Heterosteric stabilization

The interpenetrational-plus-compressional domain is also important in heterostericdly stabilized dispersions where the different stabilizing sheaths are composed of compatible polymers. Elastic repulsion can give rise to heterosteric stabilization. 

For heterosteric stabilization, we obtain for the mixing free energy in the volume element ... 

For heterosteric stabilization, the interpenetrational domain is defined by + where L2, L3=span of the stabilizing moieties and L2 > L3. 

Qualitative predictions of the theory Provided that L3 is not excessively small, the interpenetrational domain will determine the flocculation behaviour in heterosterically stabilized systems, just as it does in homosteric stabilization. In the limit of small L3, the interpenetrational-plus-compressional domain may well become important in predicting incipient instability. The elaboration of the general principles that govern heterosteric stabilization is then quite different. 

Experimental observation of selective flocculation. Croucher and Hair (1980a) have demonstrated the phenomenon of selective flocculation in mixtures of heterosterically stabilized dispersions. They prepared particles of poly(vinyl acetate) stabilized by polystyrene and polyacrylonitrile particles stabilized by polyisobutylene, both in cyclopentane. The value of X23 for polystyrene and polyisobutylene is known to be positive (Hyde and Tanner, 1968), corresponding to mutually incompatible polymers. The poly(vinyl acetate) particles stabilized by polystyrene exhibited both upper and lower critical flocculation temperatures whereas the polyisobutylene stabilized particles only flocculated on heating. 

If the dispersion medium is a good solvent for both polymers (xi 2. X13 < 2). then the system will be stable provided that Ix23l< 1—X12—Xi3- This apparently explains the heterosteric stabilization, alluded to above, that is ol rved on mixing aqueous polystyrene latices homosterically stabilized by poly(ethylene oxide) and polyacrylamide. 

It is possible, at least in principle, for X23=0, even if the two polymers are not chemically identical. In that case, heterosteric stabilization would be observed if the dispersion medium were a good solvent for both polymers. If, however. 

Note that Croucher and Hair (1980) have claimed that according to free volume theory, xiz is an order of magnitude smaller than that used by Feigin and Nappes (1978). It has subsequently been shown by Roe and Zin (1980), however, that the free volume contribution to xi3 is relatively unimportant. These authors obtained values of 23—014 for polystyrene and polybutadiene by experimental measurements of the miscibility of the two polymers, whereas the free volume contribution is only ca 0-01. Roe and Zin advocated the use of the solubility parameter approach to the calculation of j23. which is what has been adopted above. This is a further instance where the free volume theory proves inadequate at the quantitative level. This gross underestimation of X23 has unfortunately led Croucher and Hair (1980) to present erroneous values for the free energies of interaction for heterosterically stabilized systems. 

Both of the diagrams shown in Fig. 14.1 illustrate the strong repulsive interactions that can be generated in heterosteric stabilization by incompatible polymers. Indeed it is evident in this example that the 2-3 particle interactions are stronger than either the 2-2 or 3-3 interactions. In addition. Fig. 14.1b shows the appearance of a —SkT pseudo-secondary minimum in the interactional free energy of polystyrene-coated particles at 5 K below their 0-temperature. This minimum would be sufficient to ensure 2-2 homoflocculation. The 3-3 and 2-3 interactions are clearly repulsive and so the qualitative free energy calculations confirm the possibility, foreshadowed above, of the selective flocculation of one particle type in mixtures of particles sterically stabilized by different polymers. 

It is possible to neralize the preceding theories to encompass heterosteric stabilization in polymer melts. To this end, we now set out the modifications needed to allow for the polymeric nature of the dispersion medium. 

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