Kent A. Watson

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.

Fabrication and Characterization of High Temperature Resin/Carbon Nanofiber Composites

Multifunctional composites present a route to structural weight reduction. Nanoparticles such as carbon nanofibers (CNF) provide a compromise as a lower cost nanosize reinforcement that yields a desirable combination of properties. Blends of PETI-330 and CNFs were prepared and characterized to investigate the potential of CNF composites as a high-performance structural medium. Dry mixing techniques were employed and the effect of CNF loading level on melt viscosity was determined. The resulting powders were characterized for degree of mixing, thermal and rheological properties. Based on the characterization results, samples containing 30 and 40 wt.% CNF were scaled up to approximately 300 g and used to fabricate moldings 10.2 cm ′ 15.2 cm ′ 0.32 cm thick. The moldings were fabricated by injecting the mixtures at 260-280°C into a stainless steel tool followed by curing for 1 h at 371°C. The tool was designed to impart high shear during the injection process in an attempt to achieve some alignment of CNFs in the flow direction. Moldings were obtained that were subsequently characterized for thermal, mechanical and electrical properties. The degree of dispersion and alignment of CNFs were investigated using high-resolution scanning electron microscopy. The preparation and preliminary characterization of PETI-330/CNF composites are discussed.

High Temperature VARTM of Phenylethynyl Terminated Imides

Depending on the part type and quantity, fabrication of composite structures using vacuum-assisted resin transfer molding (VARTM) can be more affordable than conventional autoclave techniques. Recent efforts have focused on adapting VARTM for the fabrication of high temperature composites. Due to their low melt viscosity and long melt stability, certain phenylethynyl terminated imides (PETI) can be processed into composites using high temperature VARTM (HT-VARTM). However, one of the disadvantages of the current HT-VARTM resin systems has been the high porosity of the resultant composites. For aerospace applications, the desired void fraction of less than 2% has not yet been achieved. In the current study, two PETI resins, LaRC PETI-330 and LaRC PETI-8 have been used to make test specimens using HT-VARTM. The resins were infused into ten layers of IM7-6K carbon fiber 5-harness satin fabric at 260 or 280 °C and cured at temperature up to 371 °C. Initial runs yielded composites with high void content, typically greater than 7% by weight. A thermogravimetric-mass spectroscopic study was conducted to determine the source of volatiles leading to high porosity. It was determined that under the thermal cycle used for laminate fabrication, the phenylethynyl endcap was undergoing degradation leading to volatile evolution. This finding was unexpected as high quality composite laminates have been fabricated under higher pressures using these resin systems. The amount of weight loss experienced during the thermal cycle was only about 1% by weight, but this led to a significant amount of volatiles in a closed system. By modifying the thermal cycle used in laminate fabrication, the void content was significantly reduced (typically ∼ 3% or less). The results of this work are presented herein.

Polymer and carbon nanotube composites for space applications

Publisher Summary This chapter deals with polymer and carbon nanotube (CNT) composite materials that are potentially useful for space applications because of their unique combination of electrical, thermal, and mechanical properties. Much of the research has focused on using carbon nanotubes (CNTs) as electrically conductive additives for polymers to mitigate electrostatic charged build-up. This addresses a problem that is particularly relevant to large, deployable spacecraft composed primarily of polymer materials such as Gossamer structures. These spacecraft are constructed of compliant polymeric materials that can be folded into compact volumes for launch. Transparent, conductive coatings are needed, which can tolerate folding and subsequent deployment. Other concepts for polymer and CNT composites include lightweight, radiation shielding, high thermal conductivity matrices and coatings, and structural matrix systems. Space environment survivability is an important factor in the development of nano-composite materials. Polymer nanocomposites will suffer from environmentally induced degradation because of particulate radiation, atomic oxygen, ultraviolet radiation, thermal cling, micrometeoroid impacts, and synergistic effects because of combinations. Space environment, the effects of the space environment on polymer nanocomposites, and potential applications of polymer nanocomposites are also discussed in the chapter.

