Session D9 - Focus Session: Nanostructures in Polymers I.
FOCUS session, Monday afternoon, March 18
101-102, Indiana Convention Center

[D9.001] SINGLE-WALLED CARBON NANOTUBE / SEMICRYSTALLINE POLYMER COMPOSITE FIBERS

Single-walled carbon nanotubes (SWNTs) have demonstrated far superior mechanical, thermal and electrical properties relative to carbon fibers. Similar properties are expected for polymer/SWNT nanocomposites. We have produced SWNT-thermoplastic composites with extraordinary alignment of the nanotubes via melt processing followed by fiber melt spinning. The improved mechanical properties depend on the alignment of the nanotubes, which is controlled by the draw ratio of the composite fibers. Alignment of the nanotubes seems to reduce the electrical conductivity, because the percolation threshold of the one-dimensional conductors is reduced for straight, aligned nanotubes. Semicrystalline polyethylene and various nylons are used as the matrix and the mechanical, electrical properties are measured. The effect of the nanotubes on the crystallinity and the melting point are determined. A key factor of the composite performance is the dispersion of the nanotubes in the polymer matrix, the characterization of which is explored.

[D9.002] Multiscale Modeling of Polymer-Clay Composites

Nanosized clays, which consist of platelets with thickness 1 nm and width of 100-200 nm, are now dispersed in certain polymers, for example nylon-6 or polypropylene. This type of composite material exhibits significantly improved properties compared to that of the pure polymers. The mechanisms for these property improvements are not well understood and provide a challenge to the modeling community. We describe atomic and mesoscale lattice based methods designed to model these materials beginning at the atomic level and continuing to the macroscopic composite level. We have already successfully used these methods to study diffusion. Now we are studying the elastic properties at the microscopic level using traditional atomistic simulations and at the macroscopic level with lattice based methods. The relation between the diffusive and elastic properties is also investigated.

[D9.003] Polymer-Layered Silicate Nanocomposites with Low Coverage and Mixed Surfactants

Although polymer-layered silicate nanocomposites have been extensively studied in recent years, little is known about the exact role on morphology of the thermodynamic relationships between their components. The majority of work thus far has developed an understanding of the routes to successful fabrication of PLS nanocomposites and of their mechanical properties. The equilibrium morphology may be a function of surfactant coverage, length, and enthalpic interactions (\chi)with the intercalating homopolymer. We previously examined a system in which montmorillonite was modified with a polystyrene based surfactant, then manually mixed with PS homopolymer. For high levels of surfactant coverage of the silicate surfaces, phase separation occurred and was attributed to autophobic dewetting. Here, we present results from examination of the morphological behavior of montmorillonite modified to lower coverage with surfactant of a single molecular weight, and to full coverage with mixtures of surfactants having different molecular weights.

[D9.004] PMMA and PS / Clay Nanocomposites

We have been able to produce functionalized clay/Polymethylmethacrylate nanocompoistes using a Brabender twin screw extruder where SAXS spectra indicate a high degree of exfoliation. TEM micrographs show that 70clay is completely exfoliated, 27three platelets and 2structures. This high degree of exfoliation results in a large improvement in thermal stability and UV absorption properties. No significant changes in the modulus are observed by DMA analysis (Mettler-Toledo) below Tg, while evidence of gel formaion is observed above Tg. Cone Calorimetry tests conducted at an incident heat flux of 50kW/m2 at the National Institute of Standards and Technology clearly show that the heat release and mass loss rates are far lower and more gradual in the nanocomposite. These results are consistent with a dramatic increase in the specific heat as determined from high precision DSC (Mettler-Toledo) measurements. FTIR spectroscopy inidicates that exfoliation is driven by specific interactions between PMMA and the clay surfaces. PS/PMMA blends were also produced by this method and a significant decrease in the domain size of the phases was observed with the addition of clay. TEM micrographs indicate that the exfoliated clay platelets are positioned at the polymer interfaces thereby promoting compatibilization.

[D9.005] 3-d Orientation and Properties of Polymer Layered Silicate Nanocomposites

3-d Orientation and Properties of Polymer Layered Silicate Nanocomposites (PLSN) containing silicate (clay) layers dispersed in a polymer matrix were studied. Exfoliation of the clay platelets enhances barrier property of the nanocomposites. 3-d orientation of these clay platelets in nanocomposites plays a major role in determining final properties. Depending on the processing technique and the tensor cumulative shear the clay platelets orient in different directions. The effect of clay orientation on barrier properties is presented in this study. The effect of clay dispersion on the degree of crystallinity and crystal thickness is also presented. A relation between cumulative shear and clay orientation is proposed Polymer-Clay nanocomposite films were produced in a film blower. The effect of shear on the 3-dimensional orientation of clay platelets was studied small angle x-ray scattering (SAXS). Effects of this platelet orientation on the barrier properties were studied. Transmission electron microscopy (TEM) was used to study the exfoliation and dispersion of the clay platelets in the nanocomposite. X-ray diffraction (XRD) along with DSC, SAXS and TEM was used to study the effect of clay dispersion on the crystallization behavior of polymer in the nanocomposite.

