Control of Self-Assembling Processes of Polyamidoamine Dendrimers and Pd Nanoparticles

Abstract

We have investigated the self-assembly formed by palladium acetate (Pd(OAc)2) and polyamidoamine dendrimers (GN-NH2) in a mixed solvent of methanol and N,N-dimethylformamide. We aim to explore effects of the generation number (GN) and concentration of the dendrimers, [GN-NH2], on the self-assembly of the dendrimers (defined hereafter as “templates”) and the Pd nanoparticles inside the templates in order to clarify physical factors controlling the self-assembling mechanisms. For this purpose, we changed GN from 0 to 4 and [GN-NH2] from 0.03 to 2.7 mM, keeping the concentration of Pd(OAc)2, [Pd(OAc)2], at a fixed value of 3.6 mM. The self-assemblies of the templates and the Pd nanoparticles were investigated by means of a combined method of small-angle neutron and X-ray scattering, ultra-small-angle X-ray scattering, dynamic light scattering, and transmission electron microscopy. We elucidated the following two facts concerning the effects of increasing GN on the self-assembly under a fixed value of [−NH2] such that [Pd(OAc)2]/[−NH2] = 3.33, where [−NH2] is molar concentration of the peripheral amine group of dendrimers having varying GN. (i) The size of the templates becomes smaller, while the number density of the templates increases. This trend is well explained from the viewpoint that the effective binding force between the templates via attractive interactions of Pd(OAc)2 and amine groups of the dendrimers, which belong to different templates, becomes weak relatively to the random thermal force acting on the templates. (ii) The size of Pd nanoparticles becomes larger with increasing GN. This result can be rationalized by the following two effects: (a) increasing number fraction of Pd(OAc)2 molecules, which are easily reduced to Pd(0) atoms; (b) increasing number density of Pd(OAc)2 per single template as detailed in the text (section 4.2.1. and 4.2.3.). As for the effect of [GN-NH2], we found that the templates are formed only when [GN-NH2] falls below a critical value and thereby a sufficient amount of Pd(OAc)2 exists as the binders between the dendrimers. Otherwise dendrimers are molecularly dispersed in solution.