Cylindrical aggregate

Nonspherical aggregates (cylindrical micelles, vesicles, lamellae, etc.) were detected experimentally for a number of non-ionic block copolymers (see, e.g., [50, 136-139]) and copolymers with weakly dissociating PE block [102, 140]. Morphological transformations in non-ionic PI-fetocA -PS micelles were triggered by variations in molecular weight of the PS block [50] or by variations in the solvent composition [137, 138]. The latter studies clearly indicate the possibility of stimuli-responsive transitions (sphere —> cylinder —> vesicle) for non-ionic block copolymer aggregates in mixed organic solvents. It was also demonstrated [58] that... 

Figure C2.3.17. Model of half-cylindrical aggregates (hemimicelles) on a crystalline hydrophobic substrate, such as for tetradecyltrimethylammonium bromide on M0S2 [91], Adapted from figure 2 of [89],...

Larger aggregates seldom have spherical geometry, but tend to form cylindrical micelles. In this case, the diameter of the cylinders can usually be adjusted such that the head groups can cover their optimal head group area Uq, and the interaction free energy per surfactant reduces to the constant The size distribution for cylindrical micelles is then exponential in the limit of large N,... 

At small N, correction terms come into play, which account for the ends of the cylinders. In particular, the aggregation number of cylindrical micelles in this simple picture must always be larger than M, the most probable aggregation number of a spherical micelle. Putting everything together, the expected size distribution has a peak at M which corresponds to spherical micelles, and an exponential tail at large N which is due to the contribution of cylindrical micelles. 

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

Didodecyldimethylammonium bromide forms in the dilute-phase cylindrical reversed micelles in the range 2 < / < 8, showing an abrupt structural change at/ = 10 and forming spherical aggregates at/ > 10 [17],... 

In sodium bis(2-ethylhexyl) phosphate microemulsions, which are composed of cylindrical micelles in the dilute region, it has been observed that the formation of micellar clusters is characterized by a branched structure as the volume fraction (<1>) of the aggregates increases. At d> > 0.2, these clusters mutually overlap, forming a network expanded overall [283]. 

Vesicles [10, 11] these aggregates of insoluble natural or artificial amphiphiles in water can have various shapes (spherical, cylindrical). Depending on the preparation conditions, small unilamellar or large multilamellar vesicles can be produced. The structures meet the self-organization criterion, because they are, albeit on a long time scale, dynamic and not in thermodynamic equilibrium, which would in many cases be a macroscopically phase separated lamellar phase. 

Quartz fine aggregate and normal Portland cement were used to prepare mortar with a w/c of 0.5. A cylindrical specimen, 43 mm in diameter and 50 mm long, was cast and cured under sealed conditions for 3 days at 23 °C. The specimen was then oven dried at 105 °C for 1 day prior to exposure to lithium nitrate solution. The specimen was then placed such that the bottom of the cylinder was submerged approximately 1-2 mm into a lithium nitrate solution with Teflon tape applied to the curved surface. 

The size of carotenoid aggregates have been determined by dynamic light scattering (DLS), a noninvasive method (Santos and Castanho 1996). DLS also allows distinguishing between spherical or cylindrical aggregates. The hydrodynamic radii rH of hydrophilic carotenoids in water are given in Table 3.1. Size and molecular structure of the bolaamphiphiles crocin, 3.7, and Cardax,... 

This chapter draws a comprehensive picture of what has been done in the field of dendrimers with polymeric cores putting emphasis first on synthetic issues and then on experiments investigating the aggregation behavior of these intruiging macromolecules both in the solid state and on surfaces. Additionally, experiments will be described which show that some of these dendrimers can be considered cylindrical molecular objects. The macromolecules treated in this chapter may be considered as either dendrimers with polymeric core or alternatively dendronized polymers (or polymers with appendent dendrons) depending on whether one sees them from the vantage point of an organic or macromolecular chemist. 

Oligomeric deoxyguanosines 69 all assemble into columns in water.93 Using SANS, the average length of the columns (DP) of freshly prepared samples was determined to be around 6 nm at a concentration of 1% in D2O.88 Increase of the temperature resulted in the loss of the cylindrical aggregates. When the sodium salt was used, subsequent cooling did not result in the reformation of the cylindrical polymers. In contrast, the potassium salt did allow for reversible self-assembly. The addition of extra potassium salt even increased the DP. It binds to the inner carbonyls of the G-tetramer and stabilizes the tetramer 94... 

The amphipathic compounds shown in Scheme 2 can form a disc-like micelle(7). The shape of a molecular aggregate depends on the shape of the constituent molecules(8). For instance, conical molecules with large polar head groups prefer to form spherical micelles while cylindrical molecules tend to give flat aggregates. Trans-azobenzene is a rod-like molecule whereas the cis-form is bent. 

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