Interaction energy nonadsorbing polymer

The phase behavior of nonaqueous colloidal suspensions containing nonadsorbing polymer was investigated by Gast et al. [3] on the basis of statistical mechanics. In their theory, a second-order perturbation approach was used to calculate the free energy. Rao and Ruckenstein [4,5] examined the phase behavior of systems involving steric, depletion, and van der Waals interactions. 

State (i) represents a phenomenon that is referred to as depletion flocculation and is caused by the addition of a free nonadsorbing polymer [35]. In this case, the polymer coils cannot approach the particles to a distance A (this is determined by the radius of gyration of free polymer R f), as the reduction in entropy on close approach of the polymer coils is not compensated by an adsorption energy. Thus, the suspension particles will be surrounded by a depletion zone with thickness A. Above a CFV of the free polymer, the polymer coils are squeezed out from between the particles and the depletion zones begin to interact At this point the interstices between the particles are free from polymer coils, such that an osmotic pressure is exerted outside the particle surface (the osmotic pressure outside is higher than in between the particles), and this results in a weak flocculation [35]. A schematic representation of depletion flocculation is shown in Figure 9.11. 

Generally, the structure of the polymer-particle complex can be found from the minimization of free energy that includes the polymer-particle interaction energy, entropies of nonadsorbed monomer units and imits localized at the surface of the particle, and typically, for the system under consideration, the elastic deformation of crosslinked macromolecule. Such theoretical analysis, following the lines of [234,235], can explain the specific behavior of P(T,a) observed for the envelopes with different niunbers of crosslinks j [ 57 ]. According to [235], when the number of crosslinks is small enough, nj all jimc-... 

FIGURE 3.20 The free energy of interaction per surface site between two plates in the presence of nonadsorbing polymer at various bulk solution volume fractions q),. The left-hand figure shows the results for the 0 solvent (x = 0.5) on the right-hand side are the data for an athermal solvent (x = 0). r = 100, Xs = 0, hexagonal lattice. Reprinted from Scheutjens and Fleer (1982) with permission from Elsevier. 

As summarized in the next sections, the micropipet technique has been used to measure adhesive interbilayer interactions based on these attractions that are enhanced by depletion flocculation produced by nonadsorbent polymer, and are attenuated by short-range hydration repulsion, thermal undulations and electrostatic double-layer repulsion energies [14,15,22,25]. 

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