Adsorbed layer thickness weight

Effect of PVA Molecular Weight on Adsorbed Layer Thickness. Figure 4 shows the variation of reduced viscosity with volume fraction for the bare and PVA-covered 190nm-size PS latex particles. For the bare particles, nre(j/ is independent of and the value of the Einstein coefficient is ca. 3.0. For the covered particles, rired/ t increases linearly with tp. Table IV gives the adsorbed layer thicknesses calculated from the differences in the intercepts for the bare and covered particles and determined by photon correlation spectroscopy, as well as the root-mean-square radii of gyration of the free polymer coil in solution. The agreement of the adsorbed layer thicknesses determined by two independent methods is remarkable. The increase in adsorbed layer thickness follows the same dependence on molecular weight as the adsorption density, i.e., for the fully hydrolyzed PVA s and... 

The force-distance profiles Al, A2 appear to show the relaxed, or quasi-equilibrium limit for the interaction between the mica plates bearing the PEO in the good solvent conditions of the present study. The adsorbed layer thicknesses 6 are then about half the value of D at which onset of repulsion (A curves) is first noted. 6 thus corresponds to some 3Rg for both polymers in the present investigation, a value comparable to that obtained for hydrodynamic layer thickness of PEO absorbed on latex particles in water, for similar molecular weights, from light scattering studies. 

A schematic representation of the variation of G j, G p Ga, and Gj with surface-surface separation distance h is shown in Figure 8.4. G j increases very sharply with a decrease in h, when h<28 likewise, G i increases very sharply with a decrease in h, when h<8 and Gj versus h shows a minimum, G j , at separation distances comparable to 28. When h < 28, Gj shows a rapid increase with decrease in h. The depth of the minimum depends on the Hamaker constant A, the particle radius R, and the adsorbed layer thickness 8. G p increases with increases of A and R and, at a given A and R, also increases with a decrease in 8 (i.e., with decrease in the molecular weight, M, of the stabiliser). This is illustrated in Figure 8.5, which shows the energy-distance curves as a function of S/R. The larger the value of 5/R, the smaller the value of G j in this case, the system may approach thermodynamic stability, as occurs with nanodispersions. 

The last criterion for effective steric stabilization is to have a sufficiently thick or grafted polymer layer to screen the van der Waals attraction. An adsorbed layer thickness in the region of 5-10 nm is usually sufficient in most cases, particularly when the dispersion particle size is not too high (a few pm). With graft copolymers, a molecular weight of the side chains of the order of... 

The effect of droplet size and its distribution on the adsorbed layer thickness may be inferred from a comparison of the results obtained with the o/w emulsions with those recently obtained using polystyrene latex dispersions containing grafted PEO chains of (molecular weight 2000) (49). As discussed earlier, the viscoelastic behavior of the system (which reflects the steric interaction) is determined by the ratio of the adsorbed layer thickness to the particle radius (8/R). The larger this ratio, the lower the volume fraction at which the system changes from predominantly viscous to predominantly elastic response. With relatively polydisperse systems, ( )cr shifts to higher values when compared to monodisperse systems with the same mean size. 

Thick adsorbed layers the adsorbed layer thickness should be in the region of 5-10 nm - this means that the molecular weight of the stabdising chains could be in the region of 1000-5000. 

The structure of the adsorbed layer is described in terms of the segment density distribution, p(z). As an illustration, Fig. 5 shows some calculations by Scheutjens and Fleer [17] for loops and tails with r = 1000,4>. = 10 , and X = 0.5. In this example, 38% of the segments are in trains, 55.5% in loops, and 6.5% in tails. This theory demonstrates the importance of tails which dominate the total distribution in the outer region of the adsorbed layer. As we will discuss in the next section on experimental techniques for characterization of the adsorption and conformation of polymers at the solid liquid interface, determination of the segment density distribution is not easy and usually assigns a value for the adsorbed layer thickness 6. This increases with increase of the molecular weight of the polymer and increase of solvency of the medium for the chains. 

Figure 12.10 DLVO term for steric repulsion. The first term (the one squared in parenthesis) is simply the volume fraction of the polymer in the layer. Different expressions are obtained for the three different geometries Indicated here as the overlap volume (vq) dp = sphere diameter (m), H = distance between two objects (m), X = thermodynamic interaction parameter (estimation requires thermodynamic model) (dimensionless), M2 = molecular weight of adsorbed polymer (kg mol ), p = molar volume (m moC ), C2 = surface layer concentration of adsorbed polymer (mol m ), 8 = adsorbed layer thickness (m)...

---