Evangelos Manias

Polymer-Silicate Nanocomposites: Model Systems for Confined Polymers and Polymer Brushes

The static and dynamic properties of polymer-layered silicate nanocomposites are discussed, in the context of polymers in confined spaces and polymer brushes. A wide range of experimental techniques as applied to these systems are reviewed, and the salient results from these are compared with a mean field thermodynamic model and non-equilibrium molecular dynamics simulations.

MOLECULAR DYNAMICS SIMULATIONS OF ORGANICALLY MODIFIED LAYERED SILICATES

Molecular dynamics (MD) simulations are used to study the static and dynamic properties of 2:1 layered silicates ion exchanged with alkyl-ammonium surfactants. These systems are in the form of oligomeric alkanes grafted by cationic groups on atomically smooth crystalline layers 10 A thick and several microns wide. The organically modified layers self-assemble parallel to each other to form alternating, well-ordered organic/inorganic multilayers. By studying the systems at the experimentally measured layer separations, computer modeling directly provides the structure and dynamics of the intercalated surfactant molecules. The grafted-chain conformations are also expressed through the trans-gauche conformer ratios and transition frequencies which compare well with Fourier transform infrared spectroscopy (FTIR) experiments.

Effect of MgAl-layered double hydroxide exchanged with linear alkyl carboxylates on fire-retardancy of PMMA and PS

Alkyl carboxylate-modified layered double hydroxides (LDH) were prepared and used as nanofillers for poly(methyl methacrylate) (PMMA) and polystyrene (PS). The LDH intercalated with long-chain linear alkyl carboxylates (CH3(CH2)nCOO−, n = 8, 10, 12, 14, 16, 20) were prepared via anionic exchange of MgAl–nitrate, showing a systematic increase in basal spacing with longer alkyls. MgAl–undecenoate LDH was prepared by co-precipitation. The MgAl–LDHs were melt blended with poly(methyl methacrylate) and bulk polymerized with styrene to form nanocomposites. The dispersion of the MgAl–LDH in the polymers was investigated by transmission electron microscopy and X-ray diffraction. Thermal and fire properties were studied using cone calorimetry and thermogravimetric analysis; the thermal stability of both polymers was enhanced and a very significant reduction in the peak heat release rate was observed for almost all of the poly(methyl methacrylate) composites and a few of the polystyrene composites.

High field properties and energy storage in nanocomposite dielectrics of poly(vinylidene fluoride-hexafluoropropylene)

Poly(vinylidene fluoride) (PVDF) has generated interest for use in electrical energy storage, mostly due to its high dielectric constant compared to other polymers. There still exist challenges, such as its high energy losses, that have prevented large scale commercialization of PVDF-based capacitors, but progress is continuously being made. In this paper we explore a promising route to improve the energy storage performance of PVDF, through a synergy of HFP comonomers and of kaolinite clay nanofillers. This study shows that the addition of these high aspect ratio fillers to poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] copolymers does not increase the polar phase and, consequently, these composites exhibit markedly enhanced dielectric properties at high electric fields. Specifically, strained films of these composites exhibit reduced high field losses, markedly increased breakdown strength and, thus, large recoverable energy density values, in the range of 19 J/cm3.

Dynamical heterogeneity in nanoconfined poly(styrene) chains

Fluids in nanoscopic confinements possess a variety of unusual properties, and in particular, remarkable dynamical heterogeneities which vary on length scales as short as a fraction of a nanometer. While the surface forces apparatus provides an experimental probe of macroscopic properties of fluids in contact with atomically smooth solid surfaces, few experimental probes are available which test the microscopic origins of these heterogeneities. In this article we describe our recent efforts to apply nuclear magnetic resonance spectroscopy to nanoscopically confined poly(styrene) (PS) created by intercalation into a surface-modified fluorohectorite. A comparison between surface-sensitive cross polarization experiments with spin–echo experiments which probe the entire organic layer suggests that PS in the center of the nanopores is more mobile than the bulk at comparable temperatures, while chain segments which interact with the surface are dynamically inhibited.

