Mechanical properties of membranes

The stability of membranes against thermomechanical and chemical stresses is an important factor in determining both their short- and long-term performance. Transport and mechanical properties of membranes affect the fuel cell performance, while the lifetime of a fuel cell is mostly dependent on the thermomechanical and chemical stability of the membrane. Thermomechanical and chemical degradation of a membrane will result in a loss of conductivity, as well as mixing of anode and cathode reactant gases. 

The relationship between the structure of the disordered heterogeneous material (e.g., composite and porous media) and the effective physical properties (e.g., elastic moduli, thermal expansion coefficient, and failure characteristics) can also be addressed by the concept of the reconstructed porous/multiphase media (Torquato, 2000). For example, it is of great practical interest to understand how spatial variability in the microstructure of composites affects the failure characteristics of heterogeneous materials. The determination of the deformation under the stress of the porous material is important in porous packing of beds, mechanical properties of membranes (where the pressure applied in membrane separations is often large), mechanical properties of foams and gels, etc. Let us restrict our discussion to equilibrium mechanical properties in static deformations, e.g., effective Young s modulus and Poisson s ratio. The calculation of the impact resistance and other dynamic mechanical properties can be addressed by discrete element models (Thornton et al., 1999, 2004). 

Mechanical Properties. The mechanical properties of membranes for industrial applications are important, because of their influence on the membrane s resistance to wrinkling and pinhole formation. 

Ultrahigh molecular weight polyethylene/liquid parafiin/dibenzylidene sorbitol ter-naiy blends were studied. Dibenzylidene sorbitol fibrils nucleated UHMWPE and induced the lamellae alignment perpendicular to the flow direction by which the pore size and water permeability and mechanical properties of membrane were enhanced. ... 

In addition to the phosphoric acid content, the water uptake or atmospheric humidity, as well as the temperature influence the mechanical properties of membranes. These effects are shown in Fig. 6.7 where wta-PBI membranes were tested at 21 and 150 °C under different water activities. 

Physical-and-chemical properties of the membranes depend largely on the nature of the material, which they are formed of, and methods for their preparation. They are defined by chemical structure and composition of the polymer, which determine such important characteristics as the flexibility of the polymer chain, the origin and the energy of intermolecular interactions, as well as interactions with components of separating solutions. Physical-and-chemical and mechanical properties of membranes are largely defined by molecular structure. 

As a result, due to the pressure generated in the machine, membrane is compressed. Moving of mechanical compression energy into heat energy increases the temperature of the membrane [4], However, its value disproportionately low compared to the temperature of circulating solution, so we do not consider it. The upper temperature limit is 60 °C. We cannot take into account only this temperature limit, because it does not influence on mechanical properties of membrane, inasmuch as it does not lead to a change in the physical-and-chemical properties of membrane material. 

Many CG models have been developed and used in the past two decades, and not surprisingly, applications have been focusing primarily on the phenomenon of self-assembly and the equilibrium between phases. To some extent, all models that simplify the chemical structure of a macromolecule to focus on its physical properties can be considered as CG models of varied complexity. However, a marked distinction between these models is whether the solvent is modeled implicitly as a continuous medium interacting only with the solute, or explicitly as an ensemble of particles that also interact with each other. For brevity, we discuss only the latter kind of models because the competition between intermolecular forces is crucial to simulate self-assembly. For the purpose of modeling the mechanical properties of membranes and other known stmctures, implicit solvent models are relatively accurate [28, 29] and are typically lower in computational cost than explicit solvent models. 

The lipid composition contributes to the mechanical properties of membranes, but probably also has other functions, such as adaptation to changes in temperature (Vigh et al, 2005), but this has not yet been established for sponges. 

One of the main methods for improving the mechanical properties of linear polymers is their drawing that can be uniaxial (fibres), biaxial (films), planar symmetrical (films-membranes) etc. As a result of polymer deformation, the system changes into the oriented state fixed by crystallization. 

Inside the typical smooth muscle cell, the cytoplasmic filaments course around the nuclei filling most of the cytoplasm between the nuclei and the plasma membrane. There are two filamentous systems in the smooth muscle cell which run lengthwise through the cell. The first is the more intensively studied actin-myosin sliding filament system. This is the system to which a consensus of investigators attribute most of the active mechanical properties of smooth muscle. It will be discussed in detail below. The second system is the intermediate filament system which to an unknown degree runs in parallel to the actin-myosin system and whose functional role has not yet been completely agreed upon. The intermediate filaments are so named because their diameters are intermediate between those of myosin and actin. These very stable filaments are functionally associated with various protein cytoarchitectural structures, microtubular systems, and desmosomes. Various proteins may participate in the formation of intermediate filaments, e.g., vimentin. 

Before the first indication of the existence of cannabinoid receptors, the prevailing theory on the mechanism of cannabinoid activity was that cannabinoids exert their effects by nonspecific interactions with cell membrane lipids (Makriyannis, 1990). Such interactions can increase the membrane fluidity, perturb the lipid bilayer and concomitantly alter the function of membrane-associated proteins (Loh, 1980). A plethora of experimental evidence suggests that cannabinoids interact with membrane lipids and modify the properties of membranes. However, the relevance of these phenomena to biological activities is still only, at best, correlative. An important conundrum associated with the membrane theories of cannabinoid activity is uncertainty over whether cannabinoids can achieve in vivo membrane concentrations comparable to the relatively high concentrations used in in vitro biophysical studies (Makriyannis, 1995). It may be possible that local high concentrations are attainable under certain physiological circumstances, and, if so, some of the cannabinoid activities may indeed be mediated through membrane perturbation. 

The link from lipid properties to mechanical properties of the bilayers is now feasible within the SCF approach. The next step is to understand the phase behaviour of the lipid systems. It is likely that large-scale (3D) SCF-type calculations are needed to prove the conjectures in the field that particular values of the Helfrich parameters are needed for processes like vesicle fusion, etc. In this context, it may also be extremely interesting to see what happens with the mechanical parameters when the system is molecularly complex (i.e. when the system contains many different types of molecules). Then there will be some hope that novel and deep insights may be obtained into the very basic questions behind nature s choice for the enormous molecular complexity in membrane systems. 

Bauer, Denneler, and Wilert-Porada also studied the influence of temperature (30-120°C) and humidity (0 - 100%) on the mechanical properties of Nation 117 membrane via dynamic mechanical analysis (DMA). The mechanical behavior of Nation membranes in a humid atmosphere was observed to differ significantly from that in dry atmosphere, and the influence of water on the mechanical properties of the acid form of Nation was found to be complex. The maximum of the storage modulus ( ) as a function of humidity was shifted to higher humidity values with increasing temperature. 

Bauer, F., Denneler, S. and Wilert-Porada, M. 2005. Influence of temperature and humidity on the mechanical properties of Nafion 117 polymer electrolyte membrane. Journal of Polymer Science Part B Polymer Physics 43 786-795. 

Patri, M., Hande, V. R., Phadnis, S. and Deb, P. C. 2004. Radiation-grafted solid polymer electrolyte membrane thermal and mechanical properties of sulfonated fluormated ethylene propylene copolymer (FEP)-graft-acrylic acid membranes. Polymers for Advanced Technologies 15 622-627. 

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