Linear polymers phospholipids

Many complex systems have been spread on liquid interfaces for a variety of reasons. We begin this chapter with a discussion of the behavior of synthetic polymers at the liquid-air interface. Most of these systems are linear macromolecules however, rigid-rod polymers and more complex structures are of interest for potential optoelectronic applications. Biological macromolecules are spread at the liquid-vapor interface to fabricate sensors and other biomedical devices. In addition, the study of proteins at the air-water interface yields important information on enzymatic recognition, and membrane protein behavior. We touch on other biological systems, namely, phospholipids and cholesterol monolayers. These systems are so widely and routinely studied these days that they were also mentioned in some detail in Chapter IV. The closely related matter of bilayers and vesicles is also briefly addressed. 

The physical properties of diacetylene-containing phospholipid liposomes (25) have been investigated by D. Chapman 102 103 104> by means of UV/VIS and 13C-NMR spectroscopy. They found that short linear segments of polymer are interconnected through the glycerol backbone of the lipid in identical-chain phosphatidyl cholines, but the molecular weights of the polymers are still unknown. 

Fig. 12 Model of PDADMAC arrangement (linear rod) and a DPPA monolayer (top view). The phospholipid exhibits two aliphatic tails (black dots) that are uniformly tilted (arrows) along lattice vector a. Lattice spacing, a, corresponds to twice the repeat length of PDADMAC, b, to the polymer diameter...

Ex(30) values show a good, often linear, correlation with a large number of other solvent sensitive processes, such as reaction rates and shifts of chemical equilibria. The betaine dye (Scheme 3) and specially designed derivatives are useful molecular probes in the study of micellar interfaces, microemulsions and phospholipid bUayers, of rigid rod-Uke isocyanide polymers, and the retention behaviour in reversed-phase chromatography. In addition to its solvatochromic behaviour, the dye is sensitive to temperature ( thermosolvatochromism ) and pressure changes ( piezosolvatochromism ) and also to the presence of electrol)d es ( halosolvatochromism ). 

Around the turn of the last century, chemists were reluctant to accept the idea of rubber, starch, and cotton as long, linear chains connected by covalent bonds. A popular alternative was the idea of an associated colloidal structure. As a matter of fact, some small molecules do exhibit such behavior. Soap molecules will associate into complex liquid crystalline structures and are used as the basis for the formation of mesoscopic solids. Other surfactant molecules such as the phospholipids present in the wall of many living cells will form micelles and vesicles. However, the effective molecular weight of such structures varies with concentration and temperature, whereas the molecular weights of true polymers with covalent links do not. 

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