73 research outputs found
Rouse Chains with Excluded Volume Interactions: Linear Viscoelasticity
Linear viscoelastic properties for a dilute polymer solution are predicted by
modeling the solution as a suspension of non-interacting bead-spring chains.
The present model, unlike the Rouse model, can describe the solution's
rheological behavior even when the solvent quality is good, since excluded
volume effects are explicitly taken into account through a narrow Gaussian
repulsive potential between pairs of beads in a bead-spring chain. The use of
the narrow Gaussian potential, which tends to the more commonly used
delta-function repulsive potential in the limit of a width parameter "d" going
to zero, enables the performance of Brownian dynamics simulations. The
simulations results, which describe the exact behavior of the model, indicate
that for chains of arbitrary but finite length, a delta-function potential
leads to equilibrium and zero shear rate properties which are identical to the
predictions of the Rouse model. On the other hand, a non-zero value of "d"
gives rise to a prediction of swelling at equilibrium, and an increase in zero
shear rate properties relative to their Rouse model values. The use of a
delta-function potential appears to be justified in the limit of infinite chain
length. The exact simulation results are compared with those obtained with an
approximate solution which is based on the assumption that the non-equilibrium
configurational distribution function is Gaussian. The Gaussian approximation
is shown to be exact to first order in the strength of excluded volume
interaction, and is found to be accurate above a threshold value of "d", for
given values of chain length and strength of excluded volume interaction.Comment: Revised version. Long chain limit analysis has been deleted. An
improved and corrected examination of the long chain limit will appear as a
separate posting. 32 pages, 9 postscript figures, LaTe
A randomised double-blind placebo-controlled pilot trial of a combined extract of sage, rosemary and melissa, traditional herbal medicines, on the enhancement of memory in normal healthy subjects, including influence of age
This is the author accepted manuscript. The final version is available from Elsevier via the DOI in this recordOBJECTIVE: To evaluate for the first time the effects of a combination of sage, rosemary and melissa (Salvia officinalis L., Rosmarinus officinalis L. and Melissa officinalis L.; SRM), traditional European medicines, on verbal recall in normal healthy subjects. To devise a suitable study design for assessing the clinical efficacy of traditional herbal medicines for memory and brain function. METHODS: Forty-four normal healthy subjects (mean age 61 ± 9.26y SD; m/f 6/38) participated in this study. A double-blind, randomised, placebo-controlled pilot study was performed with subjects randomised into an active and placebo group. The study consisted of a single 2-week term ethanol extract of SRM that was chemically-characterised using high resolution LC-UV-MS/MS analysis. Immediate and delayed word recall were used to assess memory after taking SRM or placebo (ethanol extract of Myrrhis odorata (L.) Scop.). In addition analysis was performed with subjects divided into younger and older subgroups (≤ 62 years mean age n = 26: SRM n = 10, Placebo n = 16; ≥ 63 years n = 19: SRM n = 13, Placebo n = 6). RESULTS: Overall there were no significant differences between treatment and placebo change from baseline for immediate or delayed word recall. However subgroup analysis showed significant improvements to delayed word recall in the under 63 year age group (p < 0.0123) with Cohen's effect size d = 0.92. No adverse effects were observed. CONCLUSION: This pilot study indicates that an oral preparation of SRM at the selected dose and for the period of administration is more effective than a placebo in supported verbal episodic memory in healthy subjects under 63 years of age. Short- and long- term supplementation with SRM extract merits more robust investigation as an adjunctive treatment for patients with Alzheimer's disease and in the general ageing population. The study design proved a simple cost effective trial protocol to test the efficacy of herbal medicines on verbal episodic memory, with future studies including broader cognitive assessment.Support by the following organisations is gratefully acknowledged, Ross Menzies Herbal Clinic, Bodyworks Therapy Centre; Wesnes Cognition Ltd, the Royal Botanic Gardens (RBG), Kew and Dilston Physic Garden. The phytochemical analysis was gratefully funded by the Sir Jeremiah Colman Gift Trust
The effect of annealing on the elastoplastic and viscoelastic responses of isotactic polypropylene
Observations are reported on isotactic polypropylene (i) in a series of
tensile tests with a constant strain rate on specimens annealed for 24 h at
various temperatures in the range from 110 to 150 C and (ii) in two series of
creep tests in the sub-yield region of deformation on samples not subjected to
thermal treatment and on specimens annealed at 140 C. A model is developed for
the elastoplastic and nonlinear viscoelastic responses of semicrystalline
polymers. A polymer is treated an equivalent transient network of
macromolecules bridged by junctions (physical cross-links, entanglements and
lamellar blocks). The network is assumed to be highly heterogeneous, and it is
thought of as an ensemble of meso-regions with different activation energies
for separation of strands from temporary nodes. The elastoplastic behavior is
modelled as sliding of meso-domains with respect to each other driven by
mechanical factors. The viscoelastic response is attributed to detachment of
active strands from temporary junctions and attachment of dangling chains to
the network. Constitutive equations for isothermal uniaxial deformation are
derived by using the laws of thermodynamics. Adjustable parameters in the
stress-strain relations are found by fitting the experimental data.Comment: 29 pages, 14 figure
Kinetic viscoelasticity modeling applied to degradation during carbon–carbon composite processing
Kinetic viscoelasticity modeling has been successfully utilized to describe phenomena during cure of thermoset based carbon fiber reinforced matrices. The basic difference from classic viscoelasticity is that the fundamental material descriptors change as a result of reaction kinetics. Accordingly, we can apply the same concept for different kinetic phenomena with simultaneous curing and degradation. The application of this concept can easily be utilized in processing and manufacturing of carbon–carbon composites, where phenolic resin matrices are cured degraded and reinfused in a carbon fiber bed. This work provides a major step towards understanding complex viscoelastic phenomena that go beyond simple thermomechanical descriptors.United States. Air Force Office of Scientific ResearchNational Science Foundation (U.S.) (Joint U.S.-Greece Program
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