Session W44: Focus Session: Interparticle Interactions in Polymer Nanocomposites - Mechanical, Electrical, and Optical Properties

Bio-inspired Fillers for Mechanical Enhancement

An examination of natural materials has offered a new perspective on the development of multi-functional materials with enhanced mechanical properties. One important lesson from nature is the utilization of composite structures to impart improved mechanical behavior and enhanced functionality using nanofillers. A relatively unexplored expansion of this bio-inspired, nanoscale filler approach to high performance materials is the incorporation of responsive, multi-functional reinforcing elements in polymeric composites with the goal of combining superior mechanical behavior that can be tuned with additional functionality, such as sensing and bioactivity. One approach is the use of self-assembling small molecules that form uniform, one-dimensional nanostructures as an emerging class of filler components. Another pathway toward mechanical enhancement is the incorporation of stimuli-responsive and high-modulus electrospun nanofibers. We have probed the utilization of high-aspect ratio, self-assembled small molecules and responsive electrospun nanofibers as all-organic nanofillers to achieve significant modulus changes within elastomeric matrices. The influence of matrix-filler interactions and the role of hierarchical organization in these nature-inspired composites will be discussed. Potential applications in barrier technology and drug delivery have also been explored.   

Temperature sensitive mechanical properties of Graphene-epoxy nanocomposites

Carbon-based polymeric nanocomposites have potential applications including structural parts in aerospace vehicles and civil infrastructure. In this work various aspects of mechanical properties of Graphene-epoxy nanocomposites are studied at different scales. The quasi-static tensile yield stress and stiffness of the nanocomposite are larger than those of neat epoxy. While the creep response of the nanocomposite is similar to that of neat epoxy at lower stress and room temperature, a significant discrepancy is observed at high temperature and/or large stress, the nanocomposite creeping less. The fracture toughness for the nanocomposite with the optimum filler fraction is larger than the toughness of unfilled epoxy at room temperature. This difference decreases at higher temperatures. Local mechanical properties were investigated using nanoindentation and similar trends are observed.   

Mechanical Properties of Polymer Nanocomposites under Large Amplitude Deformation

Bare silica nanoparticle dispersion in polystyrene and poly(methyl methacrylate) homopolymers are found to be controlled upon changing the evaporation condition. In this study, we deformed the polymer nanocomposites at different states of particle dispersion under large amplitude oscillatory shearing (LAOS). As the structure evolved during LAOS, the TEM images and small-angle X-ray scattering results obtained for different states of deformation together with the measured moduli allowed us to relate the mechanical reinforcement and nonlinearities such as strain stiffening/softening or shear thinning/thickening to the particle-polymer and particle-particle interactions effective at nanometer dimensions. Our results show that well-dispersed system with attractive interactions between particle and polymer becomes more elastic under large shear and behaves as attractive gels.   

Polymer Nanocomposite Mechanical Properties as a Function of Nanoparticle Dispersion

Nanoparticle (NP) dispersion critically affects the properties of polymer nanocomposites (PNCs), especially mechanical properties.~ Previous work by our group and others has shown optimal material properties occur when particles form a percolated network. Particle dispersion is a function of the interaction between the polymer and nanoparticle surface, and as such is difficult to control as an independent variable. In our previous work, dispersion was controlled via polymer grafts. Thus in order to vary the particle dispersion states the length and density of the grafted chains were necessarily simultaneously varied. Here we consider bare silica nanoparticles, 14nm in diameter, in 97kg/mol poly(2-vinyl pyridine). Although the particles are bare, the dispersion can still be controlled by proper solvent casting of the material; we use varying amounts of pyridine to charge stabilize the particles in solution and thus vary the dispersion state.~ We use TEM to probe PNP structure and NP dispersion state, and rheology is used to quantify mechanical properties. We look at the recovery of the storage modulus after large amplitude oscillatory shear and use this to probe the mechanism of particle reinforcement. Because the dispersion is an independently controlled variable, we are able to more accurately quantify its effect on nanocomposites.   

