Free-energy density functions for nematic elastomers

The recently proposed neo-classical theory for nematic elastomers generalizes standard molecular-statistical Gaussian network theory to allow for anisotropic distributions of polymer chains. The resulting free-energy density models several of the novel properties of nematic elastomers. In particular, it predicts the ability of nematic elastomers to undergo large deformations with exactly zero force and energy cost—so called soft elasticity. Although some nematic elastomers have been shown to undergo deformations with unusually small applied forces, not all do so, and none deform with zero force. Further, as a zero force corresponds to infinitely many possible deformations in the neo-classical theory, this non-uniqueness leads to serious indeterminacies in numerical schemes. Here we suggest that the neo-classical free-energy density is incomplete and propose an alternative derivation that resolves these difficulties. In our approach, we use the molecular-statistical theory to identify appropriate variables. This yields the choice for the microstructural degrees of freedom as well as two independent strain tensors (the overall macroscopic strain plus a relative strain that indicates how the deformation of the elastomeric microstructure deviates from the macroscopic deformation). We then propose expressions for the free-energy density as a function of the three quantities and show how the material parameters can be measured by two simple tests. The neo-classical free-energy density can be viewed as a special case of our expressions in which the free-energy density is independent of the overall macroscopic strain, thus supporting our view that the neo-classical theory is incomplete.     

Raman spectroscopy of topiramate under high pressure conditions

Topiramate is a white crystalline solid with powerful anticonvulsant activity and is used to treat epilepsy. Drug manufacturing involves various physical and chemical processes, which may lead to the formation of an unexpected, or undesired, crystalline phase, in a phenomenon known as polymorphism. In this paper, the behavior of topiramate crystal is studied under pressures up to 10.8 GPa using Raman spectroscopy. Under the conditions employed, Raman spectra from topiramate under pressure showed no strong evidence of a phase change, although amorphization may be considered. This may be accounted for by its crystal structure, comprising oppositely running chains of topiramate molecules linked by asymmetric hydrogen bonds, which pose a kinetic barrier against a structural change. It is not yet safe to rule out the existence of topiramate polymorphs, which could possibly be obtained under special crystallization conditions, but not after compression, at least up to 10 GPa. Copyright © 2009 John Wiley & Sons, Ltd.     

Critical current degradation behaviors of ReBCO coated conductor tapes under pressurized liquid nitrogen

In the case of 2G coated conductor (CC) tapes, it has been reported that thin–thick CC tapes with IBAD substrate showed a superior electromechanical property even at smaller bending radius compared with the cases of 1G BSCCO tapes. Considering the application of CC tapes it is significant to evaluate the transport property under operating environment, because CC tapes might experience a change in operating pressure that can affect its current carrying capacity due to temperature variation and deformation. This study was focused on the I_c degradation behavior in bent CC tapes under pressurized liquid nitrogen. Differently processed YBCO and SmBCO CC tapes with IBAD substrate are used as samples. The bending strain characteristics at elevated pressure levels were evaluated by using the ρ-shaped sample holder which can induce different bending strain values at pressured state. Depressurization and thermal cycling were performed to check the reversibility of I_c in CC tapes. Vacuuming tests were also carried out to investigate the characteristics of I_c at different LN₂ temperature levels.     

BRST Invariant Theory of a Generalized 1 + 1 Dimensional Nonlinear Sigma Model with Topological Term

We give a generalized Lagrangian density of 1 + 1 Dimensional O(3) nonlinear σ model with subsidiary constraints, different Lagrange multiplier fields and topological term, find a lost intrinsic constraint condition, convert the subsidiary constraints into inner constraints in the nonlinear σ model, give the example of not introducing the lost constraint = 0, by comparing the example with the case of introducing the lost constraint, we obtain that when not introducing the lost constraint, one has to obtain a lot of various non-intrinsic constraints. We further deduce the gauge generator, give general BRST transformation of the model under the general conditions. It is discovered that there exists a gauge parameter β originating from the freedom degree of BRST transformation in a general O(3) nonlinear sigma model, and we gain the general commutation relations of ghost field. PACS numbers: 11.10.Lm; 11.30.Ly     

Generation of gluons from quark confinement

We study a model of quark confinement defined by the vanishing of colour currents. The model is shown to be equivalent to quantum chromodynamics and this equivalence is interpreted as due to the compositeness of the colour gluons. The Green’s functions of the theory are found to contain nontrivial structure only for colour singlet composites which can be identified with hadrons.     

