Polymers adsorbed layer thickness

Rheological methods of measuring the interphase thickness have become very popular in science [50, 62-71]. Usually they use the viscosity versus concentration relationships in the form proposed by Einstein for the purpose [62-66], The factor K0 in Einstein s equation typical of particles of a given shape is evaluated from measurements of dispersion of the filler in question in a low-molecular liquid [61, 62], e.g., in transformer oil [61], Then the viscosity of a suspension of the same filler in a polymer melt or solution is determined, the value of Keff is obtained, and the adsorbed layer thickness is calculated by this formula [61,63,64] ... 

PVA and TaM -for the 88%-hydrolyzed PVA. The same dependence was found for the adsorbed layer thickness measured by viscosity and photon correlation spectroscopy. Extension of the adsorption isotherms to higher concentrations gave a second rise in surface concentration, which was attributed to multilayer adsorption and incipient phase separation at the interface. The latex particle size had no effect on the adsorption density however, the thickness of the adsorbed layer increased with increasing particle size, which was attributed to changes in the configuration of the adsorbed polymer molecules. The electrolyte stability of the bare and PVA-covered particles showed that the bare particles coagulated in the primary minimum and the PVA-covered particles flocculated in the secondary minimum and the larger particles were less stable than the smaller particles. 

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. 

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

Dissolved polymer molecules can be adsorbed by polymer particles via electrostatic attractive force or hydrophobic interaction. When polyelectrolyte is adsorbed on an opposite-charge particle, the polymer molecules usually have a loop-and-tail conformation and, as a result, inversion of charge occurs. For example, sulfatecarrying particles behave as cationic ones after they adsorb poly(lysine). Then poly(-styrene sulfonate) can be adsorbed on such cationic particles and reinvert the charge of particles to anionic (14). Okubo et al. pointed out that the alternate adsorption of cationic and anionic polymers formed a piled layer of polyelectrolytes on the particle, but the increment of adsorbed layer thickness was much less than expected. This was attributed to synchronized piling of two oppositely charged polyelectrolytes (15). 

Fig. 3.31 Schematic picture of average adsorbed chain conformations in extremely dilute solution (isolated chains on the surface), dilute and semidilute solutions, and the polymer melt. The adsorbed layer thickness increases sharply with increasing concentration, mainly due to the contribution of tails. Significant tail formation occurs as soon as the chains begin to compete for surface sites. (From ref. [144])...

The adsorbed layer thickness for a polymer with general fractal dimension Xfu is derived in Problem 3.18 ... 

Chibowski, S., Zeta potential and thickness of a polymer adsorbed layer in the system dispersed sohd-electrolyte, Pol. J. Chem., (H. 1137, 1993. 

Fundamental investigation of the system at the molecular level. This requires investigations of the structure of the solid/liquid interface, namely the structure of the electrical double layer (for charge-stabiUsed suspensions), adsorption of surfactants, polymers and polyelectrolytes and conformation of the adsorbed layers (e.g., the adsorbed layer thickness). It is important to know how each of these parameters changes with the conditions, such as temperature, solvency of the medium for the adsorbed layers, and the effect of addition of electrolytes. 

As discussed in Chapter 6, complete information on polymer adsorption may be obtained if the segment density distribution can be determined - that is, the segment concentration in all layers parallel to the surface. However, such information is generally unavailable, and therefore three main parameters must be determined, namely the amount of adsorption F per unit area, the fraction p of segments in direct contact with the surface (i.e., in trains), and the adsorbed layer thickness 5. 

As a last remark on polymer adsorption, let s consider Fig. 6.3. If a polymer with a hydrodynamic radius Rg is present at low concentration, its configuration at the interface will be relatively flat with trains on the interface. After more polymer is added, the irreversible adsorption on the surface will produce an adsorbed layer thickness on the... 

Fig. 6.3. a Polymer adsorption on interface showing the surface adsorption at low concentration of polymer (q) and at higher concentration of polymer (c2). 6 is adsorbed layer thickness, b Full coverage of the interface by the adsorbed copolymer. The hydrophobic part of the copolymer is shown at the interface (A) and the hydrophilic part protruding in H20 (B). c Schematic showing the segment density distribution for polymer segments attached to the interface... 

The mixing effects described above are readily interpreted in terms of polymer solution theory, and apply when the separation between the approaching surfaces is alrout one to two adsorbed layer thicknesses. At closer approach is it likely that both mixing and elastic compression occurs. The following equation was derived by Napper for the free energy of the interaction involving both effects ... 

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... 

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