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Feb 3, 2018 - Matthew D. Gawryla 1, Eric M. Arndt 1, Miguel Sánchez-Soto 2 ID ... Keywords: aerogel; clay; composite; poly(amide-imide); mechanical ...
materials Article

Poly(Amide-imide) Aerogel Materials Produced via an Ice Templating Process Matthew D. Gawryla 1 , Eric M. Arndt 1 , Miguel Sánchez-Soto 2 1 2

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and David A. Schiraldi 1, *

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Department of Macromolecular Science & Engineering, Case Western Reserve University, Cleveland, OH 44106-7202, USA; [email protected] (M.D.G.); [email protected] (E.M.A.) Centre Catalá del Plástic, Universitat Politécnica de Catalunya. BarcelonaTech, 08022 Terrassa, Spain; [email protected] Correspondence: [email protected]; Tel.: +216-368-4243

Received: 8 January 2018; Accepted: 1 February 2018; Published: 3 February 2018

Abstract: Low density composites of sodium montmorillonite and poly(amide-imide) polymers have been created using an ice templating method, which serves as an alternative to the often-difficult foaming of high temperature/high performance polymers. The starting polymer was received in the poly(amic acid) form which can be cured using heat, into a water insoluble amide-imide copolymer. The resulting materials have densities in the 0.05 g/cm3 range and have excellent mechanical properties. Using a tertiary amine as a processing aid provides for lower viscosity and allows more concentrated polymer solutions to be used. The concentration of the amine relative to the acid groups on the polymer backbone has been found to cause significant difference in the mechanical properties of the dried materials. The synthesis and characterization of low density versions of two poly(amide-imide) polymers and their composites with sodium montmorillonite clay are discussed in the present work. Keywords: aerogel; clay; composite; poly(amide-imide); mechanical properties

1. Introduction Polymer aerogel represent a family of low density materials which are typically produced by either supercritical or freeze drying of wet gels [1,2]. The supercritical drying process developed from early work by Kistler [3], who showed that colloidal silica could be generated via a sol-gel process in an alcohol, solvent exchanged with acetone, then dried using supercritical carbon dioxide. Meador [4], Leventis [5] and others have expanded the field to generate polymer-based aerogels in a similar manner. A great advantage of the supercritical drying approach is that the exquisite structure generated in the wet gel state can be preserved in the finished aerogel, producing materials of high surface area and nanoporosity, leading to material with exceptional thermal insulative properties. Downsides to the use of the supercritical process include the handling and removal of large quantities of organic solvents, and use of capital-intensive supercritical carbon dioxide drying. The freeze drying process for producing aerogels produces low density materials whose structures are not dominated by the delicate gel structure of wet gels, but rather materials whose structures are the negatives of the frozen ice structure; polymers and fillers are forced into the grain boundaries of the ice lattice, which is then sublimed away to leave a porous material behind [6]. Because the materials produced using such an ice templating method are coarser than those generated using supercritical drying, they tend to possess lower surface areas, lack nanoporosity, and exhibit typical polymer foam-like thermal conductivities [7]. The freeze drying process benefits from the use of water as solvent and simplicity in the freeze drying process, and is amenable to a wide range of polymers, such as poly(vinyl alcohol) [8] and bio-based alginate [9], casein [10], pectin [11], gelatin [12], natural rubber [13] and hyaluronic

