Montgomery T. Shaw

Electrorheology of filled silicone elastomers

Electrorheological (ER) silicone elastomers containing particles based on silica (SiO2) and titania minerals (BaTiO3, Ba2Ti9O20, and BaTiO3/CaZrO3) were prepared and characterized. An electrical field was applied to align the particles during the cure of the silicone prepolymer. For the silicone/silica compositions, a prominent Maxwell–Wagner dispersion in the dielectric response suggested that surface conductivity of the silica particles dominated the polarization. Alignment of the particles increased the overall dielectric permittivity as well as the magnitude of the Maxwell–Wagner dispersion. Their ER response exhibited a negative deviation from a quadratic dependence on field intensity at high fields, and was accompanied by nonlinear conductivity. A highly nonlinear enhancement of the ER effect with increasing particle concentration was observed. For the silicone/titania elastomers as a class, the ER response increased with the particle’s permittivity. In the case of the silicone/BaTiO3 elastomer, the...

Mechanical Properties of Blends of HDPE and Recycled Urea-Formaldehyde Resin

The mechanical properties of blends of high-density polyethylene (HDPE) with a recycled thermosetting filler, urea-formaldehyde grit (UFG), were evaluated in the range of 0 -23% of filler by volume. Ethylene-acrylic acid (EAA) copolymers and an ionomer based on EAA were evaluated as compatibilizers. The observed tensile mod- ulus of the ionomer-treated blends was raised to three times the modulus of virgin polyethylene, whereas the modulus of the untreated blends reached double that of polyethylene. The ionomer-treated blends also showed a higher tensile strength than the blends without filler treatment. The improvement in the properties was assigned to an increased interaction between the filler and the polymer matrix.

ENHANCED ELECTRORHEOLOGICAL FLUIDS USING ANISOTROPIC PARTICLES

The electrorheological (ER) phenomenon is widely attributed to the chaining of micron-sized polarizable particles when subjected to an external electric field. It has been hypothesized that the strength of the particle–particle interactions determines the rheological properties of ER fluids. On the basis of an electrostatic polarization model, we proposed that by controlling the geometry of the particles, the dielectric properties of ER fluid can be enhanced, resulting in increased strength of the particle–particle interactions. In this work, we have developed systems featuring anisotropic particles and conducted a systematic study of the role of particle geometry in the response of ER fluids. Our findings appear to be in agreement with the electrostatic polarization model.

The phase behaviour and gelation of a rod-like polymer in solution and implications for microcellular foam morphology

Abstract The phase separation and gelation of the rod-like macromolecule, poly(γ-benzyl- l -glutamate), were studied in an effort to understand the mechanism by which microcellular materials are made via thermally induced phase separation processes. Previous workers have studied similarly prepared materials from molecules which exist as random coils in solution. The microcellular materials were formed by lowering the solution temperature until phase separation and solvent freezing occurred. The solvent was removed by vacuum sublimation. An emphasis was placed on dilute isotropic solutions (

Actuating properties of soft gels with ordered iron particles: basis for a shear actuator

Soft electrorheological (ER) gels were prepared by arranging iron particles in silicone prepolymers within magnetic fields and crosslinking the obtained structure. The gap between the particles, and consequently the conductivity, was controlled by swelling the crosslinked material with additional silicone prepolymer. Iron-particle chains were made with as little as 0.5% particles and at angles from 0° to 60° to the normal of the sample surface. The ER, transducing and actuating properties of these materials were explored using rheometry and dielectrometry. In general, the dynamic shear moduli of the aligned ER gels increased quadratically with the electric field intensity. With only 1 vol% particles, the modulus doubled to an electrical field of around 2 kV mm−1. Transducer and actuator properties were noted. With particle chain alignment of 30°, shear stresses of around 150 Pa were measured with a special sandwich-type shear fixture. The influence of tilt angle on the shear stress was found to be qualitatively consistent with the calculation based on the point-dipole approximation.

Membranes and MEAs Based on Sulfonated Poly(ether ketone ketone) and Heteropolyacids for Polymer Electrolyte Fuel Cells

Organic sulfonated polyether ketone ketoneSPEKK membranes with different ion-exchange capacities IECs, and composite membranes prepared by the addition of 20 wt % phosphotungstic acid PTA to SPEKK were used to prepare membrane electrode assemblies MEAs. The proton conductivity of the membranes increased with increasing IEC of the SPEKK, and with the addition of PTA. The proton conductivity attained at 80°C and 75% relative humidity was 20 2m S/cm. The feasibility of using SPEKK in the cathode layer of the MEAs was investigated. The electrochemically active surface areas ECAs of the SPEKK-based cathodes were lower than that of the Nafion-based cathode and decreased further as the operating relative humidity was lowered. These observations were reflected in the single-cell polarization data, which indicated that the MEAs with the SPEKK-based electrodes were outperformed by their Nafion-based counterparts. Furthermore, a mismatch in SPEKK IEC between the membrane and cathode resulted in immiscibility at the interface. While the additive stability in the composite membrane was very good, the long-term stability of the membranes was poor when compared to perfluorosulfonic acid membranes such as Nafion, with failure occurring by scission along the gasket edges of the MEA after limited operation.

