Article pubs.acs.org/Macromolecules
Polyimide Cellulose Nanocrystal Composite Aerogels Baochau N. Nguyen,† Elvis Cudjoe,§ Anna Douglas,‡ Daniel Scheiman,† Linda McCorkle,† Mary Ann B. Meador,*,‡ and Stuart J. Rowan*,§ †
Ohio Aerospace Institute, 22800 Cedar Point Road, Cleveland, Ohio 44142, United States NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135, United States § Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland, Ohio 44106, United States ‡
S Supporting Information *
ABSTRACT: Cellulose nanocrystals derived from tunicates (t-CNC) were used as a reinforcing nanofiller for polyimide aerogels. Two sets of polyimide aerogels, containing either 2,2′-dimethylbenzidine (DMBZ) or 4,4′-oxydianiline (ODA) and 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (BPDA) cross-linked with 1,3,5-tris(4-aminophenoxy)benzene (TAB), were studied. Total solids composition of the aerogels were kept constant at 7.5 wt %, with 0−13.33 wt % of the total solids being the carboxylic acid-functionalized t-CNC (tCNC−COOH) filler. The incorporation of the t-CNC− COOH, with carboxylic acid content of either 560 or 920 mmol/kg, in the polyimide aerogel networks improved both physical and mechanical properties of the final materials. Isothermal aging of t-CNC−COOH aerogel composites was also conducted at 150 and 200 °C for 24 h. Higher content t-CNC−COOH/polyimide aerogels showed less change in their density and reduced shrinkage during aging, which further emphasized the effect of the t-CNC−COOH reinforcement in retaining the structural integrity of the aerogel.
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INTRODUCTION First synthesized in 1931,1 silica aerogels are highly porous, low-density solids with high surface areas that exhibit good acoustic damping2 and low thermal and electric conductivity.3,4 They are also good candidates for use as catalyst supports,5 filtration media,6 sensors, actuators, electrodes, etc.7,8 Nevertheless, the use of silica aerogel monoliths has been restricted on account of their inherent fragility, sensitivity to moisture, and poor mechanical strength.9 By introducing amine, epoxy, or vinyl groups on the silica surface, different polymers such as polyepoxies,10 polystyrene,11 or polyureas12 can be used to reinforce the silica network. Alternatively, increasing flexibility of native silica aerogel via reduction of chemical bonds in the silica network13 or introducing flexible linking groups in the underlying silica backbones14−16 are also approaches that can be used to alter the mechanical properties of silica-based aerogels. A different class of aerogels that have drawn considerable attention are polymer aerogels. After the first polymer aerogel based on resorcinol formaldehyde (RF)17 was reported in 1989, various studies have focused on melamine formaldehyde,18 phloroglucinol-based,19,20 polyurethane,21 polyurea,22 syndiotactic polystyrene-based (sPS),23−25 and polyamide,26 to name a few. Not only do these materials display good thermal insulation, they also offer a wide range of potential uses in a number of areas, including low volatile organic chemical and gas absorption materials,20 air purification, and chemical separation/filtration.19−25 © XXXX American Chemical Society
The use of nanofillers to enhance the properties of aerogel composites is well studied. Incorporation of fillers including polyacrylonitrile fibers27 or biologically based fibers (bamboo, hemp, and ramie),28 carbon nanotubes,29 carbon nanofibers,30 cellulose nanofibrils, and graphene oxide nanosheets31−33 to enhance the strength of aerogels has been investigated over the past few years. For example, Finlay et al. demonstrated an improvement in the compressive strength and modulus in clay aerogels when reinforced with bamboo, hemp, and ramie fibers. Another example, reported by Feng et al.,27 illustrated the use of oxidized polyacrylonitrile fibers as reinforcement for carbon aerogels prepared by copyrolysis of resorcinol−formaldehyde aerogels. They showed that by impregnating the aerogels with 22.7 wt % polyacrylonitrile fibers, the shrinkage in the in-plane direction during aerogel formation went from 11% for the native aerogel to 5.2% for the composite aerogel. In other studies, silica aerogels embedded with electrospun nanofibers of PDMS-based polyurethane (TM-E2A)34 or a combination of poly(vinylidene fluoride) electrospun nanofibers and microparticles35 showed a significant improvement in flexibility in comparison to its nonreinforced counterpart. Hayase et al. also demonstrated the use of cellulose nanofibers isolated from wood powder as a filler for a polymethylsilsesquioxane Received: July 15, 2015 Revised: January 28, 2016
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DOI: 10.1021/acs.macromol.5b01573 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Scheme 1. Synthetic Route of the Polyimide/t-CNC−COOH Composite Materialsa
