Hulya Cebeci

Three-dimensional elastic constitutive relations of aligned carbon nanotube architectures

Tailorable anisotropic intrinsic and scale-dependent properties of carbon nanotubes (CNTs) make them attractive elements in next-generation advanced materials. However, in order to model and predict the behavior of CNTs in macroscopic architectures, mechanical constitutive relations must be evaluated. This study presents the full stiffness tensor for aligned CNT-reinforced polymers as a function of the CNT packing (up to ∼20 vol. %), revealing noticeable anisotropy. Finite element models reveal that the usually neglected CNT waviness dictates the degree of anisotropy and packing dependence of the mechanical behavior, rather than any of the usually cited aggregation or polymer interphase mechanisms. Combined with extensive morphology characterization, this work enables the evaluation of structure-property relations for such materials, enabling design of aligned CNT material architectures.

Effect of nanofiber proximity on the mechanical behavior of high volume fraction aligned carbon nanotube arrays

The effect of nanofiber proximity on the mechanical behavior of nanofiber arrays with volume fractions (Vf) from 1% to 20% was quantified via nanoindentation of an aligned carbon nanotube (A-CNT) array. The experimental results show that the indentation modulus for A-CNT arrays has a highly non-linear scaling with the CNT Vf, leading to modulus enhancements of up to ∼600× at Vf = 20%. Modeling illustrates that the origin of the highly non-linear trend with Vf is due to the minimum inter-CNT spacing, which is shown to be more than an order of magnitude larger than the graphitic spacing.

Effective Stiffness of Wavy Aligned Carbon Nanotubes for Modeling of Controlled-Morphology Polymer Nanocomposites

The mechanics of nano-scale fiber reinforced polymer matrices are investigated using an analytical reduction and finite element modeling to consider the effect of waviness of the reinforcing carbon nanotubes (CNTs). Nanofiber-reinforced polymer matrices are of significant interest as structural materials in and of themselves, and particularly as hybridized matrices within bulk polymer-matrix composites containing standard microndia. fibers, e.g., carbon fiber reinforced plastics (CFRP) that are used extensively in aerospace applications. Here, a representative volume element (RVE) of aligned, continuous, and wavy CNTs in a polymer matrix is modeled to deduce the waviness effects on the elastic properties of the aligned-CNT polymer nanocomposite (A-PNC) as a function of the properties of the reinforcing fibers (CNTs), including CNT type and degree of waviness. Experimental modulus data as a function of CNT volume fraction for an A-PNC RVE using an aerospace-grade thermoset epoxy is used to highlight the importance of waviness and the axial vs. bending stiffness contributions of the CNTs to RVE stiffness. Straightforward implementation for singleand multi-walled CNTs improves upon prior work that considers the filaments to have a solid, rather than hollow, cross section. The derivation of effective axial and bending stiffness for the CNT filaments utilizes proper modulus-thickness pairs for investigating more complex cases. Waviness is noted to be the dominant morphological feature controlling the elastic response of such PNCs, effectively significantly reducing stiffness relative to rule of mixtures predictions. Future work will focus on modelexperiment correlation with in-progress experimental work to characterize the full, nonisotropic, constitutive relation for A-PNCs.

Molecular Dynamics and Finite Element Investigation of Polymer Interphase Effects on Effective Stiffness of Wavy Aligned Carbon Nanotube Composites

Interphase effects have been studied for their effect on composite properties for many decades, and it is well documented that an interphase can exist in polymer composites comprised of nanofibers as well. We present a first study of interphase effects on the basic elastic response of wavy aligned carbon nanotube (A-CNT) polymer nanocomposties (PNCs). Waviness is characterized by ex situ pre-fabrication imaging of A-CNT forests and used as an input to finite element analyses of the PNCs containing an interphase region in the thermoset polymer defined using molecular dynamics (MD) simulations. The interphase thickness of ~1nm is found to be independent of crosslink density and contain regions of both higher and lower mass density than the bulk polymer. Finite element analyses of wavy single and double-wall A-CNT PNCs incorporating this interphase, allow the effective stiffness based on a representative volume element to be calculated. Waviness of the A-CNTs dominates the effective axial stiffness of the PNCs, with the interphase having a negligible effect. The interphase changes the stress and strain distribution local to the CNT ‘fiber’ and this is expected to play an important role in failure, such as CNT pullout, of the CNTpolymer system. These findings, in addition to the relatively high volume fraction (Vf) of the interphase in PNCs with high CNT Vf, suggests that the interphase may play a more important role in PNCs than in micron-scale (typical) collimated fiber composites. Future work in this area includes inelastic polymer response during nanofiber pullout.

Conductive filler morphology effect on performance of ionic polymer conductive network composite actuators

Several generations of ionic polymer metal composite (IPMC) actuators have been developed since 1992. It has been discovered that the composite electrodes which are composed of electronic and ionic conductors, have great impact on performance of ionic polymer actuators by affecting strain level, efficiency and speed. One of important factors in composite electrodes is the shape and morphology of electronic conductor fillers. In this paper, RuO2 nanoparticles and vertically aligned carbon nanotube (Va-CNT) are used as conductor fillers. Making use of unique properties of Va-CNT forests with ultrahigh volume fraction in Nafion nanocomposite, an ionic polymer actuator is developed. Ion transport speed is greatly increased along CNT alignment direction. The high elastic anisotropy, arising from the high modulus and volume fraction of Va-CNTs, enhances actuation strain while reducing the undesirable direction strain. More than 8% actuation strain under 4 volts with less than one second response time has been achieved.

Enhanced Electromechanical Responses of IPCNC Actuators

In this presentation, we will show several progresses in Ionic Polymer Conductor Network Composite Actuators (IPCNC) studies. First of all, we successfully fabricated ultra high volume fraction vertically aligned carbon nanotubes (VA-CNTs)/polymer composite electrodes which markedly improved the electromechanical performance of IPCNC actuators. The experimental results show that the continuous paths through inter-VA-CNT channels and low electrical conduction resistance due to the continuous CNTs lead to fast actuation speed (>10% strain/second). The experimental results also demonstrate that the VA-CNTs create anisotropic elastic property in the composite electrodes, which suppresses the vertical strain and markedly enhances the actuation strain (>8% strain under 4 volts). The data here show the promise of optimizing the electrode morphology in IPCNCs by the ultrahigh volume fraction VA-CNTs for ionic polymer actuators to achieve high performance.Copyright