Nanorod position and orientation in vertical cylinder block copolymer films.

The self-assembly of gold nanorods (AuNRs) of different sizes with a block copolymer (BCP) is studied. Polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) films containing P2VP functionalized AuNRs are solvent annealed resulting in a BCP morphology of vertical P2VP cylinders in a PS matrix. At the surface of the PS-b-P2VP films long AuNRs are found in the bridging and vertical states. The bridging state is where the long axis of the AuNR is parallel to the film surface, the AuNR is embedded in the film, and each end of the AuNR is at the top of nearest neighbor P2VP cylinders. The vertical state is where the AuNR is localized within a vertical P2VP cylinder, the AuNR long axis is perpendicular to the film surface and the upper tip of the AuNR is at the film surface. Short AuNRs were found in the bridging and vertical states as well as in a state not observed for the long AuNRs, the centered state. The centered state is where an AuNR has its long axis parallel to the film surface, is embedded in the film, and is centered over a vertical P2VP cylinder. Hybrid particle-field theory (HPFT) simulations modeling the experimental system predict that for the long AuNRs only the bridging state should be observed while for the short AuNRs only the bridging and centered states should be observed. Possible explanations for why the vertical state is observed in experiments despite being thermodynamically unfavorable in simulations are discussed. HPFT simulations also show that when a nanorod is in the bridging state the two cylinders it bridges remain intact and extend from the nanorod to the substrate. Further, the minority block of the BCP is shown to wet the bottom of the bridging nanorod. The bridging state is very promising for the future development of self-assembled nanoscale devices.

Alignment of Nanoplates in Lamellar Diblock Copolymer Domains and the Effect of Particle Volume Fraction on Phase Behavior.

Polymer nanocomposites (PNCs) that employ diblock copolymers (BCPs) to organize and align anisotropic nanoparticles (NPs) have the potential to facilitate self-assembling hierarchical structures. However, limited studies have been completed to understand the parameters that guide the assembly of nonspherical NPs in BCPs. In this work, we establish a well-defined nanoplate system to investigate the alignment of two-dimensional materials in a lamellar-forming poly(styrene-b-methyl methacrylate) (PS-b-PMMA) BCP with domains oriented parallel to the substrate. Monodisperse gadolinium trifluoride rhombic nanoplates doped with ytterbium and erbium [GdF3:Yb/Er (20/2 mol %)] are synthesized and grafted with phosphoric acid functionalized polyethylene glycol (PEG-PO3H2). Designed with chemical specificity to one block, the nanoplates align in the PMMA domain at low volume fractions (ϕ = 0.0083 and ϕ = 0.017). At these low NP loadings, the BCP lamellae are ordered and induce preferential alignment of the GdF3:Yb/Er nanoplates. However, at high volume fractions (ϕ = 0.050 and ϕ = 0.064), the BCP lamellae are disordered with isotropically dispersed nanoplates. The transition from an ordered BCP system with aligned nanoplates to a disordered BCP with unaligned nanoplates coincides with the calculated overlap volume fraction, ϕ* = 0.051, where the pervaded space of the NPs begins to overlap. Two phenomena are observed in the results: the effect of lamellar formation on nanoplate orientation and the overall phase behavior of the PNCs. The presented research not only expands our knowledge of PNC phase behavior but also introduces a framework to further study the parameters that affect nanoplate alignment in BCP nanocomposites. Our ability to control anisotropic NP orientation in PNCs through self-assembling techniques lends itself to creating multifunctional materials with unique properties for various applications such as photovoltaic cells and barrier coatings.

Temperature-Dependent Suppression of Polymer Diffusion in Polymer Nanocomposites.

The polymer center-of-mass tracer diffusion coefficient in athermal polymer nanocomposites (PNCs) composed of polystyrene and phenyl-capped, spherical silica nanoparticles was measured over a range of temperatures and nanoparticle concentrations using elastic recoil detection. The polymer tracer diffusion coefficient in the PNC relative to the bulk decreases with increasing nanoparticle concentration and is unexpectedly more strongly reduced at higher temperatures. This unusual temperature dependence of polymer diffusion in PNCs cannot be explained by the reptation model or a modified version incorporating an effective tube diameter. Instead we show that our results are consistent with a mechanism based on nanoparticle-imposed configurational entropy barriers.

Performance of dielectric nanocomposites: matrix-free, hairy nanoparticle assemblies and amorphous polymer-nanoparticle blends.

