Session A18: New Directions in Polymer Nanocomposites I: Structures

Multiscale Ordering in Hybrid Materials During Reaction-Induced Phase Transitions

Controlling nanoparticle and polymer organization at multiple length scales in hybrid materials has yet to be fully realized. Simply mixing nanoparticles and polymers typically leads to macroscale aggregation. To circumvent the mixing barrier associated with blending nanoparticles and polymers, and to control organization over multiple length scales, a new method, reaction-induced phase transitions (RIPT), is proposed in which monomer is polymerized in the presence of nanoparticles. Here, the presentation will describe how a mixture of oleylamine-functionalized gold nanoparticles (AuNP) dispersed and methyl methacrylate (MMA) that are polymerized lead to hybrid AuNP/poly(methyl methacrylate) (PMMA) materials with multiscale ordering. During the polymerization of MMA in the initially homogeneous AuNP/MMA mixture, the sample undergoes macrophase separation during the reaction. Although the macrophase separation occurs, AuNPs still appear well-dispersed. We find that nanoparticles aggregate when the samples are processed into films due to the removal of unreacted monomer and thermal treatment. The reported results highlight how RIPT is a new method to control the ordering of nanoparticles and polymers at both the nano and macroscales during monomer polymerization.   

Effect of nanorod physical roughness on the aggregation and percolation of nanorods in polymer nanocomposites

Polymer nanocomposites (PNCs) with nanorods as fillers are useful in energy storage, soft electronics, and photonics applications, with the PNC functionality depending strongly on the nanorods' structure within the PNCs (i.e., dispersion, aggregation, orientational alignment, percolation). Nanorods' structure in PNCs can be controlled by nanorod size (diameter, aspect ratio), chemistry, and surface functionalization. Even though many simulation studies have focused on the morphology and dynamics of nanorods in polymer melt using coarse-grained models of nanorods, studies have not demonstrated how the nanorod roughness impacts the phase behavior of nanorods in the polymer melt. In this talk, using molecular dynamics simulations, we elucidate the effect of nanorod roughness on nanorod aggregation, dispersion, and percolation in polymer nanocomposites. By choosing coarse-grained models that enable systematic variation of the nanorod roughness and by selecting purely repulsive pair-wise interactions for nanorods and polymer chains, we show how nanorod roughness affects the entropic driving forces for various PNC morphologies. Our study serves as design rules for achieving desired morphologies in PNCs with synthetic and biologically derived nanorods and nanowires that have varying extents of physical roughness.   

Superconducting Metal Gyroids: Block Copolymer Self-assembly for Mesoscale Engineering of Quantum Metamaterials

A growing body of work has demonstrated emergent properties in mesostructured quantum materials. Such quantum metamaterials have recently been generated via block copolymer (BCP) self-assembly (SA), which imparts tunable control of mesoscopic architecture. In this work we use BCP SA to structure direct polysilazanes and create mesoporous silicon oxynitride ceramics with a cubic co-continuous double gyroid structure. These ceramic templates are backfilled with molten indium under high pressures. X-ray scattering and electron microscopy demonstrate high fidelity backfilling in the resulting nanocomposites, a substantial advancement in BCP SA directed metals. Analysis of superconducting indium reveals that the BCP architecture dictates quantum properties: the superconducting coherence length decreases from 360 nm to 20 nm, roughly the diameter of a gyroid strut. This leads indium to switch from a type-I to type- II superconductor, with an enhanced critical field and evidence of vortices arrayed on the order of the gyroid lattice size. Results suggest that BCP SA approaches to quantum metamaterials opens a rich area for investigation of mesostructure-property correlations. Further, polymer solution based fabrication yields benefits beyond traditional ultrahigh vacuum based methods.   

