Predicting the Mechanical and Electrical Properties of Nanocomposites Formed from Polymer Blends and Nanorods

Imaging Orientation of a Single Molecular Hierarchical Self-Assembled Sheet: The Combined Power of a Vibrational Sum Frequency Generation Microscopy and Neural Network

In this work, we determined the tilt angles of molecular units in hierarchical self-assembled materials on a single-sheet level, which were not available previously. This was achieved by developing a fast line-scanning vibrational sum frequency generation (VSFG) hyperspectral imaging technique in combination with neural network analysis. Rapid VSFG imaging enabled polarization resolved images on a single sheet level to be measured quickly, circumventing technical challenges due to long-term optical instability. The polarization resolved hyperspectral images were then used to extract the supramolecular tilt angle of a self-assembly through a set of spectra-tilt angle relationships which were solved through neural network analysis. This unique combination of both novel techniques offers a new pathway to resolve molecular level structural information on self-assembled materials. Understanding these properties can further drive self-assembly design from a bottom-up approach for applications in biomimetic and drug delivery research.

Phase transition of a symmetric diblock copolymer induced by nanorods with different surface chemistry

We investigate the phase transition of a symmetric diblock copolymer induced by nanorods with different surface chemistry. The results demonstrate that the system occurs the phase transition from a disordered structure to ordered parallel lamellae and then to the tilted layered structure as the number of rods increases. The dynamic evolution of the domain size and the order parameter of the microstructure are also examined. Furthermore, the influence of rod property, rod-phase interaction, rod-rod interaction, rod length, and polymerization degree on the behavior of the polymer system is also investigated systematically. Moreover, longer amphiphilic nanorods tend to make the polymer system form the hexagonal structure. It transforms into a perpendicular lamellar structure as the polymerization degree increases. Our simulations provide an efficient method for determining how to obtain the ordered structure on the nanometer scales and design the functional materials with optical, electronic, and magnetic properties.

A Review of Multiscale Computational Methods in Polymeric Materials

Polymeric materials display distinguished characteristics which stem from the interplay of phenomena at various length and time scales. Further development of polymer systems critically relies on a comprehensive understanding of the fundamentals of their hierarchical structure and behaviors. As such, the inherent multiscale nature of polymer systems is only reflected by a multiscale analysis which accounts for all important mechanisms. Since multiscale modelling is a rapidly growing multidisciplinary field, the emerging possibilities and challenges can be of a truly diverse nature. The present review attempts to provide a rather comprehensive overview of the recent developments in the field of multiscale modelling and simulation of polymeric materials. In order to understand the characteristics of the building blocks of multiscale methods, first a brief review of some significant computational methods at individual length and time scales is provided. These methods cover quantum mechanical scale, atomistic domain (Monte Carlo and molecular dynamics), mesoscopic scale (Brownian dynamics, dissipative particle dynamics, and lattice Boltzmann method), and finally macroscopic realm (finite element and volume methods). Afterwards, different prescriptions to envelope these methods in a multiscale strategy are discussed in details. Sequential, concurrent, and adaptive resolution schemes are presented along with the latest updates and ongoing challenges in research. In sequential methods, various systematic coarse-graining and backmapping approaches are addressed. For the concurrent strategy, we aimed to introduce the fundamentals and significant methods including the handshaking concept, energy-based, and force-based coupling approaches. Although such methods are very popular in metals and carbon nanomaterials, their use in polymeric materials is still limited. We have illustrated their applications in polymer science by several examples hoping for raising attention towards the existing possibilities. The relatively new adaptive resolution schemes are then covered including their advantages and shortcomings. Finally, some novel ideas in order to extend the reaches of atomistic techniques are reviewed. We conclude the review by outlining the existing challenges and possibilities for future research.

Effect of carbon nanotubes on morphology evolution of polypropylene/polystyrene blends: understanding molecular interactions and carbon nanotube migration mechanisms

MWCNT migration among domains in conjunction with viscosity and elastic effects are important factors governing the morphological changes in the PP:PS blend nanocomposites.

Self-assembly of carbon nanotubes in polymer melts: simulation of structural and electrical behaviour by hybrid particle-field molecular dynamics

