Polymer exclusion zone

Due to the osmotic pressure of a polymer solution, an immersed particle will experience a force acting normal to its surface. For an isolated particle, the integral of the pressure over the entire surface gives a zero force. When the particles are closer together than the radius of the polymer, there is a polymer exclusion zone along... 

Figure 6.8 The mechanism of the depletion interaction between two particles in the presence of free polymer molecules, illustrating the exclusion zone in the region between the two particles.

Hoftyzer and van Krevelen [100] investigated the combination of mass transfer together with chemical reactions in polycondensation, and deduced the ratedetermining factors from the description of gas absorption processes. They proposed three possible cases for poly condensation reactions, i.e. (1) the polycondensation takes place in the bulk of the polymer melt and the volatile compound produced has to be removed by a physical desorption process, (2) the polycondensation takes place exclusively in the vicinity of the interface at a rate determined by both reaction and diffusion, and (3) the reaction zone is located close to the interface and mass transport of the reactants to this zone is the rate-determining step. 

The effect of exclusion on the retention volumes of macromolecules was qualitatively explained above. The pioneering work of Casassa [54] has shown that the extent of pore exclusion of macromolecules is controlled by the changes in their (conformational) entropy. The principle is explained in a simplified form in Figure 16.4a through c. A zone of polymer solution with a nonzero concentration travels along a column packed with porous particles. Initially, the concentration of macromolecules within pores is zero (Figure 16.4a). The concentration gradient outside of pore (c > 0) and within pore (c = 0) pulls macromolecules into the pores. 

When one uses gradient elution, both in the adsorption and the exclusion separation modes, it is necessary to bear in mind that the zones of different functionality may overlap which is caused by the MWD, since in both modes Kd very strongly (exponentially) depends on the length of the molecule, while the functionality dependence is given by Eq. (3.11). Moreover, gradient elution markedly limits the choice of a detector and is, therefore, applicable only for some polymers. 

Also, the theory is much more complex than just presented. Bly36 has classified the process into three mechanisms steric exclusion, restricted diffusion, and thermodynamic considerations, and the process has been thoroughly studied. The rate equation is also different for SEC. For polymers, the longer retained peaks have smaller peak dispersivities, H, than early peaks, in direct contrast to normal LC expectations. In part this is due to the fact that the smaller molecules that elute last have higher diffusion coefficients and therefore less mass transfer zone spreading. 

Figure 10.18 is a schematic representation of depletion stabilization in which the polymer is prevented from the zone of close approach between two particles. As a result of this low polymer concentration between the particles due to size exclusion, there is a lower osmotic pressure, which results in (1) an attractive force for greater than theta solvents and (2) a repulsive force for less than theta solvents. Theta solvents will be discussed in the section on the thermodynamics of polymer solutions, but first a discussion of pol3naaer properties. 

In the gel network there are zones, where the polymers interact, and large segments, where the macromolecules are randomly extended. The lattice is responsible for the elasticity and the textural strength of the product. In multicomponent gels all constituents may form separate or coupled networks, or else one component, not involved in network formation, may indirectly affect the gelling by steric exclusion of the active molecules. Such exclusion increases the concentration of the active component in the volume of the solution where the gel is formed. In gels made from the mince of squid meat at 1.5% NaCl, the added carrageenan and egg whites form separate networks that support the structure made of squid proteins, while added... 

A number of workers have reported on kinetic models for plasma polymerization. Williams and Hayes (36) first suggested that the reaction occurred exclusively on solid surfaces within the reaction zone. Initially, monomer is adsorbed onto the electrode surface, where a portion is converted to free radical species after bombardment by ions and electrons produced in the plasma. Surface radicals then polymerize with adsorbed monomer to yield the thin film product. Based on this scheme, Denaro, et. al. derived a simple rate expression which showed reasonably good agreement with deposition rate data at various pressures and power levels (16). It is, however, unrealistic to assume that the plasma polymerization reactions occur exclusively on the surface. A more likely mechanism is that both gas phase and surface reactions proceed simultaneously in plasma polymer formation. 

Actually, size exclusion should not be the only possible separation mechanism resulting in a spontaneous chromatographic salt resolution. Any type of differentiated retention of the cation or anion must also generate acidic and basic effluent zones. Thus, in the case of ammonium acetate, simple dispersive (hydrophobic) interactions of the acetate anions overbalance their partial exclusion from the Styrosorb 2 polymer phase. Here, the ammonia-enriched alkafrne firactions elute first, while the acetic acid-smelling acidic fractions elute from the column with a pronounced retention (Fig. 12.17). It should be noted that contrary to the bulky tetrabutylammonium cations, the acetate anions are barely excluded from the polymer phase and can experience hydrophobic retention in fine pores. 

The LC LC separation mode that employs narrow zone of retention promoting substance is easy to employ because the retention volume of interacting polymer can be adjusted by the time delay between barrier and sample injection. The longer time delay, the lower retention volume of interacting macromolecules because the extended time is allowed for their fast elution in the exclusion mode, before their impact with the barrier. As indicated, multiple barriers can be employed to separate three or more sample constituents such as multicomponent polymer blends, statistical copolymers of different compositions or parent homopolymers from diblock copolymers. 

Finally, the occurrence of the diffusion across the contact zone of two polymer samples with vitrified bulk indicates that the long-range segmental motions are realized in such samples not exclusively on free surfaces, as was suggested in (Kawana et al., 2001 Sharp et al., 2003) but in the contact layers as well. 

---