Elastic Moduli of Polymeric Thin Films of Nanocomposites and Blends via Buckling on Elastomeric Substrates

Mechanical properties are important for the long term durability of polymeric thin films. Unfortunately, there are very few methods for mechanical characterization of sub-micron thin films with high accuracy and repeatability. The technique of Strain-Induced Elastic Buckling Instability for Mechanical Measurements (SIEBIMM) was employed to determine the elastic moduli of nanocomposite and blend films, which were calculated from the buckling patterns generated by applying compressive stresses. In this study, polylactic acid (PLA) / Cloisite 30B nanocomposite thin films and polycaprolactone (PCL) / PLA blend thin films were prepared via spin-coating and then transferred to crosslinked polydimethylsiloxane (PDMS) flexible substrates. Results showed the strengthening effect of Cloisite 30B on PLA systems. The effect of nanoparticle concentrations and the influences of crystallinity and phase separation of blends will be presented.   

Electrical Conductivity in Transparent Silver Nanowire Networks: Simulations and Experiments

We model and experimentally measure the electrical conductivity of two-dimensional networks containing finite, conductive cylinders with aspect ratio ranging from 33 to 333. We have previously used our simulations to explore the effects of cylinder orientation and aspect ratio in three-dimensional composites, and now extend the simulation to consider two-dimensional silver nanowire networks. Preliminary results suggest that increasing the aspect ratio and area fraction of these rods significantly decreases the sheet resistance of the film. For all simulated aspect ratios, this sheet resistance approaches a constant value for high area fractions of rods. This implies that regardless of aspect ratio, there is a limiting minimum sheet resistance that is characteristic of the properties of the nanowires. Experimental data from silver nanowire networks will be incorporated into the simulations to define the contact resistance and corroborate experimentally measured sheet resistances of transparent thin films.   

Effect of Nanowire Size Dispersity and Orientation on Electrical Conductivity in Polymer Nanocomposites

We model the percolation threshold ($\phi _{c})$ and electrical conductivity of isotropic and oriented three-dimensional networks containing finite, conductive cylinders with experimentally typical (Gaussian) and engineered (bidisperse) distributions in their length and/or diameter. Our results show that narrow Gaussian distributions do not affect the threshold concentration or electrical conductivity significantly in either isotropic or oriented networks. In contrast, the addition of a small fraction of longer rods in a bidisperse system can improve the electrical properties considerably. We have also successfully extended the excluded volume percolation theory to predict $\phi _{c}$ of polydisperse networks of soft-core rods with finite-L/D by generalizing the monodisperse case and applying an empirical calibration factor from our simulations. Our analytical expression finds the critical concentration in nanocomposites with arbitrary distributions in L and/or D.   

Finite-size effects in nanocomposites: experimental and computational studies

Many proposed applications for electrically-conducting composite materials (smart textiles, e-m shielding coatings, tissue scaffolds) are nanostructured - that is, characteristic sample length scales may be similar to at least one dimension of the embedded particle. This is particularly true for long aspect-ratio particles such as nanotubes where the length of the particle can approach or exceed the thickness of a thin nanocomposite film or a nanofiber diameter. In these cases, the formation of a particle network and thus the electrical conductivity enhancement is affected by finite size effects, that is, percolation thresholds and the width of the transition to percolation differ with sample size [Stevens et al., \textit{Phys. Rev. E} \textbf{84}, 021126 (2011)]. We present experimental electrical conductivity and 3-D continuum Monte-Carlo simulation results on such finite-sized percolation effects for particles with aspect ratios of 1 to 1000 and discuss proposed scaling laws and techniques to improve conductance in the finite-size regime.   

Spectral and polarization modulation of quantum dot emission in a one-dimensional liquid crystal photonic cavity

We demonstrate spectral and polarization modulation of chemically synthesized core shell CdSe/ZnS quantum dots (QDs) embedded in a one-dimensional photonic cavity formed by a cholesteric liquid crystal (CLC) matrix. A Cano-wedge cell varies the pitch of the CLC leading to the formation of Grandjean steps. This spatially tunes the photonic stop band, changing the resonance condition and continuously altering both the emission wavelength and polarization state of the QD ensemble. Using high resolution spatially- and spectrally-resolved photoluminescence measurements we find that the emission is elliptically polarized and that the tilt of the ellipse, while dependent on the emission wavelength, additionally varies with distance across the Grandjean steps. This work opens up the possibility of designing new QD based optical devices, such as tunable single photon sources, where spatial control of wavelength and polarization of the embedded QDs would allow great flexibility and added functionalities.   

