PFPE systems

Develop an LBM scheme with generalized SRS model to accurately describe the dynamics of PFPE systems. The model is based on the mathematically simple yet physically realistic LBM models capturing the bottom level (atomistic) information. This novel formulation is based on our system for electron-phonon coupling with two states (Ghai et al., 2005), which is analogous to spin system description for endgroups. 

From the mesoscopic model described in steps 2 and/or 3, calculate physical properties including spreading, surface energy, diffusion processes, and compare the simulation results with experimental data. This can be done for pure and nanoblended PFPE systems (Figures 24d and 25d). 

In this section, we will perform the stability analysis for PFPE Zdol and Ztetraol films via the Gibbs free-energy change (AG) for the PFPE system [7] to obtain criteria for uniform, stable thin films. 

The solubility of water in CO2 is higher than in oils typically used for w/o microemulsions, and in CO2 at pressures over 200 bar there is a significant increase in water solubility as a function of temperature [57]. Hence, water partitioning into the CO2 continuous phase may be important, and a previous study using infrared spectroscopy [29] points to this for PFPE systems. The change in water solubility should have an effect on droplet size, but there are no previous reports to establish this by scattering methods hence, we performed complementary near-infrared and SANS experiments. 

PFPE, AI2O3, and X-IP were 500 mg, 200 mg, and 100 mg, respectively. The materials in Samples 2 to 4 were sufficiently mixed to ensure that the liquid PFPE and X-IP completely wet the alumina powders. These specimens were put in a closed space hlled with inert nitrogen. The flow rate of the nitrogen gas was kept at 20 milliliters per minute. The environmental temperature of the system was set at 220 ° C and the duration time was 250 minutes per sample for each individual operation procedure. PFPE used in the experiment was Z-dol and the alumina was in ultra-fine powders with chemical analytic grade purity. 

The results from TGA and FTIR tests indicate that PFPE will be more stable when X-IP is in the presence of the system... 

As a crucial factor that dominates the behavior of lubricant flow, the mobility of PFPE molecules has been studied extensively in both experiments and simulations, through observing the spreading of the lubricant on solid substrates. Investigators, including Novotny [46], O Connor et al. [47], Min et al. [48], and Ma et al. [49], in collaboration with IBM scientists, carried out systemic experimental studies on spreading... 

The interfacial tension is a key property for describing the formation of emulsions and microemulsions (Aveyard et al., 1990), including those in supercritical fluids (da Rocha et al., 1999), as shown in Figure 8.3, where the v-axis represents a variety of formulation variables. A minimum in y is observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases. Here, a middle-phase emulsion is present in equilibrium with excess C02-rich (top) and aqueous-rich (bottom) phases. Upon changing any of the formulation variables away from this point—for example, the hydrophilie/C02-philic balance (HCB) in the surfactant structure—the surfactant will migrate toward one of the phases. This phase usually becomes the external phase, according to the Bancroft rule. For example, a surfactant with a low HCB, such as PFPE COO NH4+ (2500 g/mol), favors the upper C02 phase and forms w/c microemulsions with an excess water phase. Likewise, a shift in formulation variable to the left would drive the surfactant toward water to form a c/w emulsion. Studies of y versus HCB for block copolymers of propylene oxide, and ethylene oxide, and polydimethylsiloxane (PDMS) and ethylene oxide, have been used to understand microemulsion and emulsion formation, curvature, and stability (da Rocha et al., 1999). 

Figure 8.4. Phase behavior of water/C02/surfactant systems studied to date. r=35°C, P = 414 bar and O T= 35°C, P=138 bar. PFPE COO NH4+ (Johnston et al., 1996) PFPE COO NH4+ (Zielinsky et al., 1997) di-HCF4 (Holmes et al., 1998). The one-phase microemulsion region is to the right of each curve.

Figure 8.5. Experimentally and theoretically predicted aqueous pH results for w/c PFPE COO NH4+ microemulsions and biphasic systems (no surfactant) buffered with sodium hydroxide (Holmes et al., 1999b) (T = 35 °C, P = 345 bar).

