Dangling chains, anchoring

Damman grating, SLM 798 dangling chains, anchoring 541 database Liqcryst 98 f de Broglie relationship 681 de Gennes theory... 

Another measurement on a system of copolymers consisting of polyfcthylene-oxide-fc-styrene) (PEO-PS) chains anchored to a glass surface in toluene solution was reported by Fytas et al. [18], The picture of the layer is that the PEO part attaches to the glass while the PS part dangles in the solution. The thickness of... 

Figure 4. The three main substrate classes (a) smooth surfaces on which surface molecules have a definite orientational distribution (represented surface obtained on a rubbed polyimide film [52]) (b) interpenetrable surfaces of dangling chains (c) topographies (represented grooved surface) with a favorable (left) and unfavorable director field R. In all cases, a is the macroscopic anchoring direction.

For block copolymers in which one block acts as anchor and the other block dangles into bulk solution, the conformation of the adsorbed polymer was readily determined due to the characteristic molecular structure. It was shown that the conformation is either tail-train or tail-train-tail, depending on whether the block copolymer is AB or ABA, and that the tails are more elongated than the end-to-end distance of the block chain in bulk solution. 

The above polymeric triblocks can be applied as dispersants, whereby the assumption is made that the hydrophobic PPO chain resides at the hydrophobic surface, leaving the two PEO chains dangling in aqueous solution and hence providing steric repulsion. Although these triblock polymeric surfactants have been widely used in various appHcations suspensions, some doubt has arisen as to how effective these can be. It is generally accepted that the PPO chain is not sufficiently hydrophobic to provide a strong anchor to a hydrophobic surface. 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

Polymeric surfactants can be homopolymers, random amphiphilic copolymers, and of the A-B (diblock), A-B-A (triblock), and BA (graft) types [23]. The A chain is referred to as the stabilizing chain (soluble in the medium), and the B chain is referred to as the anchor chain (insoluble in the medium with strong affinity to the surface). The simplest type of a polymeric surfactant is a homopolymer, such as PEO and poly(vinyl pyrrolidone) (PVP). Homopolymers are not the most suitable surfactants and it is better to use polymers with some groups that have affinity to the surface. The most employed copolymers are random amphiphilic copolymers, like poly(vinyl alcohol), diblocks of polystyrene-block-poly(vinyl alcohol) (PS-b-PVA), poly(ethylene oxide)-block-polystyrene (PEO-b-PS), and triblocks of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO-b-PPO-b-PEO, Pluronic) (PPO resides at the hydrophobic surface, leaving the two PEO chains dangling inaqueous solution), andpoly(ethyleneoxide)-block-polystyrene-block-poly(ethylene oxide) (PEO-b-PS-b-PEO). The graft copolymer is referred to as a comb stabilizer Atlox 4913,... 

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