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rubber [13] and hyaluronic acid [14]. These ice-templated polymer aerogels generally benefit from acid [14]. These ice-templated polymer aerogels generally benefit from incorporating smectite clays, incorporating smectite clays, such as montmorillonite or bentonite, which form exfoliated hydrogels such as montmorillonite or bentonite, which form exfoliated hydrogels at approximately 2 wt. % at approximately 2 wt.% in water, and upon freezing form well-dispersed polymer/clay composite in water, and freezing form well-dispersed polymer/clay materials [15]. Supercritical materials [15].upon Supercritical drying of polymer clay systems, acomposite process which combines elements of drying of polymer clay systems, a process which combines elements of both processes above, both processes described above, has also been reported, and is a method of producingdescribed interesting and has also been reported, and is a method of producing interesting and useful materials [16]. useful materials [16]. ◦ C; foaming Polyimides are performance polymers Polyimides are high high performance polymers often often suitable suitable for for applications applications above above 200 200 °C; foaming of polyimides is extremely and when when melting is of polyimides is extremely difficult difficult due due to to limited limited melt melt processing processing abilities abilities and melting is possible, low melt melt viscosities viscosities [17]. [17]. Many Many polyimide polyimide foams foams are are formed formed through through aa powder possible, low powder precursor precursor which expands upon heating, filling a mold of the desired shape [18]. Polyimide foams are primarily primarily which expands upon heating, filling a mold of the desired shape [18]. Polyimide foams are developed for aerospace applications which require operating temperatures from cryogenic up to to developed for aerospace applications which require operating temperatures from cryogenic up ◦ 250 °C C [19–22]. [19–22]. Polyimide Polyimide aerogels aerogels have have more more recently recently become become of of interest, interest, led led by by the the pioneering pioneering work work 250 of Meador and coworkers at NASA [23,24] and of Leventis [25]. Produced using the supercritical of Meador and coworkers at NASA [23,24] and of Leventis [25]. Produced using the supercritical drying process, drying process, these these polyimide polyimide aerogels aerogels offer offer considerable considerable promise promise in in lightweight lightweightspace/aerospace space/aerospace applications ranging ranging from Because polyimides polyimides and and their their applications from deacceleration deacceleration parachutes parachutes to to antennae. antennae. Because starting monomers are typically not soluble in water or other easily sublimable solvents, these aerogels starting monomers are typically not soluble in water or other easily sublimable solvents, these have onlyhave beenonly produced using supercritical drying. Poly(amide-imides) are alternatives to polyimides, aerogels been produced using supercritical drying. Poly(amide-imides) are alternatives to also possessing high continuous use temperature and good mechanical and chemical properties [26]. polyimides, also possessing high continuous use temperature and good mechanical and chemical Unlike polyimides, poly(amide-imides) are initially produced in a water-soluble acid) properties [26]. Unlike polyimides, poly(amide-imides) are initially produced in apoly(amic water-soluble form, which is then thermally (Figureimidized 1). Such(Figure a material then,aoffers the then, potential tothe be poly(amic acid) form, which is imidized then thermally 1). Such material offers converted into a poly(amic acid) aerogel via the freeze drying process, then thermally converted into potential to be converted into a poly(amic acid) aerogel via the freeze drying process, then thermallya finished poly(amide-imide) aerogel. Such a conversion also solves the issue of solves many aerogels converted into a finished poly(amide-imide) aerogel. Such a conversion also the issueproduced of many via ice templating, that the finished material is latently water soluble; imidized poly(amide-imide) aerogels produced via ice templating, that the finished material is latently water soluble; imidized aerogels would not be water soluble, even they were initially prepared frominitially aqueousprepared solution. poly(amide-imide) aerogels would not bethough water soluble, even though they were A recent work solution. demonstrated that work such poly(amide-imide) aerogels can be produced, and that they from aqueous A recent demonstrated that such poly(amide-imide) aerogels can be are highly efficient at removing oil from contaminated waters. In the present work, the structural produced, and that they are highly efficient at removing oil from contaminated waters. In the present and mechanical properties of these aerogels, which excellentwhich thermal, mechanical, water work, the structural and mechanical properties of exhibit these aerogels, exhibit excellentand thermal, resistance properties will be described. mechanical, and water resistance properties will be described.

® AI-30 (Solvay Advanced Polymers LLC, Alpharetta, Figure 1. Structure of poly(amide-imide) Torlon® Figure 1. Structure of poly(amide-imide) Torlon AI-30 (Solvay Advanced Polymers LLC, Alpharetta, GA, USA) in the amic acid form. GA, USA) in the amic acid form.

