Effect of Bulky Substituents in the Polymer Backbone on the Properties

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Effect of Bulky Substituents in the Polymer Backbone on the Properties of Polyimide Aerogels Rocco Peter Viggiano, Jarrod C. Williams, David A. Schiraldi, and Mary Ann Babin Meador ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b15440 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017

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ACS Applied Materials & Interfaces

Effect of Bulky Substituents in the Polymer Backbone on the Properties of Polyimide Aerogels Rocco P. Viggiano*,1, Jarrod C. Williams1, David A. Schiraldi2, Mary Ann B. Meador*,1 1. NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, Ohio 44135, United States 2. Case Western Reserve University, 2100 Adelbert Road, Cleveland, Ohio 44106, United States *Corresponding authors: [email protected], [email protected] ABSTRACT

With unique advantages over inorganic aerogels including higher strengths and compressive moduli, greater toughness and the ability to be fabricated as a flexible thin film; polymer aerogels have the potential to supplant inorganic aerogels in numerous applications. Among polymer aerogels, polyimide aerogels have high thermal stability as well as outstanding mechanical properties. However, while the onset of thermal decomposition for these materials is typically very high (greater than 500 °C), the polyimide aerogels undergo dramatic thermally induced shrinkage at temperatures well below their glass transition (Tg) or decomposition temperature, which limits use. In this study, we show that shrinkage is reduced when a bulky moiety is incorporated in the polymer backbone. Twenty different formulations of polyimide aerogels were synthesized from 3,3,´4,4´-biphenyltetracarboxylic dianhydride (BPDA) and 4,4´-

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oxydianiline (ODA) or combination of ODA and 9,9´-bis(4-aminophenyl)fluorene (BAPF), and cross-linked with 1,3,5-benzenetricarbonyl trichloride (BTC) in a statistically designed study. The polymer concentration, n-value and molar concentration of ODA and BAPF were varied to demonstrate the effect of these variables on properties. Samples containing BAPF showed a reduction in shrinkage by as much as 50% after aging at elevated temperatures for 500 hours compared to those made with ODA alone.

KEYWORDS Aerogels, Polyimides, Porous Polymers, Thermal Shrinkage, Mesoporous INTRODUCTION Aerogels are a class of nanoporous solids fabricated through forming a gel from solution in which the nanoporous architecture is developed in the wet-gel state and is then converted to the dry-solid state without compaction of the established architecture.1,2 The nanoporous framework leads to materials which possess low densities, high porosities and high internal surface areas.3 These characteristics produce materials that exhibit an array of properties including low thermal and electrical conductivities, low dielectric constants, and good acoustic dampening.4 Aerogels have been investigated as candidates for use as insulation,5,6 catalyst supports,7 filtration media,8 sensors,9 and electrodes,10,11 among other uses. Originally synthesized in 1931,12 silica aerogels have been the most widely studied,13 although inherent material properties including fragility, sensitivity to moisture and poor mechanical strength have restricted their widespread use.14 Some strategies adopted to improve the mechanical strength of silica aerogels include reinforcing the silica backbone with a conformal polymer coating.15 The introduction of epoxy, vinyl, or amine moieties to the surface of the silica network enable the production of polyepoxies,16