[D9.006] Crystalline forms in melt-crystallized syndiotactic polystyrene/clay nanocomposites

X-ray diffraction methods and polarized optical microscopy have been used to investigate the structural change of syndiotactic polystyrene/clay nanocomposites. sPS/clay nanocomposite has prepared by mixing the sPS polymer solution with the organically modified montmorillonite. Both X-ray diffraction and transmission electron microscopy results indicate that most of the swellable silicate layers are exfoliated and randomly dispersed into sPS matrix. The x-ray diffraction data also show the presence of polymorphism in sPS/clay nanocomposites, which is strongly dependent on the thermal history of sPS/clay nanocomposites from the melt and on the content of clay in sPS/clay nanocomposites. The quenching from the melt induces the crystallization into the a crystalline form and the addition of montmorillonite probably increases heterophase nucleation of a crystalline form. In this study, the effect of melting temperatures and crystallization temperatures of sPS and sPS/clay nanocomposites on their crystalline phases is discussed.

[D9.007] Inorganic Surfaces as Nucleating Agents for Semi-Crystalline Polymers

Nanocomposite hybrid materials are employed to study the nucleating effect of inorganic surfaces/fillers on semi-crystalline polymers. Namely, the structure of poly(vinyl alcohol) and poly(ethylene oxide) are comparatively studied in the presence of montmorillonite nm-thin layers. Crystalline structure, morphology, and growth were found to be determined by the inorganic/polymer interactions at the polymer/solid interface. For strongly interacting hybrids (PVA/MMT) crystallinity is promoted through specific interactions, resulting in a new crystal form with increased Tm. In contrast, for weakly interacting hybrids (PEO/MMT) the crystalline structure remains bulk-like, but spherulitic morphology and growth is hindered next to the inorganic surfaces, despite the favorable geometric conditions. Experimentally AFM, DSC, XRD, and cross polarization optical microscopy were concurrently employed for these studies. The above results were compared also with semi-crystalline polypropylene next to an organically modified montmorillonite, where the interactions are almost at theta conditions and there exists only a purely geometric effect.

[D9.008] Thermoset-Based Nanocomposites

The field of polymer-Clay nanocomposites has attracted considerable attention as a method of enhancing polymer properties and extending their utility, by using molecular or nanoscale reinforcements rather than conventional particulate field microcomposites. Layered silicates dispersed as a reinforcing phase in a polymer matrix are one of the most important forms of such inorganic-organic nanocomposites, making them the subject of intense research. While a significant amount of work has been published on thermoplastic based nanocomposites, however, comparatively few studies of thermoset-based systems have been published. Thus, our research is centered on elucidating the structure-property relationships of thermoset-based nanocomposites. We have developed a series of layered silicate/thermoset nanocomposites using several thermoset polymers (epoxies(di and tetrafunctional), cyanate esters and PMR-15 polyimide). Wide angle X-ray diffraction suggests that intercalated morphologies were obtained for the cases studied. The glass transition temperature has been found to vary as the organic modifier and its amount is varied. For difunctional epoxy samples dispersed with Cloisite 30B, a commercially available nanoclay, the Tg increased by twenty degrees upon addition of as little as 2viscoelastic behavior of these materials has also been investigated using dynamic mechanical analysis. A modest increase in the glassy storage modulus was obtained as the amount of nanoclay increased, with a significant increase in the plateau modulus. Additionally, master curves have been generated using time-temperature superposition, allowing further analysis of the effect of the nanoclay on the relaxation behavior. Activation energies calculated from Arrhenius plots increase as the clay contents increase. These effects will be discussed in the presentation. The fracture toughness increased upon addition of nanoclays while the CTE decreased. Interestingly, the onset of decomposition increased upon addition of clays, in spite of the fact that the modifier exhibits decomposition in the temperature range in which the samples were cured. This also varied with the modifier used. Correlation between the type of modifier and its effect on the dispersability, crosslink topology and the macroproperties will be discussed.

[D9.009] Molecular mechanisms of failure in polymer nanocomposites

With the emergence of synthetic methods that can produce nanometer sized fillers, resulting in an enormous increase of surface area, polymers reinforced with nanoscale particles should show vastly improved properties. Experimental evidence, however, suggests that an extrapolation of the design paradims for conventional composites cannot predict the behavior of nanocomposites. Since the origin of these differences between conventional and nanoscaled composites is not known, the design of polymer nanocomposites has still relied on principles developed during the study of conventional composites. We use a Molecular Dynamics simulation to analyze the molecular mechanisms by which nanosized filler particles reinforce polymeric matrices. We show that the mobility of the nano-filler, rather than its surface area, is key to the performance of the nanocomposite and that this mobility is a complex function of the size of the filler, the attraction between the polymer and the filler and the thermodynamic state of the matrix.

