Session V25: Polymer Composites - Nanotubes and Nanoclays

Synthetic Gecko Foot-hairs from Multiwalled Carbon Nanotubes

The mechanism that allows a gecko lizard to climb any vertical surface and hang from a ceiling with one toe has attracted considerable interest and awe for over two millennia. Recent studies have discovered that the gecko's ability to defy gravity comes from its remarkable feet and toes. Each five-toed foot is covered with microscopic elastic hairs called setae. The ends of these hairs split into spatulas which come in contact with the surface and induce enough intermolecular [van der Waals, (VdW)] forces to hold them in place. Similarly, the same VdW forces act between our two hands when they are held together, but in this case, they do not stick to each other. The reason is that the roughness of our hands prevents them from coming close to each other at separations relevant for VdW forces. On the other hand, based on the gecko's foot anatomy, if our hands were made up of tiny elastic structures that were able to deform or bend at different length scales in accordance with the contact surface and correct for the roughness, then perhaps our hands could also adhere to the surfaces we touch. In my talk, I will present the recent advances we have made in fabricating polymer surfaces with multiwalled carbon nanotube hairs with strong nanometer-level adhesion forces that are 200 times higher than those observed for Gecko foot-hairs. This fabrication process allows the flexibility to create structures that are found in nature on the Gecko's foot and offer excellent potential for applications as dry adhesives for space, microelectronics and MEMS devices. This work was done in collaboration with Betul Yurdumakan, Nachiket Raravikar and Pulickel Ajayan.   

Flow Kills Conductivity of Single Wall Carbon Nanotubes (SWNT) Composites

Most composites of polymer and single wall carbon nanotubes\textbf{ (}SWNT) reported in the literature are made by solvent casting or simple compression molding. Commercial utility of these composites requires use of precision injection molding. We have observed a unique behavior wherein the SWNT composites made by injection molding or by extrusion are insulators but upon heating become electrically conductive. This behavior appears to be the result of a relaxation phenomenon in the SWNT composite. During flow into an injection mold or through an extrusion die the well-dispersed SWNT in the polymer matrix tend to align such that they are not in contact with each other and are farther than the minimum required distance, 5 nm (1), to achieve electrical percolation through electron hopping. Upon heating the SWNT relax and either touch each other or are at a distance less than or equal to 5 nm from each other to create a percolating. \newline \newline [1] Du, F., Scogna, R, C., Zhou, W., Brand, Stijn, Fischer, J. E., and Winey, K. I., Macromolecules 2004, 37, 9048-9055.   

Fabrication and Characterization of Polyamide Nanocomposites Using Functionalized Nanotubes.

We have prepared nylon 6,10 nanocomposites with functionalized single wall nanotubes (fSWNT) using an interfacial polycondensation method previously developed in our lab. The specific functional groups (CH2)nCOCl on the sidewall of the nanotubes were designed to permit covalent bonding to the nylon matrix while fabricating the nanocomposite. Using a binary mixture of dichlorobenzene/water solvent system, we have been able to prepare nylon 6,10 and nylon 6,10 / fSWNT nanocomposites with a yield of 80{\%}. The composites were characterized using FTIR and Raman spectroscopies, optical and scanning electron microscopies, differential scanning calorimetry, and thermogravimetric analysis. The composites show good dispersion of nanotubes at the micron and submicron levels. Improved protocols have been developed to reduce the degradation of the composites at elevated temperatures. The nylon 6,10 / fSWNT nanocomposites were spun into fibers using melt fiber spinning at 240C and the mechanical properties of the fibers were studied as a function of nanotube loading.   

Electrical, Mechanical, and Morphological Characterization of Carbon Nanotube filled Polymeric Nanofibers

This work focuses on the inclusion of conductive nanotubes into polymeric matrices with the end goal of creating conductive nanocomposites. This investigation has been carried out by uniform dispersion of multi-walled carbon nanotubes in aqueous solutions of polyvinyl alcohol (PVA) and polyethylene oxide (PEO), which are inherently nonconductive polymers. To fabricate these structures we are using the electrospinning process encompassing an array of collection methods including parallel bars and a static plate. Carbon nanotubes are known to have excellent electrical conductivity and mechanical properties. This investigation shows that the inclusion of carbon nanotubes increases the electronic conduction in these polymers and enhances the mechanical properties of the composites. Dispersion of these nanotubes is the key factor in this process; gum Arabic and surfactants have been utilized for the dispersion of these nanotubes. Conductivity measurements have been carried out by two point probe method and by performing sensitive current and conductance measurements with a femtoammeter. Further morphological characterization has been performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM).\newline $^1$ Department of Textile Engineering, Chemistry, and Science \newline $^2$ Department of Physics   

Thermomechanical Properties of Polystyrene-MWNT Nanocomposites.