Self-Assembly of Hierarchical Organic Nanoparticles Controlled by Modified Surfactants

Semi-dilute nanoparticles tend to cluster in order to reduce surface area. Element crystallites form densely clustered primary particles associated with high surface area. In contrast, these larger primary particles have lower surface area and slower thermal movement and aggregate into branched mass fractals, forming a dual-level hierarchical structure. Elemental clustering can be controlled using surfactants. In this work a polyethelene oxide (PEO) based surfactant, Triton^TM X-100^®, is used to control clustering of an organic pigment. PEO displays a lower critical solution temperature (LCST) at 66 °C. It has been found that reduced miscibility in the vicinity of the LCST can be used to control clustering. For small clusters significant aggregation occurs while for sufficiently large clusters aggregation does not occur. In this way a thermally controlled hierarchical structure can be produced. The thermally tuned hierarchical structure can be locked in using chemically modified surfactant.   

Hierarchical Nanolayered Polymer/nanoparticle Composites via 3D Printing

Nanoparticle preferential alignment and oriented structures improve functional and structural properties in composites compared to those with random counterparts. Traditionally used techniques such as drop-casting, chemically modified surfaces, and external fields have been used for self-assembly, but it has several disadvantages. Thus, there is a need to develop a new approach for generating hierarchical nanoparticle structures. This research combines advanced 3D printing and evaporation-induced nanoparticles assembly for layer-by-layer deposition of 1D and 2D nanoparticles. The 1D and 2D nanoparticles were deposited onto the 3D printed substrate with microchannels via solution deposition technique. The nanoparticles trapped by microchannels experiences microfluidic forces, which lead to site-specific deposition and alignment of nanoparticles. The fabricated polymer/nanoparticle composite film showed different deposition morphologies like straight, wavy, and randomly oriented films. Furthermore, the influence of nanoparticle deposition morphology on functional properties was investigated. This novel technique shows the potential to scale up microelectronics production by 3D printing of electronic structures like interdigitated devices, FET, and circuits.   

Coarse-grained simulation studies linking physical and chemical heterogeneity in nanorods to the morphology in polymer nanocomposites

Polymer nanocomposites (PNCs) with nanorods or nanowires as fillers in a polymer matrix are used in many energy-storage, electronics, optics, and photonics applications. In these applications, the macroscopic properties of the PNC (e.g., conductivity, mechanical properties, optical response) are dependent on the nanorods’ spatial arrangement within the polymer matrix (i.e., nanorod dispersion, percolation, or aggregation with or without orientational alignment). To identify design rules for achieving desired morphologies in PNCs, we have been conducting systematic computational studies linking chemical and physical heterogeneity of the nanorod (e.g., nanorod homogenous vs. patchy functionalization, increasing physical roughness) to the structure of the nanorods in the polymer melt. In this short talk, I will present the key results from these coarse-grained molecular simulation studies comparing phase behavior in PNCs with nanorods having homogeneous vs. patchy chemical functionalization and smooth vs. rough nanorod surfaces.   

Dissipative Particle Dynamics (DPD) Simulation to understand the impact of Filler Morphology on Dispersion and Aggregation in Polymer Nanocomposites

Nanoscale fillers are widely used in polymeric systems such as natural rubber as inexpensive and effective additives for improved properties and functionality. The behavior of such fillers as well as their impact on nanocomposite efficacy, is influenced by hierarchical filler structure, the interaction between fillers-polymer matrix, and processing history. The extensive thermodynamics and kinetic histories of such systems typically result in a complex partitioning of the components and affect the polymer-filler dispersion. In this research, we perform Dissipative Particle Dynamics (DPD) simulation of polymer-irregular filler blends, to understand the hierarchical structure and dispersion over multiple lengths and timescales, while varying polymer-polymer interaction energy. In particular, we highlight the influence of filler aggregate parameters (size, fractal dimension, sticking probability) on morphology. Our results demonstrate the complex role of filler primary and aggregate structures and concentration on nanocomposite physical properties such as particle clustering, percolation threshold, and mesh size. We also explore pathways for the formation of large percolating aggregates, as a function of polymer-filler interaction, validated against small-angle x-ray scattering.   