In the first step the dianhydride is reacted with a diamine to yield the poly(amic acid), which in the second step is cross-linked with TAB and in the third step is converted into the polyimide with acetic acid and pyridine. For the composite materials the CNCs are added in the first step. a
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DOI: 10.1021/acs.macromol.5b01573 Macromolecules XXXX, XXX, XXX−XXX
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compression, and tensile properties, in combination with low thermal conductivity54 and low dielectric constant,59,60 suitable for a variety of aeronautic and aerospace applications such as lightweight substrates for antennas and insulation layers for inflatable structures for entry, descent, and landing for entry, descent and landing operations.53 Results obtained from Meador et al.54 showed that polyimide aerogels made from biphenyl-3,3′,4,4′-tetracarboxylic acid dianhydride (BPDA) and 2,2′-dimethylbenzidine (DMBZ) or 4,4′-oxydianiline (ODA) cross-linked with 1,3,5-tris(4-aminophenoxy)benzene (TAB) (Scheme 1) exhibited low density, low shrinkage, high porosity, and good mechanical and thermal properties. These polyimide aerogels could also be fabricated into rolls of flexible thin films, enabling the insulation of objects with different contours and sizes such as cryotanks or pipes. Polyimide aerogels derived from ODA as the diamine are more flexible, while DMBZ-based polyimide aerogels are higher modulus when similar densities are compared.61 This is to be expected since fully dense polyimides with a backbone made up of DMBZ/BPDA have been reported with modulus of 150 GPa,61 while those with an ODA/BPDA backbone are about 2 orders of magnitude lower in modulus.62 The modulus of polyimide aerogels tends to scale with density within a backbone family. In this paper, we report an investigation into the preparation and properties of nanocomposites of these PI aerogels with different amounts (0−13.33 wt %) of tunicate cellulose nanocrystals (t-CNC). The goal here was to understand how the addition of a nanofiller into the PI aerogels impacts their properties. In addition to the advantageous combination of properties mentioned above, t-CNCs were chosen for this study on account of the fact that they have a high tensile modulus (ca. 150 GPa similar to the DMBZ/BPDA backbone40) and are dispersible in the PI aerogel processing solvent such as N-methylpyrrolidinone (NMP). As such, no additional dispersing agent was required which allows us to examine how only the t-CNCs impact the polyimide properties. We further used either DMBZ or ODA as the diamine in the backbone of the polyimide to be consistent with previous studies, and because it is anticipated that ODA-based aerogels, having a more flexible backbone, and the DMBZ-based aerogels, with a stiffer backbone, will be impacted differently by the t-CNC filler.
(PMSQ) aerogel. They demonstrated that by adding 18 wt % of cellulose nanofibers to the composite resulted in favorable effects such as increased surface area (550−750 m2/g) and reduced density (about 0.020 g/cm3) for the composite compared to the neat PMSQ aerogel.32 Cellulose nanocrystals (CNCs) are highly crystalline nanofibers that can be isolated from a variety of renewable biosources, including a wide range of plant sources (such as wood or cotton), selected bacterial or algae sources, and sea tunicates.36 One main difference in the CNCs obtained from these sources is the dimensions of the CNCs, which depend on both the source and method of isolation. The diameters range from ca. 5 to 30 nm, and the lengths range from hundreds of nanometers to several micrometers.37−39 CNCs have several benefits, which include biosustainability, biorenewability, relatively low production cost, and low cytotoxicity.38 They exhibit low density, high surface area, low coefficient of thermal expansion, and high elastic moduli (ca. 80−150 GPa) that depend on the biosource.36−41 Recently, CNCs have been shown to result in the significant reinforcement of a wide range of polymer matrices, such as poly(styrene-co-butyl acrylate),42 poly(vinyl acetate),43poly(ethylene oxide-co-epichlorohydrin),44 poly(styrene-co-butadiene),45 polyurethane,46 and epoxy resins,47 to name a few. CNCs can also be used to access aerogels.48−51 For example, Heath et al. have prepared neat cotton CNC aerogels.48 Their study showed a linear increase in density and decrease in porosity as the CNC content of the aerogels increased. Specific surface areas were found to vary between 216 and 605 m2/g; however, there was not a clear trend in surface area with the increase in amount of CNCs. Using the average CNC dimensions from cotton, they calculated a theoretical specific surface area of the aerogel with well-dispersed CNCs, which are completely individualized with no aggregation to be 419 m2/g. Hence, the variation in surface area was attributed to aggregation between the CNCs. Yang and Cranston49 have prepared a chemically cross-linked