Session B38: Nanocomposites from Nano to Meso

Nanoparticle Ordering in Semicrystalline Polymers

One way to engineer the macroscopic properties of a crystalline polymer matrix is to place nanoparticles into them, but in an organized manner. We have recently found that NP organization can be controlled by varying the crystal growth rate. We develop a coarse-grained model to study this situation – in particular, we focus on the out-of-equilibrium dynamics of nanoparticles being pushed/engulfed by a solidification front depending on crystallization velocity $v_{s}$. Particle engulfment occurs when $v_{s}$ is higher than a critical velocity $v_{c}$. When $v_{s}$ is smaller than $v_{c}$, particles are pushed by the crystallization front and organize in a 2-D plane. Even though most models for particle engulfment consider dynamic force equilibrium at $v_{c}$, we show the system is not in equilibrium in this regime. Thus, we consider conditions for engulfment based on particle velocity with respect to crystal growth rate. Our results agree with experimental observations on anisotropic organization of nanoparticles in semicrystalline polymers driven by crystallization speed.   

Dispersion of Mixed Brush Gold Nanorods in Polymer Matrices

In this work we investigate, both experimentally and through hybrid particle/self-consistent field theoretic (hSCFT) calculations, the dispersion state of gold nanorods (AuNRs) grafted with homopolymer, bidispersed, or mixed polymer brushes. AuNRs are grafted with 11.5 kg/mol PS (HNRs), 11.5 kg/mol PS and 5.3 kg/mol PS (BNRs), or 11.5 kg/mol PS and 5 kg/mol poly(methyl methacrylate) (PMMA) (MBNRs) and cast in PS or PMMA films consisting of short to very long chains compared to the grafted brush. We further investigated the MBNR systems by varying the length of the PS brush. Overall, we find that the MBNRs dispersed markedly better than the other brush types (HNRs or BNRs) in PS matrices. We utilize hSCFT calculations, in particular potential of mean force (PMF) and brush profile calculations, to elucidate the thermodynamics of these systems. The PMFs and brush profiles exhibit similar trends for the BNRs and MBNRs where the short grafted chain forces the longer grafted chain away from the AuNR surface and promotes wetting by the matrix chains. The hSCFT calculations demonstrated qualitative trends consistent with the aggregation observed for AuNRs in PMMA matrices. Therefore, we have demonstrated that MBNR dispersion in polymer matrices is enhanced compared to the HNR and BNR cases, which extends the dispersion window for new combinations of nanorods and polymers.   

Examination of nanoparticle dispersion using a novel GPU based radial distribution function code .

We have developed a novel GPU-based code that rapidly calculates radial distribution function (RDF) for an entire system, with no cutoff, ensuring accuracy. Built on top of this code, we have developed tools to calculate the second virial coefficient (B$_{2})$ and the structure factor from the RDF, two properties that are directly related to the dispersion of nanoparticles in nancomposite systems. We validate the RDF calculations by comparison with previously published results, and also show how our code, which takes into account bonding in polymeric systems, enables more accurate predictions of g(r) than current state of the art GPU-based RDF codes currently available for these systems. In addition, our code reduces the computational time by approximately an order of magnitude compared to CPU-based calculations. We demonstrate the application of our toolset by the examination of a coarse-grained nanocomposite system and show how different surface energies between particle and polymer lead to different dispersion states, and effect properties such as viscosity, yield strength, elasticity, and thermal conductivity.   

Analysis of the kinetics of filler segregation in granular block copolymer microstructure

To realize the application of block copolymers in areas ranging from dynamically tunable photonic crystals to solid-state electrolytes, it is important to understand the role of additives in the evolution of microstructure (i.e. grain size and shape as well as distribution) during thermal processing. Previous studies have revealed the interaction of filler species (such as homopolymers or particle additives) to drive the segregation of filler into grain boundary regions, thereby stabilizing grain boundary and arresting grain growth. In this contribution we present a novel approach based on combined Ultra-Small Angle Neutron Scattering (USANS) and electron microscopy analysis (involving large area image reconstruction) to quantitatively determine the kinetics of filler segregation and its affect on grain size evolution in block copolymer blends. Calculation of the scattering length density of the grain boundary network is shown to provide detailed information about the rate and time dependence of filler segregation. For the particular case of a poly(styrene-b-isoprene)/d-polystyrene blends system is is found that 2 vol{\%} of filler segregation during the early stage of thermal annealing is sufficient to arrest grain growth.   

