Collapsed brushes

The brush thickness and the axial distance per side chain are interrelated due to the condition of fixed monomer density r in the collapsed brush as R = Vn/ 11 h. The axial tension is given by... 

In summary with this system it has been possible to observe the transition from the osmotic to a salted and then to a collapsed brush. One might suspect that these transitions are also reflected in the pressure/area isotherms. However, we have not yet been able to relate the breaks in the isotherm slopes with parameters deduced from X-ray reflectivity. This would require an even higher precision of data analysis, and precision in density determination better than 1% is difficult to achieve. We realize that the transition at nc is accompanied by a change in relaxation times as expected for a... 

Here N is the number of monomer units in the side chains and R is their end-to-end distance. In a uniformly collapsed brush below the 0 point, the binary interactions are attractive and the polymers density °c r. The thickness of the collapsed brush scales as Roc (N/h z )ir/. The axial tension is given... 

If the backbone is not stiff but flexible, an eventually resulting negative axial tension can be partly released by contraction, i.e., variation of the effective grafting density h. Moreover, under the condition of a fixed monomer density Nl h) r, minimization of the total elastic free energy of the collapsed brush has been shown to result to156... 

Curves relating the corrected retention volume to the concentration of moderator (methanol) in the mobile phase [3] are shown in Figure 4. In pure water, the hydrocarbon chains of the brush phase interact with each other and collapse onto the surface in much the same way as drops of an hydrocarbon will coalesce on the... 

In contrast, the alkane chains on the polymeric phase cannot collapse in an environment of water as they are rigidly held in the polymer matrix. Thus, the retention of the solute now continuously falls as the methanol concentration increases as shown in Figure 4. It should be pointed out that if the nature of the solutestationary phase interactions on the surface of a bonded phase is to be examined in a systematic manner with solvents having very high water contents, then a polymeric phase should be used and brush type reversed phases avoided if possible. 

In cases when the two surfaces are non-equivalent (e.g., an attractive substrate on one side, an air on the other side), similar to the problem of a semi-infinite system in contact with a wall, wetting can also occur (the term dewetting appHes if the homogeneous film breaks up upon cooHng into droplets). We consider adsorption of chains only in the case where all monomers experience the same interaction energy with the surface. An important alternative case occurs for chains that are end-grafted at the walls polymer brushes which may also undergo collapse transition when the solvent quality deteriorates. Simulation of polymer brushes has been reviewed recently [9,29] and will not be considered here. 

Everybody is familiar with a number of phenomena which indicate that the surface of a liquid is in a condition of tension, or—to use a parallel which is graphic, while incorrect in one particular—behaves as if it were composed of an elastic membrane. If a camel-hair brush is submerged in water, the hairs remain separate as they do in air, but they collapse on being S.T. I... 

It is contended that the renal slice technique measures primarily basolateral uptake of substrates or nephrotoxins, based on histological evidence of collapsed tubular lumens. This results in the inaccessibility of brush-border surfaces for reabsorptive transport (Burg and Orloff, 1969 Cohen and Kamm, 1976). This observation limits the ability of this model to accurately reflect reactions to nephrotoxins that occur as the result of brush-border accumulation of an injurious agent. Ultrastructurally, a number of alterations, particularly in the plasma membrane and mitochondrial compartments, have been shown to occur over a 2-h incubation period (Martel-Pelletier et al., 1977). This deterioration in morphology is very likely a consequence of the insufficient diffusion of oxygen, metabolic substrates, and waste products in the innermost regions of the kidney slice (Cohen and Kamm, 1976). Such factors also limit the use of slices in studying renal metabolism and transport functions. 

On large scales, the whole brush can collapse into a globule. 

The two examples of adsorbed side chain substituted macromolecules, i.e., the poly(n-butyl acrylate) brush and the tris(p-undecyloxybenzyloxo) benzoate jacketed polystyrene, demonstrate two rather complementary aspects of the interaction of such molecules with a planar surface. In the first case the two-dimension to three-dimension transition results in a cooperative collapse of an extended coil conformation to a globule. The second case shows a rather high degree ordering with a distinct orientation of the backbone in the substrate plane. Combination of both effects and partial desorption can lead to a repta-tion-hke directed motion as depicted schematically in Fig. 36. 

The brash layer thickness (dry collapsed state) obtained after seven days of polymerization time and successive soxhlet extraction was found to be approx. 10 nm and very uniform ( 0.3 nm). The uniform thickness values are provided by the homogeneous initiation, polymerization and termination reaction. Meanwhile poly(2-oxazoline) homopolymers brushes with layer thicknesses of 20 to 30 nm can be obtained [275]. 

Fig. 26 Dry thickness of poly(acryl amide) as a function of the position on the silica substrate prepared by slow ( ) and fast ( ) removal of the polymerization solution by utilizing the method depicted in Fig. 24. The inset shows the dry poly(acryl amide) thickness as a function of the polymerization time. Note that both data sets collapse on a single curve at short polymerization times. Regardless of the drain speed, the brush thickness increases linearly at short polymerization times and levels off at longer polymerization times. The latter behavior is associated with premature termination of the growing polymers...

Fig. 45 When cylindrical brushes possessing a gradient grafting density along the backbone are adsorbed on a surface, one can observe a transition from a rod-like to a tadpole conformation upon partial desorption of side chains. The end with a higher grafting density, and thus with a greater extension of the side chains, is predicted to collapse more readily than the loose end (Reproduced with permission from [173])...

Recently, Dyer used the same strategy to perform a photoinitiated synthesis of a mixed brush by using an AIBN-type initiator boimd to gold [57]. Specifically, they used initiator (24) to modify gold substrates with a binary brush composed of PS and PMMA. As Fig. 11 describes, mixed brushes will respond to the polarity of the solvent. For example, immersion into a non-selective solvent like THF brings both components to the air/hquid interface since PS and PMMA are both soluble in THF. However, immersion into a polar solvent, such as isobutanol, will selectively bring PMMA to the air/hquid interface, while the nonpolar PS collapses into the interior of the film. In contrast, immersion into cyclohexane brings PS to the air/hquid interface and PMMA is driven to the interior. The cycle is completely reversible after immersion into a nonselective solvent like THF. 

Fig. 11 Switching of a PS/PMMA brush is accomplished by immersion into various solvents. A polar solvent such as isobutanol brings PMMA to the air/liquid interface, while the PS collapses into the interior. The opposite occurs when the substrate is immersed into a nonpolar solvent such as cyclohexane. Upon immersion into a nonselective solvent, like THF, both components come to the air/liquid interface...

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