Electrical Control of Shape in Voxelated Liquid Crystalline Polymer Nanocomposites.

Liquid crystal elastomers (LCEs) exhibit anisotropic mechanical, thermal, and optical properties. The director orientation within an LCE can be spatially localized into voxels [three-dimensional (3-D) volume elements] via photoalignment surfaces. Here, we prepare nanocomposites in which both the orientation of the LCE and single-walled carbon nanotube (SWNT) are locally and arbitrarily oriented in discrete voxels. The addition of SWNTs increases the stiffness of the LCE in the orientation direction, yielding a material with a 5:1 directional modulus contrast. The inclusion of SWNT modifies the thermomechanical response and, most notably, is shown to enable distinctive electromechanical deformation of the nanocomposite. Specifically, the incorporation of SWNTs sensitizes the LCE to a dc field, enabling uniaxial electrostriction along the orientation direction. We demonstrate that localized orientation of the LCE and SWNT allows complex 3-D shape transformations to be electrically triggered. Initial experiments indicate that the SWNT-polymer interfaces play a crucial role in enabling the electrostriction reported herein.

Directed Self-Assembly of Block Copolymers for High Breakdown Strength Polymer Film Capacitors.

Emerging needs for fast charge/discharge yet high-power, lightweight, and flexible electronics requires the use of polymer-film-based solid-state capacitors with high energy densities. Fast charge/discharge rates of film capacitors on the order of microseconds are not achievable with slower charging conventional batteries, supercapacitors and related hybrid technologies. However, the current energy densities of polymer film capacitors fall short of rising demand, and could be significantly enhanced by increasing the breakdown strength (EBD) and dielectric permittivity (εr) of the polymer films. Co-extruded two-homopolymer component multilayered films have demonstrated much promise in this regard showing higher EBD over that of component polymers. Multilayered films can also help incorporate functional features besides energy storage, such as enhanced optical, mechanical, thermal and barrier properties. In this work, we report accomplishing multilayer, multicomponent block copolymer dielectric films (BCDF) with soft-shear driven highly oriented self-assembled lamellar diblock copolymers (BCP) as a novel application of this important class of self-assembling materials. Results of a model PS-b-PMMA system show ∼50% enhancement in EBD of self-assembled multilayer lamellar BCP films compared to unordered as-cast films, indicating that the breakdown is highly sensitive to the nanostructure of the BCP. The enhancement in EBD is attributed to the "barrier effect", where the multiple interfaces between the lamellae block components act as barriers to the dielectric breakdown through the film. The increase in EBD corresponds to more than doubling the energy storage capacity using a straightforward directed self-assembly strategy. This approach opens a new nanomaterial paradigm for designing high energy density dielectric materials.

Stability of Polymer Grafted Nanoparticle Monolayers: Impact of Architecture and Polymer-Substrate Interactions on Dewetting.

The stability of polymer thin films is crucial to a broad range of technologies, including sensors, energy storage, filtration, and lithography. Recently, the demonstration of rapid deposition on solid substrates of ordered monolayers of polymer grafted nanoparticles (PGN) has increased potential for inks to additively manufacture such components. Herein, enhanced stability against dewetting of these self-assembled PGN films (gold nanoparticle functionalized with polystyrene (AuNP-PS)) is discussed in context to linear polystyrene (PS) analogues using high throughput surface gradients: surface energy (20-45 mN/m) and temperature (90-160 °C). PGNs exhibit a lower surface (γp) and interfacial (γsp) energy relative to linear polymers, which results in increased thermal and energetic stability by 10-25 °C and 5-15 mN/m, respectively. This enhanced wetting-dewetting transition is qualitatively consistent with the behavior of star macromolecules and depends on the architecture of the polymer canopy. Increased film stability through canopy architecture expands the manufacturability of thin film hybrids and refines postprocessing conditions to maximize local PGN order.

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.

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.

Diffusion of nanoparticles in semidilute and entangled polymer solutions.

We studied the diffusion of gold nanoparticles in semidilute and entangled solutions of polystyrene (PS) in toluene using fluctuation correlation spectroscopy (FCS). The polymer concentration was varied from approximately 6c* to 20c*, where c* is the overlap concentration. In our experiments, the particle radius (R approximately 2.5 nm) was much smaller compared to the radius of gyration (Rg approximately 18 nm) of the chain but comparable to the average mesh size (xi) of the fluctuating polymer network. The diffusion coefficient (D) of the particles decreased monotonically with polymer concentration and it can be fitted with a stretched exponential function, D=D0 exp(-microcnu), with the value of the scaling parameter, nu approximately 0.9. At high concentration of the polymer, a clear subdiffusive motion of the particles was observed. The results were compared with the diffusion of free dyes (coumarin 480), which showed normal diffusive behavior for all concentrations.

Effect of surface curvature on critical adsorption.

We studied critical adsorption on curved surfaces by utilizing spherical nanoparticles immersed in a critical binary liquid mixture of 2,6 lutidine+water. The temperature dependence of the adsorbed film thickness and excess adsorption was determined from fluorescence correlation spectroscopy measurements of the enlarged effective hydrodynamic radius of the particles. Our results indicated that the adsorbed film thickness is of the order of correlation length associated with concentration fluctuations. The excess adsorption per unit area increases following a power law in reduced temperature with an exponent of -1, which is the mean-field value for the bulk susceptibility exponent. This has been confirmed with silica particles of two different radii, 10 and 25 nm. The results were also compared with the theoretical mean-field scaling function.

Comparing the activation energy of diffusion in bulk and ultrathin fluid films.

We have measured the activation energy (E act) of translational diffusion for a dissolved fluorescent dye in bulk and within an ultrathin liquid film formed on a solid substrate. The experiments were performed using the single-molecule sensitive technique of fluorescence correlation spectroscopy. From the temperature-dependent measurements, we have determined that the activation energy for a few nanometer thick fluid film increases by a factor of approximately 3-4 compared to bulk liquid. The results are confirmed for two distinctly different systems in regard to molecular shape, tetrakis (2-ethylhexoxy) silane and hexadecane.

Contraction and reswelling of a polymer chain near the critical point of a binary liquid mixture.

We have studied the conformation change of a flexible linear polymer chain near the critical point of a binary liquid mixture using fluorescence correlation spectroscopy, which measured the hydrodynamic radius of the chains. Our results indicate that as the critical temperature (Tc) is approached, the chain size decreases. The polymer attains its most compact conformation when the correlation length of the critical fluctuations becomes comparable to the coil size. At very close to Tc, the polymer reexpands dramatically. To our knowledge, this is the first experimental evidence supporting the prediction of Brochard and de Gennes that a polymer chain will collapse and subsequently reswell on approaching Tc.