Polyimide Aerogels Using Triisocyanate as Cross-linker - ACS

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Polyimide aerogels using tri-isocyanate as cross-linker Baochau N. Nguyen, Mary Ann Babin Meador, Daniel a Scheiman, and Linda S McCorkle ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07821 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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ACS Applied Materials & Interfaces

POLYIMIDE AEROGELS USING TRIISOCYANATE AS CROSS-LINKER Baochau N. Nguyen,1,* Mary Ann B. Meador,2,* Daniel Scheiman1 and Linda McCorkle,1 1. Ohio Aerospace Institute, 22800 Cedar Point Road, Brookpark, OH 44142 2. NASA Glenn Research Center, 21000 Brookpark Road, Cleveland OH 44135 AUTHORS EMAIL ADDRESSES: [email protected], [email protected]

ABSTRACT. A family of polyimide-based aerogels are reported using Desmodur N3300A, an inexpensive tri-isocyanate as the cross-linker. The aerogels are prepared by cross-linking amine end-capped polyimide oligomers with the tri-isocyanate. The polyimide oligomers are formulated using 2,2’-dimethylbenzidine (DMBZ), 4,4’-oxydianiline (ODA), or mixtures of both diamines, combined with 3,3´,4,4´-biphenyltetracarboxylic dianhydride (BPDA) and were chemically imidized at room temperature. Depending on the backbone chemistry, chain length, and polymer concentration, density of the aerogels ranged from 0.06–0.14 g/cm3, and BET surface areas from 350-600 m2/g. Compressive moduli of these aerogels were as high as 225 MPa, which are comparable to, or higher than, those previously reported prepared with similar backbone structures but with other cross-linkers. Because of the lower cost, commercially available cross-linker, the aerogels may have further potential as insulation for building and construction, clothing, sporting

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goods and automotive applications, although lower temperature stability may limit use in some aerospace applications.

Key words: aerogels; polyimides; tri-isocyanate cross-linker Desmodur N3300A, or HDI trimer; mesoporous; urea linkage Introduction. Due to their nanoscale pore size, low density, high porosity and high surface area, aerogels typically are good candidates for use as thermal insulation,1 catalyst supports,2 low dielectric substrates3,4 and for acoustic damping.5 While many types of aerogels share these characteristics, over the last several years, interest has turned to organic polymer based aerogels because of their superior mechanical properties compared to silica and other metal oxide aerogels. Examples of organic aerogels include those based on resorcinol/formaldehyde and melanine/formaldehyde,6,7,8 polyurethane,9,10 polyurea,11,12 and cellulose.13

More recently,

however, aerogels made from engineering polymers, such as polyimide14 and polyamide15,16 have been shown to have the best mechanical properties when compared on a same density basis, making them more attractive for a myriad of uses, including insulation for any number of aerospace applications and as lightweight substrates for antennas.17 Forms of the PI aerogel have also been shown to be moisture resistant,18,19 and resistant to shrinkage at elevated temperatures.20 Depending on the backbone chemistry, polyimide (PI) aerogels can also be cast into flexible thin films, opening up the possibility for them to be used in applications such as thermal insulation for space suits,21 or inflatable structures for entry, descent, and landing, 22 as well as insulation for industrial pipelines, protective clothing or for building and construction. Many polyfunctional amines such as 1,3,5-tris(4-aminophenoxy)benzene (TAB),23 1,3,5tris(aminophenyl)benzene

(TAPB),24

2,4,6-tris(4-aminophenyl)pyridine

(TAPP),25


2

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octa(aminophenoxy)silsesquioxane (OAPS),13 and 4,4-oxydianiline (ODA) modified graphene oxides (m-GO)26 have been used as cross-linkers to fabricate PI aerogels using anhydride endcapped PI oligomers. However, these polyamines are either custom made or expensive which impedes the scale-up process. To overcome this hindrance, low cost, commercially available alternatives have been examined. In this regard, the use of 1,3,5-benzenetricarbonyl trichloride (BTC)27 or polymaleic anhydride (PMA)28 to cross-link amine end-capped PI oligomers was recently reported. The resulting amide or maleimide linked PI aerogels exhibited comparable physical, and thermal properties, with similar morphology to those containing OAPS or TAB cross-linkers. In this study, another alternative cross-linker for PI aerogels is investigated, namely a triisocyanate, Desmodur N3300A. An aliphatic, low viscosity, solvent-free tri-isocyanate based on hexamethylene diisocyanate (HDI trimer), Desmodur N3300A is a commodity and is widely used in industry as a co-reactant in two-component adhesives, resins, or urethane coatings,29 as well as a component used in polyurethane scaffolds as biomaterials to engineer and promote bone and soft tissues.30,31,32,33 This tri-isocyanate monomer has also been used as building blocks for various polyurethane aerogels with different degrees of rigidity.34 The cost of the HDI trimer is just a fraction of the cost of BTC or PMA, making it more attractive in PI aerogel commercialization. In this study, amine-terminated end-capped PI consisting of 2,2’-dimethylbenzidine (DMBZ), 4,4’oxydianiline