Self-assembly processes of carbon nanotubes (CNTs) dispersed in different polymer phases have been investigated using a hybrid particle-field molecular dynamics technique (MD-SCF). This efficient computational method allowed simulations of large-scale systems (up to ∼1 500 000 particles) of flexible rod-like particles in different matrices made of bead spring chains on the millisecond time scale. The equilibrium morphologies obtained for longer CNTs are in good agreement with those proposed by several experimental studies that hypothesized a two level "multiscale" organization of CNT assemblies. In addition, the electrical properties of the assembled structures have been calculated using a resistor network approach. The calculated behaviour of the conductivities for longer CNTs is consistent with the power laws obtained by numerous experiments. In particular, according to the interpretation established by the systematic studies of Bauhofer and Kovacs, systems close to "statistical percolation" show exponents t ∼ 2 for the power law dependence of the electrical conductivity on the CNT fraction, and systems in which the CNTs reach equilibrium aggregation show exponents t close to 1.7 ("kinetic percolation"). The confinement effects on the assembled structures and their corresponding conductivity behaviour in a non-homogeneous matrix, such as the phase separating block copolymer melt, have also been simulated using different starting configurations. The simulations reported herein contribute to a microscopic interpretation of the literature results, and the proposed modelling procedure may contribute meaningfully to the rational design of strategies aimed at optimizing nanomaterials for improved electrical properties.

Influence of three-dimensional nanoparticle branching on the Young's modulus of nanocomposites: Effect of interface orientation

With the availability of nanoparticles with controlled size and shape, there has been renewed interest in the mechanical properties of polymer/nanoparticle blends. Despite the large number of theoretical studies, the effect of branching for nanofillers tens of nanometers in size on the elastic stiffness of these composite materials has received limited attention. Here, we examine the Young's modulus of nanocomposites based on a common block copolymer (BCP) blended with linear nanorods and nanoscale tetrapod Quantum Dots (tQDs), in electrospun fibers and thin films. We use a phenomenological lattice spring model (LSM) as a guide in understanding the changes in the Young's modulus of such composites as a function of filler shape. Reasonable agreement is achieved between the LSM and the experimental results for both nanoparticle shapes--with only a few key physical assumptions in both films and fibers--providing insight into the design of new nanocomposites and assisting in the development of a qualitative mechanistic understanding of their properties. The tQDs impart the greatest improvements, enhancing the Young's modulus by a factor of 2.5 at 20 wt.%. This is 1.5 times higher than identical composites containing nanorods. An unexpected finding from the simulations is that both the orientation of the nanoscale filler and the orientation of X-type covalent bonds at the nanoparticle-ligand interface are important for optimizing the mechanical properties of the nanocomposites. The tQD provides an orientational optimization of the interfacial and filler bonds arising from its three-dimensional branched shape unseen before in nanocomposites with inorganic nanofillers.

Controlling directed self-assembly of gold nanorods in patterned PS-b-PMMA thin films

We present a facile strategy for the directed self-assembly of gold nanorods (AuNRs) in patterned block copolymer (BCP) thin films. Parallel arrangement of AuNRs relative to the geometric confinement generated by selective removal of one block domain was achieved. Deposition of AuNRs with aspect ratios from 3.3 to 5.8 was accomplished and the alignment of the NRs within the channels was controlled primarily by capillary forces and the channel geometry. Ordered AuNR assembly in the BCP pattern can be achieved at high surface coverages, >30%, though the surface coverage depends on the aspect ratio of the NRs. Larger NRs align in the channels more readily, but pack at slightly lower densities.

Aspect-Ratio Effect of Nanorod Compatibilizers in Conducting Polymer Blends

Nanoparticles (NPs) at the interface between two different polymer blends or fluid mixtures can function as compatibilizers, thereby dramatically improving the interfacial properties of the blends or the fluid mixtures. Their compatibilizing ability is strongly dependent on their size, shape, and aspect ratios (ARs), which determines their adsorption energy to the interface as well as their entropic penalty when they are being strongly segregated at the interface. Herein, we investigated the effect of the ARs of nanorod surfactants on the conducting polymer blend of poly(triphenylamine) (PTPA) templated by polystyrene (PS) colloids. The lengths of the polymer-coated CuPt nanorods (CuPt NRs) were 5, 15, and 32 nm with a fixed width of 5 nm, thus producing three different AR values of 1, 3, and 6, respectively. For quantitative analysis, the morphological and electrical behaviors of the polymer blends were investigated in terms of the volume fraction and AR of the NRs. The dramatic change in the morphological and electrical properties of the blend film was observed for all three NR surfactants at the NR volume fraction of approximately 1 vol %. Therefore, NR surfactants with larger ARs had better compatibilizing power for a given number of NRs in the blends. Also, they exhibited a stronger tendency to be aligned parallel to the PS/PTPA interface. Also, we demonstrated the successful use of the NR surfactants in the fabrication of conducting polymer blend film that requires only minimal concentrations of conducting polymers. To the best of our knowledge, this is the first report of an experiment on the AR effect of NR compatibilizers in polymer blends.

End-to-end alignment of nanorods in thin films

A simple approach to obtain end-to-end assemblies of nanorods over macroscopic distances in thin films is described. Nanorods with aspect ratio of 8-12 can be aligned parallel to the surface in an end-to-end fashion by imposing geometric confinement via block copolymer-based supramolecular assemblies. Successful control over the orientation and location of nanorods requires a balance of particle-particle interactions and entropy associated with geometric confinement from the supramolecular framework, as well as consideration of the kinetics of assembly.

