The mechanical properties of particulate nanocomposites strongly depend upon the particle dispersion, as well as on the closely related properties in thin polymer films covering the particle surface. The length scale of such changes is relevant for the understanding of particle−particle interactions, which ultimately dominate the mechanical response. Using well-defined 44 nm diameter silica nanoparticles dispersed in poly(ethylene glycol), we focus on surface-induced changes in polymer dynamics. Using proton time-domain NMR, we distinguish three polymer phases of different mobility, i.e., a strongly adsorbed, solid-like fraction, a fraction with intermediate relaxation times and a highly mobile fraction. We explore how these fractions change as we vary polymer molecular weight from 300 to 20 000 and particle volume fraction up to 0.3. A multiple-quantum experiment enables a closer analysis of the mobile component which we show consists of two fractions, one resembling the bulk melt-like and another one showing network-like properties. We demonstrate that above a polymer molecular weight-dependent volume fraction, polymers form elastically active links between particles, resulting in the physical gelation observed in such systems. Our results provide a quantitative picture of network formation, which is described by the amount and length of network-like chains as well as heterogeneities in the polymer dynamics. We relate changes in polymer dynamics to particle microstructure obtained from small angle neutron scattering.

We establish the existence and size of adsorbed polymer layers in miscible dense nanocomposites and their consequences on microstructure and the bulk modulus. Using contrast-matching small-angle neutron scattering to characterize all partial collective structure factors of polymers, particles, and their interface, we demonstrate qualitative failure of the random phase approximation, accuracy of the polymer reference site interaction model theory, ability to deduce the adsorbed polymer layer thickness, and high sensitivity of the nanocomposite bulk modulus to interfacial cohesion. DOI: 10.1103/PhysRevLett.107.225504 PACS numbers: 61.46.Df, 61.05.fg, 61.41.+e Particle aggregation in concentrated polymer solutions, melts, and cross-linked elastomers profoundly alters the mechanical, optical, and electrical properties of nanocomposites [1][2][3][4][5]. Mechanisms of controlling the state of particle aggregation in these complex mixtures remain elusive and poorly understood [6]. Conceptual frameworks for achieving good dispersion generally rely on chemically or physically bound polymer layers [7,8] to induce a repulsive interparticle potential of mean force between nanoparticles [9,10]. Nonetheless, understanding the nature of, and what controls, the structure and properties of these layers remains an outstanding challenge in soft matter of broad importance in polymer science, colloid science, and even biological systems. Obstacles limiting progress include (i) developing experimental tools and physics-based strategies for measuring and controlling the material-specific strength of polymer segments and particle surface attraction, (ii) differentiating polymer segments adsorbed on the nanoparticle surface from bulk polymer in the dense polymer solutions or melts, and (iii) measuring the packing structure of both segments and particles over a wide range of length scales and volume fractions.In this Letter, we present experimental results addressing the above issues using contrast-matching small-angle neutron scattering techniques in conjunction with carefully designed, thermodynamically stable (miscible) concentrated ternary solutions of short-chain polymers (oligomers), nanoparticles, and solvent. Measuring the intensity of scattered neutrons as a function of particle and polymer scattering contrast allows determination of all three partial collective structure factors that quantify spatially resolved segment-segment, particle-particle, and interfacial concentration fluctuations [11]. We employ this experimental knowledge to (i) determine the adsorbed layer thickness, (ii) quantitatively test at an unprecedented level the microscopic polymer reference interaction site model (PRISM) theory [12,13], and (iii) discover strong limitations of the incompressible random phase approximation (IRPA) [14]. How interfacial cohesion can qualitatively modify the effect of particle addition on the nanocomposite bulk modulus is addressed based on the experimentally validated theory.Silica nanoparticles of diameter D ¼ 40 nm are s...

While block copolymer lithography has been broadly applied as a bottom-up patterning technique, only a few nanopattern symmetries, such as hexagonally packed dots or parallel stripes, can be produced by spontaneous self-assembly of simple diblock copolymers; even a simple square packing has heretofore required more intricate macromolecular architectures or nanoscale substrate prepatterning. In this study, we demonstrate that square, rectangular, and rhombic arrays can be created via shear-alignment of distinct layers of cylinder-forming block copolymers, coupled with cross-linking of the layers using ultraviolet light. Furthermore, these block copolymer arrays can in turn be used as templates to fabricate dense, substrate-supported arrays of nanostructures comprising a wide variety of elements: deep (>50 nm) nanowells, nanoposts, and thin metal nanodots (3 nm thick, 35 nm pitch) are all demonstrated.

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