Polymers nonadsorbing

Thus, the spacing of the chains relative to the neutral, free, swollen size of the buoy blocks is, for a given chemical system and temperature, a unique function of the solvent-enhanced size asymmetry of the diblock polymer and a weak function of the effective Hamaker constant for adsorption. The degree of crowding of the nonadsorbing blocks, measured by a decrease in the left-hand side of Eq. 28, increases with increasing asymmetry of the block copolymer. 

In recent years much attention has been paid to the stability of colloids in the presence of free, nonadsorbing polymers. It is generally found that at relatively low polymer concentrations... 

The principle of depletion is illustrated in Figure 1. If a surface is in contact with a polymer solution of volume fraction , there is a depletion zone near the surface where the segment concentration is lower than in the bulk of the solution due to conformational entropy restrictions that are, for nonadsorbing polymers, not compensated by an adsorption energy. The effective thickness of the depletion layer is A. Below we will give a more precise definition for A. 

Figure 1. Illustration of depletion effects for two plates in a solution of nonadsorbing polymer of volume fraction < >. A is the depletion thickness.

According to Equation 8, two spheres in a solution of nonadsorbing polymer attract each other. The magnitude of the attraction depends on p° (or the osmotic pressure) and on A, both factors being a function of <)>. At low i)), A is constant and p is proportional to the polymer concentration, so that -Afg increases linearly with. In more concentrated solutions, -p° increases more strongly with <)> due to higher order terms in i)), and A decreases. Since for spheres Afs is proportional to a2, the... 

Figure 1.22. Rheograms of a newtonian (4) N and two shear-thinning STl and ST2 emulsions obtained by adding 1.5 wt% ( ) and 1.9 wt% (A) of a nonadsorbing polymer in the continuous phase. (Adapted from [150].)...

Rgure 1.27. Emulsion obtained by shearing a premixed emulsion in presence of a nonadsorbing polymer. System composition sodium alginate (nonadsorbing polymer) 4 wt%, nonionic surfactant (NP7) 3 wt%, oil fraction = 30 wt%. (a) Size distribution, (b) microscopic image. (Adapted from [137].)... 

Attempts to measure the depletion force in nonadsorbing polymer medium with an SEA have failed essentially because measurements are hindered by the slow exclusion of the polymer from the narrow gap due to the large viscosity of the polymer solutions. However, depletion forces have been measured in solutions of living polymers in a semi-dilute regime by Kdkicheff et al. [50]. The... 

E. Evans and D. Needham Attraction Between Lipid BUayer Membranes in Concentrated Solutions of Nonadsorbing Polymers Comparison of Mean-Field Theory with Measurements of Adhesion Energy. Macromolecules 21, 1822 (1988). 

A. Meller and J. Stavans Stability of Emulsions with Nonadsorbing Polymers. Langmuir 12, 301 (1996). 

G.J. Fleer, J.M.H.M. Scheutjens and B. Vincent, The Stability of Dispersions of Hard Spherical Particles in the Presence of Nonadsorbing Polymer in Polymer Adsorption and Dispersion Stability, E.D. Goddard and B. Vincent (eds.), American Chemical Society, Washington DC, 1984, ACS Symposium Series 240, Chapter 16, pp. 245-263. 

Another source of attraction between two surfaces is possible when the surfaces are immersed in a solution of a nonadsorbing polymer (e.g., a polymer that does not adsorb on, or is repelled by, the surfaces). Although this force is generally weak, it can play a significant role in destabilization of colloidal particles under certain circumstances. 

Many different mobile phases have been utilized to provide the forward velocity for nonadsorbed molecules. If die mobile phase is a gas, then the technique used is gas chromatography (GC). In GC, die surface to which die molecules adsorb can be a wide variety of materials which are often prepared by coating an inert surface widi a polymer whose properties are related to its structure. In this way die surface properties and hence adsorption of die solid surface can be varied to give die best chromatographic resolution. 

The flocculation of dispersed species induced by nonadsorbing polymer molecules due to depletion forces. When solutes such as polymer molecules do not, for some reason, enter the gap between adjacent surfaces an attractive force is created between the surfaces. This depletion force arises out of the solute s ability to influence osmotic pressure in bulk but not in the gap between the surfaces. 

Fig. 1 LbL film deposition on a planar support. Immersion in polymer solutions of polycation (a) and polyanion (c). (b, d) Washing steps to remove nonadsorbed polymer molecules, (e) Structure of the LbL-assembled film...

The interactions between two parallel plates or spheres in a solution of nonadsorbing polymers assumed to be spherical have been extensively investigated experimentally,4-6 theoretically,7-18 and by simulations.14,15 The... 

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. 

Fig. 19. Polymer volume fraction profiles, plotted as log[ p(z)/

Dissolved, nonadsorbing polymer molecules must sacrifice configurational degrees of freedom to approach to surface closer than about one coil radius, an energetically unfavorable process in dilute solutions. Indeed, experiments confirm that a layer of thickness L [Pg.205]

These results demonstrate that nonadsorbing polymer can induce phase separations in colloidal systems with the nature of the phases depending primarily on the ratio of the particle and polymer sizes. Since the strength of the attraction is not necessarily a monotonic function of the polymer concentration, e.g., because of penetration of the free polymer into a grafted layer, both destabilization and restabilization are possible. 

Several recent studies demonstrate convincingly the possibilities for adjusting the rheology of colloidal dispersions through the incorporation of polymer. Here we briefly review the effects of grafted polymer, adsorbing homopolymer, and nonadsorbing polymer. The literature abounds with other and more complicated phenomena. 

With nonadsorbing polymer, rheological effects of similar magnitude accompany the phase transitions described earlier (Patel and Russel, 1989a,b). Since macroscopic phase separation takes weeks or months, rheological measurements performed within a few days on samples formulated within the two-phase region, with — Q>miJkT 2 - 20, detect a metastable structure that changes little over time. The systems respond as flocculated dispersions, but the microstructure recovers relatively quickly to a reproducible rest state after shear. Hence these weakly flocculated dispersions are quite tractable materials. 

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