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

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

Spatial Ordering of Colloids in a Drying Aqueous Polymer Droplet

We explore the role of polymer chains on deposition of colloidal particles at solid surfaces from drying aqueous drops and show that the kinetics of phase separation of colloids and polymers can be explained by spinodal decomposition of binary systems. Concentrations of polymer solutions and polymer chain lengths were varied to understand the aggregation dynamics of colloidal particles via a polymer bridging mechanism. We show that when polymer concentration in the droplet is increased, particles spatially order upon drying due to a combination of the phase separation of highly bridged particles and the Marangoni flow effect. The demonstrated effect of particle-adsorbing, water-soluble polymers on the coffee-ring formation opens up new ways of creating highly ordered, long-range patterned surfaces using a facile, template-free approach

Role of Filler Shape and Connectivity on the Viscoelastic Behavior in Polymer Nanocomposites

We compare the rheological behavior of three classes of polymer nanocomposites (PNCs) to understand the role of particle shape and interactions on mechanical reinforcement. The first two correspond to favorably interacting composites formed by mixing poly­(2-vinylpyridine) with either fumed silica nanoparticles (NPs) or colloidal spherical silica NPs. We show that fumed silica NPs readily form a percolated network at low NP volume fractions. We deduce that the NPs act as network junctions with the effectively irreversibly bound polymer chains serving as the connecting bridges. By comparing with colloidal spherical silica, which has a significantly higher percolation threshold, we conclude that the fractal shape of the fumed silica is responsible for its unusually low percolation threshold. The third system corresponds to polystyrene grafted colloidal silica nanoparticles (PGNPs) in a polystyrene matrix. These PNCs have an even lower percolation threshold probably because the grafted chains increase the effective volume fraction of the NPs. When we take these different thickness of the polymer layers in the two cases into account (i.e., grafted layer vs adsorbed layer thickness), the percolation threshold for the fumed and the grafted system occurs at similar effective loadings, but the NP network with fumed silica has a higher low-frequency plateau modulus than that formed with the PGNPs. These findings can be reconciled by the fact that the fumed silica NPs are composed of fused entities, thus ensuring that they have a higher modulus than the PGNPs where the modulus is largely attributed to interactions between the grafts. Our results systematically stress the important role of the nanofiller shape and connectivity on the mechanical reinforcement of PNCs