CNC aerogel with a high porosity of 99%. This aerogel showed an increase in compressive modulus and density as the concentration of CNCs increased as well as a bimodal pore distribution (mesopores 1 μm) and shape memory ability with more than 85% shape recovery after 80% compression. They measured a specific surface area of the CNC aerogels to be 250 m2/g, which was similar to what was reported in the literature.48 Mueller et al. recently reported the influence of CNC dimensions on the properties of nanocellulose aerogels prepared using lyophilization where poly(vinyl alcohol) was used as a binder.52 CNCs with aspect ratios of 10, 25, and 80 isolated from cotton, banana plants, and tunicates, respectively, were used in this study. Results showed a decrease in shrinkage with an increase in aspect ratio of CNC. Density was dependent on solids content and shrinkage; therefore, aerogels with high aspect ratio CNCs demonstrated low densities, resulting in an increase of solid content of the aerogel (ca. 0.03−0.16 g/cm3 for tunicate CNCs).33 One class of polymer aerogels of recent interest are crosslinked polyimide aerogels. First developed in 2011, polyimide aerogels can be prepared as either nonflaking, flexible, foldable thin films or as strong, stiff thicker parts.53−57 Polyimide aerogels are typically made via chemical imidization of poly(amic acid) gels at room temperature, followed by supercritical drying to remove solvent from the gels.58 Crosslinked polyimide aerogels offer several distinctive features such as lightweight, high surface areas, good thermal stability, good
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EXPERIMENTAL SECTION
Materials. 2,2′-Dimethylbenzidine (DMBZ), 4,4′-oxydianiline (ODA), and biphenyl-3,3′,4,4′-tetracarboxylic acid dianhydride (BPDA) were obtained from Wakayama Seika Kogya Com., Ltd. 1,3,5-Tris(4-aminophenoxy)benzene (TAB) was custom synthesized by Oakwood Chemical. N-Methylpyrrolidinone (NMP) was purchased from Tedia. Acetone, pyridine, acetic anhydride, and 2,2,6,6tetramethyl-1-piperidinyloxy (TEMPO) were purchased from SigmaAldrich. Sodium hypochlorite (NaClO, 14.5% chlorine content) was purchased from Alfa Aesar. Hydrochloric acid (HCl), sodium bromide (NaBr), and sodium chloride (NaCl) were purchased from Fisher Scientific. Sea tunicates (Styela Clava) were collected from floating docks in Warwick Cove Marina (Warwick, RI) and were cleaned to obtain mantles as described previously.42 Two different batches of tCNC−COOH were examined with surface carboxylic acid densities [COOH] of 560 and 920 mmol/kg. BPDA was dried at 125 °C under vacuum for 18 h before use. All other reagents were used without further purification. Instrumentation. All the samples were outgassed at 85 °C for 12 h under full vacuum after being supercritically dried to ensure complete removal of any solvent residue in the aerogels. Solid 13C C
DOI: 10.1021/acs.macromol.5b01573 Macromolecules XXXX, XXX, XXX−XXX
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cm/min). The films were place in closed containers to prevent/inhibit the evaporation of the solvent. A similar procedure was applied for neat polyimide aerogels. All the resulting gels were imidized and aged at room temperature for a day before the cylinders were demolded and films were peeled off the substrate. NMP in the wet gels was gradually washed out and replaced with acetone using the following routine: 75/ 25 vol % NMP/acetone, 25/75 vol % NMP/acetone, followed by an additional five washes with 100 vol % acetone. The solvent exchange was performed twice a day in 7−18 h intervals. The washed gels, in acetone, were then dried by exchanging the acetone in the gels with liquid carbon dioxide (CO2) using an Accudyne multivessel automated system. After supercritically dried, the specimens were outgassed in a vacuum oven at 85 °C for 12 h to remove any solvent residue, resulting in cross-linked polyimide aerogel composites having a density of 0.115 g/cm3. 13C solid NMR (ppm) for polyimide: 20 (−CH3), 115−150 (aromatic), and 165 (imide); for t-CNC−COOH: 35, 70− 75, 83−85, and 105−110. FT-IR (imide): 1773 (m), 1715 (s), 1500 (s), and 1371 (s) cm−1. Compression Tests. The aerogel monoliths were tested in accordance with ASTM D695-10 with the sample sizes within the range of 1.25−1.50 ratio of length to diameter. The samples were tested between a pair of compression platens on a Model 4505 Instron load frame using the Series IX data acquisition software. The platen surfaces were coated with a graphite lubricant to reduce the surface friction and barreling of the specimen. Tensile Tests. Stress−strain experiments were carried out on a Zwick-Roell Z0.5 equipped with a 100 N load cell at a strain rate of 5%/min on aerogel films with thickness of about 0.3 mm and width of about 5 mm. The data were acquired using the textXpert II software. Dynamic Mechanical Analysis (DMA). Stiffness measurements were performed on a DMA model Q800 in tensile mode at a fixed frequency of 1 Hz, 0.01 N preload force, 0.1% strain, and 125% force track. The stiffness of the aerogel films was measured by going through three heating cycles, with a heating and cooling rate of 5 °C/min (range of 20−200 °C). The data were acquired using the TA universal analysis software. Characterization. The bulk density (ρb) was calculated from the mass over the volume of the sample (cylinder). Dimensional change, or shrinkage (%), is taken as the difference between the diameters of the mold (nominally 20 mm) and the aerogel. The porosity (%) of the aerogels was determined using eq 3
