Search results for "Chemical physics"

1977

Mechanical And Thermophysical Properties Of Polymers And Chemical Structure: From Computer Simulations To Experimental Data

1988

Chemical structure is related to properties on the basis of results of computer modelling of mechanical behavior. An equation for the shift factor aT,c dependent on temperature T and concentration c is provided, connection to the chain relaxation capability noted, and potential application range of the approach discussed. Constituents of free volume are defined, and effects of individual constituents on mechanical properties.

Re-entrant glass transition in a colloid-polymer mixture with depletion attractions.

2002

Performing light scattering experiments we show that introducing short-ranged attraction to a colloidal suspension of nearly hard spheres by addition of free polymer produces new glass transition phenomena. We observe a dramatic acceleration of the density fluctuations amounting to the melting of a colloidal glass. Increasing the strength of the attractions the system freezes into another nonergodic state sharing some qualitative features with gel states occurring at lower colloid packing fractions. This reentrant glass transition is in qualitative agreement with recent theoretical predictions.

Controlling the wetting properties of the Asakura-Oosawa model and applications to spherical confinement.

2012

We demonstrate for the Asakura-Oosawa model and an extension of this model that uses continuous rather than hard potentials, how wetting properties at walls can be easily controlled. By increasing the interaction range of the repulsive wall potential acting on the colloids (while keeping the polymer-wall interactions constant) polymers begin to substitute colloids at walls and the system can be driven from complete wetting of colloids via partial wetting to complete wetting of polymers. As an application, we discuss the morphology and wetting behavior of colloid-polymer mixtures in spherical confinement. We apply the recently developed 'ensemble switch method' where the Hamiltonian is exten…

2021

Water-in-salt systems, i.e., super-concentrated aqueous electrolytes, such as lithium bis(trifluoromethanesulfonyl)imide (21 mol/kgwater), have been recently discovered to exhibit unexpectedly large electrochemical windows and high lithium transference numbers, thus paving the way to safe and sustainable charge storage devices. The peculiar transport features in these electrolytes are influenced by their intrinsically nanoseparated morphology, stemming from the anion hydrophobic nature and manifesting as nanosegregation between anions and water domains. The underlying mechanism behind this structure-dynamics correlation is, however, still a matter of strong debate. Here, we enhance the apol…

Dynamics-based assessment of nanoscopic polymer-network mesh structures and their defects.

2018

Polymer-network gels often exhibit complex nanoscopic architectures. First, the polymer-network mesh topology on scales of 1–10 nm is usually not uniform and regular, but disordered and irregular. Second, on top of that, many swollen polymer networks display spatial inhomogeneity of their polymer segmental density and crosslinking density on scales of 10–100 nm. This multi-scale structural complexity affects the permeability, mechanical strength, and optical clarity of the polymer gels, which is of central relevance for their performance in popular applications. As a result, there is a need to characterize the polymer network structures on multiple scales. On the scale of the spatial inhomo…

Mesoscopic gel at low agarose concentration in water: a dynamic light scattering study

1995

Previous work in our laboratory has shown that at very low agarose concentration in water gelation still occurs within mutually disconnected, high concentration regions generated by spinodal demixing. The freely diffusing particles obtained in these conditions are studied in the present work by depolarized dynamic light scattering and probe diffusion experiments. These particles are found to behave as large (in fact, mesoscopic) polymer fibers entangled in a continuously rearranged mesh with scaling parameters typical of partially flexible, neutral chains. The present results allow specifying the notion of mesoscopic gelation. They also reveal that the same symmetry-breaking mechanism that …

Origin of the low-frequency plateau and the light-scattering slow mode in semidilute poly(ethylene glycol) solutions

2019

A low-frequency plateau is often found in the rheological spectra of various kinds of semidilute solutions of polymers and other colloids; also, many such solutions have been reported to show slow-modes in their dynamic light scattering autocorrelation functions. Both these observations may lead to the hypothesis of weak associative network structures built by the dissolved polymer chains or colloidal building blocks. To challenge this hypothesis, we conduct a series of comparative studies on semidilute solutions of poly(ethylene glycol) by using classical rheology as well as passive microrheology based on dynamic light scattering, along with structural studies using static light scattering…

Orientation and Dynamics of Peptides in Membranes Calculated from 2H-NMR Data

2009

Solid-state (2)H-NMR is routinely used to determine the alignment of membrane-bound peptides. Here we demonstrate that it can also provide a quantitative measure of the fluctuations around the distinct molecular axes. Using several dynamic models with increasing complexity, we reanalyzed published (2)H-NMR data on two representative alpha-helical peptides: 1), the amphiphilic antimicrobial peptide PGLa, which permeabilizes membranes by going from a monomeric surface-bound to a dimeric tilted state and finally inserting as an oligomeric pore; and 2), the hydrophobic WALP23, which is a typical transmembrane segment, although previous analysis had yielded helix tilt angles much smaller than ex…

Ordering, phase behavior, and correlations of semiflexible polymers in confinement.

2021

Semiflexible polymers are ubiquitous in biological systems, e.g., as building blocks of the cytoskeleton, and they also play an important role in various materials due to their ability to form liquid-crystalline order. These rigid macromolecules are characterized by numerous (hierarchical) length-scales that define their static and dynamic properties. Confinement can promote uniform order, e.g., through capillary nematization in narrow slits, but it can also introduce long-ranged disruptions of the nematic ordering field through (unavoidable) topological defects in spherical containers. This Perspective concentrates on the theoretical description and computational modeling of such confined …