Global attractor for the nonlinear strain waves in elastic waveguides

1. IntroductionIn some problems of nonlinear wave propagation in waveguides, the illteraction of waveguides and the external medium and, therefore, the possibility of energy exchange throughlateral surface of waveguide cannot be neglected. When the energy exchange between therod and the medium is considered, for one cajse, there is a dissipation of deformation wavein the viscous external medium. The general cubic double dispersion equation (CDDE) canbe derived from Hamilton principled]:where…     

Automatic energy–momentum conserving time integrators for hyperelastic waves

An energy–momentum conserving time integrator coupled with an automatic finite element algorithm is developed to study longitudinal wave propagation in hyperelastic layers. The Murnaghan strain energy function is used to model material nonlinearity and full geometric nonlinearity is considered. An automatic assembly algorithm using algorithmic differentiation is developed within a discrete Hamiltonian framework to directly formulate the finite element matrices without recourse to an explicit derivation of their algebraic form or the governing equations. The algorithm is illustrated with applications to longitudinal wave propagation in a thin hyperelastic layer modeled with a two-mode kinematic model. Solution obtained using a standard nonlinear finite element model with Newmark time stepping is provided for comparison.     

Benzazetidines and Related Compounds: Synthesis and Potential

Benzazetidines are a class of N-heterocycles potentially very interesting for a variety of purposes, including biological applications and drug design. In the past, their high ring strain has hampered the development of trustable, general, and efficient synthetic methodologies for their preparation. In this review article, the aim is to disclose all the literature contributions about the synthesis of these compounds and the study of their reactivity, from the early examples to the most recent synthetic approaches. Recently, there has been a growth of interest for this heterocycle, driven by the publication of novel synthetic methodologies based on palladium-catalyzed intramolecular C−H amination and organocatalyzed ring-closure of 2-(N-Boc-anilino)-α-ketoesters/amides.     

Highly sensitive,stretchable and durable strain sensors based on conductive double-network polymer hydrogels

Hydrogel-based strain sensors have been attracting immense attention for wearable electronic devices owing to their intrinsic soft characteristics and flexibility. However, developing hydrogel sensors with hightensile strength, stretchability, and strain sensitivity remains a great challenge. Herein, we report a technique to synthesize highly sensitive hydrogel-based strain sensors by integrating carbon nanofibers (CNFs) with a double-network (DN) polymer hydrogel matrix comprising of a physically cross-linked agar network and a covalently cross-linked polyacrylamide (PAAm) network. The resultant nanocomposite sensors display superior piezoresistive sensitivity with a hightrue gauge factor (GF_T = 1.78) at an ultrahigh strain of 1,000%, a fast response time and linear correlation of ln(R/R₀) and ln(L/L₀) up to 1,000% strain. Most significantly, these sensors possess highmechanical strength (~0.6 MPa) and superb durability (>1,000 cycles at strain of 100%), stemming from the effective energy dissipation mechanism of the first agar network acting as sacrificial bonds and the CNFs serving as dynamic nanofillers. The combination of highstrain sensitivity and ultrahigh stretchability of hydrogel sensors makes it possible to sense both small mechanical deformations induced by human motions and large strain up to 1,000%.     

Structure-Dependent Strain Effects

Density functional theory calculations of atomic and molecular adsorption on (111) and (100) metal surfaces reveal marked surface and structure dependent effects of strain. Adsorption in three-fold hollow sites is found to be destabilized by compressive strain whereas the reversed trend is commonly valid for adsorption in four-fold sites. The effects, which are qualitatively explained using a simple two-orbital model, provide insights on how to modify chemical properties by strain design.