Effect of Graft Density on Structure and Entanglements of Model Polymer-Grafted Nanoparticle Monolayers

Neat polymer-grafted nanoparticles (PGNs) are matrix-free nanoparticle polymer composites, consisting of an inorganic core and surface grafted polymer chains. PGNs on a surface can form robust monolayers with regular interparticle spacing, of interest for a range of applications such as flexible electronic or optical devices. We use coarse-grained molecular dynamics simulations to study chain conformations and nanoscale structure in hexagonally packed PGN monolayers as a function of graft density. We analyze entanglements, including calculating their lifetimes and categorizing by whether they connect different nanoparticles. As graft density increases, we observe decreasing interpenetration of chains on neighboring PGNs, which decreases the number of interparticle entanglements per chain, and causes localization of these entanglements in interstitial regions. We also find local alignment of chains normal to the NP surface at high graft density, which is related to increased intraparticle entanglement density near the surface. We expect that understanding these relationships, and generally connecting experimentally tunable parameters to molecular-scale structure and overall material properties, can provide insight into optimal design of future materials.   

Crystallization-induced Ordering in Poly(ethylene oxide)-Silica Nanocomposites: Modifying Particle Diffusivity

Semicrystalline polymers make up over two-thirds of the world’s produced polymers and while these materials have a range of applications they are limited by their generally low modulus. We have previously shown that one strategy to improve the material modulus is to use crystallization to order silica nanoparticles into the amorphous regions of the semicrystalline hierarchy. This anisotropic ordering is controlled by the velocity of the crystal growth front relative to the diffusive rate of the nanoparticles. While the previous study has demonstrated the ability to order nanocomposites at slow enough crystallization speeds, we now wish to understand the role of particle diffusivity on the ordering process. To examine this role we used silica nanoparticles in four molecular weights of Poly(ethylene oxide) ranging from 5.4 – 46 kDa. By manipulating the molecular weight of the matrix, we primarily affect the viscosity of the melt allowing for increased particle diffusion in the lower molecular weight polymers. We then quantify the ordering using the Herman’s Orientation function on x-ray scattering images. We see that the ordering is maximized at intermediate crystal growth speeds which suggests interspherullitic segregation at very slow growth speeds.   

Modeling Pairs of Polymer-Grafted Nanoparticles (PGNs) in Solution to Determine their Phase Behavior

By grafting polymer chains onto nanoparticles (creating PGNs), one can precisely control interparticle interactions. PGNs are typically synthesized and processed in solution before use in applications such as flexible electronics where a precise spacing of inorganic particles in a robust and flexible matrix is desirable. Understanding their solution properties is crucial to control their structure during deposition and drying. We use coarse-grained molecular dynamics (MD) simulations to study the effective interactions between two PGNs in implicit solvent. Specifically, we use a Kremer-Grest type of model for graft chains and spherical nanoparticles ten times the monomer size. Monomers interact via mixed Lennard-Jones potentials; the repulsive part is kept constant while an attractive part is added with adjustable strength to consider various solvent strengths. Nanoparticle interactions are of an integrated form as though they are composed of a uniform melt density of monomers. Our MD simulations produce potential of mean force (PMF) profiles that are used in Gibbs Ensemble Monte Carlo (GEMC) simulations to determine, for each solvent strength, whether phase separation occurs and, if so, the densities of the coexisting phases. Phase diagrams for solvated PGNs will be discussed.   

Chemically grafting suppresses initial solvent-driven nonequilibrium effect on structures and properties of polymer nanocomposites

 One of the long-standing issues to achieve uniform physical properties of polymer nanocomposites (PNCs) is precise control of particle dispersions. Thus, a great deal of studies has investigated the role of microscopic parameters such as size/shape/chemistry of particles/polymers to control the dispersion and demonstrate the structure-property relationship. The investigation of these parameters of PNCs assumed that PNCs are in their equilibrium states. However, recently, processing conditions such as the preparation pathway have been found to significantly impact the final structures and properties of PNC, noting the importance of processing-induced nonequilibrium effect.

 Our previous study reported that the degree of physical adsorption of polymers in PNC can be dependent on the casting solvent, bringing a noticeable nonequilibrium effect on the structures and properties of PNCs. In this work, we investigate the casting solvent-induced nonequilibrium effect for PNCs where interfacial polymers are chemically grafted. The nonequilibrium effect depends on grafting density. It is still present at low grafting density whereas the solvent-induced structural/property variation becomes minimal at moderate grafting density, suggesting suppression of the nonequilibrium effect.