Separation mechanism factor

Toughening for whisker-reinforced composites has been shown to arise from two separate mechanisms frictional bridging of intact whiskers, and pullout of fractured whiskers, both of which are crack-wake phenomena. These bridging processes are shown schematically in Figure 13. The mechanics of whisker bridging have been addressed (52). The appHed stress intensity factor is given by ... 

Thus, a 2-D separation can be seen as 1-D displacement operating in two dimensions. The 2-D TLC separation is of no interest if selection of the two mobile phases is not appropriate. With this in mind, displacement in either direction can be either selective or non-selective. A combination of two selective displacements in 2-D TLC will lead to the application of different separating mechanisms in each direction. As an extreme, if the solvent combinations are the same (5ti = 5t2 5vi = va) or very similar (5ti 5vi 5ya), the compounds to be separated will be poorly resolved or even unresolved, and as a result a diagonal pattern will be obtained. In such circumstances, a slight increase in resolution might occur, because of an increase by a factor of V2 in the distance of migration of the zone (4). 

Multidimensional methods thus involve a combination of single mechanisms and systems. In any multidimensional (usually 2D) approach, it is desirable that each dimension be as pure as possible in terms of selectivity of the separation mechanism. In comprehensive 2D separations, the precision (or chromatographic resolution) becomes a limiting factor and is ultimately determined by the quality of the separation in both dimensions. 

To help lessen the complications of product testing, a common approach is to separate mechanical and environmental testing. The mechanical properties can be tested on the product, using a relatively simple product test rig, while environmental degradation is studied on samples of the material. For time dependent mechanical properties, the degree of mechanical deterioration of the product is then increased by a factor reflecting the... 

It is, however, also to be noted that deviations from linearity of the aforementioned plots may be readily observed, which may indicate smooth changes in the separation mechanism. If the k vs. percentage of modifier dependency is investigated over the entire or at least a wider range, [/-shaped curves may be obtained, in particular with acetonitrile as modifier. While the drop of retention factors with increase of modifier percentage at low modifier contents may follow the described RP-behavior, the trend... 

Summarizing it can be stated that the separation by gas phase transport (Knudsen diffusion) has a limited selectivity, depending on the molecular masses of the gases. The theoretical separation factor is decreased by effects like concentration-polarization and backdiffusion. However, fluxes through the membrane are high and this separation mechanism can be applied in harsh chemical and thermal environments with currently available membranes (Uhlhorn 1990, Bhave, Gillot and Liu 1989). 

In the eases reported previously, surface diffusion could have a pronounced effect and could increase the separation efficiency by a factor of 5. However, a sharp increase in the separation efficiency (a few orders of magnitude) is not to be exp>ected, due to the conflict between adsorption and mobility. A high heat of adsorption will invariably lead to a low mobility. Another drawback of surface diffusion is that it becomes less effective at high tempieratures. Therefore, it can be concluded that in order to employ surface diffusion as an eflectiye separation mechanism, the pores simuld be verx small (r iu < 3 nm) and the temperature should be kept low (T < 300°C), due to the necessary physical adsorption of the gas (Uhlhorn 1990). 

Du 1986). This reflects the importance of smaU pores in order to apply effectively capillary condensation as a separation mechanism. Uhlhom (1990) demonstrated the effect of multilayer diffusion of propylene through a modified y-alumina membrane at 0°C. The separation factor for the N2/CjHg mixture was 27, where propylene is the preferentially permeating component, while the permeability increased to 7 times the Knudsen diffusion permeability. Although this mechanism appears to be very effective because of a high separation factor and a high permeability, it is limited by the obvious need for a condensable component. This in turn restricts the applicability range, due to limits set by temperature and pressure, needed for formation of multilayers or capillary condensation. 

A number of publications (6-10) have demonstrated that the size separation mechanism In HDC can be described by the parallel capillary model for the bed Interstices. The relevant expression for the separation factor, Rj., (ratio of eluant tracer to particle mean residence times) Is given by. 

Eqn. 2.4 shows the influence of L and v on tR. Other factors which affect tR are k, which depends on the separation mechanism, and Vs and Vm which indicate the relative amounts of the stationary and mobile phase. 

The effect of simultaneous exposure to both mechanical stress and a chemical environment is often more severe than the sum of each factor taken separately. Mechanical stress, elevated temperatures, and high relative humidity can be a fatal combination for certain adhesive formulations if all occur at the same time. Add to this the possible cyclic effects of each factor, and one can easily see why it is important to understand the effects of environment on the joint. 

We have shown recently that the different separation mechanisms of HPLC and CZE preclude the definition of any retention or velocity factor for CEC that would have comparable significance to that of k in HPLC [4], However, as shown in the following section, the CEC system was defined by a combination of a retention factor that measures the retention in CEC and a velocity factor that characterizes the electrophoretic migration. Further, expressions were proposed for possible peak locators that, just like k and k, in HPLC and CZE, respectively, can serve as useful indicators of peak position on the electrochromatogram and facilitate estimation of various separation parameters such as selectivity and resolution. 

---