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polystyrene,17 and polyureas,18 respectively, to reinforce the silica backbone. However, these polymer-silica hybrid aerogels have an upper limit of thermal stability in the range of 100-150 °C.19,20,21 Often aerospace applications require materials with a higher use temperature. Since 1989, some research efforts have focused on polymer aerogel chemistries, leading to the production of resorcinol-formaldehyde (RF),22 melamine-formaldehyde,23 phloroglucinol-penolformaldehyde,24 dihydroxynapthalene-terephaldehyde,25 polyurethane,26 polyurea,27 syndiotactic polystyrene (sPS),28,29,30 polyphenyleneoxide (PPO) in the presence of nanofibrils of sPS,31 polyimide,32,33,34,35,36 and polyamide aerogels.37 Polymer aerogels possess similar low densities, high porosities and high internal surfaces areas observed in silica aerogels. Several compositions possess advantages over silica aerogels, most notably, mechanical strength. Polyimide aerogels stand out from other polymer aerogels due to their combination of excellent thermal stability, good mechanical properties, and high glass transition temperatures (Tg), as well as the ability to produce different form factors, including thin flexible films and thicker rigid substrates. With glass transition temperatures (Tg) ranging from 270-340 °C, onsets of decomposition ranging from 460-610 °C, and with compressive moduli higher than 100 MPa in some cases, polyimide aerogels possess the properties required for use in many space-based applications.34 One such application where higher temperature materials are required is for planetary entry, descent and landing systems (EDL).38 In past missions to Mars, EDL systems were used to land robotic payloads on the surface safely by employing a hard aeroshell heat shield in combination with parachutes 12-16 m in diameter. Future planned manned missions to Mars will be heavier with a larger payload necessitating more drag for landing. One proposed design is an inflatable aerodynamic decelerator with a very large diameter (30-60 m).39 This design imposes strict constraints including the use of insulation with a high degree of flexibility


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or foldability as part of a layered system. This facilitates its packing into a small space for later deployment as a lightweight heat shield and decelerator upon atmospheric entry.40 Polyimide aerogel thin films have been demonstrated as a viable part of the layered system.41 Polyimides are the polymer of choice for applications requiring high performance at elevated temperatures for long duration, such as polymer matrix composites used in aircraft engine applications.42,43,44 Thus it is surprising that while polyimide aerogels possess the same high Td and Tg as dense polyimides, aerogel cylinders exposed to elevated temperatures of as low as 150 °C and 200 °C for extended periods of time causes thermally induced shrinkage of up to 50% of their original radial diameter.36 Thermal shrinkage alters pore size and diameter, leading to higher densities, lower surface areas, and lower porosity resulting in a reduction of the thermal insulation capabilities of the aerogels. In previous studies, the highest shrinkage on aging is seen in polyimide aerogels with 2,2´-dimethylbenzidene (DMBZ) in the polymer backbone, while a combination of two diamines, 4,4´-oxydianiline (ODA) and DMBZ results in the lowest shrinkage, possibly due to disruption in the packing of polymer chains.36 A separate study shows that addition of cellulose nanocrystals (CNC) as a rigid, high aspect ratio filler in situ during the polymerization results in a reduction in shrinkage on aging with ODA containing aerogels.45 This study also demonstrated that increasing the content of CNCs reduced the degree to which the aerogels shrank during fabrication as well. Both of these studies suggest that impeding the ability of the polymer chains to rearrange at higher temperatures may result in a reduction of shrinkage. One method of inhibiting chain rearrangement may be to introduce bulky, moieties into the backbone of the polyimide. Prior research into this topic has demonstrated that the introduction of bulky aromatic


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substituents46,47,48 or pendent moieties49,50,51 along the polymer backbone disrupts packing and still maintains their outstanding thermal stability. One example is the introduction of bulky, rigid triptycene groups in the backbone of polyimides.48 In this study, Swager et al demonstrated that the rigid 3-dimensional structure of triptycenes inhibits the close packing of polymer chains and interrupts strong interpolymer interactions. The group found that the free volume imparted by the triptycene units increased the measured BET surface area of the material while maintaining the rigidity and thermal properties of a fully aromatic backbone. A bis-phenyl-fluorene is another such substituent that has been shown to maintain excellent thermal properties but disrupt polymer chain packing in polyimides.52 Wu et. al. introduced the diamine 9,9´-bis(4-aminophenyl)fluorine (BAPF), which contains the bulky bis-phenyl-fluorene side group. In comparison to ODA, BAPF is much bulkier and clearly disrupts the linearity of polymer chains reducing the potential for close packing of neighboring polymer chains as seen from energy minimized structures created using Chem3D 16.0 and shown in Figure 1. In addition, computational analysis of bond rotation of the cardo-carbon of the fluorene group and the carbon atoms on the two adjacent phenyl rings in BAPF using the dihedral driver calculation from Chem3D 16.0 shows an increased barrier to rotation compared to that around the ether linkage in ODA (170 kcal/mol compared to 157 kcal/mol at the peak). This large barrier to rotation restricts possible chain conformations and would ultimately restrict fibrillar motion and packing within the porous aerogel architecture.


