1,669 research outputs found
Influence of pH on mechanical relaxations in high solids lm-pectin preparations
The influence of pH on the mechanical relaxation of LM-pectin in the presence of co-solute has been investigated by means of differential scanning calorimetry, ζ-potential measurements and small deformation dynamic oscillation in shear. pH was found to affect the conformational properties of the polyelectrolyte altering its structural behaviour. Cooling scans in the vicinity of the glass transition region revealed a remarkable change in the viscoelastic functions as the polyelectrolyte rearranges from extended (neutral pH) to compact conformations (acidic pH). This conformational rearrangement was experimentally observed to result in early vitrification at neutral pH values where dissociation of galacturonic acid residues takes place. Time-temperature superposition of the mechanical shift factors and theoretical modeling utilizing WLF kinetics confirmed the accelerated kinetics of glass transition in the extended pectin conformation at neutral pH. Determination of the relaxation spectra of the samples using spectral analysis of the master curves revealed that the relaxation of macromolecules occurs within ~0.1 s regardless of the solvent pH
Polysaccharide Nanostructures
Polysaccharides are carbohydrate polymers where sugar units are linked
together through glycosidic linkages. In living organisms polysaccharides are the
structural polymers that provide support (e.g., cellulose in plants or chitin in
arthropods) or the sources of energy for plant development (e.g., starch).
Polysaccharides are routinely used in the food industry, most frequently as thickeners,
stabilizers of dispersions (emulsions, foams) or structuring agents of water and air
Electronic structure and the glass transition in pnictide and chalcogenide semiconductor alloys. Part I: The formation of the -network
Semiconductor glasses exhibit many unique optical and electronic anomalies.
We have put forth a semi-phenomenological scenario (J. Chem. Phys. 132, 044508
(2010)) in which several of these anomalies arise from deep midgap electronic
states residing on high-strain regions intrinsic to the activated transport
above the glass transition. Here we demonstrate at the molecular level how this
scenario is realized in an important class of semiconductor glasses, namely
chalcogen and pnictogen containing alloys. Both the glass itself and the
intrinsic electronic midgap states emerge as a result of the formation of a
network composed of -bonded atomic -orbitals that are only weakly
hybridized. Despite a large number of weak bonds, these -networks are
stable with respect to competing types of bonding, while exhibiting a high
degree of structural degeneracy. The stability is rationalized with the help of
a hereby proposed structural model, by which -networks are
symmetry-broken and distorted versions of a high symmetry structure. The latter
structure exhibits exact octahedral coordination and is fully
covalently-bonded. The present approach provides a microscopic route to a fully
consistent description of the electronic and structural excitations in vitreous
semiconductors.Comment: 22 pages, 17 figures, revised version, final version to appear in J.
Chem. Phy
Decoupling polymer, water and ion transport dynamics in ion-selective membranes for fuel cell applications
Ion conducting polymer membranes are designed for applications ranging from separation and dialysis, to energy conversion and storage technologies. A key application is in fuel cells, where the semi-permeable polymer membrane plays several roles. In a fuel cell, electrical power is generated from the electrochemical reaction between oxygen and hydrogen, catalysed by metal nanoparticles at the cathode and anode sites. The polymer membrane permits the selective transport of H+ or OH− to enable completion of the electrode half-reactions, plays a major role in the management of water that is necessary for the conduction process and is a product in the reactions, and provides a physical barrier against leakage across the cell. All of these functions must be optimised to enable high conduction efficiency under operational conditions, including high temperatures and aggressive chemical environments, while ensuring a long lifetime of the fuel cell. Polymer electrolyte membranes used in current devices only partially meet these stringent requirements, with ongoing research to assess and develop improved membranes for a more efficient operation and to help realise the transition to a hydrogen-fuelled energy economy. A key fundamental issue to achieving these goals is the need to understand and control the nature of the strongly coupled dynamical processes involving the polymer, water and ions, and their relationship to the conductivity, as a function of temperature and other environmental conditions. This can be achieved by using techniques that give access to information across a wide range of timescales. Given the complexity of the dynamical map in these systems, unravelling and disentangling the various processes involved can be accessed by applying the “serial decoupling” approach introduced by Angell and co-workers for ion-conducting glasses and polymers. Here we introduce this concept and propose how it can be applied to proton- and anion-conducting fuel cell membranes using two main classes of these materials as examples
Viscosity and viscosity anomalies of model silicates and magmas: a numerical investigation
We present results for transport properties (diffusion and viscosity) using
computer simulations. Focus is made on a densified binary sodium disilicate
2SiO-NaO (NS2) liquid and on multicomponent magmatic liquids (MORB,
basalt). In the NS2 liquid, results show that a certain number of anomalies
appear when the system is densified: the usual diffusivity maxima/minima is
found for the network-forming ions (Si,O) whereas the sodium atom displays
three distinct r\'egimes for diffusion. Some of these features can be
correlated with the obtained viscosity anomaly under pressure, the latter being
be fairly well reproduced from the simulated diffusion constant. In model
magmas (MORB liquid), we find a plateau followed by a continuous increase of
the viscosity with pressure. Finally, having computed both diffusion and
viscosity independently, we can discuss the validity of the Eyring equation for
viscosity which relates diffusion and viscosity. It is shown that it can be
considered as valid in melts with a high viscosity. On the overall, these
results highlight the difficulty of establishing a firm relationship between
dynamics, structure and thermodynamics in complex liquids.Comment: 13 pages, 8 figure
Design principles for Bernal spirals and helices with tunable pitch
Using the framework of potential energy landscape theory, we describe two in
silico designs for self-assembling helical colloidal superstructures based upon
dipolar dumbbells and Janus-type building blocks, respectively. Helical
superstructures with controllable pitch length are obtained using external
magnetic field driven assembly of asymmetric dumbbells involving screened
electrostatic as well as magnetic dipolar interactions. The pitch of the helix
is tuned by modulating the Debye screening length over an experimentally
accessible range. The second design is based on building blocks composed of
rigidly linked spheres with short-range anisotropic interactions, which are
predicted to self-assemble into Bernal spirals. These spirals are quite
flexible, and longer helices undergo rearrangements via cooperative, hinge-like
moves, in agreement with experiment
A new polymer electrolyte based on a discotic liquid crystal triblock copolymer
Peer reviewedPublisher PD
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