Ethylene pressure has been used to control the competition between isomerization (chain walking) and monomer insertion processes for ethylene coordination polymerization catalyzed by a palladium-alpha-diimine catalyst. The topology of the polyethylene varies from linear with moderate branching to "hyperbranched" structures. Although the overall branching number and the distribution of short-chain branching change very slightly, the architecture or topology of the polyethylene changes from linear polyethylene with moderate branches at high ethylene pressures to a hyperbranched polyethylene at low pressures.

A group of polyethylenes synthesized using palladium R-diimine catalysts were studied using 13 C NMR spectroscopy, intensity light scattering, dynamic light scattering, and viscometry. These catalysts are known to produce branched polyethylenes without R-olefin comonomers. The series of polymers studied were synthesized under conditions of varying ethylene pressure. The polymers are highly branched and completely amorphous and are thus soluble in common organic solvents at ambient temperatures. Light scattering determinations of the root-mean-square radius of gyration (Rg) and the molecular weight M of fractions eluting from a size exclusion chromatograph demonstrated that, at a given M, Rg decreased as ethylene pressure decreased. The hydrodynamic parameterssthe Stokes radius (RH) from dynamic light scattering and the intrinsic viscosity ([η])salso decreased. The change in Rg at a constant M results from the change in branching topology for the polymers synthesized at different ethylene pressures. The parameter Rg 2 /M varies by an order of magnitude for the polymers synthesized under ethylene pressures varying from 0.1 atm to 500 psi. However, the total branching (methyls per 1000 CH2) and the distribution of short branches (methyl, ethyl, propyl, etc.) determined by 13 C NMR remained essentially unchanged. These observations indicate the branching topology changes with polymerization pressure. Polymer topology varies from predominantly linear with many short branches at higher ethylene pressures to a densely branched, arborescent globular structure at very low ethylene pressures. Polymers synthesized at the lowest ethylene pressure studied, 0.1 atm, exhibited dilute solution parameters similar to those observed for dendrimers or many-armed stars, with R g/RH below unity, and a segment density approaching that of a hard sphere.

Organic/inorganic core shell nanoparticles have been synthesized using high K TiO(2) as the core nanoparticle, and polystyrene as the shell. This material is easy to process and forms transparent continuous thin films, which exhibit a dielectric constant enhancement of over 3 times that of bulk polystyrene. This new dielectric material has been incorporated into capacitors and thin film transistors (TFTs). Mobilities approaching 0.2 cm(2)/V.s have been measured for pentacene TFTs incorporating the new TiO(2) polystyrene nanostructured gate dielectric, indicating good surface properties for pentacene film growth. This novel strategy for generating high K flexible gate dielectrics will be of value in improving organic and flexible electronic device performance.

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The thermodynamic and hydrodynamic properties of cyclic and linear polystyrenes, ranging from 10000 to 180 OOO molecular weight, in dilute solutions of cyclohexane have been measured by small-angle neutron scattering (SANS) and dynamic light scattering. The diffusion coefficient D(c) was measured at 6A2=0 as a function of concentration c. The hydrodynamic radii were determined and found to be slightly smaller for cyclic than for linear macromolecules. The slope kD, defined by D(c) = D(O)(l + kDC), was found to be negative and was, within experimental error, independent of molecular architecture. The molecular weight dependence of the radius of gyration, the hydrodynamic radius, and kD is discussed.

The realization of the full potential for polymeric nanocomposites to manifest their entitled property improvements relies, for some properties, on the ability to achieve maximum particle–matrix interfacial area. Well-dispersed nanocomposites incorporating colloidal silica as the filler can be realized in both polystyrene and poly(methyl methacrylate) matrices by exploiting the charge stabilized nature of silica in nonaqueous solvents which act as Bronsted bases. We demonstrate that dispersions of colloidal silica in dimethylformamide are charge stabilized, regardless of organosilyl surface functionalization. When formulated with polymer solutions, the charge stabilized structure is maintained during drying until the charged double layer collapses. Although particles are free to diffuse and cluster after this neutralization, increased matrix viscosity retards the kinetics. We demonstrate how high molecular weight polymers assist in immobilizing the structure of the silica to produce well-dispersed composites. The glass transition temperatures of these composites do not vary, even at loadings up to 50 vol %.

The unusual electronic and optical properties of many electroluminescent and conducting polymers arise from extended conjugation along the polymer backbone, which can also lead to insolubility, aggregation, and gelation. Synthetic efforts to produce an optimal structure require a balance between the persistence length and the effective conjugation length for the successful implementation of these materials in photonic and electronic devices. In this study, we have investigated the solution properties of a group of poly(phenyleneethynylenes) using a variety of light scattering techniques, including polarized and depolarized intensity measurements, dynamic light scattering, and size exclusion chromatography with a multiangle light scattering detector (SEC/LS). Interpretation of light scattering in the presence of absorption, fluorescence, and optical anisotropy is discussed. The molecular weights determined by light scattering encompassed the range from 10 × 105 to 5 × 106, with the root-mean-square radius of gyration as high as 250 nm. The results may be interpreted with a wormlike chain model to yield a persistence length of about 15 nm, so that these high-M polymers are coil-like in solution, rather than “rigid rods”. This persistence length is still expected to be several times larger than the effective conjugation length.

The extensional rheological properties of dilute polymer solutions play a dominant role in many commercial processes such as air-assisted atomization. This is a high deformation rate process important in application of diverse materials such as paints, fertilizer sprays and delivery of airborne drugs. Dilute polymeric solutions which have identical values of high shear-rate viscosity (HSV) often exhibit different values of Sauter Mean Diameter (SMD) in their spray size distributions as a result of differing extensional rheological properties. We explore the atomization of a series of model Poly(ethylene oxide) (PEO) solutions dissolved in water/glycerol mixtures. Each solution is sprayed with an air-assisted spray gun under similar conditions and imaged with a commercial spray measurement system. The values of HSV for PEO solutions are close to the solvent viscosity and matched to those of typical ink or paint samples. The surface tensions of the fluids are also tuned to be very similar, however both the SMD and the droplet size distribution change considerably. For the highest molecular weight PEO systems, interconnected beads-on-string structures are observed at different positions of the spray fan. Capillary Break-up Extensional Rheometry (CaBER) can be used to measure the extensional properties of the more viscous solutions, but the well-known limitations of this approach include inertially-induced asymmetries, gravitational sagging and the very short filament lifetimes of low viscosity samples all of which constrain the range of relaxation times that can be probed. Consequently we also explore the use of Rayleigh Ohnesorge Jet Elongational Rheometry (ROJER) to probe the extensional response of these viscoelastic solutions at realistic timescales and deformation rates. A cylindrical liquid jet is excited by a piezo-actuator at a known frequency as it exits a micromachined nozzle, and stroboscopic imaging provides high temporal and spatial resolution in the break-up process. Analyzing the evolution in the jet diameter before break-up enables meaningful measurement of relaxation times down to values as small as 60 microsecond, and these values can be directly correlated with the differences in the final spray size distributions and the mean diameters. We outline a simple model for the fluid dynamics of the thinning filaments close to breakup that accurately describes the variation of the average droplet diameter as a function of the elongational relaxation time measured for each fluid. in their spray size distributions as a result of di↵ering extensional rheological properties. Studying the E↵ects of Elongational Properties onWe explore the atomization of a series of model Poly (