Polymers crosslinked by micelles

Steady Shear Solution Viscosities of Polymers Crosslinked by Micelles. 

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. 

Polymer micelle shells can also be stabilised by crosslinking the hydrophilic components. The method was first introdnced by Nystrom and co-workers [53, 54] and Sun and co-workers [55], and has resnlted in more than 80 publications to date. The systems are typically based on amphiphilic block copolymers with a hydrophilic polymer block compromised of polyacrylic acid (PAA). This forms the enter layer of the micelle. These PAA groups can then be crosslinked by amidation with a diamine, and the remaining -COOH groups can be used for further modification with imaging probes, PEG chains or targeting entities, see Figure 2.13. 

Since microgels are intramolecularly crosslinked macromolecules of colloidal dimensions, it is necessary for their synthesis to control the size of the growing crosslinked molecules. This can be achieved by carrying out polymerization and crosslinking in a restricted volume, i.e. that of a micelle or of a polymer coil. Thus, two general methods of microgel synthesis are available (1) emulsion polymerization, and (2) solution polymerization. 

A different type of microgels can be obtained by solution polymerization. Since an increase of dilution during crosslinking increases the probability of intramolecular crosslinking, the growing polymer chains in a highly dilute solution become intramolecularly crosslinked and their structure approaches that of the microgels formed within the micelles. 

Figure 6.5 Illustrations of nanoscale spherical assemblies resulting from block copolymer phase separation in solution are shown, along with the chemical compositions that have been employed to generate each of the nanostructures (a) core crosslinked polymer micelles (b) shell crosslinked polymer micelles (SCKs) with glassy cores (c) SCKs with fluid cores (d) SCKs with crystalline cores (e) nanocages, produced from removal of the core of SCKs (f) SCKs with the crosslinked shell shielded from solution by an additional layer of surface-attached linear polymer chains (g) crosslinked vesicles (h) shaved hollow nanospheres produced from cleavage of the internally and externally attached linear polymer chains from the structure of (g)...

The new connective sites provide the opportunity to destroy or disconnect other regions of the nanostructure without destruction of the entire nanoscale entity. This was demonstrated by the excavation of the core of shell crosslinked polymer micelles, by the removal of the colloid from colloidally templated... 

Precrosslinked" or "intramolecularly crosslinked" particles are micronetworks [1]. They represent structures intermediate between branched and macroscopically crosslinked systems. Their overall dimensions are still comparable with those of high molecular weight linear polymers, the internal structure of micronetworks (p-gels), however, resembles a typical network [2]. Synthesis is performed either in dilute solution or in a restricted reaction volume, e.g., in the micelles of an emulsion. Particle size and particle size distribution can be controlled by reaction conditions. Functional groups can be... 

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