Preparation and Properties of Nanocomposites from Pristine and Modified SWCNTs of Comparable Average Aspect Ratios

Low color, flexible, space-durable polyimide films with inherent and robust electrical conductivity to dissipate electrostatic charge (ESC) have been under investigation as part of a materials development activity for future NASA space missions. The use of single-walled carbon nanotubes (SWCNTs) is one means of achieving this goal. Even though the concentration of SWCNTs needed to achieve ESC dissipation is typically low, it is dependent upon purity, size, dispersion and functionalization. In this study, SWCNTs prepared by the electric arc discharge method were used to synthesize nanocomposites using the LaRCTM CP2 backbone as the matrix. Pristine and functionalized SWCNTs were mixed with an alkoxysilane terminated amide acid of LaRC TM CP2 and the soluble imide form of the polymer and the resultant nanocomposites evaluated for mechanical, thermal and electrical properties. Due to the preparative conditions for the pristine and functionalized SWCNTs, the average aspect ratio for both was comparable. This permitted the assessment of SWCNT functionalization with respect to various interactions (e.g. van der Waals, hydrogen bonding, covalent bond formation, etc.) with the matrix and the macroscopic effects upon nanocomposite properties. The results of the study are described.

Thermal Conductivity of Ultem™/Carbon Nanofiller Blends

In an effort to improve polymer thermal conductivity (TC), Ultem™ 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 Ultem™ 1000 in the melt and in solution 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 nanofillers was determined by high-resolution scanning electron microscopy (HRSEM). 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 Nanoflash™ 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.

Thermal Conductivity of Polyimide/Carbon Nanofiller Blends

In efforts to improve the thermal conductivity (TC) of Ultem™ 1000, it was compounded with three carbon based nano-fillers. Multiwalled carbon nanotubes (MWCNT), vapor grown carbon nanofibers (CNF) and expanded graphite (EG) were investigated. Ribbons were extruded to form samples in which the nano-fillers were aligned. Samples were fabricated by compression molding where the nano-fillers were randomly oriented. The thermal properties were evaluated by DSC and TGA, and the mechanical properties of the aligned samples were determined by tensile testing. The degree of dispersion and alignment of the nanoparticles were investigated with high-resolution scanning electron microscopy. The thermal conductivity was measured in two directions using the Nanoflash technique.

Effect of LEO Exposure on Aromatic Polymers Containing Phenylphosphine Oxide Groups

As part of the Materials on The International Space Station Experiment (MISSE), aromatic polymers containing phenylphosphine oxide groups were exposed to low Earth orbit for ∼4 years. All of the aromatic polymers containing phenylphosphine oxide groups survived the exposure despite the high fluence of atomic oxygen that completely eroded other polymer films such as Kapton® and Mylar® of comparable or greater thickness. The samples were characterized for changes in physical properties, thermal/optical properties surface chemistry, and surface topography. The data from the polymer samples on MISSE were compared to samples from the same batch of material stored under ambient conditions on Earth. In addition, comparisons were made between the MISSE samples and those subjected to shorter term space flight exposures. The results of these analyses will be presented.

Fabrication and Characterization of High Temperature Resin/Carbon Nanofiller Composites

As part of ongoing efforts to develop multifunctional advanced composites, blends of PETI-330 with multi-walled carbon nanotubes (MWCNTs) and carbon nanofibers (CNF) were prepared and characterized. The effect of nanofiller loading level on melt viscosity was determined. The resulting powders were characterized for degree of mixing, thermal, and rheological properties. Select samples were scaled up for processing and continuous strands of nanocomposites were extruded. Based on the characterization results, samples containing 10 and 15 wt% MWCNT and 30 and 40 wt% CNF were scaled up to ∼300 g and used to fabricate moldings 10.2 cm × 15.2 cm × 0.32 cm in size. The moldings were fabricated by injecting the mixtures at 260–280 °C into a stainless steel tool followed by curing for 1 h at 371 °C. The tool was designed to impart substantial shear during the injection process in an attempt to achieve some alignment of nanofillers in the flow direction. Moldings were obtained that were subsequently characterized for thermal, mechanical, electrical and EMI shielding properties. The degree of dispersion and alignment of nanofillers were investigated using high-resolution scanning electron microscopy. Preparation and preliminary characterization of PETI-330/MWCNT and PETI-330/CNF composites will be discussed.