Decompression studies, compression

Compression and Decompression Studies. Bovine serum albumin was spread on an isoelectric substrate (pH 5.3) and initially compressed to a pressure (tt) of 5 dynes/cm. The molecular weight of the protein, 66,590 g/mole (pH 5.35 at 26°C and co-surface of 12,000 A2/molecule) was determined with a surface micromanometer. The moving barrier compressed the monolayer at a constant rate of 1.96 X 10"2 cm/sec for 60... 

For rates of compression or decompression intermediate to these, the data fell in the region between the A and B lines. No attraction or adhesion between the PEO bearing mica surfaces was observed (within error) in any of the experiments in the present study. 

The amorphous phase appearing above 20 GPa at room temperature (see above) has also recently been studied by X-ray diffraction [135] and Raman scattering [132,133]. Serebryanaya et al. [135] identify the structure as a three-dimensionally polymerized Immm orthorhombic lattice, but find that compression above 40 GPa gives a truly amorphous structure. In contrast to the orthorhombic three-dimensional polymer structure discussed in the last section, the best fit here is found for (2+2) cycloaddition in two directions, with (3+3) cycloaddition in the third, and thus some relationship to the tetragonal phase. From the in situ X-ray data a bulk modulus of 530 GPa is deduced, about 20% higher than for diamond. Talyzin et al. [132, 133] find that this phase depolymerizes on decompression into linear polymer chains, unless the sample is heated to above 575 K under pressure. A strong interaction with the diamond substrate is also noted, such that only films with a thickness of several hundred nm are able to polymerize fully [ 132]. Hardness tests were also carried out on the polymerized films, which were found to be almost as hard as diamond and to show an extreme superelastic response with a 90% elastic recovery after indentation [133]. 

Recent CT and MRI studies showed that increased extraocular muscle volume correlates with compressive optic neuropathy as well. Although some patients show inflammation of the nerve and sheath, it has been postulated that these patients have shallow orbits or that they lack the ability to decompress anteriorly. Patients at greater risk of developing optic neuropathy are older patients with enlarged extraocular muscles and limited motility. Diabetic patients with proptosis and extraocular muscle enlargement are also more likely to develop optic neuropathy. 

Linear profiles are the simplest profiles to use for powder compressions. Typically, a sawtooth or v-shaped profile is used where the punch is extended at a constant velocity and retracts at a constant velocity. In theory, during a sawtooth profile, the punch reverses its motion instantaneously between the compression and a decompression strokes. At low speeds (e.g.. <1 mm/sec), the hydraulic response system can easily accommodate this discontinuity. However, at high speeds (>100mm/.sec), the control system may show a small lag in the position-time waveform (<10 milliseconds) as it attempts to rapidly reverse the direction of punch. The sawtooth waveform is commonly used for more fundamental compression studies (e.g.. Heckel analysis), where the desired powder volume reduction is proportional to time. It is also u.seful when evaluating instrument performance during factory acceptance testing. 

The sawtooth waveform can be augmented to create a trapezoidal profile. Like the sawtooth profile, the trapezoidal profile uses constant compression and decompression rates. However, between the compression and decompression segments an intentional dwell time is incorporated. The dwell time is often defined as the time that the moving punches remain stationary at their furthest point of travel. This type of profile is useful when studying either the effects of compression rate or dwell time on powder compression behavior. 

TiTany authors (I, 2, 3) have compared the surface behavior of macro- molecules, especially proteins, with the behavior of low molecular weight monolayers. This paper notes a series of effects that occurred when bovine serum albumin (BSA) was spread on various clean liquid surfaces and was compressed or decompressed. The transfer of the protein monolayer and of some small chain monolayers was also studied using a surface distillation technique. 

Despite the observation of profuse spectroscopic changes on compression and decompression of NaNs, the in situ high-pressure structures of the various phases remain unknown. The small sample size required by high pressure studies and large hysteresis due to the strain and stress make difficult in situ diffraction measurements. At high temperature conditions, however, the transformation from an azide to a polynitrogen phase may be more readily characterized as the requisite transformation pressures may be much lower and less subject to hysteresis. Here we describe the heating experiments on pure sodium azide and a... 

After an anecdotal report of CPR performed successfully with a plumbers helper, interest was stimulated in active decompression of the chest, allowing for negative intrathoracic pressure and increased venous return [44,45]. This technique became known as active compression - decompression (ACD-CPR), and has achieved routine clinical use in some countries, such as France. While discussion continues as to whether definitive survival is increased, studies have shown that flow, filling pressure, and valve movement (as thoracic pump) is improved with respect to standard CPR. Initial reports of significant improvement of outcome have been challenged by others who were unable to find improvements [46]. ACD-CPR... 

The polyamorphism in a—Si and a—Ge has been studied extensively via compression studies of the solid amorphous materials at ambient T, combined with MD simulations and ab initio calculations. Deb et al. compressed a porous variety of crystalline Si and observed the occurrence of PIA above 14 GPa [51]. Raman spectroscopic investigations indicated that the a—Si material produced by the PIA event was substantially different to the normal tetrahedrally bonded amorphous solid, and that it likely had its atoms in high coordination. From an examination of the optical reflectivity, the amorphous form was thought to be metallic. It was proposed that the PIA event had produced the new HDA form of a—Si, by analogy with the previous observations reported for H2O and its density driven LDA HDA polyamorphism [37,62]. During decompression the HDA a—Si sample was observed to return to its normal LDA state below approximately 4 5 GPa [51]. McMillan et al. then prepared bulk a—Si samples using a solid state chemistry approach [102], and studied the occurrence of polyamorphism at high P via in situ Raman spectroscopy and electrical conductivity measurements [103,104] (Fig. 4). 

A study of the mechanical properties ( mechanochemistry ) of immobilized enzymes (e.g. chymotrypsinogen trapped within a polyacrylamide gel) has provided a means whereby the chemical processes at a molecular level can be controlled by mechanical compression and decompression of the supporting gel (i.e. by controlling the pore size). Mechanical compression of a chymotrypsinogen-polyacrylamide gel resulted in a 20-fold increase in the diffusion-controlled tryptic activation of chymotrypsinogen, the reaction rate... 

---