Assembly of Polymer-Grafted Magnetic Nanoparticles in Polymer Melts

Hydrophobic iron oxide nanoparticles grafted with hydrophobic polymer chains of varying molecular weights and graft densities are synthesized to underpin the role of brush entanglement and dipolar forces on creating nanostructures. Grafting density on magnetic nanoparticles is controlled in grafting-to method by changing the concentration of functionalized polymer in solution. The grafting density and brush length have varied systemically to observe the changes in nanostructures. Bridging between grafted chains and dipolar forces become effective only at low grafting density and result in long chains of particles. We demonstrate experimentally that structural transition of magnetic nanoparticles is controlled with the balance between grafted chain entanglements and dipolar forces

Alignment-Assisted Networks of Polyelectrolyte-Grafted Cellulose Nanocrystals

This study aims to understand the role of polyelectrolyte grafting on the dispersed cellulose nanocrystal (CNC) rods in water through measuring transport coefficients using depolarized and polarized dynamic light scattering and by measuring the viscoelastic properties using rheometer. Rotational and translational diffusivities are found to slow down with poly(acrylic acid) (PAA)-grafted chains compared to bare CNCs. Translational diffusion is shown to remain constant between pH 3 and 9, indicating the good dispersion and stability of PAA-grafted CNC suspensions. At the overlap solution concentration, chains play a significant role in bridging the CNC and form a network, as measured with the viscoelastic properties of neutral chains. When chains are ionized by altering the pH, the higher viscosity is measured because of the hydrogen bonding between ionized and un-ionized carboxylic groups, as previously demonstrated with PAA-grafted spherical nanoparticles. We further measured the viscoelastic response of PAA-grafted CNC after applying large steady shear. The results show that CNCs with long grafts presented enhanced viscoelastic moduli, and their critical strain value decreased after large shear flow application. Short grafts, in contrast to the long grafts, did not show any changes in the viscoelastic response under shear. These results indicate that the alignment-assisted networks of PAA-grafted CNC enable better entanglements between long grafted chains at the neutral state

An Interface-Driven Stiffening Mechanism in Polymer Nanocomposites

Dynamic mechanical response in responsive and adaptive composites can be achieved either through the responsive polymer; with the chemical regulators affecting the bonding between fillers or through reversible covalent bonding. Tuning the interfaces between fillers and polymer matrix potentially plays a critical role in all these systems to enhance their adaptive responses. Here, we present that the bonding–debonding of chains on nanoparticles can be modulated under extensive periodic strains. Mechanical response of an attractive model polymer composite, poly­(methyl methacrylate) filled with silica nanoparticles, is monitored in a series of deformation–resting experiments allowing us to tune the interfacial strength of polymer. Chains that are desorbed from the surface with the oscillatory shear entangle with the free chains during the rest time. We show that periodic deformation process results in unusual stiffening of composites. Mechanical response during the recovery reveals this behavior arising from the enhancement in the entanglement of chains at interfaces. The interfacial hardening can be used in designing polymer composites with stress-sensitive interfaces to achieve new repair mechanisms for biomedical applications, and also in energy absorbing reinforced systems

Reversible Thermal Stiffening in Polymer Nanocomposites

Miscible polymer blends with different glass transition temperatures (<i>T</i>_g) are known to create confined interphases between glassy and mobile chains. Here, we show that nanoparticles adsorbed with a high-<i>T</i>_g polymer, poly­(methyl methacrylate), and dispersed in a low-<i>T</i>_g matrix polymer, poly­(ethylene oxide), exhibit a liquid-to-solid transition at temperatures above <i>T</i>_g’s of both polymers. The mechanical adaptivity of nanocomposites to temperature underlies the existence of dynamically asymmetric bound layers on nanoparticles and more importantly reveals their impact on macroscopic mechanical response of composites. The unusual reversible stiffening behavior sets these materials apart from conventional polymer composites that soften upon heating. The presented stiffening mechanism in polymer nanocomposites can be used in applications for flexible electronics or mechanically induced actuators responding to environmental changes like temperature or magnetic fields

Effect of Ionic Groups on Polymer-Grafted Magnetic Nanoparticle Assemblies

Conductivity in ionomer melts is governed by the density of conducting ions and ionic aggregation within low dielectric polymers. New material design strategies are needed to direct ion aggregation by utilizing low ion densities that will improve ion conductivity in polymer composite films. Here, we report the dispersion of ionomer-grafted magnetic nanoparticles (NPs) in polymers to explore their potential in energy applications. Iron oxide NPs coated with a uniform silane layer are grafted with polystyrene (PS) chains and are randomly sulfonated to various extents. We examine the interplay between ionic interactions and chain repulsion by varying the ion concentration and length of grafted chains. Transmission electron microscopy and small-angle X-ray scattering results show that ion-containing polymer-grafted NPs form highly ordered chain-like structures below 3 mol % sulfonation in bulk at two particle loadings (5 and 15 wt %). Moreover, increasing grafted chain length leads to long-range spacing correlations between sulfonated strings. This strategy to create discrete and connected highly ordered string nanostructures can be used as a means of controlling the ion aggregation and transport in polymer nanocomposites

