Session G33: Polymer Nanocomposites: Structure and Property

Silicone-Iron Oxide Nanocomposite Encapsulants for Common Mode Noise Reduction in Switching Power Electronics

The use of polymer encapsulants continues to permeate the electronics industry as a way to improve the reliability of packaging solutions, and the incorporation of nanomaterials presents an area of potential advancement. Management of electromagnetic noise represents a design challenge in power electronics that can be aided through the use of nanoparticle-enhanced encapsulants. Aluminum heat sinks often create common mode noise from AC disturbances generated by switching power supplies. Mitigation of this noise in modern electronics results in large increases in device efficiency; nanoparticles which have an intrinsic susceptibility to electromagnetic fields could add another dimension to encapsulant functionality by improving noise shielding. In the study presented herein, iron oxide nanoparticles were incorporated into a silicone rubber matrix to create an electomagnetically-susceptible nanocomposite in an effort to reduce common mode noise. After applying the nanocomposites to a prototype heat sink, a noticeable reduction in common mode noise was observed with just a relatively small amount of nanomaterial. Introducing this simple concept to modern designs will aid in improving noise management in power electronics devices.   

Flexible Textured Nanocomposite for Energy Harvesting Applications

The present study provides a method for designing and fabricating a multifunctional flexible absorber (flexorb) that can be used as a coating material for ultra-high absorption applications. Untextured nanocomposite (UNC) is synthesized by reinforcing 1 wt. % of nanofillers [carbon nanotubes (CNTs), zinc oxide nanorods and iron nanoparticles] in polydimethylsiloxane (PDMS). Homogeneous mixture of these nanoparticles led to an ultra-high absorption ≥ 96 % from 300-2000 nm. Subsequently, texturing on the surface of UNC, enhance the absorption ≥ 99 % in entire UV-Vis-NIR wavelength range. This increase in absorption is due to the multiple scattering of incident beam along with the surface texture. In addition to this CNTs reinforced in flexorb provides better impedance matching which results in lower reflection losses from flexorb/air interface. Moreover, the textured nanocomposite exhibited a better tensile, tear, shear and peel-off strength than pure PDMS and UNC. The flexorb has an advantage of flexibility which ensures that it can be deformed in variety of shapes without any damage or creasing failure. Such a ultra-black textured nanocomposite may find many applications such as suppression of stray light in telescopes, thermal imaging sensors, spectroscopy and energy harvesting.   

Nanoparticle Assembly Modulated by Biobased Polymers and Its Coating Application

We study how nanoparticles assemble in a dried film containing biobased polymers. Different nanoparticles were dispersed with hydroxyethyl cellulose (HEC) and hydroxyethyl starch (HES). It was discovered that polymer morphology has a profound influence on the assembly structures of nanocomposite. In addition, an unexpected highly fractal network structure assembled by nanoparticles were formed in HEC matrix with different types of nanoparticles. Electron microscopy and small X-ray scattering offer the detailed analysis of the structures. Based on these results, we further developed a biobased waterborne coating system to effectively block the UV radiation while maintaining the transparency using ZnO nanoparticles. Our new coating materials can lead to coating films ~ 100 times thinner than the previously reported coating systems of similar UV-blocking performance. The same concept can be applied to other polymer nanocomposite systems. The study opens the opportunity of utilizing more sustainable biobased materials for high-value functional coating applications.   

Coarse-grained molecular dynamics simulations on mechanical properties of polymer composites for bulk heterojunction solar cells

We investigate mechanical properties of polymer composites of poly(3-hexylthiophene) (P3HT) and fullerene C₆₀ using coarse-grained molecular dynamics (CGMD) simulations, where the P3HT monomer unit and C₆₀ are represented by three CG beads and a single CG bead. Pure P3HTs with the degrees of polymerization (DPs) of 50, 100, and 150 exhibit almost identical tensile moduli, while the tensile strength increases with the DP. We quantify an increase in the number of molecular chain entanglements resulting from increasing DP, which in turn enhances the tensile strength. Meanwhile, the decomposition of molecular interactions contributing to stress indicates that the tensile modulus is primarily determined by non-bonded potentials and bond length potentials, almost independent of the chain entanglements. Furthermore, the addition of C₆₀ leads to higher tensile modulus and hence more brittle behavior of the composite in accordance with experiments. We find that an increase in C₆₀ mass fraction further inhibits molecular chain entanglements, leading to a significant reduction of the tensile strength. Meanwhile, an increase in the tensile modulus mainly originates from an increase in non-bonded interactions associated with C₆₀.   

