Joseph G. Smith

Ionic polymer-metal composites (IPMCs) as biomimetic sensors, actuators and artificial muscles - a review

This paper presents an introduction to ionic polymer-metal composites and some mathematical modeling pertaining to them. It further discusses a number of recent findings in connection with ion-exchange polymer-metal composites (IPMCs) as biomimetic sensors and actuators. Strips of these composites can undergo large bending and flapping displacement if an electric field is imposed across their thickness. Thus, in this sense they are large motion actuators. Conversely by bending the composite strip, either quasi-statically or dynamically, a voltage is produced across the thickness of the strip. Thus, they are also large motion sensors. The output voltage can be calibrated for a standard size sensor and correlated to the applied loads or stresses. They can be manufactured and cut in any size and shape. In this paper first the sensing capability of these materials is reported. The preliminary results show the existence of a linear relationship between the output voltage and the imposed displacement for almost all cases. Furthermore, the ability of these IPMCs as large motion actuators and robotic manipulators is presented. Several muscle configurations are constructed to demonstrate the capabilities of these IPMC actuators. This paper further identifies key parameters involving the vibrational and resonance characteristics of sensors and actuators made with IPMCs. When the applied signal frequency varies, so does the displacement up to a critical frequency called the resonant frequency where maximum deformation is observed, beyond which the actuator response is diminished. A data acquisition system was used to measure the parameters involved and record the results in real time basis. Also the load characterizations of the IPMCs were measured and it was shown that these actuators exhibit good force to weight characteristics in the presence of low applied voltages. Finally reported are the cryogenic properties of these muscles for potential utilization in an outer space environment of a few Torrs and temperatures of the order of -140 degrees Celsius. These muscles are shown to work quite well in such harsh cryogenic environments and thus present a great potential as sensors and actuators that can operate at cryogenic temperatures.

Dispersion of single wall carbon nanotubes by in situ polymerization under sonication

Single wall nanotube reinforced polyimide nanocomposites were synthesized by in situ polymerization of monomers of interest in the presence of sonication. This process enabled uniform dispersion of single wall carbon nanotube (SWNT) bundles in the polymer matrix. The resultant SWNT-polyimide nanocomposite films were electrically conductive (antistatic) and optically transparent with significant conductivity enhancement (10 orders of magnitude) at a very low loading (0.1 vol%). Mechanical properties as well as thermal stability were also improved with the incorporation of the SWNT.

Preparation and Characterization of Polyimide/Organoclay Nanocomposites

Abstract Organically modified montmorrillonite clay, containing a long chain aliphatic quarternary ammonium cation, was used to prepare polyimide/organoclay hybrids. Several approaches were examined in an attempt to achieve fully exfoliated nanocomposites. These included simple mixing of the clay in a pre-made high molecular weight poly(amide acid) solution; simple mixing followed by sonication of the organoclay/poly(amide acid) solutions; and the preparation of high molecular weight poly(amide acid)s in the presence of the organoclay dispersed in N-methyl-2-pyrrolidinone (NMP). The best results were obtained using the in-situ polymerization approach. The resulting nanocomposite films (both amide acid and imide), containing 3–8% by weight of organoclay, were characterized by differential scanning calorimetry (DSC), dynamic thermogravimetric analysis (TGA), transmission electron microscopy (TEM), X-ray diffraction (XRD) and thin film tensile properties. A significant degree of dispersion was observed in the nanocomposite films of the amide acid and the imide. After thermal treatment of amide acid films to effect imidization, in both air and nitrogen, the films were visually darker than control films without clay and the level of clay dispersion appeared to have decreased. In the latter case, the separation between the layers of the clay decreased to a spacing less than that present in the original organoclay. These observations suggest that thermal degradation of the aliphatic quarternary ammonium cation occurred likely during thermal treatment to effect imidization and solvent removal. These thermal degradation effects were less pronounced when thermal treatment was performed under nitrogen. The polyimide/organoclay hybrid films exhibited higher room temperature tensile moduli and lower strength and elongation to break than the control films.

Rapid, solventless, bulk preparation of metal nanoparticle-decorated carbon nanotubes.