2. Results and Discussion 2. Results and Discussion Freeze drying of aqueous solutions of the poly(amic acid) form of the poly(amide-imides) (PAIs) Freeze drying of aqueous solutions of the poly(amic acid) form of the poly(amide-imides) (PAIs) in in the presence of diethylaminoethanol (which increases the polymer solubility in water) readily the presence of diethylaminoethanol (which increases the polymer solubility in water) readily produced produced mechanically-fragile monoliths, which when thermally treated for several hours◦at 200 °C mechanically-fragile monoliths, which when thermally treated for several hours at 200 C 3under under vacuum, produced robust structures with densities in the range of 0.07–0.19 g/cm . The vacuum, produced robust structures with densities in the range of 0.07–0.19 g/cm3 . The concentration concentration of amine in the preparation solution was found to have little effect on the visual of amine in the preparation solution was found to have little effect on the visual appearance of the appearance of the PAI clay composites. The composites all exhibited lamellar structures (Figure 2) PAI clay composites. The composites all exhibited lamellar structures (Figure 2) independent of amine independent of amine concentration. The structures of freeze dried aerogels have been previously concentration. The structures of freeze dried aerogels have been previously reported, and range reported, and range from cellular to lamellar solids, depending primarily upon the solute from cellular to lamellar solids, depending primarily upon the solute concentrations and freezing concentrations and freezing temperatures employed [6,14]. Increasing the concentration of amine temperatures employed [6,14]. Increasing the concentration of amine3 increased the density of the 3 increased the density of the composites slightly about 0.085 g/cm and 0.075 g/cm just over3 3 andfrom 3 to just 3 and to composites slightly from about 0.085 g/cm 0.075 g/cm over 0.1 g/cm 0.08 g/cm 0.1 g/cm3 and 0.08 g/cm3 for the AI-50 and AI-30 composites respectively (Figure 3). The moduli of for the AI-50 and AI-30 composites respectively (Figure 3). The moduli of AI-50 composites were AI-50 composites were generally higher than that of AI-30 for all amine concentrations, most likely generally higher than that of AI-30 for all amine concentrations, most likely due to the more rigid due to the more rigid m-phenylene diamine monomer incorporated into the AI-50 backbone. Within experimental error, the compressive moduli did not change with increasing amine concentration for

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m-phenylene diamine monomer incorporated into the AI-50 backbone. Within experimental error, Materials 2018, 11, 234 3 of 10 the compressive moduli did not change with increasing amine concentration for the AI-50 composite Materials 2018, 11, 234 3 of 10 materials (Figure 4). The AI-30 aerogel composites showed a slight increase in modulus at the highest the AI-50 composite materials (Figure 4). The AI-30 aerogel composites showed a slight increase in the AI-50 compositeour materials (Figure The that AI-30 aerogel composites a slight increase in amine concentration; prior work has 4). shown mechanical properties showed increase geometrically with modulus at the highest amine concentration; our prior work has shown that mechanical properties modulus at thesohighest amine concentration; our prior work density has shown that mechanical properties aerogel density, these increases most likely reflect the modest increases observed. Toughness, increase geometrically with aerogel density, so these increases most likely reflect the modest density increase geometrically with aerogel these increases likely reflect the modest density the amount of energy absorbed by adensity, materialsofor a given strainmost value, were generally independent increases observed. Toughness, the amount of energy absorbed by a material for a given strain value, increases observed. Toughness, the amount of5). energy absorbed bybe a material forthen a given strain value, of the solution amine concentrations (Figure It can generally concluded that the amine were generally independent of the solution amine concentrations (Figure 5). It can generally be were generally independent of the solution amine concentrations (Figure 5). It can generally be processing used to the achieve high solutionhigh concentrations had little concludedaid then that aminerelatively processing aidpoly(amide-imide) used to achieve relatively poly(amide-imide) concluded then thehad amine used to achieve relatively high poly(amide-imide) effect on the final that aerogel products. solution concentrations littleprocessing effect on theaid final aerogel products. solution concentrations had little effect on the final aerogel products.