Effect of Attapulgite Nanorods and Calcium Sulfate Microwhiskers on the Reaction-Induced Phase Separation of Epoxy/PES Blends

The influence of two kinds of mesoscale inorganic rod fillers, nanoscale attapulgite and micron-sized CaSO4whisker, on the reaction-induced phase separation of epoxy/aromatic amine/poly- (ether sulfone) (PES) blends has been investigated by optical microscopy (OM), scanning electron microscopy (SEM), and time resolved light scattering (TRLS). By varying the PES concentration and curing temperature, we found that the incorporation of attapulgite and CaSO4had dramatic impact on the phase separation process and the final phase morphology of blends. In blends at higher content than critical concentration, the process of phase separation was retarded by the incorporation of nanoscale fillers but accelerated by that of the micron-sized fillers, mainly due to the enhanced viscoelastic effect and the preferential wettable effect, respectively. Meanwhile both mesoscale fillers could change the cocontinuous phase structure of blends with lower PES content than critical concentration into PES-rich dispersed structure due to the surface affinity of fillers to epoxy matrix.

Direct nanorod assembly using block copolymer-based supramolecules

Developing routes to control the organization of one-dimensional nanomaterials, such as nanorods, with high precision is critical to generate functional materials since the collective properties depend on their spatial arrangements, interparticle ordering, and macroscopic alignment. We have systematically investigated the coassemblies of nanorods and block copolymer (BCP)-based supramolecules and showed that the energetic contributions from nanorod ligand-polymer interactions, polymer chain deformation, and rod-rod interactions are comparable and can be tailored to disperse nanorods with control over inter-rod ordering and the alignment of nanorods within BCP microdomains. By varying the supramolecular morphology and chemical nature of the nanorods, two highly sought-after morphologies, that is, nanoscopic networks of nanorods and nanorod arrays parallel to cylindrical BCP microdomains can be obtained. The supramolecular approach can be applied to achieve morphological control in nanorod-containing nanocomposites toward fabrication of optical and electronic nanodevices.

Engineering nanostructured polymer blends with controlled nanoparticle location using Janus particles

Janus particles are used on a multigram scale for the blend compatibilization of two polymers in a twin screw mini-mixer. It is shown that the Janus particles can be located exclusively at the interface of the two polymer phases despite the high temperature and shear conditions. The domain sizes of the dispersed phase decrease with increasing content of Janus particles. The decrease is yet ongoing for high contents of Janus particles. Furthermore, the biphasic particles exhibit an ordered arrangement at the interface. Thus, the approach demonstrates that a nanoscopic structuring of the interface can be achieved under macroscopic processing conditions. The structural order occurs on two levels. The first is the complete adsorption at the interface and the second is the lateral ordering at the interface. The strong adsorption at the interface is explained in terms of the increased desorption energy of Janus particles. Secondly, the compatibilization efficiency is critically compared to state-of-the-art compatibilizers. The efficiency of the Janus particles is found to be superior as compared to block copolymer-based compatibilizers. The efficiency gap between Janus particles and block copolymer compatibilizers widens for larger amounts added.

Two-dimensional confinement of nanorods in block copolymer domains

Gold nanorods (NRs) self-orient during the self-assembly of a symmetric poly(styrene-b-methyl methacrylate) (PS-b-PMMA) film. The NRs are selectively sequestered and confined in lamellar PMMA domains, which are narrower than the NR length. This confinement orients 71% of NRs within +/- 5 degrees of the lamella plane. During solvent annealing, a gradient in the concentration of NRs is observed. This route to produce alternating layers containing conducting NRs separated by dielectric domains has the potential for fabricating self-assembled nanodevices.

High modulus nanopowder reinforced dimethacrylate matrix composites for dental cement applications

Results from the study of a novel, high modulus nanopowder filled resin composite are presented. This composite is developed to serve (1) as a high stiffness support to all-ceramic crowns and (2) as a means of joining independently fabricated crown core and veneer layers. Nanosized Al(2)O(3) (average particle size 47 nm) reinforcement provides stiffness across joins. Two systems are examined: Al(2)O(3) with 50:50 bis-GMA:TEGDMA monomers (ALBT) and Al(2)O(3) with pure TEGDMA (ALT). To obtain higher filler levels, surfactant is used to aid mixing and increase maximum weight percent of nanopowder filler from 72 to 80. The loading level of Al(2)O(3) has significant effects on composite properties. The elastic modulus for cured ALBT systems increases from 4.6 GPa (0 wt % filler) to 29.2 GPa (80 wt % filler). The elastic modulus for cured ALT systems increases from 3.0 GPa (0 wt % filler) to 22.9 GPa (80 wt % filler). Similarly, ALBT hardness increases from 200 MPa (0% filler) to 949 MPa (80 wt % filler), and ALT hardness increases from 93 MPa (0% filler) to 760 MPa (80 wt % filler). Our results indicate that with a generally monodispersed nanosized high modulus filler relatively high elastic modulus resin based composite cements are possible.

