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44641-G10
Synthesis of Metal-Filled Carbon Nanoparticle Chains from Polyacrylonitrile Functional Magnetic Nanoparticles

Jeffrey Pyun, University of Arizona

Advances in FY 2007-2008:  Research efforts for this project have focused on developing synthetic methods to prepare multi-gram quantities of functional, polymer coated ferromagnetic nanoparticles.  PRF funded efforts from FY2006-2007 have demonstrated the ability to scale-up the synthesis of polystyrene coated ferromagnetic cobalt nanoparticles to 1-gram quantities, which represented a significant advance in the field.  To date, numerous methods have been reported for the preparation of metallic, metal oxide and semiconductor nanocrystals and colloids.  However, the vast majority of these methods afford nanomaterials on the milligram scale (tens to hundreds), which greatly limits exploration of new applications for these materials.  We have examined this problem and have focused efforts on multi-gram scale synthesis of the functional polymer surfactant and the final polymer coated ferromagnetic nanoparticle.  This work has proven successful and is a key first step to enabling the preparation of functional materials, namely 1-D carbon nanowires.
                Very early demonstrations on the synthesis of polymer coated ferromagnetic nanoparticles was reported in the 1960’s by investigators at Chevron using a simple terpolymer of poly(methyl methacrylate-random-ethyl acrylate-random-vinylpyrrolidone) (P(MMA-r-EA-r-VP).  While these nanoparticles were prepared on large scale and exhibited ferromagnetic properties, we observed that these materials readily flocculated out of reaction mixtures that could not be handled, or redispersed in organic media.  Both the promise and the limitations of this system spurred our interest to more deeply investigate this synthetic approach to enable our needs for a large scale synthesis of ferromagnetic nanoparticles that was amenable to versatile functionalization.  For the first time, we embarked on a synthesis of P(MMA-r-EA-VP) terpolymers with varying compositions of VP and correlated these properties with different particle forming conditions via the thermolysis of dicobaltoctacarbonyl (Co₂(CO)₈).  Terpolymers were synthesized by the simple free radical copolymerization of MMA, EA and VP using conventional radical initiators (e.g., AIBN) of high molar mass (M_n = 200,000 g/mol; M_w/M_n = 1.67) and targeted compositions (PMMA 40-mol%; PEA 55-mol%; PVP 5-mol%).  While these terpolymers were did not possess precise molar mass and low polydispersity, these heterogeneities did not affect the formation of ferromagnetic CoNPs, and furthermore could be prepared on 20-100 g scales.  Our earlier reports using polystyrene surfactants of precise molar mass was prepared using a controlled radical polymerization (i.e., ATRP) and could be prepared on 10-g scales.  However, terpolymers prepared from conventional radical polymerizations significantly cut the number of synthetic steps to prepare polymeric surfactants and in significant larger quantities.  Using this terpolymer surfactant in the thermolysis of Co₂(CO)₈ for the first time, we have synthesized uniformly sized 30 nm, 40 nm and 50 nm polymer coated ferromagnetic nanoparticles with enhanced magnetic properties (i.e., higher magnetization and coercivity).  TEM images of terpolymer coated CoNPs reveal the formation of self-assembled nanoparticle chains spanning over 10 microns in length held together via magnetic association and polymer shell-intergitation. (Fig. 1)  These materials have been synthesized on 2-3 g scales

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