Figure 8.6. Emulsion stability contours (log of emulsion stability in seconds) for 50/50 (mass) C02-water (0.01 M NaCl) systems with 10.7 mM PFPE COO NH4+ (g/mol = 2500) surfactant. w/c emulsions O (c/w) emulsions. Dotted line indicates the phase boundary of the surfactant in C02 cross-hatched region indicates highly flocculated emulsions (Lee et al., 1999b).

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. 

Here, we adopted a spin analogy/lattice gas model, or SRS model, as shown in Fig. 1.28(a), which represents an oversimplified molecular structure yet still captures the essence of the molecule-surface interactions for describing SME profiles. Similar techniques using the Ising model to study other physical systems have been investigated [148,149,160] however, none of the literature deals with the simulation of PFPE lubricant dynamics described here. 

Simulations of PFPE Zdol include three steps (1) the generation of film, (2) relaxation of the spin system, and (3) the spreading process. These steps are described carefully by Ma [52],... 

We assumed that all polymer chains in the system have the same number of beads, that is, that they are monodisperse. The number of beads Np in each polymer chain is chosen as Np =6, 10, or 16. PFPEs have rigid fluorocarbon backbone units connected via ether bonds, which give flexibility to the chain while keeping PFPEs stable in a liquid state at room temperature. The beads in our model reproduce the rigid units and are connected only via their... 

The need to extend the liquid range of perfluorinated systems to very high molecular weights was satisfied by the important introduction of perfluoropolyethers (PFPEs) [72]... 

Fig. 1 (A) The molecular structure of PFPEs and its simplified model and (B) The spreading profiles as time progresses (t = 0 to ti) from thought experiment (i) Z and (ii) Zdol. The coordinate system is represented in B(i). (View this art in color at www.dekker.com.)...

In addition to w/c microemulsions, o/c microemulsions may be formed for systems with strong surfactant adsorption. The area occupied by PFPE-C00 NH4 at the interface between 600 molecular weight polyethylene glycol (PEG) md CO2 is 440 per molecule based upon measurement of the interfacial tension versus surfactant concentration [21]. This surface coverage is sufficient for microemulsion formation as was verified with phase behavior measurements. Only 0.55 wt% of 600 molecular weight polyethylene glycol is soluble in CO2 at 45 °C and 300 bar. With the addition of 4wt% PFPE-C00 NH4 surfactant, up to 1.8 wt% is solubilized. The additional PEG resides in the core of the microemulsion droplets, consistent with the prediction from the adsorption measurement. 

The conductivity and high-frequency dielectric constant of an emulsion are both indicators of which phase (water or CO2) is the continuous one. If CO2 is the continuous phase in an emulsion, the emulsion conductivity will be extremely low. In contrast, if water is the continuous phase, substantial conductivity will be observed, provided that ionic solutes are present. Likewise, the dielectric of a CO2-continuous emulsion should be similar to that of CO2 (ca. 1.2), whereas the dielectric of a water-continuous emulsion should approach that of water (ca. 80). Emulsions were created by using a high-pressure emulsifier (Emulsiflex, Avestin) to provide high-shear mixing for systems containing CO2, water, and Mn(PFPE)2 surfactant as shown in Figure 2.4-8. 

Remarkably, the C02/water/Mn(PFPE)2 emulsion system is CO2-continuous up to 50% water by volume. As shown in Figure 2.4-9, conductivities for the entire emulsion are as low as six orders of magnitude less than those of the aqueous phase, clearly suggesting that the emulsions are CO2-continuous. Dielectric data support this assertion as well. The measured dielectric constants for the emulsion remain not far above those of CO2, from 1.6 to 1.7, even at 50% by volume water. These opaque emulsions look like white milk. 

Scattering techniques provide the most definite proof of micellar aggregation. Zielinski et aL (34) employed SANS to study the droplet structures in these systems. Conductivity measurements (35) and SANS (36) were also used to study droplet interactions at high volume fraction in w/c microemulsions formed with a PFPE-COO NH4 surfactant (MW = 672). Scattering data were successfully fitted by Schultz distribution of polydisperse spheres (see footnote 37). A range of PFPE-COO NH/ surfactants were also shown to form w/c emulsions consisting of equal amount of CO2 and brine (38-40). 

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