Stretching experiments

A piston driven stretching device has been developed by Koch et al. Stretching occurred in a fraction of a second. The sample can be heated by a stream of hot air. For stretching velocities up to 0.75 mm s , a stretching device has been developed by Holland Moritz and Stach which can also be used for real time fourier transformed infrared (FTIR) experiments. Both the force and elongation are transferred to a PDF 11/24 computer by a CAMAC based voltage/frequency converter (Fig. 31). 

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The shear viscosity, especially as measured with capillary rheometers characterized by high shear rates, is hardly sensitive to material structure since the investigator usually has to deal with the substantially destroyed structure in the molten sample. Melt stretching experiments would normally provide much more information [33]. 

Kotlikojf Sometimes we see a substantial lag other times it is shorter than 500 ms. I don t know the answer. Because of the number of differences between a single-cell stretch experiment and real physiology, I don t know whether this explains things such as pressure-induced vascular tone. One thing that will be informative here is if the RyR2 knockouts are viable and we can look at them. 

Earlier attempts to use the AFM for mechanically stretching chromatin fibers have run into a rather unexpected artifact. Long native chromatin fibers isolated from chicken erythrocytes, or fibers assembled in vitro from purified histones and relatively short, tandemly repeated DNA sequences were deposited on mica or glass surfaces and pulled with the AFM tip [69,70]. In such stretching experiments the scanning of the sample in the x- and y-direction used for imaging was disabled, and the cantilever-mounted tip was allowed to move only in the z-direction, i.e., upwards and downwards, away and towards the surface. When the AFM tip is pushed into the sample, it may attach to the sample by non-specific adsorption upon retraction it stretches the sample and force-extension curves are recorded (see Fig. lb for an explanation of a typical force curve). 

Nucleosome-nucleosome interaction potentials can be calibrated by comparison with the characteristics of liquid crystals of mononucleosomes at high concentrations. Under suitable conditions, nucleosome core particles form a hexagonal-columnar phase with a distance of 11.55 1 nm between the columns and a mean distance of 7.16 0.65 nm between the particles in one column [44,46]. These distances may be assumed to correspond to the positions of the minima of an attractive internucleosomal potential. The depth of the interaction potential (i.e., the binding energy per nucleosome) was estimated in the stretching experiments of Cui and Bustamante [66] to 2.6-3.4 kT. A slightly lower potential minimum of 1.25 kT is obtained by a comparison of the stability of the nucleosome liquid crystal phase with simulations [50]. 

Hummer and Szabo" demonstrated that in single molecule stretching experiments, the JE provides an expression for the work at different times, whereas from an experimental point of view it is of more interest to know the free energy difference between states at different extensions of the molecule. They show how this can be obtained and apply it in experiments. 

Thermodynamics of DNA interactions from single molecule-stretching experiments 02ACR159. 

Fig. 3.73 Top Schematic of the combined AFM - stretching experiment and bottom (15 pm)2 AFM images of LDPE at (a) 1.5% elongation, (b) 17.5% elongation, and (c) 27.5% elongation. Reproduced with permission of John Wiley Sons, Inc. from [151]. Copyright 2001. John Wiley Sons, Inc.

The coke formation in methanol to hydrocarbons conversion over zeolite H-MFI was also studied with UV-RS. To distinguish between the signals that correspond to CH deformations and to CC stretches, experiments were carried out with deuterated methanol (CD3OH). The bands assignments used in this study are summarized in Table 2. It is concluded that cyclopentadienyl species are intermediates in the formation of polyaromatic hydrocarbons. By comparison with pure polynuclear aromatics UV-RS spectra, it is suggested that coke... 

Williams, M.C., Rouzina, L, Bloomfield, VA. 2002. Thermodynamics of DNA interactions from single molecule stretching experiments. Acc. Chem. Res. 35 159-166. 

This model was subjected to a wide range of loading conditions. A single set of model parameters is sufficient to describe the main features of isometric, isotonic, and quick release and stretch experiments. The model is sensitive to mechanical disturbances, consistent with experimental evidence from muscle force curves [34], aequorin measurements of free calcium ion [35], and high-speed x-ray diffraction studies [36]. The model is also consistent with sarcomere length feedback studies [29] where reduced internal motion delays relaxation. 

In a stretching experiment a test specimen is placed under tension, causing the length to increase and the cross-sectimi to decrease, until finally it breaks. For these stress-strain measurements the test specimen has shoulders at both ends, such that the break occurs in the desired place, namely at the position of lowest cross-section. 

The measured results of a stretching experiment are normally treated with the empirical Mooney-Rivlin relation (Mooney 1940 Rivlin 1949), as given by... 

The yield point in engineering stretching experiments is always located shortly above point B. The position of the critical strain at the point C, at which the critical stress that starts destructing the crystal blocks is achieved, depends on the interplay between the entanglement density of the amorphous... 

With the advent of nanotechnology, interest in the physics of small systems far from equilibrium has strongly increased. Suddenly, tiny systems in which thermal fluctuations prevail could be easily conceived and realized. Mechanically driven transformations, as carried out by single-molecule stretching experiments... 

Fig. 16 and B-values derived from stretching experiments, we can make some estimates. First, at the same crosslink density of 10%, the compression modulus B of E60/40 is about a third of that for E70/30. If the value S 50 nm, characteristic of the low temperature state of E70/30, is divided by 3 we arrive at = 29 nm, which is close to the saturated correlation length S 27 nm of E60/40. Second, a reasonable value of the percolation limit of the present elastomers is Co = 0.04. Then, neglecting a possible temperature dependence of the modulus B, the ratio should be (6) 2.4. Considering first E70/30, taking 5% 150 nm (at the transition point to nematic phase) and io% — 50 nm at low temperatures, we arrive at a ratio = 3, close to our estimate. However, for E60/40 we find... 

Using gelatin as a model cross-linked elastomer, its rubber elasticity can also be demonstrated by a simple stretching experiment. Thin slices of the... 

However, in the stretching experiments, we have to consider also cavitation. Cavitation creates an additional phase, characterized by zero electron density. Therefore, in the deformation range between 200% and 400-500% our systems consist of four phases, y form, mesophase, amorphous compraient and nanocavities. At strain higher than 400-500 % the y form is almost completely transformed into mesophase, and only three phases (mesophase, amorphous component and nanocavities) should be considered. The scattering invariant for this three-phase system (that is, for deformations e above the threshold limit at which transformation of y form is almost complete) can be written as (Glatter and Kratky 1982) ... 

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