Flower micelle

Figure 5.16 Model for associations of telechelic polymers as a function of increasing concentration. For strong associations, isolated flower micelles form just above the critical micelle concentration (CMC), which is often around 2 to 10 ppm (Winnick and Yekta 1997). At higher concentrations, the flowers are expected to be connected by bridges. (From Winnik and Yekta 1997, with permission from Current Chemistry Ltd.) 1997 Current Opinion in Colloid -i- Interface Science.

Two types of poly(2-vinyl pyridine-b-styrene-b-2-vinyl pyridine) triblock copolymers were synthesised by anionic living polymerisation. These polymers formed monodispersed micelles in toluene or toluene/cyclohexane mixture. Poly(2-vinyl pyridine) sequences in the core part of the polymer micelles were crosslinked with 1,4-diiodobutane. After crosslinking, no macrogelation was observed. The morphology of the crosslinked products did not correspond with that of the original triblock copolymer. All products were polystyrene spheres and each of them had one poly(2-vinyl pyridine) core in its centre. It was therefore concluded that the poly(2-vinyl pyridine) core-PS shell type flower microgels were synthesised by crosslinking of the flower micelles in solution. 27 refs. 

In general, the CMC of the flower micelles is very low it can be as low as 10 polymer wt%. At low concentrations, intramolecular flowers dominate. With an increase in the polymer concentration, loops dissociate and have more chance to form an open association with many bridges, thus eventually leading to gelation. The sharpness of the transition depends on the association constant and the aggregation number of the micelles. 

Figure 10.8(b) shows the volume fraction (/>fl of flower micelles plotted against the total polymer concentration. The loop parameter is varied from curve to curve for the multiplicity in the allowed range 5 fi rises from zero is CFMC. Since it slowly increases in the figure, the conventional method to identify CMC (the population curve of the micelles bends most sharply) is not directly applicable. As a rough estimate, we here employ a simple criterion that the concentration where the absolute value of tpn reaches a certain threshold value is CFMC. The actual value of the... 

In some aqueous polymer solutions, hydration is noncompetitive with association. For instance, in solutions of telechelic polymers, main chain hydration only indirectly affects the end-chain association. There is interference only in the region very close to the chain end. Dehydration and chain collapse start near the core of the flower micelles in the form of heterogeneous nucleation. The solutions with such coexisting hydration and association turn into gels on cooling (low-temperature gelation), while they phase separate at high temperatures. 

In the solutions of homopolymers, the onset temperature of phase separation coincides with the coil-globule transition temperature. In contrast, telechelic PNIPAM solutions have a cloud-point temperature that is several degrees lower than the collapse temperature. The solution becomes turbid by the scattering of the light due to the formation of aggregates (of flower micelles) whose size is comparable to the wavelength of the light. Thus, the LCST splits from the collapse transition temperature. 

Since the critical micelle concentration is reported to be extremely small (c < 10 wt%), the spinodal curves for solutions of telechelic polymers with concentration lower than 1 wt% are expected to be substantially modified due to the formation of flower micelles. Within the present tree approximation, however, it is not sufficient to study the formation of flower micelles the critical point is identical to the crossing point of the spinodal curve and the sol-gel transition curve. 

Rg. 10.17 (a) Dissociation of intramolecular bonds, and (b) dissociation of intramolecular flower micelles,... 

In 2005, an amphiphiUc ABA triblock polymer capped with Cgo at both ends was prepared by Gan et al. via the ATRP strategy (Figure 7.9) [71]. C o was successfully incorporated at both ends of the polymer chain via azido cycloaddition. In aqueous solution, 57 aggregates to form flower micelles that can potentially be used as carriers for drug/gene deUvery application. 

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