Hydrophobically modified chains

Interpretation of NRET Data Conformations of Hydrophobically Modified Chains... 

This chapter describes the most important fluorescence studies of the behavior of polymer chains in solutions, gels, and melts and in the bulk amorphous state. Because the book contains several chapters devoted to specific systems and methods, we try to avoid duphcities concerning, e.g., self-assembled core-shell nanoparticles, the behavior of hydrophobically modified chains, and PCS studies of polymers in solutions, and turn our attention to other aspects. Nevertheless, it was sometimes necessary to mention some topics treated in the other chapters. 

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

Fig. 3.5 Representation of a scheme of an experiment (upper set of drawings) and the obtained experimental results presented as AFM images (middle part) and cross-sectional profiles (bottom) that provides evidence of silica nucleation and shell formation on biopolymer macromolecules. Scheme of experiment. This includes the following main steps. 1. Protection of the mica surface against silica precipitation. It was covered with a fatty (ara-chidic) acid monolayer transferred from a water substrate with the Langmuir-Blodgett technique. This made the mica surface hydrophobic because of the orientation of the acid molecules with their hydrocarbon chains pointing outwards. 2. Adsorption of carbohydrate macromolecules. Hydrophobically modified cationic hydroxyethylcellulose was adsorbed from an aqueous solution. Hydrocarbon chains of polysaccharide served as anchors to fix the biomacromolecules firmly onto the acid monolayer. 3. Surface treatment by silica precursor. The mica covered with an acid mono-...

A new class of amphiphilic, surface-active graft copolymers, hydrophobically modified hydroxyethyl celluloses (HM-HEC s), are comprised of a cellulose backbone with short polyethylene oxide (PEO) and grafted alkyl side chains. They are excellent steric stabilizers of 0/W emulsions. 

The hydrogen bonds in aliphatic alcohol clusters can be modified in a systematic, yet subtle, way by replacing hydrogen atoms of the alkyl group by fluorine atoms [248, 249]. This leads to only modest changes in spatial extension, but it introduces polarity into the hydrophobic alkyl chains. Despite their polarity, the fluorine atoms are not considered to be attractive hydrogen bond acceptors [250]. Huorinated alkanes have quite remarkable properties that can be related to this combination of polarity and weak hydrogen bond propensity. Alcohols with... 

Stereo view of the sixty propane minima (thick lines) obtained with the modified force field (see text) on the surface of the A peptide chain (medium lines) of the GCN4 leucine zipper (PDB code 2ZTA). Although the peptide chain was removed during the MCSS procedure, its backbone and hydrophobic side chains are also drawn (thin lines) to show how the propane minima match the aliphatic groups of chain B. Hydrophobic residues are labeled at their Ca atom.Five clusters of propane minima that do not match the hydrophobic side chain of the helix involved in the interhelical interactions are labeled from A (top right) to E (bottom center) and discussed in the text. 

Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles. 

The hydrophobe modification of acrylic acid represents an important class of hydrophobe-modified thickeners prepared by a chain-growth free-radical process. They differ slightly from other examples in that these products are generally cross-linked. 

As described previously for dansylleucine-modified CDs, the hydrophobic side chain of leucine affected the guest binding of the fluorescent CDs. To examine the presence of hydrophobic units near the dansyl moiety, monensin-incorpo-rated dansyl-L-lysine modified (3-CD (41) was prepared as an environment-re-... 

Inclusion properties of molecular nanotubes composed of crosslinked a-cyclodextrin was investigated [47], Induced circular dichroism was used to probe the formation and dissociation of complexes between the nanotubes and azobenzene modified polyethylene glycol), either with or without a hydrophobic alkyl chain. The inclusion complex between the nanotubes and polymers formed at room temperature, and the polymers dissociated from the nanotubes with increasing temperature. 

In a dilute solution, when the polymer is in a coil state (Fig. 6a), the diffusion of hydrophobic particles into the coil is normally faster than the chemical reaction [53]. In this case, the local concentration of particles H inside the coil is practically the same as in the bulk. Therefore, we expect that at the initial stage, the reaction will lead to a random copolymer some of the P monomeric units will attach to H reagent and thereby they will acquire amphiphilic (A) properties P + H —A (Fig. 6b). As long as the number of modified A units is not too large, the chain remains in a swollen coillike conformation (Fig. 6b). However, when this number becomes sufficiently large, the hydrophobically modified polymer segments would tend to form... 

Fig. 6 Schematic representation of the hydrophobic modification of a hydrophilic polymer in a solvent, which is selectively poor for hydrophobic modifier and modified chain segments. The modifying agent and hydrophobic monomers are shown in gray...

Fig. 7 Typical snapshot of a hydrophobically modified copolymer. Hydrophilic chain segments are shown in green and hydrophobic side groups in red...

Fig. 8 Average number fraction of blocks consisting of l neighboring amphiphilic monomers occurring in a 256-unit copolymer chain hydrophobically modified in a solvent having different selectivity for modifying agent. Adapted from [25]...

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