Polyimide Aerogels as Lightweight Dielectric Insulators for Carbon

Jun 4, 2019 - The specimens were cut and polished to make sure that the top and bottom .... The resulting aerogel was further vacuum-dried at 75 °C o...
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Cite This: ACS Appl. Polym. Mater. 2019, 1, 1680−1688

Polyimide Aerogels as Lightweight Dielectric Insulators for Carbon Nanotube Cables Haiquan Guo,*,† Oliver S. Dewey,‡ Linda S. McCorkle,† Mary Ann B. Meador,§ and Matteo Pasquali‡ †

Ohio Aerospace Institute, 22800 Cedar Point Road, Cleveland, Ohio 44142, United States Rice University, Houston, Texas 77005, United States § NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135, United States Downloaded via UNIV OF SOUTHERN INDIANA on July 27, 2019 at 03:09:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Replacing copper as the central conductor with carbon nanotube (CNT) yarn has potential for greatly reducing the weight of data and power cables for any number of aerospace and automotive applications. Polyimide aerogels, with lightweight and low dielectric constants approaching one, have promise to even further reduce the weight of the cable as the dielectric insulation layer. In this paper, polyimide aerogels containing 4,4′-bis(4aminophenoxy)biphenyl (BAPB) in the backbone and two different cross-linkers (1,3,5-triaminophenoxybenzene, TAB, or poly(isobutylene-alt-maleic anhydride), PMA-D) were examined as potential coatings on CNT yarns. The effects of polyimide concentration and different solvent combinations on the quality of the coating were also examined. The best quality coatings were obtained with aerogels made by using PMA-D as the cross-linker, high polymer concentration, and a mixture of solvents. This work demonstrates the feasibility of using polyimide aerogels as the dielectric layer for lightweight CNT data and power cables. More improvements need to be made in the future, including scale up of the coating process, coating quality, and durability. KEYWORDS: lightweight, dielectric insulator, cables, CNT yarns, polyimide aerogel, BAPB

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electrical conductivity.6,7 The solution spinning method is scalable which, in conjunction with improved mechanical and electrical properties, is making commercialization of this technology feasible.8 Likewise, direct spinning of CNT fibers from a CNT growth furnace and densified produces similar properties.9 Using CNT yarns to replace traditional copper wires could decrease the weight of the cables by as much as 70%.5,6 CNT solutions in superacid have recently been used to replace the outer conductor in coaxial cables, reducing the weight of those cables by 97% of the outer conductor mass.10 This combination of scalability, properties, and demonstrated weight savings makes a CNT inner conductor of significant interest. To best preserve the properties of a CNT conductor, a lightweight and flexible dielectric insulator must be identified. The insulation layers for CNT yarns should provide good electrical insulation and be mechanically strong and moisture resistant. Polymers have been previously demonstrated as insulation layers for CNT yarns. For example, Alvarez et al. used Zetpol 2000 hydrogenated nitrile butadiene rubber

INTRODUCTION Much research has been dedicated over the years to reducing the weight of aerospace launch vehicles, aircraft, and automobiles. It may be surprising that so much of a vehicle mass is in the data and power cables. For example, the Space Shuttle had over 230 miles of data and power cables.1 For future launch vehicles, such as the Orion, the avionics mass accounts for half of the vehicle mass, and over half of that mass is associated with the wires. The Boeing 777 has ∼4000 pounds of copper data and power cables.2 In a midsize car, there are about 55 pounds of copper wires, which is the third heaviest component in a car, right behind the chassis and engines,3,4 and this proportion is increasing due to the transition to autonomous vehicles and electric or hybrid powertrains. Thus, a reduction in wire mass will have a large return on investment for reducing mass for launch vehicles, aircraft, and automobiles.5 Carbon nanotube (CNT) yarns or fibers are promising materials to replace the traditional copper wires used for the data and power cables because they are lightweight and have good mechanical strength and electrical conductivity. Specifically, CNT fibers produced using a solution spinning method have been shown to have very high mechanical strength and © 2019 American Chemical Society

