Compression rates

Neumann has adapted the pendant drop experiment (see Section II-7) to measure the surface pressure of insoluble monolayers [70]. By varying the droplet volume with a motor-driven syringe, they measure the surface pressure as a function of area in both expansion and compression. In tests with octadecanol monolayers, they found excellent agreement between axisymmetric drop shape analysis and a conventional film balance. Unlike the Wilhelmy plate and film balance, the pendant drop experiment can be readily adapted to studies in a pressure cell [70]. In studies of the rate dependence of the molecular area at collapse, Neumann and co-workers found more consistent and reproducible results with the actual area at collapse rather than that determined by conventional extrapolation to zero surface pressure [71]. The collapse pressure and shape of the pressure-area isotherm change with the compression rate [72]. 

BirdsalkJ. C., Graph Finds Compression Rate Quickly, Hydrocarbon Processing V. 47, p. 153, Jan. (1968). 

Fig. 13 Force/area curves of dipalmitoylphosphatidyl choline monolayers spread on pure water at 25°C (solid line) and 45°C (dashed line). The compression rate is 7.2 A2/molecule per minute. The shape of the isotherms is identical for homochiral and heterochiral films.

Fig. 17 Surface pressure/area isotherms for the compression and expansion cycles of racemic (dashed line) and enantiomeric (solid line) stearoylserine (A), stearoyl-alanine (B), stearoyltryptophan (C), and stearoyltyrosine methyl esters (D) on a pure water subphase at 25°C carried out at a compression rate of 7.1 A2/molecule per minute. Arrows indicate the direction of compression and expansion.

Fig. 19 Surface pressure/area isotherms for the compression/expansion cycle of enantiomeric (dashed line) and racemic (solid line) SSME monolayers on pure water subphase at (a) 20°C, (b) 25°C, (c) 30°C and (d) 40°C. The compression rate is 29.8 A2/ molecule/min. Reprinted with permission from Harvey et al., 1989. Copyright 1989 American Chemical Society.

Fig. 22 Surface pressure/area isotherms for the compression cycles of stearoyltyrosine on a buffered pH 6.86 subphase carried out at a compression rate of 19.24 A2/molecule per minute at 16,19,22,25,28, 31, and 34°C. Reprinted with permission from Harvey et ah, 1990. Copyright 1990 American Chemical Society.

Fig. 45 Surface pressure/area isotherms for the compression cycle of 12-ketooctadecanoic acid (A) and octadecanoic acid (B) on a buffered subphase (AR hydrochloric acid pH 4.0) at 30°C carried out at a compression rate of 2.0-3.0 A2/molecule per minute.

When an ideal monolayer is compressed, A = A - vt, where t is the time after the experiment begins. At t = 0, A = A, the uncompressed area of the monolayer, and u A = n RT at equilibrium. From equations (2) and (3), a simp e°rela ionship for the surface pressure as a function of area, compression rate and time is obtained ... 

Figure 3. Effect of subphase on the tc-A curve of PhDA2-8 film. The subphase temperature was 5.0°C and the compression rate was 7.5(A2/molecule)/min. The subphase composition was as follows ...

Figure 6 shows the effects of compression rate on the ji-A curve for the PhDA2-8 thin film at air/water interface. Accompanied with the increase in the compression rate, the hump becomes more significant and the maximum surface pressure of the hump shifts toward the larger surface area. It is to be noted that the region with zero surface pressure appears only with appropriate compression rates of 3 - 7.5 (A2/molecule)/min as in (d), (e), and (f). 

As has been mentioned above, the 7t-A curves of the PhDA2-8 thin film show the existence of the zero pressure region after the formation of the overshoot hump. It is noteworthy that the remarkable overshoot hump and the subsequent zero surface pressure are observable only in a particular range of compression rates, suggesting that the inclusion of the effect of non-... 

Figure 6. Effect on the compression rate on the PhDA2-8 Jt-A curve for the subphase with SxlO M CdCl2 and 5xlO sM KHC03.

Figure 7.1. Surface pressure-area plot for polymer monolayer measured at 20°C and at a compression rate 2 x 10 nm/s for each side group.

The stability of films, even in thermodynamically metastable states, may be tested by stopping the barrier drive at intervals during compression of the film. If there is no drop in film pressure after several minutes, it is unlikely that the it-A relationships up to that point will be dependent on the compression rate. Figure 12 shows... 

Figure 11. Effect of variation of compression rate on a film that shows kinetic effects of surface packing curve I, tenfold variation of rate in curve II. From Thompson (101).

Figure 33. w-A isotherms of the chiral Af-a-methylbenzylstearamides on 3, 6, and lOA H2SO4 at 25 1°C. Compression rate, 7.2 A /molecule min initial molecular area, 120 A. Solid lines depict behavior of racemate broken lines depict behavior of enantiomers. From Thompson (101). 

As shown in Fig. 4.1, resin feedstocks have a considerable level of interparticle space that is occupied by air. This level of space and thus the bulk density of the feedstock depend on the temperature, pressure, pellet (or powder) shape, resin type, and the level and shape of the recycle material. For a specific resin feedstock, the bulk density Increases with both temperature and the applied pressure. Understanding the compaction behavior of a resin feedstock is essential for both screw design and numerical simulation of the solids-conveying and melting processes. Screw channels must be able to accommodate the change in the bulk density to mitigate the entrainment of air and the decomposition of resin at the root of the screw. Typically, screw channels are set by using an acceptable compression ratio and compression rate for the resin. These parameters will be discussed in Section 6.1. 

Conventional transition sections are constructed by simply decreasing the depth of the channel in the down-channel direction. The amount and rate of the depth change sets the performance of the melting process and the removal of entrained air that resides between the feedstock pellets or powders. The compression ratio sets the amount of compression while the compression rate sets the rate of the compression. The compression ratio and compression rate are calculated as follows for conventional-flighted transition sections ... 

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