In this work we have studied the thermomechanical properties of polystyrene nanocomposites-MWNTs (Multi-walled carbon nanotubes) concentration. A number of different combinations of the matrix and the filler concentrations are being analyzed to study the effect on glass transition temperature using differential scanning calorimetry and a dynamic mechanical analyzer. FE-SEM (Field Emission Scanning Electron Microscope) is being used to study the dispersion and interaction of nanotubes in the matrix. We show that the thermomechanical properties of these materials are very similar to composites with SWNT, thus suggesting that the tube stiffness may not play an important role in this context.   

Carbon Nanotube Composites from Modified Plant Oils

Carbon nanotubes (CNTs) with their impressive mechanical properties are ideal reinforcement material. Acrylated epoxidized soy oil (AESO) has been previously shown to have favorable interactions with carbon nanotubes. CNTs mixed into AESO, both with and without styrene as a co-monomer, using mechanical shear mixing showed dispersion only on the micron level, resulting in modest mechanical property improvements. Greater improvements were seen, especially in the rubbery modulus, when the resin's viscosity was kept high, either through a reduction of the styrene content, or by curing at a lower temperature. CNTs were also dispersed via sonication in methyl methacrylate. The resulting dispersion was then mixed with AESO. The resulting composites showed better CNT dispersion, with no micron-sized aggregates, as verified using SEM and optical microscopy. The mechanical properties also showed greater improvement.   

Carbon Fibers from Chicken Feather Keratin

As the availability of synthetic and fossil-fuel based resources is becoming limited, bio-based materials offer an environmentally friendly alternative. Chicken feathers remain a huge agricultural waste. The feathers are comprised of approximately 97{\%} keratin, but are currently used only to enrich animal feed. However, this usage is becoming a problem with the spread of diseases such as Bovine Spongiform Encephalopathy, commonly called ``Mad Cow Disease.'' The hollow, microcrystalline, oriented keratin feather fibers offer a novel, low cost approach to producing carbon fibers through controlled pyrolysis. Carbonized feather fibers (CFF) were prepared by first heating to 225 $^{o}$C (below the melting point)in N$_{2}$ for 26 hours to crosslink and stabilize the fiber structure; then carbonization occurred by increasing the temperature to 450 $^{o}$C for two more hours. The resulting CFF were hollow, stiff and strong and had an affine 80{\%} weight loss, which is near the theoretical value for the C-content of keratin. Initial studies showed that a composite with the CFF and an epoxidized soybean oil (AESO) gave an improved fiber modulus E$_{CFF}$ of order 13.5--66.1 GPa. With continued research, the goals are to increase the stiffness of the feathers to 100 GPa, while increasing the strength in the range of 5-10 GPa.   

Enhanced Dispersion in Polymer Nanocomposites by Optimized Hydrogen Bonding

The dispersion of the minor phase in a multi-component polymer system can readily lead to nonlinear enhancement of material properties. In any multi-component polymer system, including polymer nanocomposites, understanding and control of the dispersion of the nanofiller in the polymer matrix is critical to rationally design and create a \textit{useful} new material. This presentation will discuss the work in our group to optimize the specific interactions between components of polymer nanocomposites. We will discuss results that indicate that the optimization of intermolecular interaction between components provides a controllable mechanism to improve the dispersion of nanoscale fillers in a polymer matrix and that the improved dispersion correlates directly to improved thermal, mechanical, and electrical properties.   

Supercritical CO$_{2}$ Processing: A Novel Method for Producing Highly Dispersed Clay-Polymer Nanocomposites

The key challenges in producing high performance nanocomposites are separation of the layered filler into individual platelets, preservation of large aspect ratios and promotion of polymer-filler interactions. By exploiting the ability of scCO$_{2}$ to diffuse into the layered filler and solvate/swell polymers we have been able to disperse the nanoclay and facilitate polymer-clay interactions even with Na$^{+}$ montmorillonite. Nanocomposites are prepared by processing the pristine nanoclay and compounding it with an appropriate polymer or processing clay/polymer simultaneously. SEM and WAXD results show significant clay layer separation and preservation of platelet aspect ratios. A high degree of dispersion is also observed in TEM images of these nanocomposites. The facilitation of polymer-clay interactions during the scCO$_{2}$ process results in higher glass transition temperatures and improved dynamic moduli for these systems. For example, a 5wt{\%} 93A/PS nanocomposite has a 10 degree increase in T$_{g}$ and G' becomes nearly independent of frequency at low shear rates where it demonstrates an order of magnitude increase.   