Figure 2. SEM (Scanning Electron Microscope) image of the microstructure of a 5 wt.% Na+-MMT/2.5 Figure 2. SEM (Scanning Electron Microscope) image of the microstructure of a 5 wt. % wt.% (Nanocor Inc. PGW grade, Chicago, IL, USA) AI-30 composite with 2 equivalents of amine. + -MMT/2.5 wt. % (Nanocor Inc. PGW grade, Chicago, IL, USA) AI-30 composite with 2 equivalents Na Figure 2. SEM (Scanning Electron Microscope) image of the microstructure of a 5 wt.% Na+-MMT/2.5 of amine. wt.% (Nanocor Inc. PGW grade, Chicago, IL, USA) AI-30 composite with 2 equivalents of amine.

Figure 3. Change in density of 5 wt.% Na+-MMT/2.5 wt.% AI-30 composites as a function of amine concentration. + -MMT/2.5 +-MMT/2.5 Figure3.3. Change in NaNa wt.% AI-30 composites as a function of amine Figure in density density of of 55 wt.% wt. % wt. % AI-30 composites as a function of concentration. amine concentration.

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+-MMT/2.5 + -MMT/2.5 Figure 4. Compressive moduli of 55 wt.% Na wt.% AI-30 composites as a function of amine Figure wt. %Na Na wt.AI-30 % AI-30 composites as a function of +-MMT/2.5 Figure4.4. Compressive Compressive moduli of 5 wt.% wt.% composites as a function of amine concentration. amine concentration. concentration.

+-MMT/2.5 wt.% AI-30 composites as a function of Figure 5. Toughness at 10% strain of 5 wt.% Na +-MMT/2.5 + -MMT/2.5 Figure5.5.Toughness Toughnessatat10% 10% strain 5 wt.% Na wt.% AI-30 composites composites as as aafunction functionofof Figure strain ofof 5 wt. % Na wt. % AI-30 amine concentration. amineconcentration. concentration. amine

Materials containing containing only only polymer, polymer, i.e., i.e., no no clay, clay, were were created created from from the the 5 wt.% wt.% solutions solutions of of AI-30 AI-30 Materials Materials containing only polymer,the i.e.,effect no clay, were created from the 5 5wt. % solutionsthe of AI-30 and AI-50 in order to investigate of clay on the properties. Increasing amine andAI-50 AI-50 in order to investigate theofeffect ofthe clay on the Increasing properties.theIncreasing the amine and in order to investigate the effect clay on properties. amine of concentration concentration proved detrimental to the AI-50 composites above six equivalents amine. The concentration proved detrimental to the AI-50 composites above six equivalents of amine. The proved detrimental tosuch the AI-50 composites above six equivalents ofthe amine. The plasticizing effect plasticizing effect of a large excess of amine could contribute to foam-like structure which is plasticizing effect of such a large could excess contribute of amine could contribute to structure the foam-like structure which is of such athe large excess of amine to The the foam-like which is seen in the seen in higher amine concentrations (Figure 6). foam-like structures were very brittle and seen inamine the higher amine concentrations (Figure 6). Thestructures foam-likewere structures were very brittle and higher (Figure The foam-like brittle and unable to unable to to be be concentrations mechanically tested tested or6). measured for density density due due to to the the very non-uniform shape. AI-30 unable mechanically or measured for non-uniform shape. AI-30 be mechanically tested or measured for density due to the non-uniform shape. AI-30 materials did materials did did not not show show the the same same level level of of bubble bubble formation formation and and therefore therefore useful useful materials can can be be materials not showup thetosame level of bubble formation and therefore useful materials can be materials created up to at created at least 10 equivalents of amine. The internal structure of the polymer-only materials is created up to at least 10 equivalents of amine. The internal structure of the polymer-only materials is much smoother smoother and and more more uniform uniform than than that that seen seen when when clay clay is is included included (Figure (Figure 7). 7). When When it it was was much included the clay is the main component of the system, 66 wt.% in the final, dried material, and included the clay is the main component of the system, 66 wt.% in the final, dried material, and

