Preparation of Ni-g-polymer core–shell nanoparticles by surface-initiated atom transfer radical polymerization
Graphical abstract
Introduction
In recent years, because of the unique magnetic properties of nanocrystalline metallic nanoparticles such as Fe, Co and Ni, considerable attention has been devoted to the synthesis of these materials. In particular, Ni nanoparticles have received great interest for their potential applications in many diverse fields, including magnetic recording media, biomedical materials, catalysis, and electro-conductive materials. Numerous different physical and chemical preparative routes have been developed to synthesize Ni nanoparticles, such as pyrolysis [1], sputtering [2], microemulsion [3], [4], aqueous [5], [6] and nonaqueous [7], [8], [9] chemical reduction, sonochemical deposition [10], and polyol methods [11].
Because of the anisotropic dipolar attraction, Ni nanoparticles have strong tendency to aggregate into large clusters, thus lose their specific magnetic properties associated with the single-domain nanostructures. This greatly limits the uses of Ni nanoparticles in various applications. In order to overcome this problem, it is essential to do some modification work on Ni nanoparticle surface to prevent aggregation. In most preparation methods of Ni nanoparticles, capping agents like long chain alkyl acids, amines and phosphates were always applied to control growth of nanoparticles; and at the same time, to prevent them from aggregation.
Compared to small molecular capping agents, polymeric shells have their unique advantages. Because of flexibility in control of polymer composition and functionality, polymer shells are not only able to protect nanoparticles from aggregation, but also readily endow nanoparticles with interesting functionalities. Among many approaches for coating nanoparticles with polymer shells, surface-initiated polymerization techniques have recently become very popular choices, especially surface-initiated atom transfer radical polymerization (si-ATRP). By this method, polymer chains are in situ grafted from initiator molecules previously immobilized onto nanoparticle surfaces. The most significant advantage of this “grafting from” method is its ability to produce dense polymer brushes, with grafting densities ranged from 0.1 to 0.7 chains/nm2. Moreover, ATRP is a well-established controlled radical polymerization technique and can offer good control over polymer molecular weight, and thus polymeric shell thickness, allowing preparation of polymer shells with low polydispersities.
Although si-ATRP has been employed to graft polymers from various nanoparticles, such as SiO2 [12], [13], Au [14], MnFe2O4 [15], etc, no successful experiments have been reported on modification of nanocrystalline metallic nanoparticles. Unlike other types of materials, metal surfaces are highly reactive in electrochemical and acid/base reactions that complicate ATRP reactions. Red-ox deactivation of ATRP catalyst is a particular risk. In our previous study [16], using triethoxysilane-based initiator and iron catalyst, we succeeded in grafting polymer brushes from flat Ni and Cu surfaces. In this paper, we report the first successful study on surface modification of pristine Ni nanoparticles by si-ATRP. A combination of ligand exchange and condensation reaction was employed to covalently immobilize triethoxysilane-based ATRP initiators onto Ni nanoparticle surface. Various types of polymers, like poly(methyl methacrylate) (PMMA) and poly(n-isopropylacrylamide) (PNIPAM), were grafted in situ from the immobilized initiators. After polymer grafting, both dispersion and stability of the Ni nanoparticles in solvents were greatly improved. The grafted polymer shells were very effective to prevent the Ni nanoparticles from aggregation. The improved dispersion and stability, as well as good compatibility with polymer matrices, are of great benefit to preparation of high quality Ni nanoparticle/polymer composite materials.
Section snippets
Materials
Nickel acetylacetonate [Ni(acac)2] (95%), hexadecylamine (HDA) (98%), trioctylphosphine oxide (TPPO) (99%), sodium borohydride (99%), o-dichlorobenzene (anhydrous, 99%), acetic acid (≥99.7%), 3-aminopropyltriethoxysilane (99%), 2-bromoisobutyryl bromide (98%), triethylamine (>99%), iron(II) bromide (98%), iron(III) bromide (98%), triphenylphosphine (TPP) (99%), toluene (anhydrous,99.8%), N,N-dimethylformamide (DMF) (anhydrous, 99.8%), n-isopropylacrylamide (NIPAM) (97%) were purchased from
Immobilization of initiator on Ni nanoparticle
Ligand exchange reaction is an effective way to modify nanoparticle surface with ATRP initiators. For metal oxide nanoparticles, carboxylic acid-based initiators are often used to replace the ligands on nanoparticle surfaces, followed by in situ polymerization. However, this method has a drawback. The carboxylate bond between polymer chain and nanoparticle surface is not strong enough to achieve a stable linkage. The polymer chains are readily dissociated from the nanoparticle surface through
Conclusions
Ni nanoparticles have been modified with triethoxysilane-containing ATRP initiators without any aggregation through the combination of ligand exchange and condensation reaction. PMMA and PNIPAM were successfully grafted from the modified Ni nanoparticles by si-ATRP technique. The iron catalyst system offered good control over the polymerization and did not impose corrosion threat to the nanoparticles. After grafted with polymers, the Ni nanoparticles possessed greatly improved dispersion and
Acknowledgement
We thank the Emerging Materials Knowledge (EMK) program of the Ontario Centre of Excellence (OCE) for supporting this work. We thank Dofasco Inc. for donating some of the metal samples and assistance in the corrosion analysis. We also thank the Canada Foundation of Innovation (CFI) for supporting our lab equipment and facilities.
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