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Figure 1: Three-dimensional ball and stick models for the monomers ODA and BAPF and the conformation of the polyimide chain comprised of both monomers. Herein, we use the BAPF as a way of disrupting chain packing in the polyimide aerogel with the idea of reducing the shrinkage occurring on aging at elevated temperature. Thus, we present the synthesis and characterization of novel polyimide aerogels containing BAPF, as shown in Scheme 1. Polyimide aerogels based on 3,3´,4,4´-biphenyltetracarboxylic dianhydride (BPDA) and 4,4´-oxydianiline (ODA) cross-linked with 1,3,5-benzenetricarbonyl trichloride (BTC) have been fully characterized in previous studies36 and thus are used as the control in this study. A statistically designed experimental study using twenty formulations of polyimide aerogels is carried out to understand the effect of the variables on their properties. Variables include use of BAPF in place of up to 50 mol % ODA, the number of formulated repeat units, n, in the oligomer backbone (20 to 40) and the amount of polymer in solution (7 to 10 wt %). The resultant aerogels are characterized and the effects on the density, porosity, mechanical properties and shrinkage during aging are empirically modeled. EXPERIMENTAL SECTION


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MATERIALS Acetic anhydride (AA), triethylamine (TEA), and 1,3,5-benzenetricarbonyl trichloride (BTC) were purchased from Sigma-Aldrich (3050 Spruce Street, St. Louis, MO 63103) and used without further purification. Anhydrous N-methylpyrrolidone (NMP) was purchased from Tedia (1000

Tedia

Way,

Fairfield,

OH

45014).

4,4´-Oxidianiline

(ODA),

9,9´-bis(4-

aminophenyl)fluorene (BAPF), and biphenyl-3,3,´4,4´-tetracarboxylic dianhydride (BPDA) were obtained from Chriskev, Inc. (13920 W. 108th Street, Lenexa, KS 66215). BPDA was dried at 125 °C in vacuum for 24 h before use. GENERAL Nitrogen-adsorption porosimetry was obtained using an ASAP 2000 surface area/pore distribution analyzer (Micrometrics Instrument Corp.). Skeletal density of the specimens was determined using a Micrometrics Accupyc 1340 helium pycnometer.

Thermogravimetric

analysis (TGA) was carried out with a TA model 2950 HiRes instrument. Infrared spectroscopy was acquired using a Nicolet Nexus 470 FTIR spectrometer. A Bruker Avance 300 spectrometer was used to obtain solid

13

C NMR spectra using cross polarization and magic angle spinning

(CP-MAS) at a rate of 11 kHz. Spectra were externally referenced to the carbonyl of glycine (176.09 ppm relative to tetramethoxy silane). Scanning electron microscopy (SEM) was used to obtain micrographs of the platinum plated aerogels using a Hitachi S-4700 field emission microscope.

Compression testing was performed following ASTM D695-10 as previously

described.53 Experimental design and analysis were conducted using Design Expert, version 9, from StatEase, Inc. (Minneapolis, MN, USA). A face-centered, central composite design including three variables was used. The concentration of BAPF (0 to 50 mol % with the amount of ODA being


