Session B18: Structure and Dynamics in Polymer Nanocomposites

Externally Activated, Thermodynamically Governed Dispersion Behavior of Silica Nanoparticles in PMMA

Dispersion of nanoparticles is a topic of significant interest to the industrial, government, and academic communities as a route for creating new materials that combine the properties of the matrix polymer with functional properties of the dispersed particulate phase. We have previously demonstrated that the inclusion of a Diels-Alder linkage in the ligands use to compatibilize nanoparticles allows external control over the miscibility of those particles in the polymer matrix. Above a known temperature range, the ligands separate at the Diels-Alder linkage site, changing the miscibility of the particle. Here we are investigating the dispersion of silica particles approximately 40 nm in diameter in a PMMA matrix, using SAXS, TEM, and optical microscopy to characterize the dispersion of the particles. Annealing the samples causes in a change in the morphology of the materials, with the formation of larger aggregates and depletion zones. The optical clarity of the PMMA is substantially degraded by this process.   

Single Chain Mean Field Theory (SCMF) in Polymer Nanocomposites

Recent experimental work in our group has shown that grafting the surface of inorganic nanoparticles with polymer brushes improves the dispersion of the particles in a polymer matrix. Since improved dispersion is critical to the enhancement in material properties of the resultant nanocomposite, we are motivated to study the thermodynamics of the interface structure in the polymer particle system since the interface itself is a major determinant of nanocomposite properties. We will present the results from recent and ongoing SCMF studies on a model polymer nanocomposite system with grafted particles and explore the thermodynamics of the particle/chain miscibility region along with explicit knowledge of the polymer chain structures (both matrix and grafted). Dependence on various parameters such as the molecular weights of the grafted and matrix chains respectively, the density of the grafted chains and the particle size will also be explored.   

Improved Mechanical Properties by Grafting Nylon 6, 10 to Single Wall Carbon Nanotubes

We have prepared nylon 6, 10 nanocomposites using functionalized single wall carbon nanotubes (SWNT) using our interfacial \textit{in situ }polycondensation method. To improve mechanical properties, SWNT were functionalized with specific groups --(CH$_{2})_{n}$COCl with n of 4 or 9 to facilitate covalent bonding between the SWNT and the nylon matrix via alkyl segments. After synthesis, the SWNT/nylon 6,10 nanocomposites were melt spun into fibers. Relative to nylon 6,10 prepared in the same manner, the 1wt{\%} SWNT/nylon 6,10 composite shows significant increases in tensile modulus (160{\%}), strength (160{\%}) and toughness (140{\%}) with only modest reduction in the strain to break. The alkyl segments at the SWNT/nylon 6,10 interface appear to be critical to observed improvements.   

Mechanical properties of nanocomposite systems

We employ molecular simulations to investigate the alteration of the mechanical properties upon addition of nanoparticles to a polymer matrix in the melt and the glass regime. In the glass regime, the formation of a stiffer glassy layer is apparent in the vicinity of the nanoparticles. Between particles an increase of the population of local high moduli domains suggests that a second mechanism of property improvement is the formation of a glassy network percolating throughout the material. The distribution of local moduli and the correlation of the non-affine displacements provides interesting insights into the inhomogeneity and the fragility of the nanocomposite and these systems. In the melt regime, relaxation times and viscosity are found to be higher for the nanocomposite systems than the pure polymer. Calculation of chain bridges between the particles is compared to the chain length to examine the bridge formation hypothesis. A primitive path analysis is performed to investigate the effect of inclusions on the entanglement length; results suggest that the entanglement length decreases upon addition of nanoparticles.   

Morphological dependence on the conductance of polymer/MWNT electrospun mats

Porous, electrically conducting structures are an invaluable resource for improving and developing technologies such as electrostatic dissipating filters and tissue scaffolds. Fibrous mats of electrospun polymers doped with multi-walled carbon nanotubes (MWNT) are an ideal candidate for such materials. A characterization of the morphological effect on conductance for these mats is presented here. Changes in the percolating behavior were investigated under the influence of different processing parameters. Alterations of fiber size, alignment, and construction (single component vs core-sheath bicomponent) were performed. For each set of processing parameters, electrospun mats were produced with varying carbon nanotube doping levels above and below the anticipated percolating region. Multiple parameters describing the percolation were calculated and compared for each of the processing regimes.   