Localization of Individual Nanoparticle in the Perforated Lamellar Phase of Self-assembled Block Copolymer Driven by Entropy Minimization

Although precisely controlled microdomains of block copolymers (BCP) provide an excellent guiding matrix for multiple nanoparticles (NPs) to be controllably segregated into a desired polymer block, localization and positioning of individual NPs have not been demonstrated. Here, we report a unique one-to-one positioning phenomenon of guest Au NPs in the host BCP microdomains; each of polystyrene-functionalized Au NPs is embedded within the perforation domain of hexagonally perforated lamellar (HPL) morphology of poly(dimethylsiloxane-$b$-styrene) BCP. The local minimization of free energy achieved by the placement of Au NPs into the center of the perforation domain is theoretically supported by the self-consistent field theory (SCFT) simulation. We propose a novel design principle for more precisely controllable nanocomposites by developing a new route of NP arrangement within a polymer matrix.   

Predicting the dynamics and thermodynamics of nanoparticles in block copolymers

In applications involving polymer nanocomposites, controlling the dispersion of the nanoparticles is one of the most critical aspects of their design. For example, optimal mechanical properties are typically found when particles are maximally dispersed, while varying the interparticle spacing on the nm length scale can tune the optical properties of a composite. In all of these cases, the distribution of nanoparticles is a complex interplay of entropic and energetic interactions between the matrix polymers, nanoparticles, particle surface-grafted polymers, and even the processing conditions. Recently, my group has been extending the polymer field theory framework to enable the study of inhomogeneous polymer nanocomposites. The framework has the advantage of being computationally efficient and able to treat anisotropic particles (nanorods) and explicit surface chemistry, such as grafted nanoparticles. In this talk, I will describe some of our recent results with the method studying the distribution and interactions between nanoparticles in block copolymer matrices. First, I will show how we have quantified the interactions between nanoparticles and block copolymer grain boundaries. Second, I will describe our more recent efforts using non-equilibrium methods to study the role of processing, such as solvent annealing, on the distribution of nanoparticles in block copolymer thin films.   

A Novel Method to Characterize Nanorod Orientation and Aggregation in Polymer Nanocomposites

Gold nanorods provide an ideal system for the systematic change of optical properties through changes in the rod aspect ratio. Furthermore, the dispersity and orientation of the nanorods within a polymer matrix greatly affects the optical properties of the composites. Here, we use spectroscopic ellipsometry to characterize the properties of nanocomposite thin films. The optical properties of the nanorod are modeled as an effective index of refraction for a disordered meta-material. This effective medium index is then related to the longitudinal surface plasmon resonance (LSPR) of the nanorods. The degree of birefringence in the LSPR frequency, as determined by variable angle ellipsometry measurements, can help determine the average orientation of the rods in the thin film as well as the degree of aggregation. With this method, one can quickly and accurately define the average orientation and average aggregation of nanorods within a nanocomposite with a single measurement. Ellipsometry also allows us to perform \textit{in-situ} variable temperature measurements to monitor properties such as nanoparticle shape and the glass transition temperature of the matrix.   

Microstructure of 3D-Printed Polymer Composites Investigated by Small-Angle Neutron Scattering

Polymer composites printed from the large scale printer at Manufacturing Demonstration Facility at Oak Ridge National Laboratory have been investigated by small-angle neutron scattering (SANS). For the Acrylonitrile Butadiene Styrene (ABS)/Carbon Fiber (CF) composites, the microstructure of polymer domains and the alignment of CF have been characterized across the layer from the printed piece. CF shows strong anisotropic alignment along the printing direction due to the flow of polymer melt at the nozzle. Order parameter of the anisotropy which ranges from -0.11 to -0.06 exhibits strong correlation with the position within the layer: stronger alignment near the layer interface. It is also confirmed that the existence of CF reduces the polymer domain correlation length significantly and reinforces the mechanical strength of the polymer composites. For the Epoxy/nano-clay platelet composites, the effect of processing condition, nozzle size, and the addition of the another filler, Silicon Carbide (SC), have been investigated by SANS. Nano-clay platelet shows strong anisotropic alignment along the printing direction as well. Order parameter of the anisotropy varies according to nozzle size and presence of the SC, and difference disappears at high Q region.   