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given as 100 – mol % BAPF), total polymer concentration (7 to 10 wt %), and number of repeat units, n, in the amine end-capped oligomers (20 to 40) were varied. A total of twenty separate batches of aerogels were produced as summarized in Table 1, including six repeats of the center point in the design. Runs were carried out in a random order. Empirically generated data were analyzed using multiple linear regression. A full quadratic equation was developed, including all two-way interactions, for each response. Backwards stepwise regression analysis was performed on the model to eliminate statistically insignificant terms (p > 0.1). PREPARATION OF BAPF CONTAINING POLYIMIDE AEROGELS Polyimide gels were synthesized using the dianhydride BPDA, the diamines ODA and BAPF, and BTC the cross-linking agent in the polar aprotic solvent, NMP, as shown in Scheme 1. Imidization was carried out chemically at room temperature. The aerogels were formulated to have amine end-caps on the polyimide oligomers and n formulated repeat units (ranging from 20 to 40) using n equivalents of BPDA and n + 1 equivalents of diamine. The oligomers were cross-linked with BTC to form gels. Table 1 outlines the variables used to prepare each sample, along with the measured properties. As an example, the procedure for the synthesis of sample 6 from Table 1 consisting of 50 mol % ODA and 50 mol % BAPF, a formulated n value of 20 and a polymer concentration of 10 wt % is as follows: To a solution of ODA (1.906 g, 9.52 mmol) in 87.5 mL of NMP was added BPDA (5.335 g, 18.13 mmol) and the mixture was stirred for two hours then placed in a sonic bath for 5 minutes to dissolve. BAPF (3.3172 g, 9.52 mmol) was then added and the solution was further stirred until homogenous.

Afterwards, acetic anhydride (13.69 mL) was added and stirred until

homogenous, followed by TEA (2.53 mL). After ten minutes, a solution of BTC (0.1605 g, 0.604 mmol) in 10 mL NMP was added to the solution while stirring. Immediately after mixing,


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the solution was poured into syringe molds that were covered with Parafilm. The solution gelled after approximately 20 minutes. The gels were then aged for 24 hours in the molds, after which time they were extracted into a solution of 75 v/v % NMP/25 v/v % acetone and allowed to soak overnight. Afterwards, the solvent was replaced by a solution of 25 v/v % NMP/75 v/v % acetone and the gels were allowed to soak for another 24 hours, followed by four more solvent exchanges in 100 % acetone in 24 hour intervals. The gels were then supercritically dried using liquid CO2 extraction, followed by drying under vacuum at 75 °C overnight to ensure all solvent had been removed from the samples. The resultant aerogels had a density of 0.1337 g/cm3 and a porosity of 90.6%. Solid 13C NMR (ppm): 165, 143, 135, 125, 65. FTIR (δ): 1716, 1502, 1363, 1236, 1103, 1076. RESULTS AND DISCUSSION Polyimide aerogels were fabricated according to Scheme 1 using variables outlined in Table 1. For fabricating aerogels with a mixture of ODA and BAPF, the ODA was dissolved first in NMP followed by addition of all of the BPDA. This should produce a solution of mostly the n = 1 oligomer, BPDA-ODA-BPDA, plus some excess BPDA. Addition of BAPF should produce polyamic acid oligomer with a largely alternating arrangement of BAPF-(BPDA-ODA-BPDABAPF). It has been shown in previous studies that fabricating the polymers using this method produces aerogels of more uniform porosity, while dissolving the two diamines together before adding dianhydride may result in phase separation and a hierarchical pore structure.36 To the polyamic acid solution is added triethylamine and acetic anhydride. Triethyl amine is a non-nucleophilic base used to abstract a proton from the amic acid intermediate facilitating ring closure to the imide. Acetic anhydride acts as a water scavenger. Once the imidized oligomers form, the cross-linker BTC, dissolved in 10 mL of NMP, is added. For samples containing only


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ODA and BPDA, gelation occurs in 20 minutes. For samples containing 50 mol % BAPF, gelation time more than doubles to 45 to 60 minutes. The increased gelation time may be due to the bulky side groups disrupting the ability for the polyimide chains to pack and increasing their solubility, or may be due to the lower reactivity of the amine compared to ODA. The variables used to synthesize the aerogels are shown in Table 1, along with density, porosity, surface area, compression properties, and the onset of decomposition for each formulation. Porosity (Π) was calculated using equation 1. ఘ

Π = 100 × ቀ1 − ఘ್ቁ ೞ

(1)

where ρb is the bulk density and ρs is the skeletal density determined by helium pycnometry. Shrinkage given in Table 1 is the initial shrinkage as fabricated calculated based on the diameter shrinkage from the initial diameter of the cylindrical mold and diameter of aerogel post supercritical drying.