Demands to increase the stored energy density of electrostatic capacitors have spurred the development of materials with enhanced dielectric breakdown, improved permittivity, and reduced dielectric loss. Polymer nanocomposites (PNCs), consisting of a blend of amorphous polymer and dielectric nanofillers, have been studied intensely to satisfy these goals; however, nanoparticle aggregates, field localization due to dielectric mismatch between particle and matrix, and the poorly understood role of interface compatibilization have challenged progress. To expand the understanding of the inter-relation between these factors and, thus, enable rational optimization of low and high contrast PNC dielectrics, we compare the dielectric performance of matrix-free hairy nanoparticle assemblies (aHNPs) to blended PNCs in the regime of low dielectric contrast to establish how morphology and interface impact energy storage and breakdown across different polymer matrices (polystyrene, PS, and poly(methyl methacrylate), PMMA) and nanoparticle loadings (0-50% (v/v) silica). The findings indicate that the route (aHNP versus blending) to well-dispersed morphology has, at most, a minor impact on breakdown strength trends with nanoparticle volume fraction; the only exception being at intermediate loadings of silica in PMMA (15% (v/v)). Conversely, aHNPs show substantial improvements in reducing dielectric loss and maintaining charge/discharge efficiency. For example, low-frequency dielectric loss (1 Hz-1 kHz) of PS and PMMA aHNP films was essentially unchanged up to a silica content of 50% (v/v), whereas traditional blends showed a monotonically increasing loss with silica loading. Similar benefits are seen via high-field polarization loop measurements where energy storage for ∼15% (v/v) silica loaded PMMA and PS aHNPs were 50% and 200% greater than respective comparable PNC blends. Overall, these findings on low dielectric contrast PNCs clearly point to the performance benefits of functionalizing the nanoparticle surface with high-molecular-weight polymers for polymer nanostructured dielectrics.

Excluded volume model for the reduction of polymer diffusion into nanocomposites.

An analytic model for the slowing down of polymer chain diffusion in nanocomposites attributable to excluded volume effects is presented. The nanocomposite is modeled as an ensemble of cylinders through which the polymer chains diffuse. The reduction of polymer diffusion in each cylinder is equated with the reduction of diffusion for a sphere through a cylinder. The distribution of cylinder diameters within the ensemble is determined from statistical mechanical theories based on the packing of spherical particles. For low loadings of spherical particles in nanocomposites, this model results in a master curve for the reduced diffusion coefficient. With no adjustable parameters, the model agrees with recent data for tracer diffusion measurements in polymer nanocomposites at low loading.

Universal Scaling of Polymer Diffusion in Nanocomposites.

Nanoparticles are new and valuable additives that can favorably tune thermomechanical, electric, optical, and magnetic properties of polymeric materials. The addition of nanoparticles can also enhance or slow down polymer dynamics depending on the mixture thermodynamics and key length scales, namely, nanoparticle size, interparticle spacing (ID), and the polymer radius of gyration (Rg). Presently, a framework for understanding how nanoparticles affect polymer dynamics is not available, in part, because of a lack of wide-ranging experimental studies. Here, tracer diffusion is studied in model nanocomposites containing silica nanoparticles grafted with either polymer brushes (soft nanoparticles) or short ligands (hard nanoparticles). Over a wide range of tracer molecular weights and nanoparticle loadings, the normalized diffusion coefficient collapses onto a universal curve for both soft and hard nanoparticles when plotted against a confinement parameter, defined as ID/Rg, which accounts for tracer penetration into the brush. These experimental results provide new insights into the fundamental principles required to construct predictive models of polymer dynamics in nanocomposites.

Dielectric breakdown in silica-amorphous polymer nanocomposite films: the role of the polymer matrix.

The ultimate energy storage performance of an electrostatic capacitor is determined by the dielectric characteristics of the material separating its conductive electrodes. Polymers are commonly employed due to their processability and high breakdown strength; however, demands for higher energy storage have encouraged investigations of ceramic-polymer composites. Maintaining dielectric strength, and thus minimizing flaw size and heterogeneities, has focused development toward nanocomposite (NC) films; but results lack consistency, potentially due to variations in polymer purity, nanoparticle surface treatments, nanoparticle size, and film morphology. To experimentally establish the dominant factors in broad structure-performance relationships, we compare the dielectric properties for four high-purity amorphous polymer films (polymethyl methacrylate, polystyrene, polyimide, and poly-4-vinylpyridine) incorporating uniformly dispersed silica colloids (up to 45% v/v). Factors known to contribute to premature breakdown-field exclusion and agglomeration-have been mitigated in this experiment to focus on what impact the polymer and polymer-nanoparticle interactions have on breakdown. Our findings indicate that adding colloidal silica to higher breakdown strength amorphous polymers (polymethyl methacrylate and polyimide) causes a reduction in dielectric strength as compared to the neat polymer. Alternatively, low breakdown strength amorphous polymers (poly-4-vinylpyridine and especially polystyrene) with comparable silica dispersion show similar or even improved breakdown strength for 7.5-15% v/v silica. At ∼15% v/v or greater silica content, all the polymer NC films exhibit breakdown at similar electric fields, implying that at these loadings failure becomes independent of polymer matrix and is dominated by silica.