Sedimentation-resistant electrorheological fluids based on PVAL-coated microballoons

Sedimentation is often a problem in electrorheological (ER) fluids featuring solid particles suspended in a low-density hydrocarbon oil. This problem was addressed by synthesizing particles comprising silica microballoons coated with PVAL using a salt-induced coacervation process. The ER performance of the fluids based on these particles was equivalent to prototypical commercial fluids, both with respect to current leakage and shear stress under steady simple shear flow. For comparing diverse fluids as to these practical characteristics, a dimensionless ER effectiveness number, Er, was proposed: Er = σγ/EJ where σ is the shear stress, γ is the shear rate, E is the electric field, and is the current. The resulting uniform coatings were also found to impart a degree of resistance to breakage.

Fabrication and mechanical properties of PLLA/PCL/HA composites via a biomimetic, dip coating, and hot compression procedure.

Currently, the bone-repair biomaterials market is dominated by high modulus metals and their alloys. The problem of stress-shielding, which results from elastic modulus mismatch between these metallic materials and natural bone, has stimulated increasing research into the development of polymer-ceramic composite materials that can more closely match the modulus of bone. In this study, we prepared poly(l-lactic acid)/hydroxyapatite/poly(ε-caprolactone) (PLLA/HA/PCL) composites via a four-step process, which includes surface etching of the fiber, the deposition of the HA coating onto the PLLA fibers through immersion in simulated body fluid (SBF), PCL coating through a dip-coating process, and hot compression molding. The initial HA-coated PLLA fiber had a homogeneous and continuous coating with a gradient structure. The effects of HA: PCL ratio and molding temperature on flexural mechanical properties were studied and both were shown to be important to mechanical properties. Mechanical results showed that at low molding temperatures and up to an HA: PCL volume ratio of 1, the flexural strain decreased while the flexural modulus and strength increased. At higher mold temperatures with a lower viscosity of the PCL a HA: PCL ratio of 1.6 gave similar properties. The process successfully produced composites with flexural moduli near the lower range of bone. Such composites may have clinical use for load bearing bone fixation.

Self-reinforced composites of hydroxyapatite-coated PLLA fibers: fabrication and mechanical characterization.

Self-reinforced composites (SRCs) are materials where both the matrix and fiber-reinforcing phase are made up of the same polymer. Improved bonding can be achieved with self-reinforced composites compared to traditional dual-polymer, fiber-reinforced composites owing to the identical chemistry of the components in SRCs. Bonding between the fiber and matrix phase is an important factor in applications where mechanical stability is required, such as in the field of bone repair. In this study, we prepared bioabsorbable poly(L-lactic acid)/hydroxyapatite (PLLA/HA) self-reinforced composites via a three-step process that includes surface etching of the fiber, the deposition of the HA coating onto the PLLA fibers through immersion in simulated body fluid (SBF), and hot compaction molding. Although coated with a layer of HA, self-reinforced composites were successfully generated by hot compaction. The effects of compaction time (15 and 30 min), compaction temperature (140, 150, 155, 160, 165, and 170 °C), and HA wt% (0, 5, 10, and 15 wt%) on flexural mechanical properties were studied. Mechanical test results indicated that in unfilled (no HA) PLLA SRCs, compaction time and temperature increased the flexural modulus of the composites tested. Based on the results obtained for unfilled composites, a single compaction time and temperature condition of 15 min and 170 °C were selected to study the effect of HA loading on the composite mechanical properties. HA was successfully loaded onto the fibers at 0, 5, 10, and 15 wt% before hot compaction and was found to significantly increase flexural modulus (P=0.0001). Modulus values ranged from 8.3 GPa±0.5 (0 wt% HA) to 9.7 GPa±0.6 (15 wt% HA). Microscopy results suggest that the HA in these composites forms a nodular-like structure along the fibers, which allows polymer-polymer contact yet prevents longitudinal shear. The procedure used successfully generated composites with flexural moduli near the lower range of bone that may have a possible clinical use for load-bearing bone-fixation devices.

Viscosity Behavior of Liquid Crystals

Abstract The literature on the viscosity behavior of liquid crystals is reviewed with emphasis on the experimental results. These results are discussed in terms of the mesophase structure. Discussion of polymeric liquid crystals has been excluded since they have recently been reviewed.