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least 10 equivalents of amine. The internal structure of the polymer-only materials is much smoother and more uniform is Materials 2018, 11, 234 than that seen when clay is included (Figure 7). When it was included the clay 5 of 10 Materials 2018, 11, 234 5 of 10 the main component of the system, 66 wt. % in the final, dried material, and therefore the polymer acts primarily as a binder the clay as particles without the clay in thewithout system,the theclay polymer free to therefore the polymer acts to primarily a binder to the clay particles in theissystem, therefore the polymer acts primarily as a binder to the clay particles without the clay in the system, form smooth layers and in the case of AI-30 these layers are highly flexible due to their dimensions. the polymer is free to form smooth layers and in the case of AI-30 these layers are highly flexible due thetheir polymer is these free toThe form smooth layerspolymer-only and in the case ofamine AI-30increases these layers highly flexible due The density of polymer-only increases with content, again due to the plasticized to dimensions. density ofmaterials these materials withare amine content, again to their dimensions. The density of these polymer-only materials increases with amine content, again polymer being able to polymer shrink more easily 8); densities of these aerogels show greater due to the plasticized being able(Figure to shrink more easily (Figure 8); densities of thesevariability aerogels due to the plasticized polymer being able to shrink more easily (Figure 8); densities of these aerogels than those of typical freeze-dried materials [15] freeze-dried because of the complex[15] interactions show greater variability than those of typical materials because of of plasticization the complex show greater variability than those of typical freeze-dried materials [15] because of the complex and crosslinking, effects not and studied in detail to date in systems. Withtovariability in densities interactions of plasticization crosslinking, effects notsuch studied in detail date in such systems. interactions of plasticization and crosslinking, effects not studied in detail to date in such systems. comes some variability in modulus, as this mechanical property is highly sensitive to polymer density. With variability in densities comes some variability in modulus, as this mechanical property is highly With variability in densities comes some variability in modulus, as this mechanical property is highly As the amine contentdensity. was increased, the modulus showed aand general increasing sensitive to polymer As the amine contentand wastoughness increased,also the modulus toughness also sensitive to polymer density. As the amine content was increased, the modulus and toughness also trend (Figures 9 and 10). AI-30 and(Figures AI-50 exhibited drastic the amine content increases, showed a general increasing trend 9 and 10). AI-30increases and AI-50asexhibited drastic increases as showed a general increasing trend (Figures 9 and 10). AI-30 and AI-50 exhibited drastic increases as gaining over 10 times the stiffness. the amine content increases, gaining over 10 times the stiffness. the amine content increases, gaining over 10 times the stiffness.

Figure 6. Image of 5 wt.% Torlon®®AI-50 materials with (A) 6 and (B) 10 equivalents of amine. Figure % Torlon Torlon® AI-50 AI-50materials materialswith with (A) (A) 66 and and (B) (B) 10 10 equivalents equivalents of of amine. amine. Figure 6. 6. Image Image of of 55 wt. wt.%

Figure 7. SEM image of the microstructure of a 5 wt.% AI-30 Composite with 2 equivalents of amine. Figure 7. SEM image of the microstructure of a 5 wt.% AI-30 Composite with 2 equivalents of amine. Figure 7. SEM image of the microstructure of a 5 wt. % AI-30 Composite with 2 equivalents of amine.

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Figure 8. Density a function aminecontent content for for aa 55 wt.% polymer material. Figure 8. Density as aasfunction ofofamine wt. % polymer material. Figure 8. Density as a function of amine content for a 5 wt.% polymer material. Figure 8. Density as a function of amine content for a 5 wt.% polymer material.

Figure 9. Modulus as a function of amine content for a 5 wt.% polymer material.

Figure 9. Modulus a functionof ofamine amine content material. Figure 9. Modulus as as a function contentfor fora a5 5wt.% wt. polymer % polymer material. Figure 9. Modulus as a function of amine content for a 5 wt.% polymer material.