(ODA),

or

mixture

of

the

both

DMBZ

and

ODA,

and

3,3´,4,4´-

biphenyltetracarboxylic dianhydride (BPDA) are cross-linked with N3300A to produce gels which are dried using supercritical CO2 extraction to give the aerogels. In addition to varying the amine used in the backbone, two other variables are studied, including total concentration of polymer in the solution (5, 7.5, and 10 wt%) and number of formulated repeat units of oligomer between cross-


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links, n (20, 30, and 40). The effect of these variables on the physical, mechanical, and thermal properties, morphologies, and isothermal aging of the aerogels at 150 °C and 200 °C will be discussed. The PI aerogels are also be compared to those made with BTC and PMA cross-linkers with the same oligomer backbones.

Experimental. Materials. 2,2’-Dimethylbenzidine (DMBZ), 4,4’-oxydianiline (ODA), and 3,3’,4,4’biphenyltetracarboxylic dianydride (BPDA) were purchased from Wakayama Seika Kogya Com., Ltd.,. N-methyl pyrrolidinone (NMP) was purchased from Tedia. Triethylamine (TEA), acetic anhydride (AA), and acetone were obtained from Sigma Aldrich. Desmodur N3300A was kindly supplied by Bayer Material Science and was used as received. General. The cross-linked PI aerogels were dried using supercritical CO2 extraction using an Accudyne multivessel automated system, and were further outgassed at 80 oC under full vacuum to remove any solvent residue. Skeletal densities of the dried samples were measured using a Micromeritics Accupyc 1340 helium pycnometer. Dimensional change, or % shrinkage, was taken as the difference between the diameters of the mold (nominally 20 mm) and the dried aerogel cylinder. Brunauer-Emmett-Teller (BET) surface areas and Barrett-Joyner-Halenda (BJH) pore size distribution were determined from a Micromeritics ASAP 2020 chemisorption apparatus. The specimens used weighed from 0.0100 to 0.0180 g, and were degassed at 85 oC for another 12 hours under full vacuum before the chemisorption test. Thermal gravimetric analysis (TGA) was performed at a temperature ramp rate of 10 oC/min from room temperature to 750 oC under N2, using a TA model 2950 instrument. Solid

13

C NMR spectroscopy was obtained from a Bruker

Avance-300 spectrometer. Micrographs were collected using a Hitachi S-4700-11 field emission scanning electron microscope (SEM). The porosity (%) of the aerogels was determined using eq 1


4

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Porosity = (1 – ρb / ρs) × 100 %

(1)

where ρs is the skeletal density and ρb is the bulk density.

Compression and tensile tests. Aerogel samples were tested following the ASTM D695-10 standard for compression on a model 4505 Instron with a 2.25 kN load cell at a separation rate of 0.05 inch per minute. Sample sizes were 1.25 – 1.50 ratio of diameter to length and both ends were sanded to be parallel and smooth. The samples were placed between a pair of compression platens which were coated with a graphite lubricant to reduce the surface friction and barreling of the specimen. The samples were compressed to 80 % strain. The data was collected and analyzed using the Series IX data acquisition software. Tensile properties were compiled in accordance to ASTM D638 standard on dog-bone specimens using the Tytron 250. The specimens were pulled at the test speed of 0.05 inch per minutes until failure. Isothermal aging study. All aerogel cylinders nominally 0.5 cm to 1.0 cm in height were sanded using a 320 grit sand paper. Two sets of samples were isothermally aged either at 150 °C or 200 °C for up to 500 h in a conventional oven in air and removed at intervals to weigh and measure the dimensions of the test samples. Statistical analyses. Data for physical, mechanical properties, and BET surface areas are listed in Table 1. The study was conducted using a statistical experimental design and data was analyzed using Design Expert 9 software from Stat-Ease, Inc. The experimental design employed was based on a face-centered central composite design with 20 separate formulations made including 6 repeats of the center point of the design. Three variables included polymer concentration from 5 to 10 wt%, oligomer repeat units, n of 20 to 40, and concentration of 0 to 100 mol% DMBZ (with the amount of ODA given as 100 – mol % of DMBZ). Analyses was done using backward stepwise