Large Volume Self-Organization of Polymer/Nanoparticle Hybrids with Millimeter Scale Grain Sizes using Brush Block Copolymers

The lack of sufficient long-range order in self-assembled nanostructures is a bottleneck for many nanotechnology applications. In this work, we report that exceptionally large volume of highly ordered arrays (single grains) on the order of millimeters in scale can be rapidly created through a unique innate guiding mechanism of brush block copolymers (BBCPs). The grain volume is over 1 billion times larger relative to that of typical self-assembled linear BCPs (LBCPs). The use of strong interactions between nanoparticles (NPs) and BBCPs enables the high loadings of functional materials, up to 76 wt{\%} (46 vol{\%}) in the target domain, while maintaining excellent long-range order. Overall this work provides a simple route to precisely control the spatial orientation of functionalities at nanometer length scales over macroscopic volumes, thereby enabling the production of hybrid materials for many important applications.   

Tuning the interactions between nanoparticles in block copolymer domains

Block copolymer nanocomposites have the potential to become a platform for new materials with improved thermal, electrical, or optical properties compared to neat polymers. However, it is critical to control the dispersion of the nanoparticles in the block copolymer matrix, and thus it is important to understand how nanoparticles interact with each other within block copolymer domains. In this work, we use a polymer nanocomposite field theory (PNC-FT) that was recently developed in our group to study the interactions of nanoparticles within both cylindrical and lamellar block copolymer structures. We find that the nanoparticles induce a curvature in the A-B interface in the block copolymer, which plays a significant role in the interparticle interactions, leading to a non-monotonic potential of mean force between the particles. This effect becomes more pronounced as the nanoparticle size increases. Finally, we will also present results showing the effect of nanoparticle surface functionality (polymer grafting) on the interparticle interactions.   

Novel Polymer Nanocomposites Resulted from Melt Processing of Polystyrene-Based Substrates Coated with Layer-by-Layer Assemblies

The novel polymer nanocomposites (PNCs) prepared through two steps of coating polystyrene-based substrates with layer-by-layer (LBL) deposition of montmorillonite and alternative polyelectrolyte layers of polyethyleneimine and polyethylene terephthalate ionomer, followed by their cyclic melt pressing, demonstrated particular morphologies. Transmission electron microscopy images at high magnification scales showed the occurrence of swollen intercalation and flocculated exfoliations of clay platelets, down to a few nanometer thickness, inside and sometimes out of LBL assemblies crushed portions. In fact, intercalation and exfoliation of clay platelets, established in LBL assemblies, increased by shear applied through their repetitive melt pressing. Additionally, x-ray diffractometry traces confirmed the aforementioned increase in clay intercalation. These high aspect ratio LBL assemblies portions formed highly tortuous labyrinths, which may work as scavenging centers to promote barrier properties of the PNCs against transport of gases like oxygen and carbon dioxide. It is despite spontaneously low interaction between hydrophobic styrenic groups and almost hydrophilic natural clay and moderate efficiency of cyclic pressing for providing intensive shear stress on samples.   

Stabilization of PS/PLA cocontinuous blends by interfacial graphene

Reduced graphene oxide (r-GO) is known to be effective in increasing the conductivity of cocontinuous polymer blends with a lower electrical percolation threshold. However, little is known regarding the localization and dynamics of r-GO along with morphology change during annealing. In this study, we develop a facile method to stabilize the polystyrene (PS)/polylactic acid (PLA) cocontinuous blends with r-GO jammed at interface. In this method, the non-functionalized GO is premixed with PLA via solvent method, and then reduced in-situ at 210oC to obtain a PLA/r-GO polymer composite. This composite is further mixed with PS via batch melt compounding. We observe the migration of r-GO from the PLA phase to the interface during annealing. The interfacial r-GO suppresses the coarsening of cocontinuous morphology and increases the conductivity of the filled polymer blend. Moreover, we systematically investigate the relationship between r-GO localization, rheological and conductivity change during annealing of r-GO filled PLA/PS blends.