer dynamics: the role of ergodicity time and length scale. At the end, we suggest that a similar scenario can be applicable also to other molecular systems with slow intra-molecular degrees of freedom and to chemically complex systems where the time scale of chemical fluctuations can be longer than the time scale of structural relaxation.

Dynamics of biological macromolecules: not a simple slaving by hydration water.

We studied the dynamics of hydrated tRNA using neutron and dielectric spectroscopy techniques. A comparison of our results with earlier data reveals that the dynamics of hydrated tRNA is slower and varies more strongly with temperature than the dynamics of hydrated proteins. At the same time, tRNA appears to have faster dynamics than DNA. We demonstrate that a similar difference appears in the dynamics of hydration water for these biomolecules. The results and analysis contradict the traditional view of slaved dynamics, which assumes that the dynamics of biological macromolecules just follows the dynamics of hydration water. Our results demonstrate that the dynamics of biological macromolecules and their hydration water depends strongly on the chemical and three-dimensional structures of the biomolecules. We conclude that the whole concept of slaving dynamics should be reconsidered, and that the mutual influence of biomolecules and their hydration water must be taken into account.

A broad glass transition in hydrated proteins.

We performed Raman and Brillouin scattering measurements to estimate glass transition temperature, T(g), of hydrated protein. The measurements reveal very broad glass transition in hydrated lysozyme with approximate T(g) approximately 180+/-15 K. This result agrees with a broad range of T(g) approximately 160-200 K reported in literature for hydrated globular proteins and stresses the difference between behavior of hydrated biomolecules and simple glass-forming systems. Moreover, the main structural relaxation of the hydrated protein system that freezes at T(g) approximately 180 K remains unknown. We emphasize the difference between the "dynamic transition", known as a sharp rise in mean-squared atomic displacement at temperatures around T(D) approximately 200-230 K, and the glass transition. They have different physical origin and should not be confused.

Influence of pressure on quasielastic scattering in glasses: relationship to the boson peak.

We report unexpectedly strong variations in the quasielastic scattering (QES) intensity in glasses under pressure. Analysis of the data reveals strong correlations between pressure-induced changes in the QES intensity and the intensity of the boson peak. This observation emphasizes a direct relationship between these two components of the fast dynamics. In addition, we observe changes of the QES spectral shape that can be interpreted as pressure-induced variations in the underlying energy landscape.

How rigid are viruses.

Viruses have traditionally been studied as pathogens, but in recent years they have been adapted for applications ranging from drug delivery and gene therapy to nanotechnology, photonics, and electronics. Although the structures of many viruses are known, most of their biophysical properties remain largely unexplored. Using Brillouin light scattering, we analyzed the mechanical rigidity, intervirion coupling, and vibrational eigenmodes of Wiseana iridovirus (WIV). We identified phonon modes propagating through the viral assemblies as well as the localized vibrational eigenmode of individual viruses. The measurements indicate a Young's modulus of approximately 7 GPa for single virus particles and their assemblies, surprisingly high for "soft" materials. Mechanical modeling confirms that the DNA core dominates the WIV rigidity. The results also indicate a peculiar mechanical coupling during self-assembly of WIV particles.

Protein and solvent dynamics: how strongly are they coupled?

Analysis of Raman and neutron scattering spectra of lysozyme demonstrates that the protein dynamics follow the dynamics of the solvents glycerol and trehalose over the entire temperature range measured 100-350 K. The protein's fast conformational fluctuations and low-frequency vibrations and their temperature variations are very sensitive to behavior of the solvents. Our results give insight into previous counterintuitive observations that protein relaxation is stronger in solid trehalose than in liquid glycerol. They also provide insight into the effectiveness of glycerol as a biological cryopreservant.

Slow relaxation process in DNA.

A dynamic transition at temperatures ∼200-230K is observed in manyhydrated bio-polymers. It shows up as a sharp increase of the mean-squaredatomic displacements above this temperature range. We present neutronscattering data of DNA at different levels of hydration. The analysis showsthat the dynamic transition in DNA is related to a slow relaxation processin the MHz-GHz frequency range. This slow relaxation process iscompletely suppressed in the dry DNA sample where no dynamic transitionwas observed. The nature of the slow process is discussed. We ascribe it toa global relaxation of DNA molecule that involves cooperative motion ofmany base-pairs and backbone.

Slow relaxation process in DNA at different levels of hydration.

There are many speculations about the dynamic transition observed in hydrated bio-polymers at temperatures T ∼ 200 - 230 K being an important factor for enabling of their functions. The transition shows up as a sharp increase of atomic mean-squared displacements above this temperature. The nature of the dynamic transition is not yet clear. Using inelastic neutron scattering we show in this Note that the transition in DNA is related to the appearance of a slow relaxation process. Decrease in the hydration level suppresses the process and the dynamic transition. It is found that, in terms of dynamics, the decrease in water content is similar in effect to a decrease in temperature. The obtained results support the idea that the dynamic transition is mediated by the water of hydration since bulk water has a dynamic transition around the same temperature.