The impact of surface modification on magneto-functional iron oxide – polyethylene oxide nanocomposites

Abstract

Remote triggering of smart materials such as shape memory polymers and nanocomposites for drug delivery are research areas of continued interest for magneto-functional nanocomposites. Magnetically susceptible nanoparticles (NPs) generate heat when exposed to an alternating magnetic field (AMF), making these NPs an ideal candidate for use in smart nanocomposites. While a wide range of nanoparticle chemistries have been studied as ferrofluids in various liquid carrier media, the behavior of these nanoparticles in solid state polymers is not widely understood. This research studies the dependence of heat generation mechanisms and interfacial interactions on nanocomposite structure and morphology, nanoparticle surface coating and nanoparticle concentration in iron oxide (Fe3O4) - poly(ethylene oxide), PEO, nanocomposites. Nanoparticles were coated with surfactants and polymers to improve dispersion and magnetic properties. In this work we first focused on the impact of surface coating of iron oxide (Fe3O4) NPs on magnetic volume reduction, structure, magnetic heating efficiency and mechanical properties of poly(ethylene oxide), PEO, nanocomposites. Uncoated, poly(ethylene glycol), PEG, coated and amine coated 10–nm–diameter Fe3O4 NPs were dispersed at concentrations less than 1% by weight in PEO. Although loaded at low concentration these nanocomposites displayed excellent values for intrinsic power loss especially at low concentrations. We found that dispersion of nanoparticles was strongly related to the character of the surface coating. Uncoated nanoparticles formed large aggregates which led to a significant decrease in the heat generation capabilities. The surface coatings also strongly impacted the magnetic phase reduction. Amine coated nanoparticles had the least magnetic phase reduction. All nanoparticles showed unexpectedly higher heating efficiencies in PEO than when dispersed in water due to decreased magnetic volume loss. Aggregation was determined to be the dominant factor for decreased heating efficiency. Calorimetry experiments explored the impact of the nanoparticles on crystallinity and nucleation rates. Nanoindentation was used to evaluate the mechanical properties via stress relaxation and creep experiments. Amine coated nanoparticles were found to improve the moduli of the nanocomposites. Low concentrations of nanoparticles led to increased relaxation and decreased creep compliance whereas high concentrations had no effect on relaxation and increased creep compliance. The relevance of five rheological models was evaluated. Stress relaxation was best modeled by a power law or logarithmic based model whereas the creep was best modeled by a Generalized Maxwell model. In the second part of this work, single core aminosilane coated 10–nm–diameter Fe3O4 NPs were dispersed at concentrations less than 2% by weight in PEO matrices with varying molecular weights. Altering the matrix molecular weight of the matrix polymer allows for consistent intermolecular interactions between the NP surface groups and the PEO in order to determine relative importance of Brownian and Neel relaxation processes. Increased matrix molecular weight above the polymer matrix entanglement molecular weight led to decreased heat generation efficiency that was consistent with decreases in the nanoparticle magnetic volume determined via vibrating sample magnetometry. Brownian and Neelian relaxation mechanisms were proven to be present despite the high viscosity of the matrix media. Dynamic polymer relaxation modes such as the Reptation or Rouse models were found to be inactive.