Received: March 13, 2019 Accepted: June 4, 2019 Published: June 4, 2019 1680

DOI: 10.1021/acsapm.9b00241 ACS Appl. Polym. Mater. 2019, 1, 1680−1688

Article

ACS Applied Polymer Materials

Scheme 1. Chemical Structures of the Precursors and the Synthesis of Polyimide 3D Network Made with (a) TAB CrossLinker and (b) PMA-D Cross-Linker

(HNBR) as the insulation layer on CNT yarns by dip coating.11 Recently, polyimide aerogels with vastly improved mechanical properties over other aerogels12,13 have been

reported, and their use was demonstrated as low dielectric substrates in lightweight antennas.14 Because of their low dielectric constants and ability to form flexible films, these 1681

DOI: 10.1021/acsapm.9b00241 ACS Appl. Polym. Mater. 2019, 1, 1680−1688

Article

ACS Applied Polymer Materials

Mechanical Characterization. The specimens were cut and polished to make sure that the top and bottom surfaces were smooth and parallel. Samples were conditioned at room temperature for 48 h prior to testing. The diameter and length of the specimens were measured before testing. The specimens were tested with sample sizes close to the 1:1.25 ratio of diameter to length. The specimens were marked with several black dots vertically and horizontally with a black sharpie. The distances between the black dots will change during the compression test and indicate the strain change. The samples were tested between a pair of compression plates with an Instru-Met load frame, 200 lbf load cell, by using an Aramis GOM 3D optical measuring system and MTS TestWorks 4.0. All testings were performed at nominal room temperature conditions and at a crosshead speed of 0.05 in/min as dictated by the ASTM guidelines. The aerogels were crushed to 80% strain or the full capacity of the load cell (whichever occurred first). The Young’s modulus was taken as the initial linear portion of the slope of the Aramis stress−strain curve. Rheological Characterization. Polyimide sols were characterized by using shear and oscillatory rheology in an ARES G2 rheometer. To minimize the effects of solvent evaporation, a Couette concentric cylinder geometry with a Deutsches Institut für Normung (DIN) bob was used and covered with a cap. Polyimide sols were tested in this geometry. The samples were prepared by using the methods described below and pulled into a plastic syringe 2.5 min after addition of the TEA. This allowed time for the sol to homogenize under stirring. The syringe was used to inject the sols into the bottom of the geometry cup, and the DIN bob was lowered to the operating gap. Immediately after loading, a cover was placed on the cup to inhibit solvent evaporation. At this time, the rheological testing was started. For all sols tested, a strain amplitude sweep was performed to determine the linear viscoelastic (LVE) range. Osciallatory tests were performed within the LVE range to ensure that the forming structure of the sol was not altered by the test itself. Steady shear tests were performed at several strain rates to measure sol viscosity. For gelling sols, Fourier transform mechanical spectroscopy (FTMS) was used to evaluate the sol properties at several oscillation frequencies simultaneously.21 A combined oscillation of 20, 40, and 60 rad/s was used. Gelation times were determined by identifying the crossover points between the storage (G′) and loss (G′′) moduli at each frequency tested for a given sample. The gel point was taken to be the average of the three crossover points, one from each of the tested frequencies. The G′/G′′ crossover closely corresponds with the true gelation point, which occurs when the tan δ (tan δ = G′′/G′) is frequency independent.22 For experimental reasons, the point at which tan δ crosses over could not be determined for lower viscosity sols, as the high oscillation amplitudes required to measure the response clearly would exceed torque limits on the rheometer during gelation. Water Uptake Experiment. The samples were weighed and forced to be fully immersed into water in a closed jar for 24 h. Then the samples were taken out of the water, the surface water of the aerogels was wiped off, and the samples were weighed again. The water uptake of the aerogel was calculated via the equation w − w0 water uptake% = × 100% w0 (2)