Figure 10. Toughness as a function of amine content for a 5 wt.% polymer material. Figure 10. Toughness as a function of amine content for a 5 wt.% polymer material. Figure 10. Toughness as a function of amine content for a 5 wt.% polymer material.

Figure 10. Toughness as a function of amine content for a 5 wt. % polymer material.

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As produced via the freeze-drying process, the poly(amic acid) form of the poly(amide-imide) aerogels was, was, not not surprisingly, surprisingly,generally generallysensitive sensitivetotomoisture. moisture.When When these structures were placed these structures were placed in in a container water,they theyofofcourse coursefloated floatedinitially, initially,but but ultimately ultimately dissolved dissolved back back into solutions a container of of water, similar to those used to produce the frozen starting C, a remarkable starting materials. materials. Upon curing at 200 ◦°C, change in this property occurred, and the previously hydrophilic poly(amic acid) aerogels became thoroughly hydrophobic hydrophobic poly(amide-imide) poly(amide-imide) materials. materials. Exposed Exposed to to water water for up to a year, the aerogels were essentially unaffected. Thus, it is possible to produce a hydrophobic, water-stable, low density “foam” “foam” from from aqueous aqueous solution. solution. Anisotropic produced by freeze dryingdrying the aqueous precursorprecursor solutions Anisotropic poly(amide-imides) poly(amide-imides)were were produced by freeze the aqueous in the mixed plastic/metal mold. The difference in thermal conductivity of the metal and polymeric solutions in the mixed plastic/metal mold. The difference in thermal conductivity of the metal and sides of thesides mold to preferential formation of seed crystals on the metal which which when polymeric of led the mold led to preferential formation of seed crystals on thesurface, metal surface, followed by lamellar growthgrowth of ice crystals risegave to anisotropic structuresstructures and mechanical properties when followed by lamellar of ice gave crystals rise to anisotropic and mechanical (Figure 11 (Figure and Table 1. As can be seen the from moduli Table 2, 50-fold in moduli properties 11 and Table 1. As canfrom be seen theinmoduli in aTable 2, adifference 50-fold difference in was typical comparing the ice growth transverse directions directions for these structures. With such moduli waswhen typical when comparing the iceand growth and transverse for these structures. anisotropy mechanical is possible toitproduce a stiff, temperature stable, insulating With such in anisotropy in properties, mechanicalit properties, is possible to high produce a stiff, high temperature material which in material its perpendicular is flexibledirection and can is beflexible bent around structures, as stable, insulating which in direction its perpendicular and can be bent such around pipes and motor housings. structures, such as pipes and motor housings.

Figure 11. Mold and aerogel monoliths from anisotropic freezing. Figure 11. Mold and aerogel monoliths from anisotropic freezing. Table 1. Anisotropic PAI aerogel compressive moduli. Table 1. Anisotropic PAI aerogel compressive moduli. Horizontally Aligned, First Horizontally Aligned, First Compression Compression Cycle Cycle Compressive moduli, 71 ± 17 KPa Compressive 71 ± 17 moduli, KPa

Horizontally Aligned, Second Horizontally Aligned, Second Compression Compression Cycle Cycle 67 ± 15 67 ± 15

Vertically Aligned, First Compression Vertically Aligned, First Compression Cycle Cycle 4500 ± 370

Vertically Aligned, Second Compression Vertically Aligned, Second Compression Cycle Cycle 3200 ± 390

4500 ± 370

3200 ± 390

3. Materials and Methods 3.1. Materials Sodium Montmorillonite (Na+-MMT) (Nanocor Inc. PGW grade, Chicago, IL, USA), and Torlon® AI30, Torlon® AI-50 (Solvay Advanced Polymers LLC, Alpharetta, GA, USA), diethylaminoethanol (DEAE) (99.5%, Sigma Aldrich, St. Louis MO, USA) were used as received. Deionized water was