Nanoparticle Templating of Ultrathin and High Density 6 nm Pore Arrays

Active layers for next-generation water filtration membranes require large area and uniform arrays of nanometer-sized pores. We demonstrate an alignment-free method to transform freely suspended nanoparticle (NP) monolayer sheets into mechanically robust, high density 6 nm pore arrays using electron beam irradiation and subsequent wet chemical etching. As the secondary electrons responsible for ligand crosslinking originate from the NPs themselves, our substrate-free approach benefits from a variable crosslink density throughout the ~10 nm film thickness. We apply this general strategy to two NP systems—gold NPs with alkanethiol ligands and Fe₃O₄ NPs coated with oleic acid—to fabricate nanostructured polymer films with either thiol or carboxylate pore chemical functionalities.   

Gas Transport of Self-assembled Polymer Nanocomposites with Binary Nanoparticle Size

Covalently grafting organic (hydrophobic) polymer chains to inorganic (hydrophilic) nanoparticles is a facile means of addressing their inherent immiscibility. We have previously shown that membranes comprised of such grafted nanoparticles (GNPs) exhibit remarkable increases in gas permeabilities relative to that of the neat polymer. Here, we go beyond these previous works and combine GNPs of two different core sizes (14 nm and 50 nm) to probe the role of NP size polydispersity on NP structuring and therefore properties. Structurally, we find that the GNPs form self-assembled structures that fill space. We present the self-assembled morphologies and measured gas transport properties of these polymer nanocomposites with binary core sizes.   

Polymer Composites of Two-Dimensional Layered Materials for Structural Applications

Polymer composites of two-dimensional (2D) layered nanostructures such as graphene, graphene oxide (GO) and h-BN have promising potential in future technologies. However, although it has been more than a decade polymer composites of 2D-layered materials have been investigated, still fundamental challenges remain for practical applications. Currently, poor load transfer, good interfacial engineering and dispersion of layered materials in polymer matrix remain as a big challange that needs to be overcome for the commercialization of these materials. 2D-layered materials offer significant advantages for developing thermally stable with robust mechanical properties, high electrical and thermal conductivities toward building ultra-lightweight multifunctional composites. Here, we report the development of highly porous, ultralight-weight and flexible polymer composite of 2D-layered structures for high temperature environmental applications. Physical, chemical and interfacial interactions of various polymer and 2D-layered structures will be discussed with the support of fully atomistic reactive molecular dynamics simulations that discloses unusual interface interactions between polymer matrix and 2D-layered structures.   

Mesostructured Metal Superconductors via Block Copolymer Nanocomposites: Quantum Metamaterials from Soft Matter

Three-dimensionally mesostructured superconductors have unique properties compared to thin film or bulk analogues, but their investigation has been limited by the lack of facile synthesis methods. Though largely unexplored in this application, block copolymers (BCPs) can serve as structure-directing agents to tune material architectures and, by extension, properties over scales on the order of characteristic lengths in superconductors (10s of nm). Hybrid BCP-inorganic self-assembly provides a route to mesostructured NbN superconductors, but complex materials chemistry prevents comparison with bulk equivalents. In recent work we demonstrate a versatile approach to mesoscale ordered metal superconductors via self-assembly of BCP nanocomposites. Results indicate quantum metamaterials behavior, with evidence that properties such as the transition temperature, coherence length, and critical field change substantially in a 3D periodic mesostructure (e.g., the bicontinuous double gyroid). Future work will investigate emergent phenomena such as angle-dependent magnetization behavior, and expansion of our technique to other metals could lead to novel photonic, electronic, and/or catalytic properties.   

Experiments and Simulations of Nanoplate String Assembly in Lamellar Diblock Copolymer

Within poly(styrene-b-methyl methacrylate) (PS-b-PMMA) block copolymer (BCP) lamellae, oriented nanoplates grafted with polyethylene glycol assemble into aligned strings at small interparticle separations. These assemblies are studied using X-ray scattering, electron microscopy, and hybrid particle/self-consistent field theory (hSCFT) simulations. The insertion of a nanoplate in a BCP microdomain is expected to produce a local domain bulge as the PS/PMMA interface distorts to optimize conformational chain entropy. 2D simulations of the equilibrium BCP structure show bulge formation around the nanoplates. As a function of particle separation, the potential of mean force (PMF) reveals a global minimum corresponding to an interparticle spacing of 7.0 nm, in good agreement with experimentals, 6.42 nm. The PMF calculation exhibits an activation barrier due to the high curvature penalty between two nanoplates at a separation of 21.7 nm. Ultimately, nanoplate strings form to optimize free energy contributions from interfacial area and chain stretching. The effect of BCP molecular weight on the PMF will also be presented.   