Effect of MWNT and Carbon Nanofiber Orientation in Polymer Nanocomposites on Electrical Conductivity

Poly(methyl methacrylate) (PMMA) composites were prepared by a coagulation method containing multi-wall carbon nanotubes (MWNT) or carbon nanofibers (CNF). The critical concentrations for electrical conductivity are 0.25wt{\%} and 4.0wt{\%} for MWNT and CNF composites, respectively. These isotropic composites were melt spun into fibers to align the fillers and the extent of alignment was quantified using x-ray scattering methods to measure the azimuthal full width at half-maximum (FWHM). The electrical conductivity shows a strong dependence on filler alignment and the presence of a critical alignment (FWHM$_{c})$ indicates a percolation threshold with respect to alignment. The critical alignment shifts to lower FWHM (more aligned) as the MWNT loading increases. We are also exploring the filler alignment and electrical conductivity of nanocomposites with both MWNT and CNF.   

Controlling the Average and Local Glass Transition Temperatures of PMMA-SWCNT Nanocomposites

The presence of nanofillers in a polymer matrix can drastically alter the chain segmental mobility and limit the polymer conformations, changing the properties of the system. Here, fluorescence is used to measure local and average Tg's in poly(methyl methacrylate) (PMMA)/single-walled carbon nanotubes (SWCNT) nanocomposites. SWCNT functionalized with amide groups via chemical modification of carboxyl groups introduced on the carbon nanotube surface, allow for covalent bonding with PMMA. In this study, three side groups, (CH$_2$)$_{12}$, a phenyl ring and (CH$_2$)$_2$, were used in the amide functionalized SWCNT. The PMMA-grafted SWCNT was blended with bulk PMMA at different compositions to form polymer nanocomposites. The results show that, relative to the bulk Tg of PMMA, the increases in local and average Tg of PMMA-SWCNT nanocomposites, ranging from 3 K to 32 K, correlate with the length and flexibility of the side group on the amide functionalized SWCNT.   

The Aging Effect on Segmental Dynamics of PMMA brushes Studied by Incoherent Neutron Scattering

We have performed quasi-elastic neutron scattering experiments on nanocomposites comprised of a PMMA homopolymer mixed with silica particles on which a PMMA brush is attached. The PMMA brushes were 55k in molecular weight with a grafting density of 0.3 chains/nm$^2$, while we considered two matrices, 20k and 200k PMMA homopolymers, respectively. (The particle weight fraction was varied in the range 0-0.25 in a series of samples.) In all cases there are very strong aging effects, emphasizing the long equilibration times that these hybrid materials require. After a week of equilibration at 150$^{\circ}$C, we have found that in the case of wet brushes (i.e., the 20k matrix) the mean-square displacement only changes weakly with decreasing particle concentration. In contrast, the MSD decreased strongly with increasing particle concentration in the case of non-wetting matrices. These results stress that confinement effects (which increase with increasing particle content) play a secondary role to the effect of wetting behavior in determining the thermomechanical properties of these nanocomposites.   

Polymer Diffusion in Single Wall Nanotube / Polystyrene Nanocomposites

In addition to providing exception properties, single wall carbon nanotubes (SWNT) enable fundamental studies of polymer diffusion. SWNT and small SWNT bundles have high aspect ratios ($>$ 30:1) and diameters on the order of nanometers, such that nanocomposites containing $\sim $ 1wt{\%} SWNT provide a fixed set of nanoscale obstacles. Here we will report our first results of polymer diffusion in these materials. A SWNT / polystyrene nanocomposite is prepared by a coagulation method and compression molding. Trace diffusion experiments are performed using deuterated polystyrene (dPS) where the concentration profile is characterized by forward recoil spectrometry (FRES) as a function of diffusion time and temperature. The SWNT filler, particularly with the creation of a SWNT network, inhibits the diffusion of dPS and the diffusion coefficient decreases by one order of magnitude in the nanocomposite.   