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3. Materials and Methods 3.1. Materials Sodium Montmorillonite (Na+ -MMT) (Nanocor Inc. PGW grade, Chicago, IL, USA), and Torlon® AI-30, Torlon® AI-50 (Solvay Advanced Polymers LLC, Alpharetta, GA, USA), diethylaminoethanol (DEAE) (99.5%, Sigma Aldrich, St. Louis MO, USA) were used as received. Deionized water was prepared using a Barnstead RoPure reverse osmosis system (RoPure, Barnsted, Van Nuys, CA, USA). The Torlon® AI-30 chemical structure is given in Figure 1; the AI-50 product differs from AI-30 in that the methylene dianiline monomer is replaced with at 0.7/0.3 mixture of 4,40 -oxydianiline/m-phenylene diamine [27]. 3.2. Clay/Torlon® Composites Aqueous solutions of Torlon® AI-30 and Torlon® AI-50, were prepared by mixing a calculated amount of diethylaminoethanol and deionized water (Table 2. Values for the moisture content and acid number of the Torlon® AI-30 and Torlon® AI-50 powders were assumed to equal 65 wt. % and 125 mg KOH/g respectively, as per the manufacturer’s literature [28]. The amine solution was then heated to 95 ◦ C and the polymer slowly added. 10 wt. % clay gels were prepared from Na+ -MMT and deionized water using a model MC2 (Waring, Stamford, CT, USA) mini laboratory blender on high speed. 50 mL of the polymer solution was then slowly added to 55 g of the clay gel while stirring. After mixing, the samples were placed in 18 mL polystyrene vials and frozen in a solid carbon dioxide/ethanol bath. Freeze drying was carried out using a Advantage Model EL-85 (Virtis, Warminster, PA, USA) lyophilizer with an initial shelf temperature of −10 ◦ C, which increases to 25 ◦ C after full vacuum is attained. 5 wt. % polymer solutions were also frozen in the same manner to obtain clay free materials, tmade entirely of poly(amide-imide). After freeze drying, all samples were removed from the vials and cured in a vacuum oven at 200 ◦ C under vacuum for several hours to convert the poly(amide amic acid) to poly(amide imide) and to remove DEAE from the samples. In the case of directionally-frozen, anisotropic aerogels, a complex mold composed of an aluminum bottom and polystyrene sides was used. As the freezing solution in mold was lowered into the low temperature batch, ice crystals first nucleated on the metal surface, giving rise to largely unidirectional lamellar ice crystallization. Table 2. Poly(amide-imide) solution compositions. Stoichiometric Multiplier

Deionized Water (g)

DEAE (g)

Polymer Powder (g)

2 4 6 8 10

83.1 80.5 77.9 75.3 72.7

2.611 5.222 7.833 10.444 13.055

14.286 14.286 14.286 14.286 14.286

3.3. Characterization Compression samples were cut from the dried monoliths using a band saw such that the sample height was less than or equal to the width (approximately 2 cm). Compression tests were carried out using an model 5565 (Instron, Norwood, MA, USA) universal testing machine with a crosshead speed of 1mm per minute. Stress strain curves were plotted, and the moduli were measured from the initial linear portions of the curves. Densities were calculated from the mass of each sample and their dimensions. Scanning electron microscopy imaging was performed on a XL-30 ESEM (Phillips, Amsterdam, The Netherlands).

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4. Conclusions Using the high temperature poly(amide-imide) polymers Torlon® AI-30 and AI-50, low density foam-like materials can be created which exhibit useful mechanical properties. Increasing the amount of amine counter ion in the polymer solution has a larger effect when there is no clay in the material. The clay stabilizes the material and provides a support structure as the amine is being removed during curing. The higher amine concentrations appear to plasticize the polymer and therefore when creating polymer-only materials an upper limit exists. Acknowledgments: Support from the Case Western Reserve University Provost’s office is greatfully acknowledged. Author Contributions: For research articles with several authors, a short paragraph specifying their individual contributions must be provided. D.S. and M.G. conceived and designed the experiments; E.A. and M.G. performed the experiments; M.G., E.A., M.S. and D.S. analyzed the data. Conflicts of Interest: Coauthors Gawryla and Schiraldi are inventors on patents which could overlap with some of the content of this paper, and which conceivably could be commercialized.

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