Temperature critical flocculation

As another criterion of stability, a critical flocculation temperature(OFT) was measured. The measurement of CFT was carried out as follows the bare latex suspension was mixed with the polymer solution of various concentrations at 1+8 °C by the same procedure as in the adsorption experiments. Then, the mixture in a Pyrex tube(8 ml, U.0 wt %) was warmed slowly in a water bath and the critical temperature at which the dispersion becomes suddenly cloudy was measured with the naked eye. 

Figure 8. Relationship between critical flocculation temperature(CFT) and concentration of HPC for polystyrene latex.

For both kinds of polymer-grafted particles, flocculation was induced either by changing the temperature or by adding a nonsolvent for the stabilising polymer. In this way critical flocculation temperature (c.f.T) and critical flocculation vol. fractions of non-solvent (c.f.v.) values were obtained, in general as a function of . 

Figure 3. Critical flocculation temperature (T) versus log (particle volume fraction ) for the two Si02 g PDMS dispersions in bromocyclohexane O, S15/PDMS5 x, S15/ PDHS3.

Figure 2 The upper critical flocculation temperature is shown plotted against the applied pressure. refers to the PMMA latex while, o refers to the PVAc latex. The solid line represents 9 conditions for a solution of PIB + 2-methylbutane while the dashed line represents the theoretically calculated UCFT as a function of pressure.

Figure 5 shows the variation of Xg with temperature at two C values (0.20 and 0.25 mol dm 3). In both cases Xg is essentially zero until a critical temperature is reached, above which xg increases rapidly with increasing temperature reaching a maximum above which there is a tendency forXg to fall again with further increase in temperature. The critical temperature corresponding to the abrupt increase inTg is 20 and 25°C for C equal to 0.25 and 0.20 mol dm, respectively This temperature may be identified with the critical flocculation temperature (CFT) of the concentrated dispersion. 

FIG. 13.14 Critical flocculation temperature of aqueous poly(vinyl acetate) dispersion stabilized by poly(oxyethylene) indicated by a sharp change in absorbance with temperature. (Redrawn with permission from D. H. Napper, in Colloid and Interface Science (M. Kerker, R. L. Rowell, and A. C. Zettlemoyer, Eds.), Academic Press, New York, 1977.)... 

TABLE 13.3 Critical Flocculation Temperatures for Various Polymer-Solvent Systems, with 0 Temperatures Included for Comparison... 

Discuss polymer-colloid interactions and steric stability from a thermodynamic perspective. What is enthalpic stabilization What is entropic stabilization What is the critical flocculation temperature (CFT) ... 

Larpent and Tandros [102] prepared microlatex particles by polymerization of PEO-MA macromonomer with MMA, styrene, and vinyl acetate. The nonionic latexes are very stable, giving no flocculation up to 6 mol dm 3 NaCl or CaCl2 and a critical flocculation concentration (CFC) of 0.6 mol dm 3 for Na2S04 or MgS04 was estimated. Charged latexes are less stable than the nonionic ones. The CFC of all latexes are determined as a function of electrolyte concentration. With the nonionic latexes, however, the critical flocculation temperature (CFT)... 

Table 8.4 Classification of sterically stabilised dispersions and comparison of critical flocculation temperatures (c.f.t) with theta-temperatures112 (By courtesy of Academic Press Inc.)...

The current theories of steric stability (3-6) predict that provided the particles are well-covered and the polymer is well-anchored particles bearing non-ionic polymers should flocculate at or near the 0-point of the stabilising chains. The available experimental date ( 3, 7 9 8) confirm this result in as much as critical flocculation temperatures and pressures have been found to correlate tolerably well with the relevant 0-points for a wide range of systems. Where the correlation has been less than satisfactory the discrepancy has often been understandable in terms of multiple anchoring, selective adsorption of lyophobic blocks, or other specific effects (9, 10). 

