Flocculation concentration, critical free polymer

Weak flocculation occurs above a critical free polymer concentration which decreases with Increase of molecular weight, of the free polymer. The weeik... 

Recent studies in our laboratory (3) using polystyrene latex dispersions stabilised by the above "comb" dispersing agent have shown that polymers such as poly(ethylene oxide) induce wealc flocculation above a certain critical concentration of free polymer which was dependent on the molecular weight of the chain. 

The foregoing interpretation of the nature of the two phases is valid at concentrations of free polymer above the critical value necessary to induce the separation into two phases. Below this critical value, the interpretation of the two phases is quite different. One phase is a liquid, the dispersion medium the other is thQ latex particles. Again, no information is available from the phase rule vis-d-vis the state of aggregation of the dispersed particles in the dispersion medium. This latter analysis is presumably also applicable to dispersions subject to flocculation by the advent of free polymer. 

It is apparent that up to a critical volume fraction Vj of free polymer, the particles displayed long-term stability (1/1F=0). Any further increase in the free polymer concentration resulted in the onset of instability, which was manifested by an increase in l/W. In all cases, however, l/W reached a maximum value and then declined. Flocculation was thus not evident at high concentrations of free polymer. It usually occurred only over a finite range of free polymer concentrations. This lack of flocculation at higher concentrations of free polymer is the phenomenon of depletion stabilization mentioned in the prefatory remarks to this chapter. It will be considered in detail in the next chapter. 

Flocculation was observed to be reversible, either by shear or by dilution of the dispersion medium to below the critical concentration of free polymer. In the former instance, redispersion was only temporary in that the floes reformed on cessation of the shearing action. In the latter case, redispersion was permanent. 

The critical concentration of free polymer that must be added to cause flocculation decreased with increasing molecular weight (see Fig. 16.9). The value of Cj depended upon the molecular weight with an exponent of -0-7. This value is quite different from the exponent (- 0-25) originally reported by de Hek and Vrij (1979) for nonaqueous silica dispersions stabilized by stearyl alcohol and flocculated by polystyrene. The value measured by Sperry et al. is... 

Depletion flocculation theories all predict qualitatively the observed dependence of the critical concentration of free polymer required for flocculation on both the molecular weight of the free polymer and the particle radius. All of the observations published to-date point to (i) higher molecular weight free polymer being a more potent flocculant than lower molecular weight species, and (ii) larger particles being more readily flocculated than smaller ones. 

This is expected since the extrapolation procedure usually overestimates the yield value. The data of Figs. 3 and 4 show a rapid increase in yield value above a critical PEO concentration, 4. This concentration corresponds to the critical flocculation concentration of the free polymer. However, since the rise in Tg did not occur at a sharp value, the latter was taken as the intersection point at which the extrapolated hprizontal and vertical lines meet. This gave values of 4 of 0.02 0.002, O.OItO.001 and O.OOStO.OOl for PEO with of 20,000, 35,000 and 90,000 respectively. Moreover, the 4p values obtained from the two sets of rheological results were almost the scime within the error of locating p. 

This is achieved by the addition of free (nonadsorbing) polymer in the continuous phase [26]. At a critical concentration, or volume fraction of free polymer,, weak flocculation occurs as the free polymer coils become squeezed-out from between the droplets. This is illustrated in Figure 10.27, which shows the situation when the polymer volume fraction exceeds the critical concentration. 

Another method of reducing sedimentation is to employ the principle of depletion flocculation (described in Chapter 7). The addition of free (non-adsorbing) polymer can induce weak flocculation of the suspension, when the concentration or volume fraction of the free polymer ( p) exceeds a critical value denoted by p. Asakura and Oosawa reported the first quantitative analysis of the phenomenon [96]. They showed that when two particles approach to a separation that is smaller than the diameter of the free coil, polymer molecules are excluded from the interstices between the particles, leading to the formation a polymer-free zone (depletion zone). Figure 14.16 shows this for the situation below and above. ... 

Another method of reducing creaming or sedimentation is to induce weak flocculation in the emulsion system. This may be achieved by controlling some parameters of the system, such as electrolyte concentration, adsorbed layer thickness and droplet size. These weakly flocculated emulsions are discussed in the next section. Alternatively, weak flocculation may be produced by addition of a free (non-adsorbing) polymer. Above a critical concentration of the added polymer, polymer-polymer interaction becomes favourable as a result of polymer coil overlap and the polymer chains are squeezed out from between the droplets. This results in a polymer-free zone between the droplets, and weak attraction occurs as a result of the higher osmotic pressure of the polymer solution outside the droplets. This phenomenon is usually referred to as depletion flocculation [59] and can be applied for structuring emulsions and hence reduction of creaming or sedimentation. 

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