Molecular fluids, high-density

Via, J. C., Braue, C. L., Taylor, L. T. (1994) Supercritical fluid fractionation of a low molecular weight high density polyethylene wax using carbon dioxide, propane and propane modified carbon dioxide /ia/. Chem. 66, 603. 

Molecularly thin lubricant film is an important application of nanoscale confined polymeric fluids, and is the focus of this chapter. Ultrathin lubricant films are necessary in high-density data storage to increase the reliability and performance of hard-disk drive (HDD) systems [2-4]. Spinoff and intermittent contact between the slider (or head) and the lubricated disk [ultrathin perfluoropolyether (PFPE) films are applied to the disk s carbon-overcoated surface, as shown in Fig. 1.1] cause loss and reflow of the lubricant film. The relevant HDD technology is summarized briefly in the end-of-chapter Appendix Section A.I, which provides an overview of how certain information technology devices are controlled by nanoscale chemistry. 

It is possible, however, to determine a relationship exists between the fluid pressure and its density, thus allowing indirect measurement and control of the density. For an ideal gas, the relationship is simply, PV/RT= 1, where Fis the molar volume (reciprocal of the molar density). From the molecular weight of the gas (M), the mass density, p, can be calculated as M/V. The simple equation breaks down at the high densities characteristic of supercritical fluids, but, the work of Pitzer et allows the ideal gas law to be extended... 

We must point out two related limitations of the LE theory. First, it applies to small-molecule LCs and to LCPs in the limit of vanishing strain rate. This is because the LE theory uses a vector n to represent the orientation state of the fluid, tacitly assuming that the molecular orientation distribution stays at its equilibrium state. This is reasonable when the molecular relaxation time is much shorter than the characteristic time of the flow. Second, the theory does not allow orientational defects, which would be singularities in the n field. In reality, LCs and LCPs tend to have a high density of defects. ° Near the defect core, large spatial gradients distort the molecular orientation distribution, thus invalidating the LE theory. 

HMX (1,3,5, 7-tetranitro-l, 3,5,7-tetraazacyclooctane) is widely used as an ingredient in various explosives and propellants. A molecular solid at standard state, it has four known pol5miorphs, one of which, the 8 phase is comprised of six molecules per unit cell, as depicted in Fig. 10. We study the chemical decomposition of the dense fluid of this phase by conducting a high-density and temperature (p = 1.9 g/cm, T = 3500 K) quantum mechanical based molecular dynamics simulation. [50] To our knowledge, this is the first reported ab initio based/molecular dynamics study of an explosive material at extreme conditions for extended reaction times of up to 55 picoseconds, thus allowing the formation of stable product molecules. 

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