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regression to remove insignificant terms from the model (p < 0.1). Standard deviations reported are from multiple linear analysis of the pooled data. Synthesis of N3300A cross-linked polyimide aerogels. The aerogels were prepared as shown in Scheme 1, where n is the formulated number of repeat units in the oligomers between crosslinks. In a typical reaction, a diamine (either DMBZ or ODA) or a 50:50 mol% mixture of both DMBZ and ODA, was first dissolved in NMP. BPDA, in powder form, was added and stirred until all was dissolved. N3300A, dissolved in NMP, was then added to the oligomer solution. Once the solution became homogeneous, acetic anhydride and triethylamine were added in sequence. Gelation occurred between 30 to 50 min, depending on the formulation. Higher polymer concentration gelled faster. As an example, aerogel prepared in experiment #5 is based on 100 mol% of ODA with polymer concentration of 10 wt% and 20 formulated repeat units. The synthesis procedure is as followed: ODA (2.17 g, 10.5 mmol) was first dissolved in 40.0 ml NMP. BPDA (3.03 g, 10.0 mmol) was then added. The reaction was stirred at room temperature until BPDA was totally dissolved. The cross-linker, Desmodur N3300A, (0.173 g, 0.335 mmol), dissolved in 5.0 ml NMP, was mixed into the poly(amic acid) solution for about 1 min or until a homogenous solution was obtained. Acetic anhydride (7.80 mml, 80.0 mmol) was then added, followed by triethylamine (2.90 ml, 20.0 mmol), and then the solution was poured into cylindrical and dog-bone shaped molds. Viscosity increased with time, and the solution gelled in 30 min. The gels were sealed to avoid evaporation of solvent and allowed to age at room temperature for a day. The NMP in the wet gels was gradually removed by solvent exchange with acetone by first soaking gels in 75 v/v% NMP in acetone, then 25 v/v% NMP in acetone, followed by four more immersions in 100 % fresh acetone in half day increments. The gels, in acetone, were converted into aerogels using supercritical CO2 extraction. The resulting aerogels were outgassed in a vacuum oven at 80


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°C overnight before being characterized and tested to remove any residual solvent. The crosslinked aerogels had an opaque, bright yellow appearance with density of 0.139 g/cm3, were 91 % porous and had a BET surface area of 430 m2/g. CPMAS NMR (ppm): 165.0, 153.2, 143.8, 130.6, 123.6 and 222.6. FTIR (cm-1): 1774, 1718, 1662, 1618, 1503, 1421, 1373, 1291, 1281, 1172, 1117, 1083, 1015, 882, 834, 737. Synthesis of model compounds. Gels made from two oligomers composed of ODA/BPDA with n of 1 and 10, respectively and cross-linked with N300A were also synthesized. All gelled in approximately 30 min and were aged overnight. These model compounds were washed well with acetone, dried at room temperature, and then further outgassed under full vacuum at 80 °C overnight. For n = 1 gel,

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C CP-MAS NMR (ppm): 169.0, 165.9, 156.8, 150.7, 140.3, 132.2,

124.8, 116.7, 41.8, and 23.3. FT-IR (cm-1): 1773, 1714, 1660, 1611, 1500, 1423, 1370, 1245, 1172, 1117, 1082, 1015, 882, 834, 765, and 737. For n = 10 gel, 13C CP-MAS NMR (ppm): 165.6, 153.8, 144.0, 131.2, 124.4 and 23.3. FT-IR (cm-1): 1774, 1717, 1620, 1500, 1420, 1374, 1290, 1241, 1170, 1116, 1087, 1015, 880, 829, 764 and 738.


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Scheme 1. Synthesis of polyimide aerogels cross-linked with Desmodur N3300A (or HDI trimer)

Results and Discussion. The PI aerogels were fabricated by first making amine end-capped PI oligomers by using n equivalents of dianhydride and n + 1 equivalents of diamine, where n is the formulated number of repeat units. These oligomers are cross-linked with Desmodur N3300A, as shown in Scheme 1.