Molecular engineering of graft and matrix polymers for tuning grafted particle dispersion in polymer nanocomposites: A theory and simulation study

In polymer nanocomposites (PNCs) comprised of grafted nanoparticles in a polymer matrix, tailoring the graft and matrix polymers is a way to tune the effective inter-particle interactions and morphology. In this talk we present our work using molecular simulations and theory showing how increasingly attractive graft-matrix interactions affect the interpenetration of matrix and graft chains (termed as grafted layer wetting) and the dispersion/aggregation of grafted particles in the matrix. Past work by our group on similar systems has shown that wetting/dewetting and dispersion/aggregation are two distinct phase transitions, former a continuous one and the latter a first-order transition as a function of graft-matrix interactions. In this work we find that as the graft-matrix attraction increases, the graft chains extend and matrix chains increasingly wet the grafted layer, leading to larger and harder grafted particles compared to analogous PNCs with athermal graft-matrix interactions. Simultaneously, the PNC structure changes from an aggregated/dispersed morphology dictated by the entropic limit to a dispersed morphology due to favorable weak graft-matrix attraction, and finally, to a correlated fluid of hard grafted particles at stronger graft-matrix attraction   

Polymer grafted nanoparticles on Polymer Films: Entropic and Enthalpic effects on strcture and dynamics

Nanoparticle based ultra-thin membranes have been shown to have remarkable mechanical properties while also possessing novel electrical, optical or magnetic properties, which could be controlled by tailoring properties at the level of individual nanoparticles. Here we report thermal stability and the corresponding microscopic dynamics of polymer supported ultra-thin membranes comprising of self-assembled, ordered grains of polymer grafted nanoparticles having tunable mechanical properties. The initially ordered membranes show distinct pathways for temperature induced disordering depending on membrane flexibility as well as on interfacial entropic and enthalpic interactions with the underlying polymer thin film. We also observe contrasting temperature dependence of microscopic dynamics of these membranes depending on whether the graft polymer-substrate polymer interactions are predominantly entropic or enthalpic in nature. Our results suggest that apart from their varied applications, the soft nanoparticle-polymer hybrid membranes are a playground for rich physics involving subtle entropic and enthalpic effects along with the nanoparticles softness, which eventually determine their thermo-mechanical stability   

Gradient-Based Explicit Theoretical Framework for Simulation of Block Copolymer-Nanoparticle Co-assembly

The co-assembly of dispersed nanoparticles in block copolymers is a promising avenue for creating ordered structures at the nanoscale and materials with unique properties. Consequently, the theoretical understanding of this phenomenon is of great interest. In this talk, we present a novel theoretical framework for the simulation of the block copolymer-nanoparticle co-assembly. In this approach, we explicitly keep track of each particle in a sharp fashion while describing the polymer material using the Self-Consistent Field Theory. We consider particles of arbitrary shapes and allow the particle's surface affinity for different polymer components to be variable in space. The relaxation of block copolymer-nanoparticle mixtures to (meta) stable configurations is performed using a gradient-type approach based on the analytically derived expression for the virtual work of nanoparticles or, in other words, the full derivative of the system's energy with respect to the particles's orientations and positions. We provide several benchmark examples to demonstrate the capabilities of the proposed framework.   

Relating Entanglements and Toughness in Model Polymer-Grafted Nanoparticles

Polymer-grafted nanoparticles (PGNs) are a means to create precisely structured inorganic-organic hybrid materials. The graft length and graft density are key parameters that control interparticle spacing and other structural and mechanical properties. To guide materials design, we use coarse-grained molecular dynamics (MD) simulations to relate these parameters to structure, entanglements, and mechanical properties. We consider moderate to high graft density PGNs, which do not have large bare surface regions and are stable in the melt state in a hexagonally packed monolayer on a smooth attractive surface. As intuitively expected, grafts on adjacent PGNs are more interpenetrated in lower graft density systems. We define the interparticle entanglements (involving grafts originating from two different particles) and analyze these using both a topological and a time averaging method. We find that lower graft density (increased interpenetration) leads to increased interparticle entanglements per chain and increased toughness in both the melt and glassy state. The relationship between entanglement type and location and toughness will also be discussed.