A rapid, solventless method is described for the decoration of carbon nanotubes with metal nanoparticles. The straightforward two-step process utilizes neither reducing agents nor electric current and involves the dry mixing of a precursor metal salt (e.g., a metal acetate) with carbon nanotubes (single- or multi-walled) followed by heating in an inert atmosphere. The procedure is scalable to multigram quantities and generally applicable to various other carbon substrates (e.g., carbon nanofiber, expanded graphite, and carbon black) and many metal salts (e.g., Ag, Au, Co, Ni, and Pd acetates). As a model system, Ag nanoparticle-decorated carbon nanotube samples were prepared under various mixing techniques, metal loading levels, thermal treatment temperatures, and nanotube oxidative acid treatments. These nanohybrids were characterized by a variety of microscopic and spectroscopic techniques. For example, X-ray diffraction and scanning electron microscopy indicated that the average size of the Ag nanoparticles has little to do with the thermal treatment temperature but can be easily controlled by varying the Ag loading. Raman spectroscopy illustrated both the metal-nanotube electronic interactions and the surface enhancement effect from the Ag nanoparticle attachment. High-resolution transmission electron microscopy captured the in situ salt-to-metal conversion events on the nanotube surface. The mechanistic implications from the characterization results are discussed.

Polyimides from 2,3,3',4'-biphenyltetracarboxylic dianhydride and aromatic diamines

The present invention relates generally to polyimides. It relates particularly to novel polyimides prepared from 2,3,3′,4′-biphenyltetracarboxylic dianhydride and aromatic diamines. These novel polyimides have low color, good solubility, high thermal emissivity, low solar absorptivity and high tensile strength.

Chemistry and properties of imide oligomers end-capped with phenylethynylphthalic anhydrides

Abstract A series of phenylethynyl-terminated imide oligomers were prepared by the reaction of aromatic dianhydride(s) with a stoichiometric excess of aromatic diamine(s) at calculated number average molecular weights of 1500–9000 g mol −1 and end-capped with phenylethynylphthalic anhydrides in N -methyl-2-pyrrolidinone. Unoriented thin films cured in flowing air to 350°C exhibited tensile strengths and moduli of 105.5–139.3 MPa and 2.8–3.2 GPa at 23°C, respectively, with good retention of properties at 177°C. Stressed film specimens exhibited excellent resistance to a variety of solvents after a 2 week exposure period at ambient temperature. One phenylethynyl-terminated imide oligomer was selected for more extensive evaluation and gave high fracture toughness, adhesive and composite properties. The chemistry, physical and mechanical properties of these materials are discussed.

Oligomers and Polymers Containing Phenylethynyl Groups

2. DISCUSSION....................................................................................................... 209 2.1. Polymers Containing Pendant Phenylethynyl Groups............................... 209 2.2. Oligomers Terminated with Phenylethynyl Groups .................................. 214 2.3. Oligomers Terminated with Multiple Phenylethynyl Groups ................... 223 2.4. Oligomers Containing Pendant Phenylethynyl Groups ............................. 224 2.5. Curing of Phenylethynyl Groups ............................................................... 226

High Temperature Transfer Molding Resins: Laminate Properties of PETI-298 and PETI-330

Two phenylethynyl terminated oligomers designated PETI-298 and PETI-330 were developed at the NASA Langley Research Center and have emerged as leading candidates for composite applications requiring high temperature performance (i.e. ≥ 288 °C for 1000 hours) combined with the ability to be readily processed into composites without the use of an autoclave or complex or lengthy cure or postcure cycles. These high performance/high temperature composites are potentially useful on advanced aerospace vehicles in structural applications and as aircraft engine components such as inlet frames and compressor vanes. The number designation (i.e. 298, 330) refers to the glass transition temperature in degrees Centigrade as determined on neat resin cured for 1 hour at 371 °C. The resins are processable by non-autoclave techniques such as resin transfer molding (RTM), vacuum assisted RTM (VARTM) and resin infusion (RI). Both resins exhibit low complex melt viscosities (0.1-10 poise) at 280 °C and are stable for ≥ 2 hours at this temperature. Typically, the resins are melted, de-gassed and infused or injected at 280 °C and subsequently cured at 371 °C for 1-2 hours. Virtually no volatiles are evolved during the cure process. The resin synthesis is straightforward and has been scaled-up to 25 kg batches. The chemistry of PETI-298 and PETI-330 and the RTM AS-4 and T-650 carbon fabric laminate properties, and those of BMI-5270 for comparison, are presented.

