Two-Component Self-Assemblies: Investigation of a Synergy between Bisurea Stickers

It is of interest to develop two-component systems for added flexibility in the design of supramolecular polymers, nanofibers, or organogels. Bisureas are known to self-assemble by hydrogen bonding into long supramolecular objects. We show here that mixing aromatic bisureas with slightly different structures can yield surprisingly large synergistic effects. A strong increase in viscosity is observed when a bisurea with the sterically demanding 2,4,6-trimethylbenzene spacer is combined with a bisurea bearing no methyl group in position 2 of the aromatic spacer (i.e., 4-methylbenzene or 4,6-dimethylbenzene). This effect is the consequence of a change in the supramolecular assembly triggered by the composition of the mixture. The mixture of complementary bisureas forms rodlike objects that are more stable by about 1 kJ/mol and that are thicker than the rodlike objects formed by both parent systems

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

Self-Assembly of Monodisperse versus Bidisperse Polymer-Grafted Nanoparticles

We systematically compare the dispersion and self-assembly of silica nanoparticles (NPs) grafted with either a sparse monomodal long chain length polystyrene (PS) brush or a bimodal brush comprised of a sparse grafting of long PS chains and a dense carpet of short poly­(2-vinylpyridine) (P2VP) chains. These two different types of NPs are placed in pure PS matrices of varying molecular weights in a series of experiments. We first show that NP dispersion is generally improved in the case of bimodal brushes. More interestingly, at low PS grafting densities the bimodal brushes give different self-assembled structures relative to the monomodal brushes; we conjecture that the presence of the short P2VP chains in the bimodal brush reduces the effective core–core attractions and thus allows these bidisperse NPs to display self-assembly behavior that is less likely to be kinetically trapped by the strong intercore attractions that control the behavior of monomodal NPs. In this low PS grafting density limit, where we expect the spatial coverage of the brush to be the most nonuniform, we find the formation of “vesicular” structures that are representative of highly asymmetric (“tadpole”) surfactants. Our results therefore show that reducing the inter-NP attractions gives rise to a much richer ensemble of NP self-assemblies, apparently with a smaller influence from kinetic traps (or barriers)

Control of the Pore Texture in Nanoporous Silicon via Chemical Dissolution

The surface and textural properties of porous silicon (pSi) control many of its physical properties essential to its performance in key applications such as optoelectronics, energy storage, luminescence, sensing, and drug delivery. Here, we combine experimental and theoretical tools to demonstrate that the surface roughness at the nanometer scale of pSi can be tuned in a controlled fashion using partial thermal oxidation followed by removal of the resulting silicon oxide layer with hydrofluoric acid (HF) solution. Such a process is shown to smooth the pSi surface by means of nitrogen adsorption, electron microscopy, and small-angle X-ray and neutron scattering. Statistical mechanics Monte Carlo simulations, which are consistent with the experimental data, support the interpretation that the pore surface is initially rough and that the oxidation/oxide removal procedure diminishes the surface roughness while increasing the pore diameter. As a specific example considered in this work, the initial roughness ξ ∼ 3.2 nm of pSi pores having a diameter of 7.6 nm can be decreased to 1.0 nm following the simple procedure above. This study allows envisioning the design of pSi samples with optimal surface properties toward a specific process

Polymer Chain Behavior in Polymer Nanocomposites with Attractive Interactions

Chain behavior has been determined in polymer nanocomposites (PNCs) comprised of well-dispersed 12 nm diameter silica nanoparticles (NPs) in poly­(methyl methacrylate) (PMMA) matrices by Small-Angle Neutron Scattering (SANS) measurements under the Zero Average Contrast (ZAC) condition. In particular, we directly characterize the bound polymer layer surrounding the NPs, revealing the bound layer profile. The SANS spectra in the high-<i>q</i> region also show no significant change in the bulk polymer radius of gyration on the addition of the NPs. We thus suggest that the bulk polymer conformation in PNCs should generally be determined using the high <i>q</i> region of SANS data

Tunable Multiscale Nanoparticle Ordering by Polymer Crystallization

While ∼75% of commercially utilized polymers are semicrystalline, the generally low mechanical modulus of these materials, especially for those possessing a glass transition temperature below room temperature, restricts their use for structural applications. Our focus in this paper is to address this deficiency through the controlled, multiscale assembly of nanoparticles (NPs), in particular by leveraging the kinetics of polymer crystallization. This process yields a multiscale NP structure that is templated by the lamellar semicrystalline polymer morphology and spans NPs engulfed by the growing crystals, NPs ordered into layers in the interlamellar zone [spacing of O (10–100 nm)], and NPs assembled into fractal objects at the interfibrillar scale, O (1–10 μm). The relative fraction of NPs in this hierarchy is readily manipulated by the crystallization speed. Adding NPs usually increases the Young’s modulus of the polymer, but the effects of multiscale ordering are nearly an order of magnitude larger than those for a state where the NPs are not ordered, i.e., randomly dispersed in the matrix. Since the material’s fracture toughness remains practically unaffected in this process, this assembly strategy allows us to create high modulus materials that retain the attractive high toughness and low density of polymers