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Scheme 1. Synthesis of cardo-diamine containing polyimide aerogels


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Table 1. Formulations and Properties of Cardo-Diamine Containing Polyimide Aerogels

BAPF (mol. %)

Bulk Density (g/cm3)

Porosity (%)

Shrinkage (%)

BET Surface Area (m2/g)

Sample

n

Polymer Conc. (wt. %)

1

30

7.0

50

0.0884

93.7

16.7

402

7.60

596

2

30

10

50

0.128

91.0

14.9

479

12.9

601

3

30

7.0

25

0.0986

95.3

16.1

476

7.70

595

4

30

8.5

50

0.0862

94.1

12.2

442

7.80

599

5

30

8.5

25

0.111

92.4

14.8

465

9.60

598

6

20

10

50

0.134

90.6

14.2

427

19.2

607

7

30

10

25

0.177

89.2

20.4

395

30.1

599

8

40

10

0

0.163

90.1

18.1

376

26.3

598

9

30

8.5

25

0.115

93.5

14.3

464

12.6

594

10

20

7.0

50

0.0541

96.4

6.02

464

-

626

11

30

8.5

0

0.157

90.6

19.7

391

22.2

606

12

20

8.5

25

0.106

93.9

12.1

477

9.20

603

13

20

7.0

0

0.111

94.4

16.6

431

10.4

601

14

40

7.0

0

0.141

92.0

22.2

360

14.1

591

15

40

7.0

50

0.0629

96.4

8.00

439

-

593

16

40

8.5

25

0.126

92.9

16.0

457

12.8

590

17

30

8.5

25

0.132

92.5

18.1

449

13.2

612

18

40

10

50

0.134

91.5

12.8

414

19.9

585

19

20

10

0

0.161

90.5

17.9

404

28.5

598

20

30

8.5

25

0.118

91.8

16.0

464

10.7

605

Modulus (MPa)

Onset of Decomposition (°C)


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Figure 2 shows the 13C solid state NMR spectra of two representative samples from the study formulated with n of 40 and 10 wt % polymer concentration, made with no BAPF (sample 8, spectrum a) and 50 mol % BAPF (sample 18, spectrum b). The sharp peaks at 165.3 ppm (imide carbonyl, peak a) and 143 ppm (quarternary aromatic, peak c), and the broad peaks from 115 to 138 (non-quaternary aromatics) are present in both spectra, as expected. The peak at 153 ppm (quarternary aromatic attached to oxygen, peak b) also appears in both spectra but is larger in spectrum a since this sample contains twice the amount of ODA. There are two peaks present exclusively in spectrum b that are assigned to carbons in BAPF, at 141 ppm (quarternary aromatics in flourene unit, d) and 65 ppm (aliphatic carbon in flourene unit, e).

These

assignments are in good agreement with previously reported polyimides made using BAPF.54

Figure 2.

13

C solid state NMR spectra of representative aerogels from the study: (a) sample 8 (0

mol % BAPF, 10 wt %, n=40); (b) sample 18 (50 mol % BAPF, 10 wt % n=40).


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Scanning electron micrographs (SEM) of representative aerogels made with 7 wt % polymer concentration are shown in Figure 3. As with other polyimide aerogels, the architecture is composed of an open cellular porous network consisting of a homogenous fibrillar network. Formulations containing higher polymer concentrations have a similar appearance to the 7 wt % samples shown here, but appear slightly less porous. In general, samples containing no BAPF are similar in morphology to those containing 50 mol % BAPF.