Oxygen plasma-resistant phenylphosphine oxide-containing polyimides and poly(arylene ether heterocycle)s: 2

Phenylphosphine oxide-containing poly(arylene ether imide)s, poly(arylene ether quinoxaline)s, poly(arylene ether benzoxazole)s and poly(arylene ether benzothiazole)s were prepared by reacting the appropriate difluoro heterocyclic compound with bis(4-hydroxyphenoxy-4′-phenyl)phenylphosphine oxide. The polymers exhibited glass transition temperatures from 209 to 255°C and inherent viscosities from 0.35 to 1.34 dl g−1. Thin-film tensile properties measured at room temperature and 177°C exhibited tensile strengths of 10.2–15.8 and 6.0–9.0 ksi (∼ 70.3–108.9 and ∼ 41.4–62.1 MPa), respectively, and tensile moduli of 340–381 and 204–365 ksi (∼ 2.34–2.63 and ∼ 1.41–2.52 GPa), respectively. Unoriented thin films of these phenylphosphine oxide-containing polymers were subsequently exposed to a radiofrequency-generated oxygen plasma under vacuum along with Kapton® HN. To assess the resistance of the materials to the oxygen plasma, the weight losses of the films were monitored as a function of exposure time. Phenylphosphine oxide-containing poly(arylene ether benzoxazole)s and poly(arylene ether benzothiazole)s exhibited weight-loss rates that were 38–190 (1–2 orders of magnitude) times slower than that of Kapton® HN. Phenylphosphine oxide-containing poly(arylene ether quinoxaline)s exhibited weight-loss rates only slightly slower (1–7 times) than those of Kapton® HN. The changes in surface chemistry of the exposed films were subsequently examined using X-ray photoelectron spectroscopy. In most cases, the phosphorus and oxygen near the surface exhibited increases in relative concentration and the photopeaks shifted towards higher binding energies. These changes are indicative of the formation of phosphate-type species. In addition, their limiting oxygen indices were calculated from char yields at 850°C in nitrogen utilizing a reported method. For the most part, the incorporation of phenylphosphine oxide groups did not substantially increase the limiting oxygen indices.

Thermal Conductivity of UltemTM/Carbon Nanofiller Blends

In an effort to improve polymer thermal conductivity (TC), UltemTM 1000 was compounded with nano-fillers of carbon allotropes. As-received and modified multiwalled carbon nanotubes (MWCNTs), vapor-grown carbon nanofibers (CNF) and expanded graphite (EG) were investigated. Functionalization of MWCNTs was performed to improve the TC compatibility between the resin and MWCNTs. It was postulated that this may provide an improved interface between the MWCNT and the polymer which would result in enhanced TC. The nano-fillers were mixed with UltemTM 1000 inthemeltandinsolution at concentrations ranging from 5 to 40 wt.%. Ribbons were extruded from the blends to form samples where the nano-fillers were aligned to some degree in the extrusion direction. Samples were also fabricated by compression molding resulting in random orientation of the nano-fillers. Thermal properties of the samples were evaluated by differential scanning calorimetry (DSC) and thermal gravimetric analyzer (TGA). Tensile properties of aligned samples were determined at room temperature. As expected, increased filler loading led to increased modulus and decreased elongation with respect to the neat polymer. The degree of dispersion and alignment of the nano-fillers was determined by high-resolution scanning electron microscopy (HRSEM). The HRSEM of the ribbons revealed that the MWCNTs and CNFs were predominantly aligned in the flow direction. The TC of the samples was measured using a NanoflashTM instrument. Since the MWCNTs and CNF are anisotropic, the TC was expected to be different in the longitudinal (parallel to the nanotube and fiber axis) and transverse (perpendicular to the nanotube and fiber axis) directions. The largest TC improvement was achieved for aligned samples when the measurement was performed in the direction of MWCNT and CNF alignment (i.e. longitudinal axis). Unaligned samples also showed a significant improvement in TC and may be potentially useful in applications when it is not possible to align the nano-filler. The results of this study will be presented.