Polyethylene nanocomposite dielectrics: Implications of nanofiller orientation on high field properties and energy storage

Nanocomposite formation, through the incorporation of high aspect ratio nanoparticles, has been proven to enhance the dielectric properties of thermoplastic polymers, when the mitigation of internal charges and the nature of the interfacial regions are properly adjusted. Here, we explore polyethylene/montmorillonite nanocomposites, and we specifically investigate how to impart desirable dielectric behavior through controlled nanoscale texturing, i.e., through control of the spatial arrangement of the high aspect ratio nanofiller platelets. In particular, it is shown that filler alignment can be used to improve the high electric-field breakdown strength and the recoverable energy density. The origins of the improved high field performance were traced to improved charge-trapping by a synergy of nanofillers and polar maleic anhydride (MAH) groups—introduced via polyethylene-MAH copolymers—as templated by the inorganic nanofillers. Further, it is conclusively demonstrated that the alignment of the two-dimensional nanoparticles has a measurable positive effect on the breakdown strength of the materials and, consequently, on the maximum recoverable energy density.

High-and low-field dielectric characteristics of dielectrophoretically aligned ceramic/polymer nanocomposites

Polymer/ceramic composites with controlled spatial distribution of fillers are synthesized, and the corresponding changes in their properties are studied. Using dielectrophoretic assembly, we create anisotropic composites of aligned BaTiO3 particles in silicone elastomer and study their electrical properties as a function of ceramic volume fraction and composite morphology. These structured composites show an increase in the permittivity compared to composites with the same composition and randomly dispersed (0–3) fillers. This study emphasizes the important role of conductivity, permittivity, and, particularly, local cluster distribution in controlling high-field dielectric behavior. Designed anisotropy in dielectric properties can provide unexampled paradigms for the development of high energy density materials and gain important insights into the mechanisms that control dielectric breakdown strengths and nonlinear conduction at high fields in polymer/ceramic composites.

Dynamics of poly(ethylene oxide) in nanoscale confinements: A computer simulations perspective

Molecular Dynamics simulations are used to explore the effect of severe –1 nanometer– confinement on the short-time dynamics of poly(ethylene oxide) (PEO). Bulk and intercalated systems have been atomistically modeled to comparatively illustrate their differences. In particular, we aim to trace the molecular level mechanisms responsible for the counter intuitive distribution of relaxation times recorded by solid state 2H NMR for the C–H bond reorientations in confinements. Computer simulations complement the experiments and reveal that factors such as local density inhomogeneities, proximity of Li+, and disorder in the system, combine to determine the PEO segmental dynamics. In contrast with the respective bulk PEO systems, where a clear transition from distinct solid to liquid like dynamics takes place with increasing temperatures, for the nanoscopically confined chains there persists a coexistence of fast and slow segmental relaxations over the same temperature range.

Epoxy-based nanocomposites for electrical energy storage. I: Effects of montmorillonite and barium titanate nanofillers

Polymer nanocomposites prepared by epoxy reinforced with high permittivity barium titanate (BT) fillers or high aspect ratio montmorillonite (MMT) fillers exhibited marked changes in their high electric field properties and their relaxation dynamics, depending on the nanoparticle type and concentration, the nanoparticle size, and the epoxy matrix conversion. We investigated epoxy resin composites based on organically modified montmorillonite (oMMT) or BT (BaTiO3) nanoparticles in order to delineate the effects of the high aspect ratio of the MMT and the high permittivity of the BT particles. We also explored the potential benefits of the synergy between the two fillers in systems consisting of epoxy and both oMMT and BT particles. It was observed that the nature of the organic–inorganic interfaces dominate the glass transition temperature and the dielectric properties of these composites. Specifically, using dielectric relaxation spectroscopy, we probed the local dynamics of the polymer at the interfaces....

State of Water in Perfluorosulfonic Ionomer (Nafion 117) Proton Exchange Membranes

The nature of water in acid-form Nafion 117 was quantified at several hydration levels by dielectric relaxation spectroscopy. Two independent experimental setups were used to collect complex dielectric permittivity spectra at low frequencies (0.01 Hz to 1 MHz at -80 to 25°C) and in the microwave region (0.40-26 GHz at 25-45°C). We directly observed the states of water, manifested through three population averages with distinctly resolved dynamical behaviors, and their changes with temperature and hydration level. The fastest process observed was identified as the cooperative picosecond relaxation of free (isotropic, bulklike) water, whereas the slowest process (microsecond relaxation times) corresponded to water molecules strongly bound to the charged sulfonic groups. A third type of water was also observed, also characterized by picosecond relaxation times, close to and about three times slower than those of bulk water, which was attributed to loosely bound water and may contain substantial dynamical heterogeneities.