Self-Seeding Of Oligo(P-Phenylenevinylene)-B-Poly(2-Vinylpyridine) Micelles: Effect Of Metal Ions

Self-seeding is a useful protocol for generating elongated fiberlike block copolymer micelles of uniform length starting with micelle fragments obtained by sonication of long polydisperse micelles. In this process, the suspension of micelle fragments is heated in the selective solvent to temperatures where nearly all the fragments dissolve. The surviving fragments serve as seed nuclei for micelle growth when the solution is cooled. While there is evidence that the nature and length of the corona polymer plays a role in determining the characteristic dissolution temperature (T-c) of the micelle fragments and other aspects of the self-seeding process, our understanding of these effects are limited. Here, we examine self-seeding of fiberlike micelle fragments in ethanol of an oligo(p-phenylenevinylene)-b-poly(2-vinylpyridine) (OPV5-b-P2VP(42), the subscripts denote degrees of polymerization) block copolymer in which we introduced different amounts of divalent metal ions (Cu2+, Zn2+ Mn2+, and Cd2+) to promote complexation without affecting the packing of the micellar core. We found that the resistance of seed micelles of OPV5-b-P2VP(42) toward dissolution increased significantly as the percentage of metal ions to pyridyls was increased from 0 to about 5% and then showed a more gradual increase as the metal ion content was increased to 20%. These phenomena can be interpreted in terms of the formation of noncovalent cross-linking within the P2VP layer. Metal complexation should lead to contraction of the dimension of the corona chains, decreasing corona chain repulsion and the tension exerted on the core by the stretched P2VP chains in the metal-free micelles. Cross-linking would also require additional energy to break interchain coordinating bonds between pyridyls and metal ions for both the detachment of unimers from micelles and fragmentation of micelles. Metal ion complexation also reduces the mobility of P2VP chains. This effect appears to slow the addition rate of the free unimer to the surviving seed micelles in the cooling/aging step in the self-seeding process.