Self-assembly of silicon quantum dot clusters in polymer nanocomposites

We measure the influence of polymer-driven silicon nanocrystal (SiNC) self-assembly on the photoluminescent stability of SiNC clusters. Coexisting phases of varying nanoparticle density are identified in annealed SiNC-polymer nanocomposites, and the local photobleaching kinetics are measured under varied exposure to atmospheric oxygen. Increased particle packing and decreased oxygen exposure both contribute to improvements in cluster photostability, with Monte Carlo simulations of ensemble photobleaching clarifying the critical role of nanoparticle packing. The simulations further demonstrate the potential importance of nanoparticle interactions in dictating the photo-response of the self-assembled SiNC clusters.   

Utilizing photothermal heating by metal nanoparticles within polymer composites

Photothermal heating by metal nanoparticles have been extensively researched in solution environments for applications such as cancer treatment and drug delivery, but very few have explored photothermal heating in solids, such as metal nanoparticle-polymer composites. When metal nanoparticles are excited by light resonant with the particle's surface plasmon, non-radiative relaxation efficiently generates heat. Thus, this photothermal effect facilitates~\textit{in situ}~thermal processing of polymeric materials via externally-controllable light excitation.~By embedding fluorophores in the composite, a sensitive relative fluorescence approach can be utilized to dynamically monitor the average temperature within the sample as it is thermally processed. With modest light intensities and dilute nanoparticle concentrations, controllable temperature changes of several hundred degrees Celsius have been achieved.~ We discuss various cooling mechanisms and their respective effect on the heating process. The spatial specificity and temperatures achieved can potentially be used for triggering phase transitions, cross-linking, or enabling region-specific chemical reactions within a polymeric material. ~   

What determines photoluminescence and quenching when fluorophores in a polymer matrix?

A model system composed of fluorophore (pyrene), dispersed in a polymer matrix (polystyrene, PS) is investigated in order to find the relation between the structure of pyrene assembly in the PS matrix and its fluorescence/quenching. It has been shown that the pyrene disperses differently in the polymer matrix as it is prepared by different processes (namely, electrospin, solution cast and spincoat) with same composition. The difference can result in drastically variation of fluorescence response and quenching efficiency in presence of DNT. Our preliminary data indicate that the salt (tetrabutylammonium hexafluorophosphate, TBAP) applied to electrospinning process plays a crucial role in excimer fluorescence in the emission spectra, while various polymer matrices (e.g., PS and poly-methyl methacrylate, PMMA) yield similar fluorescence without TBAP. X-ray diffraction data suggests that strong dimer fluorescence of the sample may relate to the alignment of pyrene crystal structure, with a lattice length of 8.4 {\AA}. Moreover, a C$^{13}$ solid-state NMR result seems to indicate that the mobility of pyrene in the PS matrix of electrospin system is lower for samples with higher quenching efficiency.   

Microtextured Omniphobic Surfaces by Solution Spraying of Fluorodecyl POSS/PMMA Blends

We present a simple technique to prepare various micro-structured surfaces by spray coating a polymer blend of poly(methyl methacrylate) (PMMA) and the low surface energy molecule 1H,1H,2H,2H-heptadecafluorodecyl polyhedral oligomeric silsesquioxane (fluorodecyl POSS) using an air brush with a pressurized nitrogen stream. The sprayed surface morphology can be systematically tuned from a spherical or corpuscular microstructure to beads-on-string and a fiborous non-woven mesh, nearly identical to structures obtained by electrospinning similar PMMA/fluorodecyl POSS solutions. A semi-empirical framework is used to develop an operating diagram to predict the surface morphology produced during the simple spraying technique based on the polymer solution concentration and molecular weight. The presence of the low-surface-energy POSS molecules at the surface combined with the re-entrant microtextured features confers super-liquid-repellent properties to the spray-coated substrate, which are characterized by advancing and receding contact angle measurements with liquids of a range of surface tensions.