NMR spectroscopy was performed with a Bruker Avance-300 spectrometer outfitted with a 4 mm solids probe using crosspolarization and magic angle spinning at 11 kHz. Pore structures of samples were viewed under a Hitachi S-4700-11 field emission scanning electron microscope (SEM) after samples were sputtercoated with 5 nm platinum. Aerogel monoliths were viewed under an FEI Technai F30 transmission electron microscope (TEM) at 300 kV after samples were microtomed and stained with 2 wt % uranyl acetate. Aerogel films were solution cast using a ChemInstrument EZ Coater EC-100. Brunauer−Emmett−Teller (BET) surface area and pore distribution were obtained using a Micromeritics ASAP 2020 chemisorption system. Samples for chemisoption, ranging from 0.0100 to 0.0200 g, were degassed at 85 °C for another 12 h period before being analyzed. The skeletal density was determined using a Micromeritics Accupyc 1340 helium pycnometer. The t-CNC− COOH were dispersed in NMP, using a Misonix Ultrasonic liquid processors S4000 sonicator. Thermal gravimetric analysis (TGA) was performed using a TA model 2950 instrument. Small specimens of 1.5−8.5 mg were heated from room temperature to 750 °C at a temperature ramp rate of 10 °C/min under N2 at 60 cm3/min through the balance and 40 cm3/min through the furnace for a total of 100 cm3/min. Titrations of the t-CNC−COOH were performed using an Accumet AR50 dual channel pH/ion/conductivity meter (Fisher Scientific, Pittsburgh, PA). Isolation of t-CNCs (t-CNC−OH). The t-CNC−OH was isolated using previously published procedures43 (see Supporting Information for experimental details). Synthesis of Carboxylic Acid-Functionalized t-CNCs (t-CNC− COOH). The t-CNC−OH were oxidized in 89% yield using TEMPO, NaBr, and NaClO following procedures from previous publications with minor modifications in the amount of NaClO added.43 Determination of carboxylic acid content was conducted using conductometric titrations (see Supporting Information for experimental details and Figure S1 for titration graphs). FTIR: 3660−3200 (broad, s), 2800 (broad, m), 1600 (broad, weak), 1500−1200 (broad, weak), and 1200−900 cm−1 (s). Synthesis of Polyimide Composite Aerogels. Polyimide aerogels were fabricated by first making poly(amic acid) oligomers in solution end-capped by anhydrides, cross-linking with TAB, and then imidizing using acetic anhydride and pyridine. Formulated molecular weights (FMW) of the oligomers were derived by a ratio of anhydride to amine of (n + 1) to n, where n is the number of repeat units of diamine and dianhydride in the oligomer. Previous studies have shown that n usually has only a small effect on aerogel properties; thus, in this case n = 30 was used for all of the formulations. Based on this, formulated molecular weight of the cross-linked polyimides was calculated according to eq 1. The total aerogel composite weight was kept constant at 7.5 wt % total solid, with 0−13.33 wt % of t-CNC− COOH incorporated eq 2.
porosity % =
(1)
PI wt % + t ‐CNC−COOH wt % = 7.5 wt %total concentration in solution
1/ρb
× 100 (3)
where ρs is the skeletal density. Isothermal Aging. Thermal aging of the aerogels was done at 150 and 200 °C for 24 h to study the behavior of aerogels at higher temperature. Aerogel monoliths were used and two specimens from each formulation were prepared and sanded to a final thickness ranging from 4 to 7 mm. The specimens, each in a glass vial, were placed in preheated ovens. At the end of the aging period, they were cooled to room temperature. Their final weight and dimension were measured for density, shrinkage, and porosity. Further evaluation of aged aerogels includes NMR, SEM, and BET. Statistical Analysis. Design Expert 8.1 software from Stat-Ease, Inc., was used for analyzing data. Formulations having either DMBZ or ODA used as the amine were prepared with t-CNC−COOH concentration ranging from 0 to 13.33 wt % with t-CNC−COOH charge densities of either 560 or 920 mmol/kg. A quadratic model was considered, including first-order terms for all three variables (amine type, concentration, and charge density), second-order effect of concentration, and all two-way interaction terms, in analyzing the data. Terms not deemed statistically significant were removed from the model one by one using backward-stepwise regression until all terms remaining had p-values of
