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(N-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.

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.

Dynamics of the interphase region between matrix and bound polymers on nanoparticles is important to understand the macroscopic rheological properties of nanocomposites. Here, we present neutron scattering investigations on nanocomposites with dynamically asymmetric interphases formed by a high-glass transition temperature polymer, poly(methyl methacrylate), adsorbed on nanoparticles and a low-glass transition temperature miscible matrix, poly(ethylene oxide). By taking advantage of selective isotope labeling of the chains, we studied the role of interfacial polymer on segmental and collective dynamics of the matrix chains from subnanoseconds to 100 nanoseconds. Our results show that the Rouse relaxation remains unchanged in a weakly attractive composite system while the dynamics significantly slows down in a strongly attractive composite. More importantly, the chains disentangle with a remarkable increase of the reptation tube size when the bound polymer is vitreous. The glassy and rubbery states of the bound polymer as temperature changes underpin the macroscopic stiffening of nanocomposites.