Modeling self-assembly and phase behavior in complex mixtures

Using a variety of computational techniques, I investigate how the self-assembly of complex mixtures can be guided by surfaces or external stimuli to form spatially regular or temporally periodic patterns. Focusing on mixtures in confined geometries, I examine how thermodynamic and hydrodynamic effects can be exploited to create regular arrays of nanowires or monodisperse, particle-filled droplets. I also show that an applied light source and chemical reaction can be harnessed to create hierarchically ordered patterns in ternary, phase-separating mixtures. Finally, I consider the combined effects of confining walls and a chemical reaction to demonstrate that a swollen polymer gel can be driven to form dynamically periodic structures. In addition to illustrating the effectiveness of external factors in directing the self-organization of multicomponent mixtures, the selected examples illustrate how coarse-grained models can be used to capture both the equilibrium phase behavior and the dynamics of these complex systems.

Nanoparticle Polymer Composites: Where Two Small Worlds Meet

The mixing of polymers and nanoparticles is opening pathways for engineering flexible composites that exhibit advantageous electrical, optical, or mechanical properties. Recent advances reveal routes to exploit both enthalpic and entropic interactions so as to direct the spatial distribution of nanoparticles and thereby control the macroscopic performance of the material. For example, by tailoring the particle coating and size, researchers have created self-healing materials for improved sustainability and self-corralling rods for photovoltaic applications. A challenge for future studies is to create hierarchically structured composites in which each sublayer contributes a distinct function to yield a mechanically integrated, multifunctional material.

Modeling the morphology and mechanical properties of sheared ternary mixtures

Through a combination of simulation techniques, we determine both the structural evolution and mechanical properties of blends formed from immiscible ternary mixtures. In this approach, we first use the lattice Boltzmann method to simulate the phase separation dynamics of A/B/C fluid mixtures for varying compositions within the spinodal region. We also investigate the effect of an imposed shear on the phase ordering of the mixture. We assume that the fluid is quenched sufficiently rapidly that the phase-separated structure is preserved in the resultant solid. Then, the output from our morphological studies serves as the input to the lattice spring model, which is used to simulate the elastic response of solids to an applied deformation. These simulations reveal how the local stress and strain fields and the global Young's modulus depend on the composition of the blend and the stiffness of the components. By comparing the results for the sheared and unsheared cases, we can isolate optimal processing conditions for enhancing the mechanical performance of the blends. Overall, the findings provide fundamental insight into the relationship between structure, processing, and properties for heterogeneous materials and can yield guidelines for formulating blends with the desired macroscopic mechanical behavior.

Newtonian fluid meets an elastic solid: coupling lattice Boltzmann and lattice-spring models

We integrate the lattice Boltzmann model (LBM) and lattice spring model (LSM) to capture the coupling between a compliant bounding surface and the hydrodynamic response of an enclosed fluid. We focus on an elastic, spherical shell filled with a Newtonian fluid where no-slip boundary conditions induce the interaction. We calculate the "breathing mode" oscillations for this system and find good agreement with analytical solutions. Furthermore, we simulate the impact of the fluid-filled, elastic shell on a hard wall and on an adhesive surface. Understanding the dynamics of fluid-filled shells, especially near adhesive surfaces, can be particularly important in the design of microcapsules for pharmaceutical and other technological applications. Our studies reveal that the binding of these capsules to specific surfaces can be sensitive to the physical properties of both the outer shell and the enclosed fluid. The integrated LBM-LSM methodology opens up the possibility of accurately and efficiently capturing the dynamic coupling between fluid flow and a compliant bounding surface in a broad variety of systems.

Using nanoparticles to create self-healing composites

The need for viable materials for optical communications, display technologies, and biomedical engineering is driving the creation of multilayer composites that combine brittle materials, such as glass, with moldable polymers. However, crack formation is a critical problem in composites where thin brittle films lie in contact with deformable polymer layers. Using computer simulations, we show that adding nanoparticles to the polymers yields materials in which the particles become localized at nanoscale cracks and effectively form "patches" to repair the damaged regions. Through micromechanics simulations, we evaluate the properties of these systems in the undamaged, damaged, and healed states and determine optimal conditions for harnessing nanoparticles to act as responsive, self-assembled "band aids" for composite materials. The results reveal situations where the mechanical properties of the repaired composites can potentially be restored to 75%-100% of the undamaged material.