Interest in dispersing inorganic nanoparticles (NPs) in polymeric matrices continues to increase as a viable and facile means by which to enhance the mechanical, optical, electrical, and magnetic properties of organic polymers. While many studies have focused on using the (co)polymer matrix to spatially modulate the NPs, the overarching objective of the present proposal is to demonstrate the feasibility of using field aligned surface functionalized magnetic nanoparticles (NPs) to direct phase orientation and therefore generate new multiphase polymer morphologies with potentially new anisotropic properties. The effects of magnetic NP chains formed in homogeneous polymer mixtures by externally applied, uniform magnetic fields, on the nano/microstructure during (micro)phase separation will be explored in two systems: polymer blends and block copolymers composed of polystyrene and poly(methyl methacrylate) (PS/PMMA). We hypothesize that, due to the formation of NP chains along the field direction, the blends will form highly elongated phase domains or possibly layers instead of conventional spheroidal domains, whereas the lamellae in the copolymer will align with the NPs to form large grain sizes (with a reduced number of defects). The NP surfaces with native long chained aliphatic functional groups are expected to agglomerate at the interface in both systems and thus serve as stabilizing sites. This prediction will be verified using magnetic (Co and Fe3O4) NPs under magnetic fields of up to 20 kOe to induce formation of linear NP chains. These NP chains, responsible for observed ferrofluid behavior, are predicted to template formation of the interface through their pinning behavior. To explore a different regime of thermodynamic behavior, NPs will be functionalized with oligostyrene (OS) or oligo(methyl methacrylate) (OMMA) to impart selective solubility in one of the polymer species. The field aligned nano/microstructures will be compared for magnetic NP chain formation at the interface versus within one of the (micro)domains. Binary core Au/Fe3O4 NPs will also be prepared. Owing to the distinct Au and Fe3O4 surface chemistries, each half will be independently functionalized one half with OPS, and the other with OMMA, to form Janus NPs. The possibility of altering interfaces with oriented Au/Fe3O4 NPs will be explored. Multiscale molecular dynamics simulations complementing experiments will be performed to (i) provide a theoretical framework of the dynamics of NP chain formation and (ii) guide experimental design by establishing operational thresholds.
Intellectual Merit:
The proposed research will significantly enhance the current understanding of the role that NPs can serve to pin or align phase boundaries in heterogeneous polymer systems. Use of magnetic-field aligned NP chains to template formation of oriented lamellar phase boundaries will be of interest for further studies and applications, since there exists a great need to achieve greater control over morphological orientation in multiphase polymer systems.
Broader Impact:
The results from the proposed research will be incorporated into two courses at NCSU. In addition to enriching the undergraduate and graduate curriculum through student participation, we plan outreach programs to local students at the K-12 levels through the Kenan Fellows Program at NCSU. The PIs will endeavor to attract underrepresented and minority students from the RISE (incoming freshman students) and Pack Promise (low income students provided with tuition wavers, research positions, and academic support) programs at NC State. The PIs have successful experience recruiting exemplary students from such student groups.
In this project, several novel phenomena in nanoparticle polymer composites were investigated. When polymer solutions containing gold nanorods were formed into nano/microfibers through electrospinning, nanoscale alignment of gold nanorods within individual fibers was observed. Macroscale alignment of the fibers using already established methods thus gave macroscale alignment of the gold nanorods, which exhibited polarization-dependent optical properties. Applying magnetic fields during electrospining of polymer fibers containing magnetic nanoparticles caused the nanoparticles to undergo chaining within the fibers, which affected the magnetic properties. In a related study, magnetic nanoparticles dispersed in a fluid monomer assembled into chains when a magnetic field was applied. Polymerization of the monomer resulted in a composite of magnetic nanoparticle chains embedded within a polymer, which exhibited enhanced magnetic anisotropy along the chaining direction. When magnetic nanoparticles were dispersed in polystyrene/poly(methyl methacrylate) blends through solvent casting in an applied magnetic field, assembly of magnetic nanoparticle chains affected the morphology. The polymer domains were elongated in the direction parallel to the magnetic nanoparticle chains. Complementary modeling studies revealed that the polarity of the solvent used in solvent casting plays an important role in determining the morphology of polystyrene/poly(methyl methacrylate) diblock copolymer nanocomposite materials. Magnetic field-directed self-assembly of magnetic nanoparticles in polymers has promise for the development of complex polymer morphologies with wide-ranging potential applications, including anisotropic property development, adhesion, molecular separations, and photovoltaics. Students and faculty participated in several outreach activities, including Nanodays and Materials Camp.