Polyacrylic acid was chosen as the stabilising moiety because there is a substantial body of literature on its solution properties and because Napper (12) has demonstrated a correlation between critical flocculation temperature and 0-temperature for particles stabilised by copolymers of unionised PAA. Also, since PAA is a weak acid the degree of ionisation can be varied by titration with base. 

VI Flocculation Studies, Critical flocculation temperatures were measured using suspensions of 0.6% v/v latex containing the appropriate amount of sodium chloride. Differing degrees of neutralisation were produced by the addition of NaOH to stock solutions and checked by conductometric titration with base in the absence and presence of added HC1. Flocculation temperatures (CFT) were determined by visual observation at a cooling/heating rate of M).5 C s. The flocculation was generally reversible provided that the latices had not been left in the flocculated state for more than a few minutes or so. 

Figure 6. Critical flocculation temperatures (CFVs) for a latex as a function of the degree of neutralization (a) in 0.83M, 1.1 OM, and 1.38M NaCl on heating (, A, M) and on cooling (O, A, Q), respectively.

TABLE 10.6 Comparison of Critical Flocculation Temperatures (CFT) with 0 Temperatures for Some Aqueous and Nonaqueous Dispersions"... 

Filler particles can be modified to decrease flocculation. Kaolin particles modified by a graft of poly(ethylene oxide) showed an increase in the upper critical flocculation temperature. Stabilization of particle dispersion was due to an enhanced steric stabilization." ... 

When >0.5, becomes negative (attractive) this, combined with the van der Waals attraction at this separation distance, produces a deep minimum causing flocculation. In most cases, there is a correlation between the critical flocculation point and the 0-condition of the medium. A good correlation is found in many cases between the critical flocculation temperature (CFT) and the 0-temperature of the polymer in solution (with both block and graft copolymers the 0-temperature of the stabilising chains A should be considered) [2]. A good correlation was also found between the critical volume fraction (CFV) of a nonsolvent for the polymer chains and their 0-point under these conditions. In some cases, however, such correlation may break down, and this is particularly the case for polymers that adsorb by multipoint attachment. This situation has been described by Napper [2], who referred to it as enhanced steric stabilisation. 

For example, the emulsion will flocculate at a temperature (referred to as the critical flocculation temperature CFT) that is equal to the 0-temperature of the stabilising chain. The emulsion may flocculate at a critical volume fraction (CFV) of a nonsolvent, which is equal to the volume of nonsolvent that brings it to a... 

In the presence of 0.1 mol dm Na2S04, G remained constant up to 40 °C, above which it was increased with further increases in temperature this temperature is denoted as the critical flocculation temperature (CFT). The CFT decreases with increases in electrolyte concentration, reaching 30°C in 0.2 and 0.3moldm Na2S04. The reduction in CFT with increases in electrolyte concentration was... 

This can be achieved for stericaUy stabihsed suspensions, when the medium for the chains becomes a 0-solvent This occurs, for example, on heating an aqueous suspension stabilised with PEO or poly(vinyl alcohol) chains. Above a certain temperature (the 0-temperature), which depends on the electrolyte concentration, flocculation of the suspension will occur, and the temperature at which this occurs is defined as the critical flocculation temperature (CFT). 

Neville and Hunter [6] studied the flocculation of poly(methylmethacrylate) (PMMA) latex stabilised with poly(ethylene oxide) (PEO). Flocculation was induced by the addition of electrolyte and/or an increase in temperature. Figure 21.12 shows the variation of with increase of temperature at constant electrolyte concentration. In the figure, it can be seen that increased with increases of temperature, reaching a maximum at the critical flocculation temperature (CFT), and then decreased with further increase in temperature. The initial increase was due to the flocculation of the latex with increase of temperature, as the result of a reduction in solvency of the PEO chains with the increase in temperature. The... 

CFT Critical flocculation temperature. See Critical Coagulation Temperature. 

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