Hydrophobic interactions, polymer-water

The interpolymer association is a distinctively different property of exciplex forming polymers from that of excimer forming polymers. The reason must be attributed to the presence of the ground state interaction in the former polymers. Interpolymer excimer formation is, however, facilitated by the aid of hydrophobic interaction in water. Polyionenes bearing anthryl groups (J2) form both inter- and intrapolymer excimer in water (2s). The excimer intensity decreases with increasing hydrophobic interaction. All experimental results indicate that weak intermolecular interactions almost undetectable in small molecule systems are amplified enormously in polymer systems in dilute solutions. 

Polymer Adsorption. The driving force for adsorption typically is the en-thalpic interaction between the interface and the polymer segments. Exceptions include aqueous solutions near hydrophobic substrates. In that case, the driving force may include the hydrophobic interaction between water and the surface. The enthalpic interactions may be of various forms such as hydrogen bonding, inter-facial tension, van der Waals attraction, polar interactions, and electrostatic attractions. This enthalpic interaction is offset by the loss in conformational entropy... 

These four types of forces are responsible for the adaptive behavior of smart gels. The different forces come into play when the network of polymer chains composing a gel is disturbed, (a) Charged ionic regions can attract or repel each other, (b) Nonpolar hydrophobic regions exclude water, (c) Hydrogen bonds may form from one chain to another, (d) Dipole-dipole interactions can attract or repel chains. 

In the case of water-soluble polymers, there is another factor that has to be taken into account when considering solubility, namely the possibility of hydrophobic interactions. If we consider a polymer, even one that is soluble in water, we notice that it is made up of two types of chemical species, the polar functional groups and the non-polar backbone. Typically, polymers have an organic backbone that consists of C—C chains with the majority of valence sites on the carbon atoms occupied by hydrogen atoms. In other words, this kind of polymer partially exhibits the nature of a hydrocarbon, and as such resists dissolution in water. 

Hydrophobic interactions of this kind have been assumed to originate because the attempt to dissolve the hydrocarbon component causes the development of cage structures of hydrogen-bonded water molecules around the non-polar solute. This increase in the regularity of the solvent would result in an overall reduction in entropy of the system, and therefore is not favoured. Hydrophobic effects of this kind are significant in solutions of all water-soluble polymers except poly(acrylic acid) and poly(acrylamide), where large heats of solution of the polar groups swamp the effect. 

The hydrophobic interaction results in the existence of a lower critical solution temperature and in the striking result that raising the temperature reduces the solubility, as can be seen in liquid-liquid phase diagrams (see Figure 5.2a). In general, the solution behaviour of water-soluble polymers... 

Maldotti (96) studied the kinetics of the formation of the pyrazine-bridged Fe(II) porphyrin shish-kebab polymer by means of flash kinetic experiments. Upon irradiation of a deaerated alkaline water/ethanol solution of Fe(III) protoporphyrin IX and pyrazine with a short intense flash of light, the 2 1 Fe(II) porphyrin (pyrazine)2 complex is formed, but it immediately polymerizes with second-order kinetics. This can be monitored in the UV-Vis absorption spectrum, with the disappearance of a band at 550 nm together with the emergence of a new band due to the polymer at 800 nm. The process is accelerated by the addition of LiCl, which augments hydrophobic interactions, and is diminished by the presence of a surfactant. A shish-kebab polymer is also formed upon photoreduction of Fe(III) porphyrins in presence of piperazine or 4,4 -bipyridine ligands (97). 

Random distribution of a significant number of hydrophilic NVIAz units along the polymer chain could result in uniform hydrophilization. This, in turn, could lead to a loss of ability for the coil-globule transition, which is caused by the hydrophobic interactions. As a result, such copolymers should be water-soluble over a wide temperature range. 

Xu QH, Gaylord BS, Wang S, Bazan GC, Moses D, Heeger AJ (2004) Time-resolved energy transfer in DNA sequence detection using water-soluble conjugated polymers the role of electrostatic and hydrophobic interactions. Proc Natl Acad Sci USA 101 11634-11639... 

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