8

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This is followed by addition of acetic anhydride as water scavenger and triethylamine as the catalyst for chemical imidization reaction which occurs at room temperature. After gelation, the solvent in the gels was exchanged into acetone and then the gels were dried using supercritical CO2 extraction as previously described. To verify that the cross-linker N3300A reacts with the diamine end caps, model compounds were made using two oligomers formulated to be n = 1 and n = 10 using ODA as the diamine and BPDA as dianhydride, and cross-linked with N3300A. Figure 1a shows the NMR spectrum of the n = 1 oligomer cross-linked with N3300A. Peaks at 169.0 and 165.9 are assigned to the imide carbonyls (A). The two peaks may indicate that some of the anhydride did not react with amine but instead reacted with the isocyanate to form imide as suggested by Leventis et al.35 The peaks at 156.8 ppm can be assigned to the urea carbonyl (B), while the peak at 150.7 ppm can be assigned to the carbonyls in the isocyanurate ring (C)36,37,12 as well as the aromatic ether carbons on ODA (C) while the peak at 116.7 belongs to the aromatic carbons of ODA next to the ether carbons. The peak at 140.3 can be assigned to the aromatic carbons attached to the imide rings (D) while those at 132.2 (E) and 124.7 (F) ppm belong to all of the other aromatic carbons. In addition, peaks at 41.8 and 23.3 ppm belong to the methylene carbons in the N3300A, the former being from the methylene next to nitrogen (H) and the latter from the four methylenes in the middle (I). In comparison, in the spectra shown in Figure 1b for the n = 10 oligomer reacted with N3300A, and in Figure 1c for the n = 20 oligomer reacted with N3300, as the oligomer length increases, the peaks belonging to the cross-linker become very small, as expected. Only one imide peak is seen and the peaks due to cyanurate and urea carbonyl are swamped by the ODA ether peak. Aerogels made using DMBZ as the amine have similar NMR spectra with the exception of the peaks derived from the amine moieties.


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F

E G A

I BC D

a.

H

b.

c.

Figure 1. NMR spectra of samples a) model compound made by reacting an n = 1 oligomer with N3300A, b) model compound made by reacting an n = 10 oligomer with N3300A, c) n = 20 aerogel from run 5 Listed in Table 1 are physical and mechanical properties of the N3300A cross-linked PI aerogels produced in the study along with the variables used to fabricate them. Empirical models of shrinkage, density and porosity of the aerogels vs polymer concentration, and polymer chain length, n, are graphed in Figure 2 (a-c). Results obtained from these models are very similar to those previously reported for aerogels cross-linked with BTC.27 Hence, it can be concluded that the cross-linker has little effect on the physical properties of the aerogels. Rather, properties are


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controlled more by the backbone chemistry, polymer concentration, and the repeating unit, n. Thus as seen previously, shrinkage and density were found to increase as polymer concentration and n increased while the opposite trends were seen for porosity. Aerogels composed of 50 mol% DMBZ and 50 mol% ODA exhibited the lowest density and shrinkage, followed by 100% DMBZ formulations. Those containing 100 mol% ODA (0 % DMBZ) in the backbone exhibited the highest density and shrinkage. DMBZ DMBZ-ODA ODA

DMBZ DMBZ-ODA ODA 0.18

21

0.15

10

Repe ating u

25

20

5

nit, n

R2 = 0.9831 stdv. = 0.004

35

30 25 Repe ating unit, n

b)

20

5

7 6

er c

6

8

0.00

er c

30

10 9

Po lym

35

7

on c.,

8

R = 0.9180 stdv. = 1.11

6

0.06 0.03

wt %

9 2

0.09

Po lym

12

0.12

wt %

15

on c.,

18

9

a)

Density, g/cm

3

24

Shrinkage, %

DMBZ DMBZ-ODA ODA 100

Porosity, %

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


95

90

85 25

Re

pe

ati

ng

35

un

it,

c)

6

2

30

n

R = 0.9798 stdv. = 0.30

40

8

t% ,w

. onc er c

9 10

7

5

lym

Po

Figure 2. Empirical models for a) shrinkage, b) density, and c) porosity vs polymer concentration and n


11


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Scanning electron micrographs (SEM) of selected aerogels from this study are shown in Figure 3 (a-c). Similar to other PI aerogels cross-linked with TAB,23 OAPS,14 BTC27 and PMA,28 differences in morphologies of the aerogels depend on the polymer backbone. Those made with at least 50 mol% DMBZ in the backbone (Figure 3 a-b) appear to have a fibrous structure, while the pore network of the ODA-based aerogel (Fig. 3c) is denser and polymer strands are wider, again perhaps due to the greater shrinkage seen with aerogels made using ODA.