Design of Ion-Containing Polymer-Grafted Nanoparticles for Conductive Membranes

While sulfonated polymers are commonly used in membranes for fuel cells and water filtration applications, challenges of controlling ionic aggregation and understanding morphology effects on conductivity and transport still remain. In this work, we investigate the aggregation of copolymer-grafted nanoparticles that are designed to form conductive structures with low sulfonation amounts of chains. We demonstrate that long grafts of polystyrene chains with sulfonated end groups form side-by-side aggregated strings and retain their structures in ionic liquid, 1-hexyl-3-methylimidazolium bis­(trifluoro­methyl­sulfonyl)­imide, [HMIM]­[TFSI]. Transmission electron tomography results revealed that these aggregates are monolayers of particles at low sulfonations and planar-like networks at 3 mol % sulfonation in the ionic liquid. Organization of magnetic nanoparticles with the polymer grafting approach is shown, for the first time, to enhance conductivity upon incorporation of an ionic liquid

Elastic Properties of a Protein–Polymer-Grafted Surface

Surfaces grafted with poly­(methyl methacrylate) (PMMA) and streptavidin were synthesized through click chemistry to investigate the role of surface stiffness on protein adsorption as the hydrophilic and hydrophobic surface coverage of the substituents vary. Surface topographies coupled with the nanoindentation results indicated that, with the appropriate selections of polymer coverage and chain length, the extent of non-specific protein adhesion could be controlled by the hydrophobic interactions between PMMA, biotin, and streptavidin. It was shown that, when the molecular weight and stiffness of PMMA was close to that of streptavidin, patchy PMMA morphologies were obtained, which help inhibit the non-specific adsorption of streptavidin

Elastic Properties of Protein Functionalized Nanoporous Polymer Films

Retaining the conformational structure and bioactivity of immobilized proteins is important for biosensor designs and drug delivery systems. Confined environments often lead to changes in conformation and functions of proteins. In this study, lysozyme is chemically tethered into nanopores of polystyrene thin films, and submicron pores in poly­(methyl methacrylate) films are functionalized with streptavidin. Nanoindentation experiments show that stiffness of streptavidin increases with decreasing submicron pore sizes. Lysozymes in polystyrene nanopores are found to behave stiffer than the submicron pore sizes and still retain their specific bioactivity relative to the proteins on flat surfaces. Our results show that protein functionalized ordered nanoporous polystyrene/poly­(methyl methacrylate) films present heterogeneous elasticity and can be used to study interactions between free proteins and designed surfaces

Programmable Light-Controlled Shape Changes in Layered Polymer Nanocomposites

We present soft, layered nanocomposites that exhibit controlled swelling anisotropy and spatially specific shape reconfigurations in response to light irradiation. The use of gold nanoparticles grafted with a temperature-responsive polymer (poly(<i>N</i>-isopropylacrylamide), PNIPAM) with layer-by-layer (LbL) assembly allowed placement of plasmonic structures within specific regions in the film, while exposure to light caused localized material deswelling by a photothermal mechanism. By layering PNIPAM-grafted gold nanoparticles in between nonresponsive polymer stacks, we have achieved zero Poisson’s ratio materials that exhibit reversible, light-induced unidirectional shape changes. In addition, we report rheological properties of these LbL assemblies in their equilibrium swollen states. Moreover, incorporation of dissimilar plasmonic nanostructures (solid gold nanoparticles and nanoshells) within different material strata enabled controlled shrinkage of specific regions of hydrogels at specific excitation wavelengths. The approach is applicable to a wide range of metal nanoparticles and temperature-responsive polymers and affords many advanced build-in options useful in optically manipulated functional devices, including precise control of plasmonic layer thickness, tunability of shape variations to the excitation wavelength, and programmable spatial control of optical response

Structure and Entanglement Factors on Dynamics of Polymer-Grafted Nanoparticles

Nanoparticles functionalized with long polymer chains at low graft density are interesting systems to study structure–dynamic relationships in polymer nanocomposites since they are shown to aggregate into strings in both solution and melts and also into spheres and branched aggregates in the presence of free polymer chains. This work investigates structure and entanglement effects in composites of polystyrene-grafted iron oxide nanoparticles by measuring particle relaxations using X-ray photon correlation spectroscopy. Particles within highly ordered strings and aggregated systems experience a dynamically heterogeneous environment displaying hyperdiffusive relaxation commonly observed in jammed soft glassy systems. Furthermore, particle dynamics is diffusive for branched aggregated structures which could be caused by less penetration of long matrix chains into brushes. These results suggest that particle motion is dictated by the strong interactions of chains grafted at low density with the host matrix polymer