Chain Formation of PNIPAM coated Magnetic Nanoparticles in an External Magnetic Field and the Effect of Temperature

Abstract

Over the past decade, magnetic field-induced self-assembly of magnetic inorganic nanoparticles has been studied extensively for particles having different sizes and compositions. However, relatively little attention has been devoted to how the surface chemistry of polymer nanoparticles affects their self-assembly properties. Self-assembly of nanoparticles becomes more important because novel collective and exceptional properties can be produced in the ordered array of nanoparticles [1-3]. More recently, methods using external fields such as magnetic fields have emerged as key methods to direct the assembly of inorganic nanoparticles effectively [4, 5].In this work, we present the first direct observation of polymer nanoparticle chain formation in a magnetic field based on temperature induced surface properties of the PNIPAM layer. The colloidally stable PNIPAM coated Iron Oxide Nanoparticles were successfully synthesized by using 7-nm magnetic iron oxide nanoparticles embedded in a PNIPAM-g-PGMA shell. This temperature responsive polymer (PNIPAM-g-PGMA) was synthesized by a simple polymerization reaction at 80 oC involving the anchoring of poly(N-isopropylacrylamide) (PNIPAM) chains onto the surface of poly(glycidyl methacrylate) (PGMA). The PGMA was used because it has been explicitly shown to have the epoxide functional groups groups [6] located in the loops and tails of the core macromolecule.This work was concerned with the capability of thermoresponsive polymer nanoparticles forming chains in an external magnetic field at 25 oC and 45 oC. Chain length and width information was measured using SEM and TEM. The time-series experiment performed enabled the measurement of proton transverse relaxation rates with the magnitude of the fractional reduction in R2 being observed to decrease as the concentration of iron in polymer nanoparticles decreased for both temperatures. At 45 oC, we obtained more nanosized chain-like structures of PNIPAM-g-PGMA-NPs, which contain wider and longer chain-like substructures, owing to the hydrophobic properties of PNIPAM chain as shown in Fig. 1 (c). It is important to note that the magnetic properties of all the synthesized nanoparticles presented as superparamagnetic even after the surface was coated with PNIPAM- PGMA. The relevance of this work is that many applications of PNIPAM- PGMA coated magnetic nanoparticles are likely to involve the particles being exposed to strong magnetic fields. As such, a knowledge of the rates of chain formation and sizes of chains could be critical to predicting the behaviour of the flow of the particles in the blood stream, for example. **