Figure 3.

Comparison of SEM images for polyimide aerogels fabricated with a polymer

concentration of 7 wt%: (a) 0 mol % BAPF, n = 40 (sample 14) and (b) 50 mol % BAPF, n = 40 (sample 15). Figure 4 shows the empirical models for shrinkage occurring during fabrication (standard deviation = 1.46%, R2 = 0.91) and density (standard deviation = 0.01 g/cm3, R2 = 0.89) of all of the samples in the study. The plot of the shrinkage as a function of the n-value and BAPF concentration (cardo-diamine) for each of the three polymer concentrations (Figure 4a), shows that increasing the concentration of BAPF significantly reduces shrinkage but more so at lower polymer concentrations.

Samples containing 0 mol % BAPF shrank approximately 20%,


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whereas samples containing 50 mol % BAPF shrank as little as 5%. The reduction in shrinkage may be due to the effect of BAPF on the chain packing as seen in previous polyimides studied. The polymer chains may adopt a kinked conformation with less tightly packed networks that keep the aerogel from shrinking during processing. The n-value has a small though significant effect on the observed shrinkage. Since the degree of shrinkage affects the density of each sample, it is expected that the density model shown in Figure 4b would be very similar to Figure 4a. Increasing the fraction of BAPF does cause a significant decrease in density as expected. However, there is no significant synergistic affect between polymer concentration and BAPF concentration as was seen with shrinkage. In other words, density increases with increasing BAPF concentration to the same extent for all polymer concentrations. This may indicate that lower concentrations lead to a loss of some low molecular weight material during washing and supercritical drying, especially at higher BAPF concentrations.

7 wt % 8.5 wt % 10 wt %

25 Shrinkage, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 15 10 5 25 30 n

a)

35 40

50

40

30

20

10

0

F, % BAP


15


7 wt % 8.5 wt % 10 wt %

3

0.20 Density, g/cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.15 0.10 0.05 0.00 25 30 n 35 40

b)

50

40

30

20

F, BAP

10

0

%

Figure 4. Graphs showing (a) the shrinkage (%) as a function of n-value and (b) the BAPF content (%) for the three levels of polymer concentration. Braunauer-Emmett-Teller (BET) surface areas were obtained on all of the samples using nitrogen sorption. The empirical model for surface area, Figure 5 (standard deviation = 7.3, R2 = 0.98), shows several distinct trends. The surface area significantly decreases with increasing polymer concentration and increasing n.

In addition, surface area increases with BAPF

concentration but reaches a predicted maximum at 30 mol %. It is expected that BAPF would increase the microporosity and therefore the surface area of the polymer strands by disrupting chain packing, but it may be that increasing BAPF beyond 35% causes an increase in larger pores sizes.


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480 460 7 wt % 8.5 wt % 10 wt %

440 420 400 380 360 340

%

50 40 30 20 10

35

30 n

BA PF ,

m2 /g

500

Surface Area,

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25

20

0

Figure 5. Graphs of the empirical model of surface area (m2/g) as a function of n-value and BAPF content (%) for the three levels of polymer concentration. Figure 6 shows the pore size distributions for representative samples, whose SEM images are shown in Figure 3. The sample made with 50 mol % BAPF possesses a wider pore size distribution centered at 44 nm, but also has a small peak at 10 nm and more pores below 8 nm in size than the aerogels made using no BAPF. The sample made with 100% ODA has a narrower pore size distribution centered at 20-25 nm. The increase in small scale porosity which was also observed in other formulations containing BAPF may arise in the same way that Swager et. al. observed an increased surface area and an increase in free volume within the polymer by introduction of bulky triptycene groups (vide supra). This indicates that the addition of BAPF produces a significant number of small pores compared with samples containing 0% BAPF, but at the same time the reduced shrinkage observed during processing also introduces an increase in the number of larger pores, accounting for the wider pore distribution in the BAPF containing samples.