aerogels can potentially provide an insulation layer that would be lighter than typical polymers used in conventional cables. Previous studies of cross-linked polyimide aerogels have mainly focused on cross-linkers with amine groups, including octa(aminophenyl)silsesquioxane (OAPS)15 or 1,3,5-triaminophenoxybenzene (TAB),1−3 as well as 2,4,6-tris(4-aminophenyl)pyridine (TAPP)16 or 1,3,5-tris(aminophenyl)benzene (TAPB)17 to cross-link anhydride end-capped oligomers. However, all of those cross-linkers are either expensive or not commercially available. Recently, we reported aerogels made using commercially available cross-linkers, such as 1,3,5benzenetricarbonyl trichloride (BTC),18 triisocyanates,19 and poly(maleic anhydride) (PMA)20 to cross-link amine-terminated polyimide oligomers. Mechanical properties of the resulting polyimide aerogels were similar to TAB and OAPS cross-linked aerogels and depend on the backbone chemistry of the oligomers. The main goal of this work is to develop lightweight polyimide aerogel dielectric insulation and methods for applying the aerogel to CNT yarns with the potential to further reduce the weight of data and power cables. A statistical experimental design was performed to explore two crosslinkers (poly(isobutylene-alt-maleic anhydride), PMA-D and TAB) at different polyimide concentrations (8−12% w/w). The cross-linkers were chosen because they are known to produce aerogels with higher moisture resistance, with the right combination of diamine and dianhydride. Moreover, different solvent combinations (0−25% w/w tetrahydrofuran (THF) in N-methylpyrrolidone (NMP)) were studied to evaluate whether lower boiling THF in combination with NMP could improve fiber wetting, sol viscosity, and pot life for coating the CNT yarns. THF should also evaporate during the coating process and thereby may decrease gel time. As shown in Scheme 1, polyimide oligomers were made from biphenyl3,3′,4,4′-tetracarboxylic dianydride (BPDA) as dianhydride and 4,4′-bis(4-aminophenoxy)biphenyl (BAPB) as diamine and cross-linked by using either TAB or PMA-D. The effects of cross-linker, polymer concentration, and solvent mixture used to fabricate the aerogels will be discussed.

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EXPERIMENTAL SECTION

Materials. BPDA was purchased from UBE America Inc. (Livonia, MI). BAPB was purchased from OmniSpecialty Corporation (Teaneck, NJ). Poly(isobutylene-alt-maleic anhydride) (PMA-D), HPLC grade N-methyl-2-pyrrolidone (NMP), anhydrous acetic anhydride, and triethyleneamine (TEA) were purchased from Millipore Sigma (St. Louis, MO). 1,3,5-Triaminophenoxybenzene (TAB) was custom-made at Triton Systems (Chelmsford, MA). All reagents were used without further purification. BPDA was dried at 125 °C in a vacuum for 24 h before use. CNT yarns were obtained from Nanocomp Technologies, Inc. (Merrimack, NH). General. Attenuated total reflectance (ATR) infrared spectroscopy was obtained using a Nicolet Nexus 470 FT-IR spectrometer. Scanning electron micrographs were obtained using a Hitachi S-4700 field emission scanning microscope after sputter coating the samples with gold. The samples were outgassed at 80 °C for 8 h under vacuum before running nitrogen adsorption porosimetry with an ASAP 2000 surface area/pore distribution analyzer (Micromeritics Instrument Corp.). The skeletal density was measured by using a Micromeritics Accupyc 1340 helium pycnometer. Using bulk density (ρb) and skeletal density (ρs) measured by helium pycnometry, we calculated the percent porosity via eq 1:

porosity = (1 − ρb/ ρs ) × 100%

where w is the weight of the aerogel samples after absorbing water and w0 is the original weight of the aerogel samples before soaking. Preparation of Aerogel Monoliths. PMA-D cross-linked poly(amic acid) was formulated in NMP by using a molar ratio of diamine to dianhydride of (n + 1) to n. TAB cross-linked poly(amic acid) oligomer was formulated in NMP by using a molar ratio of dianhydride to diamine of (n + 1) to n. BAPB was used as diamine and BPDA was used as dianhydride to form the polyimide oligomer backbone. In this paper, n is 25. Preparation of PMA-D Cross-Linked Aerogel Monoliths. A sample procedure for an oligomer (n = 25) made with BAPB with total polymer 12 and 25% w/w THF is as follows: To a stirred solution of BAPB (2.6435 g, 7.176 mmol) in 22.08 mL of NMP and

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DOI: 10.1021/acsapm.9b00241 ACS Appl. Polym. Mater. 2019, 1, 1680−1688

