Communication: Unusual dynamics of hybrid nanoparticles and their binary mixtures

Abstract

We present the results on the evolution of microscopic dynamics of hybrid nanoparticles and their binary mixtures as a function of temperature and wave vector. We find unexpectedly a nonmonotonic dependence of the structural relaxation time of the nanoparticles as a function of the morphology. In binary mixtures of two of the largest nanoparticles studied, we observe re-entrant vitrification as a function of the volume fraction of the smaller nanoparticle, which is unusual for such high diameter ratio. Possible explanation for the observed behavior is provided. Soft colloids—particles with a core shell structure, soft, tunable, and have interactions—have been widely studied over the past few years due to their interesting structural and dynamical phase behavior.1–7 Block copolymer micelles,5 star polymers,6–8 and microgel particles3,4 constitute few examples of such materials, which have been extensively studied. However, hybrid nanoparticles9–11 having an inorganic core and organic or polymeric shell, which also belong to the same class of materials, have not received as much attention. This is slightly surprising since these hybrid particles have gained tremendous importance due to their ability to form self-assembled ordered structures with very interesting optical, electrical, or thermomechanical properties.1,2,12–14 However, very little information is available regarding the role of core material shape and size as well as the morphology of grafted chains in determining the properties of such assemblies. Recent effort9,10,15 has focused on understanding the conformation of the grafted polymer chains’ polymer brushes as a function of various parameters such as grafting density, chain length, etc. However, no effort has been made in understanding their dynamics, especially for concentrated solutions or melts where these hybrid nanostructures are expected to form gel, glass, or crystalline phases similar to that observed for block copolymer5 or star polymers.6,16 Recently, we have shown11 how temperature and wave vector dependent dynamics in such hybrid nanoparticles can show counterintuitive behavior as compared to the related star polymer melts.8 Here, we significantly extend that work to present the results on the dynamics of such particles with different functionality f as well as their binary mixtures. We observed a nonmonotonic dependence of structural relaxation time on f as well as a re-entrance behavior in their binary mixtures as a function of the volume fraction ϕ. We also provide plausible explanations for the observed behavior, which could be subject to further experimentation and theoretical modeling. The results presented here are based on hybrid particles with core of gold nanoparticles and corona of thiol terminated polystyrene (PST) synthesized by a method described earlier.11,17 Both the core size as well as the grafting density of PST chains on the gold surface were varied to obtain hybrid particles with various sizes. Binary mixtures of two such hybrid particles, having different sizes, were prepared by mixing them in tetrahydrofuran solutions in various fractions and subsequent evaporation of the solvent, followed by drying in vacuum. Subsequently, the powder samples were annealed at 150 °C for 24 h under a vacuum of better than ∼5×10−4 mbar. Small angle x-ray scattering (SAXS) measurements on powders of all the samples were performed using a laboratory source at a wavelength of 1.54 Å (Nanostar, Bruker), while x-ray photon correlation spectroscopy (XPCS) measurements were performed at the Advanced Photon source in Argonne National Laboratory at the 8-ID beamline with monochromatic x rays with energy of 7.35 keV collimated to 20 μm×20 μm.15,17 The scattered beam was recorded on charge coupled device camera (Princeton Instruments) located at a distance of 3550 mm from the sample. In Fig. 1(a), the SAXS data on powders of the various hybrid nanoparticle samples are shown. We have used estimates of the core radius R0 of the polymer capped gold nanoparticles from transmission electron microscope (TEM) (Ref. 18) and SAXS and their compositions from thermogravimetric analysis to calculate the functionality f for each of our samples. While the high q(>0.1) SAXS data provide information about the core size, modeling of low q data allows the extraction of the corona radius Σ.