Evidence of re-entrant behavior in polymer-nanoclay systems

Polymer-clay systems are of interest in a variety of applications, including nanocomposites, personal care products, and oil field products. Rheology and dynamic light scattering capture re-entrant behavior of laponite-polymer systems. Neat laponite at basic pH and concentrations of 2 weight percent or greater forms a viscoelastic isotropic solid due to electrostatic repulsions. This phase appears to be a colloidal glass of laponite particles. We show that that addition of low molecular weight poly(ethylene oxide) (PEO) melts the glass due to a depletion force. The depletion force speeds up dynamics in the system resulting in a low viscosity solution. A re-entrant viscoelastic solid is formed with the addition of high molecular weight PEO due to the polymer chains bridging between laponite particles. In addition we present non-linear rheological behavior below and above the transition. As expected the transition from a low to high viscosity solution scales with size of the polymer mean square end-to-end distance and gap between laponite particles.   

Organoclay Networking in Polypropylene-Clay Nanocomposites

We report on networking of organoclays and its influence on the rheology of polypropylene-clay nanocomposites. Samples are blended using either a twin screw extruder (TSE) or a single screw extruder with \textit{in situ} addition of supercritical CO$_{2}$ (SCCO$_{2})$. Blends contain proportional amounts (3:1) of maleic anhydride functionalized PP and organically modified montmorillonite (Cloisite$^{\mbox{{\textregistered}}}$ 15A, Southern Clay Products) at several clay loadings in a base PP resin (MFI 12 g/10 min at 230\r{ }C). Small-amplitude oscillatory shear (SAOS) and steady shear flow were utilized to probe clay morphology with varying annealing time. In SAOS experiments repeated over several hours, the terminal behavior of the TSE samples became increasingly solid-like; in steady shear, the magnitude of the viscosity overshoot increased with annealing time. The single-screw/SCCO$_{2}$ materials at the same clay loadings differed little rheologically from the neat resin. Finally, network formation kinetics accelerated with increasing temperature, and sufficiently large deformations irreversibly weakened the network structure.   

Morphology and Gas Barrier of Polystyrene-Clay Nanocomposites

Gas barrier of polymer clay nano-composites are often found to be substantially smaller than expected, especially in the case of exfoliated polymer clay nano-composites where the improvement of gas barrier should be especially noticeable. This fact remains a subject of great controversy and calls for fundamental understanding. Model intercalated and exfoliated polystyrene/montmorillonite (MMT) systems were prepared by in-situ polymerization method, and their oxygen barrier was examined as a function of NaMMT content and related to nano-composite morphology studied by TEM. It was shown that apparent exfoliation of mineral layers, as confirmed by WAXS, not necessary was associated with their homogeneous dispersion. Agglomerates containing 3-6 practically parallel exfoliated layers were observed instead, and this morphology was responsible for poor gas barrier. In contrast, intercalated polymer clay nano-composites exhibited an interesting phenomenon of layer stair-case sliding leading to gas barrier properties better than expected. Gas barrier in both cases was successfully modeled by applying the Nielsen model which was modified to take into account layer agglomeration.   

Rheology of Supercritical CO$_{2 }$ dispersed Polymer/Clay Nanocomposites

Effective dispersion of the fillers in a polymer matrix has been a key challenge in the field of nanocomposites. Supercritical carbon dioxide (scCO$_{2})$ appears , PS/clay, The nanocomposites are characterized using WAXD, SEM, TEM, Rheology and DSC. The high degree of dispersion achieved through sc-CO$_{2}$ appears to result in an order of magnitude increase in the rheological properties of PS, associated with an increase in the T$_{g}$ of around 13\r{ }C, at 10{\%} clay loading. These moduli improvements are significant better than those obtained with conventional, chemically-modified intercalated clay nanocomposites. The degree of enhancement in the properties appears to be strongly dependant on the polymer-clay interactions, and how it is promoted by the supercritical fluid. In the case of PDMS nanocomposites, where the clay-polymer interactions were weak, the modulus increase at low frequencies (for sc-CO$_{2}$ processed system) was only a factor of 2. In the case of PVME- I30P clay nanocomposites, the modulus increase was substantial even at moderate loadings and dispersions, perhaps to be hydrogen-bonding interactions. The clay and the polymer orientation and interactions in these nanocomposites are also being probed using rheo-optical FTIR spectroscopy.