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9.28 mL of THF was added BPDA (2.0300 g, 6.900 mmol). The mixture was stirred until all BPDA was dissolved, and a solution of PMA-D (0.0856 g, 4 mmol) in 2.080 mL of NMP was added. The resulting sol was stirred for 15 min, after which acetic anhydride (5.416 mL, 57.40 mmol) and then TEA (1.000 mL, 7.174 mmol) were added. The sol was continuously stirred for 10 min and then poured into a 20 mL syringe mold (2 cm in diameter), prepared by cutting off the needle end of the syringe and extending the plunger all the way out. The sol gelled within 40 min. The gels were then aged in the mold for 1 day before being extracted into 75% v/v NMP in acetone solution to soak for 24 h to remove acetic acid and TEA. The solvent within the gels was then gradually exchanged to acetone in 24 h intervals starting with 75% v/v NMP in acetone, followed by 25% v/ v NMP in acetone and finally three more times with 100% v/v acetone. The gels were placed in a supercritical fluid extraction chamber in acetone and washed with liquid CO2; then CO2 was converted into a supercritical state, and gaseous CO2 was slowly vented out. The resulting aerogel was further vacuum-dried at 75 °C overnight. The dry aerogel made with PMA-D cross-linker and BAPB produced in this way has a density of 0.179 g/cm3. Preparation of TAB Cross-Linked Polyimide Aerogel Monoliths. A sample procedure for a TAB cross-linked aerogel (n = 25) made with BAPB with total polymer 12 and 8% w/w THF is as follows: To a stirred solution of BAPB (2.5603 g, 6.950 mmol) in 27 mL of NMP and 2.97 mL of THF was added BPDA (2.1266 g, 7.228 mmol). The mixture was stirred until all BPDA was dissolved, and a solution of TAB (0.074 g, 0.1852 mmol) in 2.533 mL of NMP was added. The resulting sol was stirred for 15 min, after which acetic anhydride (5.456 mL, 57.82 mmol) and then TEA (1.007 mL, 7.225 mmol) were added. The sol was continuously stirred for 10 min and then poured into a 20 mL syringe mold (2 cm in diameter), prepared by cutting off the needle end of the syringe and extending the plunger all the way out. The gels which formed within 30 min were aged in the mold for 1 day before extracting into 75% v/v NMP in acetone solution to soak for 24 h to remove acetic acid and TEA. The solvent within the gels was then gradually exchanged to acetone in 24 h intervals starting with 75% v/v NMP in acetone, followed by 25% v/v NMP in acetone and finally three more times with 100% v/v acetone. The gels were then placed in a supercritical fluid extraction chamber in acetone and washed with liquid CO2; then CO2 was converted into a supercritical state, and gaseous CO2 was slowly vented out. The resulting aerogel was further vacuum-dried at 75 °C overnight. The dry aerogel made with TAB and BAPB has density of 0.242 g/cm3. Coating Polyimide Aerogel on CNT Yarns. Polyimide sol was prepared as previously described and stirred. An ∼1 in. CNT yarn was used to test when the sol reached a high enough viscosity to form a thin coating on the CNT yarns. Then the sol was poured into a flask as shown in Figure S3. A CNT yarn was pulled through several rollers; the flask containing polyimide sol was collected on another roller after gelation. Afterward, the coated CNT yarn was washed and dried with supercritical liquid CO2 dried as previously described. Statistical Design and Analysis. For the samples prepared in Table S1, experimental design and analysis were conducted using Design Expert Version 10, available from Stat-Ease, Inc. (Minneapolis, MN). Multiple linear regression analysis was used to derive empirical models to describe the effect of each of the process variables studied on measured properties. Varying factors include polymer concentration (8−12% w/w), THF concentrations (0−25% w/w), and two types of cross-linkers (TAB and PMA-D). An I-optimal design strategy was used to select a minimum number of experiments while still achieving the lowest average predictive variance across the design space. A full quadratic model including all main effects, second-order effects, and all two-way interactions was entertained, and continuous variables were orthogonalized (transformed to −1 to +1 scale) before analysis. Terms deemed to not be significant in the model (