Bimodal distribution techniques

Conclusions. Table II compares some characteristics of the semicontinuous and batch techniques. According to 1H NMR and GPC data the semicontinuous technique produces polymers with well defined symmetrical end groups, while the batchy polymerization may yield once-fired and unfired chain ends. The M Mn values of polymers obtained in semicontinuous runs are close to theoretical, whereas those harvested in conventional batch polymerization exhibit broader molecular weight distributions due to changing [I] and [M], presence of once-fired and/or unfired chain ends, and insufficient reactor control. The latter circumstance may even result in bimodal distributions. 

By applying a steric sub technique (Giddings et al., 1980), field-flow fractionation was shown to have the potential to separate stable polysaccharide suprastructures in greater than hydrocolloidal diameters. Moon and Giddings (1993) used the procedure to size starch granules into a bimodal distribution of mass greater and less than 10 pm. 

A fundamental advantage of the frequency response method is its ability to yield information concerning the distribution of molecular mobilities. For example, a bimodal distribution of diffusivites, which is difficult to detect by conventional sorption measurements, leads to two different resonances [49], Moreover, from an analysis of the frequency response spectrum it is even possible to monitor molecular diffusion in combination with chemical reactions [45]. As in conventional sorption experiments, however, the intrusion of heat effects limits the information provided by this technique for fast adsorption-desorption processes [50]. 

For now, the bimodal distribution may be an artifact. The two lifetimes can be considered as lower and upper bounds of the pore size distribution. This technique is the only one available that can provide non-destructive depth profiles without sample preparations other than mounting them in the vacuum system. Depth profiled lifetime data are currently being collected. This is practical due to the high data acquisition rate of 3TO3 to 104 lifetime events per second, depending on the implantation depth. 

Unilamellar vesicles are usually formed from lipid dispersions with sonication. Thus the suspension is metastablc, and vesicles aggregate to form a multilamellar structure. The decay time of sonicated phosphatidylcholine vesicles, as measured by DLS, showed a bimodal distribution 50. The hydrodynamic radius estimated from the smaller decay time was consistent with the values obtained by other experimental techniques such as ultracentrifugation. 

There is a need to be able to calculate interparticle distance and other parameters from geometric models of bimodal and conventional systems. This paper explains how computer programs for the placement of spheres in a three-dimensional space have been modified to accommodate bimodal distributions. Examples of models composed of simple bimodal particle distributions are presented to illustrate the technique. 

Figure 3 depicts the results of experiments with DPs of 200-500. The average DPs of the components on the left were 244 and 357. That means a ratio of 1.5 between the two maxima. However, the gap between the last fraction of the first component and the first fraction of the second component extends only from 270 to 330. Refractionation of this mixture (lower left) demonstrates clearly that this technique can reveal a bimodal distribution, even if the two maxima are as close to one another as they are in this example. On the other hand, the refractionation yields a fraction lying exactly in the gap at P = 300. This, however, does not prove that there is material at this DP it is possible that the cutting of the second fractionation takes away parts of the two mixing components, yielding a bimodal fraction. If there is a larger... 

A pronounced bimodal distribution of chain conformations in a micelle that contains about 10 chains is something that one would hardly expect in an equilibrium system, even though bimodal distributions have been tentatively proposed for some other equilibrium polymer systems [145]. To elucidate the studied problem, we performed a series of simulations for modified micellar shells using the MC technique for neutral systems and the combined MC-mean field simulation. The simulations are described later (see Sect. 4.2). 

Secondly, when a protein exists in a conformational heterogeneity, a deuterated peptide often exhibits a bimodally distributed isotope pattern (such as in the case of Figure 7.8c) the peak with less deuterium represents the more folded conformation, while the peak with more deuterium represents the less folded conformation. This is a unique advantage of HX-MS relative to most other biophysical techniques, which usually observe an average structural property. 

The method is of particular value for demonstrating the presence of a bimodal distribution, with peaks of concentrations of molecules of high and low affinity. (The Gauss error function assumes a single peak with decreasing concentration of molecules on each side.) Werblin and Siskind used their computational techniques to demonstrate the continued presence of low affinity antibody, up to one year after initial immunization, coexisting with antibody of high affinity present after the first few weeks. As with other techniques, the lower limit of affinity detectable is dependent on the antibody concentration. 

For example, in a recent work (13), ion exchange was proposed as a technique to remove emulsifier from latexes for electron microscopy, to improve the quality of the electron micrographs. The poly(vinyl chloride-co-ethyl acrylate) latex subjected to ion exchange was comprised of a bimodal distribution of particle sizes the average... 

The power of the GPC technique in providing full distributions is illustrated in several ways. Bimodal distributions have been revealed in products of mechanochemistry (see, e.g.. Fig. 2.7), and the data are directly amenable to mathematical modeling [27-30]. 

---