Origin of dynamical properties in PMMA-C$_{60}$ nanocomposites

To develop further insight into the mechanism(s) by which nanoscale fillers influence the properties of polymer nanocomposites (PNCs), we evaluate the thermal and viscoelastic behavior of a model PNC, narrow molecular weight distribution PMMA into which C$_{60}$ fullerene particles are incorporated. Differential scanning calorimetry and dynamic mechanical analysis measurements indicate systematic increases of the PNC T$_{g}$ over the C$_{60}$ composition range. Oscillatory shear rheological measurements reveal an increase in the longest relaxation time of the polymer chains accompanies this change in T$_{g}$. An assessment of particle dispersion within the polymer host suggests that the changes in the material properties are due to polymer-particle interfacial interactions, as confinement of polymer molecules between filler particles is unlikely. The interfacial interactions lead to an increase in the local friction of the system, which ultimately suppresses polymer dynamics. The suppression of local polymer dynamics is evaluated using incoherent neutron scattering.   

Instabilities in Nanoporous Media

We studied Rayleigh instabilities in thin polymer films confined within nanoporous alumina templates. Thin films of PMMA were generated by filling cylindrical nanopores of an anodic aluminum oxide (AAO) membrane with a PMMA solution in chloroform followed by solvent evaporation. Undulations in the film thickness were observed that were induced by a Rayleigh instability when the PMMA nanotubes were annealed above the glass transition temperature (Tg). The amplitude of the undulations increased with time and eventually bridged across the cylindrical nanopore in the AAO membrane, resulting in the formation of polymer nanorods with periodically encapsulated holes. A similar behavior was observed when PMMA films were confined within carbon nanotubes (CNT). The Rayleigh instabilities in these confined geometries offer a novel means of controlling and fabricating the polymer nanostructures. These compartmentalized nanorods may have potential applications as delivery devices.   

Dynamic Consequences of the Fractal Network of Nanotube - Poly(ethylene oxide) Nanocomposites

In this work, SWNTs are successfully dispersed in a PEO matrix with the aid of an anionic surfactant. A geometric percolation at $\sim $ 0.1 vol {\%} of SWNTs ($\phi _{c})$ is observed and indicates a good state of dispersion of tubes with an effective aspect ratio of $\sim $ 650. At compositions ($\phi )$ well above the percolation threshold, the melt state rheological properties of the nanocomposites are dominated by the self-similar fractal network of the nanotubes (verified by scattering measurements) and demonstrate `time-temperature-concentration' superposition. The scaling of the network elastic strength, G $\left( {\propto \left( {\phi -\phi _c } \right)^{3.4}} \right)$, and critical strain for the onset of shear-thinning, $\gamma _{c} \quad \left( {\propto \left( {\phi -\phi _c } \right)^{-1.9}} \right)$, reveal a bond-bending mechanism to bear stress as expected from the strong short-range interactions between nanotubes. The onset of non-linearity and the damping behavior of the network show concentration invariance when represented against the local strain experienced by the network elements, with the onset occurring at a local strain value of 0.1.   

Gelation of Freely Associating Single-Wall Carbon Nanotube Networks

We report on the rheological evolution of a model filamentous network comprised of a semidilute dispersion of Single-Wall Carbon Nanotubes (SWNT). Using microrheology, we follow the gelation of a surfactant stabilized SWNT suspension from an initial sol of contacting but unbonded SWNT, endpointing in a gel network with a finite, zero-frequency elastic modulus. The SWNT network exhibits all the hallmarks of a critical sol-gel transition, including divergence of the viscosity as the gel point is approached from below, emergence of a finite elastic shear modulus above the gel point, and power law scaling of the viscosity and shear modulus below and above the gel point, respectively. The viscoelastic moduli exhibit a remarkable collapse under time-cure superposition, a footprint of self-similarity in inter-tube bond connectivity. Additionally, we present a scheme to spatially map rheological inhomogeneities in the network during gelation.   

Preparation and rheology of supercritical CO$_{2}$-based 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. Our results indicate that scCO2 can not only disperse nanoclays in polymers, it can also significantly enhance clay-polymer interactions.