While incorporation of nanoparticles in a polymer matrix generally enhances the physical properties, effective control of the nanoparticle/polymer interface is often challenging. Here, we report a dramatic enhancement of the mechanical properties of polymer nanocomposites (PNCs) using a simple physical grafting method. The PNC consists of low molecular weight poly(ethylene glycol) (PEG) and silica nanoparticles whose surfaces are modified with dopaminemodified PEG (DOPA-mPEG) brush polymers. With DOPA-mPEG grafting, the nanoparticle surface can be readily altered, and the shear modulus of the PNC is increased by a factor of 10 5 at an appropriate surface grafting density. The detailed microstructure and mechanical properties are examined with small-angle X-ray scattering (SAXS) and oscillatory rheometry experiments. The attractive interactions between particles induced by DOPA-mPEG grafting dramatically improve the mechanical properties of PNCs even in an unentangled polymer matrix, which shows a much higher shear modulus than that of a highly entangled polymer matrix.

Shear is an effective method to create long-range order in micro-or nanostructured soft materials. When simple shear flow is applied, particles or polymer microdomains tend to align in the shear direction to minimize viscous dissipation; thus, transverse alignment (so-called log-rolling) is not typically favored. This is the first study to report the transverse alignment of cylinder-forming coil−coil block copolymers. Poly(styrene-bmethyl methacrylate), PS−PMMA, where the PS blocks form the matrix, can adopt a metastable PMMA hemicylindrical structure when confined in a thin film, and this hemicylindrical structure can orient either along the shear direction or transverse to the shear direction depending on the shearing temperature. A monolayer of PS−PMMA forming full cylinders exhibits logrolling alignment. This unusual log-rolling behavior is explained by the low chain mobility of the cylinder-forming PMMA block at low temperatures, which is the critical quantity determining the direction of shear alignment.

Localized surface plasmon resonance (LSPR) effect relies on the shape, size, and dispersion state of metal nanoparticles and can potentially be employed in many applications such as chemical/biological sensor, optoelectronics, and photocatalyst. While complicated synthetic approaches changing shape and size of nanoparticles can control the intrinsic LSPR effect, here we show that controlling interparticle interactions with silica-coated gold nanoparticles (Au@SiO NPs) is a powerful approach, permitting wide range of optical bandwidth of gold nanoparticles with great stability. The interparticle interactions of Au@SiO NPs are controlled through concentration-, temperature-, and time-dependent polymer-induced interactions. The polymer-induced interactions modulate the state of particle dispersion, resulting an effective plasmonic shift by more than 200 nm. We further explore the microstructure of particle aggregation and explain mechanisms of plasmonic shift based on the results of small-angle X-ray scattering (SAXS) and discrete dipole approximation (DDA) calculation. We show that an effective control of LSPR behavior is now available through trapped aggregation of Au@SiO NPs with temperature variation. We anticipate that the suggested strategy can be employed in many practical applications such as optical bioimaging and optoelectronic devices.

Good particle dispersion in polymer nanocomposites (PNCs) is often hampered by autophobic dewetting where the matrix polymers are expelled from the grafted polymer, generally believed to result in increased particle aggregation and enhanced mechanical properties in dilute particle regime. However, we found that autophobic dewetting with highly extended short-chain polymers improves/disrupts particle dispersity, strongly dependent on particle volume fraction. Under strong autophobic condition given with the high-molecular-weight ratio between the matrix, P, and grafted polymer, N, (P/N ≫ 1), silica nanoparticles grafted with dopamine-modified poly­(ethylene glycol) (DOPA-mPEG) brush polymer are dispersed in the PEG matrix by varying the surface grafting rate. In the dilute particle regime, we found that increasing grafting rate ironically improves particle dispersion and reduces the shear modulus as dewetted polymers cannot bridge the particles. In the concentrated particle regime, on the contrary, particles become more aggregated and the corresponding mechanical strength increases with grafting rate as a denser particle network is formed by depletion attractions. Investigating the microstructures, dynamics, and rheological properties of PNCs with small-angle X-ray scattering, time-domain proton NMR, and oscillatory rheometry experiments, respectively, this study provides additional design guidelines for controlling the detailed structure and properties of PNCs.

The ability to control the degree of particle dispersion in polymer solutions has been a long-standing subject in colloidal science. While a generally accepted principle is that nonadsorbing polymers can induce depletion attraction, which is mostly temperature independent, the effects of adding adsorbing polymers are still poorly understood. In this study, we investigated the effects of adsorbing polymers on the temperature-dependent stability of nanoparticles. The model systems consisted of silica nanoparticles in low-molecularweight poly(ethylene glycol) solutions. The detailed microstructures were determined with small-angle X-ray and neutron scattering measurements, while the dynamics of the temperature-dependent microstructures of the nanoparticles and polymers were probed with diffusing-wave spectroscopy. It was found that a poor solvent for polymer could drive adsorbed polymers to leave the particle substrate and return to the bulk solution due to a complicated interaction with surface, while the loss of the steric layer causes the nanoparticles to aggregate at elevated temperatures.