[D9.010] ORDERED ORGANOSILICATE NANOCOMPOSITES TEMPLATED BY BLOCK COPOLYMERS

Nanostructured organosilicates with controlled morphologies have been of great interest for many potential applications, including microelectronics and photonics. One approach to create ordered nanostructures is to use block copolymers as templates by taking advantage of the microphase separation. Periodic hexagonal array of cylinders in organosilicates at a length scale of 10-20 nm are formed with film thickness approaching one millimeter. The self-assembly process of the block copolymers is monitored and studied by solid-state proton NMR, TEM and SAXS. It is found that the microstructure and phase behavior of the nanocomposites are very sensitive to the polymer-matrix interface and interaction parameter between different blocks. A layered structure is formed in the system we studied to minimize the interfacial energy. Understanding of the templating mechanism will provide a new route for non-lithographic nanopatterning and incorporating functionalities for photonic applications.

[D9.011] Metal Nanoparticle Arrays from Diblock Copolymer Templates

We use diblock copolymer thin film templates as masks for generating metal nanoparticle arrays composed of a wide variety of materials. The resulting nanoparticle diameters and nearest-neighbor separations can be controlled by varying polymer molecular weights, and in this experiment we present 20 nm diameter nanostructures on a 40 nm pitch. Our straightforward fabrication process consists of electron-beam deposition of metal into the template holes, followed by polymer dissolution to produce nanoparticle arrays over large wafer areas (up to 8 inch diameters). This procedure is analogous to the metal lift-off technique of standard photolithography, except the final structures are smaller than can be achieved with conventional means. We have demonstrated the flexibility of our process by producing metal nanoparticle arrays composed of Ti, Au, AuPd, and Co. We will present our current efforts aimed toward semiconductor and magnetic recording applications.

[D9.012] Block Copolymer Templates for Optical Materials and Devices

Block copolymers can act as super-lattices for creating novel optical structures. We have fabricated block copolymer photonic crystals from one, two and three dimensionally periodic systems and have enhanced their dielectric properties towards creating complete 3D band gaps. By using carefully selected blends of linear and star block copolymers, we are able to create hierarchical blends which exhibit precise molecular positioning of fluorescent molecules. We are exploring these unique patterning capabilities of block copolymer systems for the formation of ordered arrays of optically active components within a photonic crystal. Precise location of both fluorescent and nonlinear components within block copolymer photonic crystals affords new opportunities for creating low threshold, upconverting and array lasers as well as optical modulators and other photonic devices.

[D9.013] Effect of nanoscopic particles on microphase ordering of diblock copolymer/particle mixture

We investigate microphase ordering of asymmetric diblock copolymer and nanoscopic particles using numerical self-consistent field theory and a scaling theory based on the strong segregation theory. We find that the characteristic interaction between the blocks and the particle and the size of the particle play a critical role in the outcome of the equilibrium morphology. For example, the addition of a small volume fraction of relatively large non-selective particles, which are comparable in size to the block domains, can induce a phase transition to a new morphology that may differ from the structure expected for the pure diblock of specific composition. These findings provide guidelines for tailoring desired morphologies of diblock/particle mixtures.

[D9.014] Self-Consistent Field Theory for a Binary Hard Sphere/Diblock Copolymer Systems

We present a self-consistent field theory for a composite system of diblock copolymers and binary hard sphere filler particles. By choosing appropriate volume fractions of large and small spheres, it is found that the particles phase separate based on size differences within the diblock copolymer microstructure. This indicates that the spheres can be manipulated at the nanoscopic length scale through self-assembly to make structures such as functionally gradient materials. We also present morphologies for binary sphere systems in which the particles are of the same size but different chemical species. We demonstrate that again intricate structures can be achieved through nanoscopic self-assembly.

[D9.015] RIGID PORE STRUCTURE FROM HIGHLY SWOLLEN POLYMER GELS

We have discovered that a structure with open pores self-assembles naturally when crystallizing a highly swollen polymer gel. More importantly, the resulting pore structure affords a unique combination of desirable properties such as high void fraction, high purity surface, mechanical strength, and retention the shape of a polymer specimen while increasing its size (and lowering its average density). The new concept is demonstrated here with polyethylene, one of the least expensive commercial polymers. The pore sizes vary between 10nm and 10mm, depending on crystallization conditions. New results will be shown.

Part D of program listing