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2.5 0% BAPF 25% BAPF 50% BAPF

2.0 3

Pore Volume, cm /g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.5 1.0 0.5 0.0

1

10

100

1000

Pore Diameter, nm

Figure 6. Plot of the pore volume (cm3/g) as a function of pore diameter (nm) for sample 14 containing 7 wt % polymer concentration, 100% ODA, and an n-value of 40 and sample 15 containing 7 wt % polymer concentration, 50% ODA/50% BAPF, and an n-value of 40. Compression testing was performed on all of the aerogels in the experimental design study. Typical stress-strain curves are shown in Figure 7, from representative samples made using 10 wt % polymer concentration, formulated n = 40, and no BAPF (sample 8) and 50 mol % BAPF (sample 18). The Young’s modulus is defined as the initial linear region of the stress-strain curve.


18

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10 0% BAPF 50% BAPF

8 Stress, MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 4 2 0 0.0

0.2

0.4

0.6

0.8

Strain

Figure 7. Typical stress-strain curves for the compression of polyimide aerogel sample 8 (ρ = 0.163 g/cm3) and sample 18 (ρ = 0.134 g/cm3) (both formulations are made with n = 40 and 10 wt % polymer concentration). The empirical model for modulus (standard deviation = 0.1 MPa, R2 = 0.84), shown in Figure 8a, indicates a strong dependence on polymer concentration and concentration of BAPF, but no significant effect of n over and above random error. The model shows that increasing the polymer concentration increases the modulus, while increasing the fraction of BAPF decreases the modulus. However, as seen in Figure 8b, when density effects are accounted for in the plot of modulus as a function of density, increasing the content of BAPF actually slightly increases the modulus. Thus, the decrease in modulus with increasing BAPF seen in the model is actually due to the decrease in the density of the aerogels.


19


Model Data

Modulus, MPa

100

10

1 10 9 Po lym er 8 co nc .,

a)

30

40

50

20 %

7

10 0

,% PF A B

100

Modulus, MPa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

10

0% Cardo diamine 25% Cardo diamine 50% Cardo diamine 1 0.08 0.09 0.1

b)

0.2 Density, g/cm3

Figure 8. Graph (a) showing the empirical model for the compressive modulus (MPa) of the aerogels as a function of polymer concentration (%) and content of BAPF (%) and (b) the log-log plot of modulus vs. density as a function of polymer concentration and BAPF concentration. Figure 9a shows TGA plots of representative aerogel samples from the study made using no BAPF (sample 8) and 50 mol % BAPF (sample 18). Incorporating BAPF into the polymer backbone slightly reduces the onset of decomposition temperatures from 598 °C to 585 °C.


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Page 21 of 37

Use temperature is not limited, however, by the decomposition of the polymer backbones. Rather it was found that during aging at temperatures as low as 150 °C, the polyimide aerogels made with ODA or DMBZ in the backbone shrink. To assess the effect of aging on aerogels made with varying amounts of BAPF, isothermal aging was carried out for 500 hours on all compositions at temperatures of 150 °C and 200 °C. The samples were removed from the oven at 24, 100 and 500 hours during the course of the study to measure changes in weight, density and diameter shrinkage. As with previous studies, the most significant changes in shrinkage and density occur within the first 24 hours of the aging study and plateaued throughout the remainder of the study. Figure 9b shows the same two representative samples of aerogel as shown in Figure 9a made with no BAPF and 50 mol % BAPF. Due to shrinkage, the density of the aerogel made with no BAPF increases from 0.16 g/cm3 to 0.36 g/cm3 in the first 24 hours and remains fairly constant after that. In contrast, the density of the aerogel made using 50 mol % BAPF increased from 0.14 g/cm3 to 0.19 g/cm3 over the 500 hours. Very little weight loss (

Effect of Bulky Substituents in the Polymer Backbone on the Properties

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