Association Strength of Metal Ions with Poly(4-vinylpyridine) in horganic/Poly(4-vinylpyridine)-b-poly(ε-caprolactone) Hybrids

A series of inorganic/poly(4-vinylpyridine)-b-poly(epsilon-caprolactone) (P4VP PCL) hybrids were prepared to examine the phase behavior of hybridization. Various metal ions including Au3+, Cu2+, Cu+ and Ag+ ions were used for the hybridization. As demonstrated by transmission electron microscopy, the phase transformation of self-assembled nanostructures can be easily induced by adding small amount of metal ions due to the association of metal ions with P4VP block that profoundly creates the extra volume in P4VP microclomain (i.e., gives rise to the significant increase of effective excluded volume) beside the Ag+ ions. The variation in the effective excluded volume (relevant to the degree of domain swelling, as evidenced by small-angle X-ray scattering) is strongly dependent upon the association strength of the metal ions with the P4VP block, as determined by Fourier transform infrared spectroscopy. The association strength for the formation of the hybrids follows the order of Au3+ > Cu2+ and Cu+ > Ag+. Accordingly, the degree of domain swelling in the hybrids increases with the enhancement of association strength following the order of Au3+ > Cu2+ and Cu+ > Ag+. Furthermore, as demonstrated in the hybrids of Au nanoparticles and P4VP-PCL, a dramatic decrease of the association strength can be found in the hybrids after reduction. Consistent with the theoretical prediction, the reduction of metal ions causes the alleviation of the association strength. The association effect is also dependent upon the particle size; the larger the particle size is, the weaker the association will be. Consequently, the accommodation of the metal nanoparticles within the P4VP microdomain is justified by the size of the metal nanoparticles.