Displacement, adsorbed polymer layers

Assuming that the W/O emulsion behaves as a near hard-sphere dispersion, it is possible to apply the Dougherty-Krieger equation [7, 8] to obtain the effective volume fraction, 4>. The assumption that the W/O emulsion behaves as a near hard sphere dispersion is reasonable as the water droplets are stabihsed with a block copolymer with relatively short PHS chains (of the order of lOnm and less). Any lateral displacement of the polymer will be opposed by the high Gibbs elasticity of the adsorbed polymer layer, and the droplets will maintain their spherical shape up to high volume fractions. 

Effect of adsorbed polymer on the double-layer. Because of the presence of adsorbed train segments, the double layer is modified. The zeta-potential, , is displaced because the adsorbed polymer displaces the plane of shear. The parameters for describing adsorbed polymers are the fraction of the first layer covered by segments, 0, and the effective thickness, A, of the polymer layer, The insert gives the distribution of segments over trains and loops for polyvinyl alcohol adsorbed on silver iodide. Results obtained from double layer and electrophoresis measurements. 

Some mention should be made concerning the state of the polymer at the beginning of TLC experiment. Normally the sample is applied to the plate from solution in a relatively nonpolar solvent. This solvent is then evaporated, and the chromatographic plate is eluted with the chosen eluent. The drying step results in a polymer deposit which would be difficult to characterize it is not simply a precipitate, and it probably is not a simple adsorbed (multi) layer. Redissolution and entry into the mobile phase under displacement conditions occur in a minute or less (13). 

The drawback of the described adsorbents is the leakage of the bonded phase that may occur after the change of eluent or temperature of operation when the equilibrium of the polymer adsorption is disturbed. In order to prepare a more stable support Dulout et al. [31] introduced the treatment of porous silica with PEO, poly-lV-vinylpyrrolidone or polyvinylalcohol solution followed by a second treatment with an aqueous solution of a protein whose molecular weight was lower than that of the proteins to be separated. Possibly, displacement of the weakly adsorbed coils by the stronger interacting proteins produce an additional shrouding of the polymer-coated supports. After the weakly adsorbed portion was replaced, the stability of the mixed adsorption layer was higher. 

For the calculation of AhP we must count contacts within the surface layer, and between the surface layer and the neighbouring layer (which has bulk composition). This calculation is simplified very much if we assume that at the critical point the surface is almost saturated with displacer (i.e. g 1), since not only the polymer, but also the still more weakly adsorbing solvent will have been almost completely displaced. Before exchange, we have a displacer molecule at the surface, and a segment in the solution, giving contributions to the mixing energy h = X Jx 0 and ... 

One of the most important parameters in the S-E theory is the rate coefficient for radical entry. When a water-soluble initiator such as potassium persulfate (KPS) is used in emulsion polymerization, the initiating free radicals are generated entirely in the aqueous phase. Since the polymerization proceeds exclusively inside the polymer particles, the free radical activity must be transferred from the aqueous phase into the interiors of the polymer particles, which are the major loci of polymerization. Radical entry is defined as the transfer of free radical activity from the aqueous phase into the interiors of the polymer particles, whatever the mechanism is. It is beheved that the radical entry event consists of several chemical and physical steps. In order for an initiator-derived radical to enter a particle, it must first become hydrophobic by the addition of several monomer units in the aqueous phase. The hydrophobic ohgomer radical produced in this way arrives at the surface of a polymer particle by molecular diffusion. It can then diffuse (enter) into the polymer particle, or its radical activity can be transferred into the polymer particle via a propagation reaction at its penetrated active site with monomer in the particle surface layer, while it stays adsorbed on the particle surface. A number of entry models have been proposed (1) the surfactant displacement model (2) the colhsional model (3) the diffusion-controlled model (4) the colloidal entry model, and (5) the propagation-controlled model. The dependence of each entry model on particle diameter is shown in Table 1 [12]. 

Corrosion protection of metal ware by plastics containing contact inhibitors can be exercised according to the schema in Fig. 1.15, b. An anticorrosion plastic element 3 contains an inhibitor in the polymer matrix micropores. Element 3 contacts the protected metal surface 1. Corrosive medium 2 penetrates into the contact gaps and wets the article surface. Corrosion inhibitor 4 either spontaneously or under some effect, e.g. of mechanical forces, releases from the micropores of the polymer matrix into the contact gaps and adsorbs on the metal article surface. In case of mutual displacement of the contacting article 1 and element 3, spreading of the adsorption layer over the article surface is accelerated. 

Surface modification with hydrophilic polymers, such as poly(ethylene oxide) (PEO), has been beneficial in improving the blo( compatibility of polymeric biomaterials. Surface-bound PEO is expected to prevent plasma protein adsoiption, platelet adhesion, and bacterial adhesion by the steric repulsion mechanism. PEO-rich surfaces have been prepared either by physical adsorption, or by covalent grafting to the surface. Physically adsorbed PEO homopolymers and copolymers are not very effective since they can be easily displaced from the surface by plasma proteins and cells. Covalent grafting, on the other hand, provides a permanent layer of PEO on the surface. Various methods of PEO grafting to the surface and their effect on plasma protein adsorption, platelet adhesion, and bacterial adhesion is discussed. 

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