Erkan Senses

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(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 Poissons 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.

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.

Microscopic Chain Motion in Polymer Nanocomposites with Dynamically Asymmetric Interphases.

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.

Reversible Thermal Stiffening in Polymer Nanocomposites.

Miscible polymer blends with different glass transition temperatures (Tg) are known to create confined interphases between glassy and mobile chains. Here, we show that nanoparticles adsorbed with a high-Tg polymer, poly(methyl methacrylate), and dispersed in a low-Tg matrix polymer, poly(ethylene oxide), exhibit a liquid-to-solid transition at temperatures above Tgs 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.

Tuning mechanical properties of nanocomposites with bimodal polymer bound layers

We show that a bound layer composed of short and long chains can be exploited to regulate the elastic moduli of bulk polymer nanocomposites at same particle loadings and dispersion states. The bound layer thickness on particles with high coverage of long chains is reduced with oscillatory deformation in a model attractive nanocomposite system. Reversibility of the bound layer is, thus, possible for the short chains in the interphase. Compositional dynamic heterogeneity in the interphase has subsequent effects on the fragility of composites. With increasing amount of adsorbed long chains, the fragility index systematically improves from moderate to high values. Our results suggest that the interphase layer between adsorbed chains and free matrix directly governs the reinforcement in poly(methyl methacrylate)–silica nanocomposites and can be dynamically altered under large shear.

Mechanistic model for deformation of polymer nanocomposite melts under large amplitude shear

We report the mechanical response of a model nanocomposite system of poly(styrene) (PS)-silica to large-amplitude oscillatory shear deformations. Nonlinear behavior of PS nanocomposites is discussed with the changes in particle dispersion upon deformation to provide a complete physical picture of their mechanical properties. The elastic stresses for the particle and polymer are resolved by decomposing the total stress into its purely elastic and viscous components for composites at different strain levels within a cycle of deformation. We propose a mechanistic model which captures the deformation of particles and polymer networks at small and large strains, respectively. We show, for the first time, that chain stretching in a polymer nanocomposite obtained in large amplitude oscillatory deformation is in good agreement with the nonlinear chain deformation theory of polymeric networks.

Nanoscale Particle Motion in Attractive Polymer Nanocomposites

Using x-ray photon correlation spectroscopy, we examined the slow nanoscale motion of silica nanoparticles individually dispersed in an entangled poly (ethylene oxide) melt at particle volume fractions up to 42%. The nanoparticles, therefore, serve as both fillers for the resulting attractive polymer nanocomposites and probes for the network dynamics therein. The results show that the particle relaxation closely follows the mechanical reinforcement in the nanocomposites only at the intermediate concentrations below the critical value for the chain confinement. Quite unexpectedly, the relaxation time of the particles does not further slow down at higher volume fractions-when all chains are practically on the nanoparticle interface-and decouples from the elastic modulus of the nanocomposites that further increases orders of magnitude.