500 nm

500 nm

a)

b)

500 nm

c)

Figure 3. SEM images of PI aerogels made using 7.5 wt % polymer and n = 30, with a) 100 mol% DMBZ (sample 20), b) 50 mol% DMBZ and 50 mol% ODA (sample 10), and c) 100 mol% ODA (sample 17) The use of Desmodur N3300A as a cross-linker does affect the pore structure of these crosslinked PI aerogels compared to those having other cross-linkers. Pore size distribution of the aerogel, characterized by BJH analyses, is presented in Figure 4 graphed as specific pore volume vs pore diameter. The graph shows that the pore distribution becomes narrower as the amount of DMBZ used to make the aerogel increases. In addition, the mode of the distribution is lower for 100 mol % DMBZ (~ 70 nm) compared to 100 % ODA (100 nm). All of these values are larger, compared to those cross-linked with OAPS,14 TAB,23 PMA28 and BTC,27 which typically have pore diameter peaks around 20-40 nm. Nevertheless, the N3300A cross-linked aerogels exhibit high BET surface area as shown in the empirical model of the BET surface area in Figure 5. These results are similar to those cross-linked with BTC that have the same oligomer backbone.27 It is


12

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also observed that increasing the ODA content, and polymer concentration results in lower surface area, similar to previous studies. The number of repeat units, n, also has a small effect on surface area, especially when ODA content is increased. 3.0 DMBZ DMBZ-ODA ODA

3

Pore volume, cm /g

2.5

7.5 wt%, n = 30

2.0 1.5 1.0 0.5 0.0 2

4.5

10

20

45

100

200

Pore diameter, nm

Figure 4. Plot of pore volume vs. pore diameter of aerogels composed of 100 mol% DMBZ (sample 20), 100 mol% ODA (sample 17) and 50 mol% DMBZ and 50 mol% ODA (sample 8)

DMBZ DMBZ-ODA ODA 600 550 2

BET, m /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


500 450 400 350 40

300

35

2

6

R = 0.8889 stdv. = 21.28

Po lym e

7 r co

8 nc. ,

9 wt%

10

20

30 25

u ing

,n

nit

at pe

Re

Figure 5. Empirical model of BET surface area graphed vs polymer concentration and n Compression and tensile testing of the N3300A cross-linked PI aerogels were performed on cylindrical and dog-bone shaped specimens, respectively. Shown in Figure 6a are typical stress-


13


strain curves for compression of the PI aerogels made of 100 mol% DMBZ, 50 mol% DMBZ and 50 mol% ODA, and 100 mol% ODA at 7.5 wt% solid. As found in previous studies with other cross-linkers,14-16, 23 the compressive modulus and strength are also dependent on the stiffness or flexibility of the polymer backbone. Aerogels made with 100 mol% DMBZ exhibited the highest Young modulus, even though their density is lower. 10 DMBZ DMBZ-ODA ODA

Stress, MPa

8

7.5 wt%, n = 30 6 4 2 0 0.0

0.2

0.4

0.6

0.8

Strain

a) DMBZ DMBZ-ODA ODA

DMBZ DMBZ-ODA ODA

nit, n

20

5

c)

0.1

R = 0.9928 stdv.(log) = 0.029

35 30 Repe 25 ating unit, n

20

5

nc .,

8

2

0.01

7 6

wt %

10 9

co

6

25

Po lym er

30 Repe ating u

nc .,

7

co

8 1

wt %

10 9

1

er

10

35

b)

ain Stress @ 5% str

100

10

Po lym

, MPa

R2 = 0.9463 Stdv. (log) = 0.11 odulus, MPa Compression m

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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Figure 6. Graphs of a) typical stress-strain curves for compression of PI at 7.5 wt% solid and empirical models for a) compression modulus, b) compression strength and c) stress vs polymer concentration and n


14

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Presented in Figures 6b and 6c are the empirical models for Young’s modulus, and compressive strength (stress at 5% strain) of the cross-linked PI aerogels. As expected, modulus and strength increase with higher polymer concentration since density also increases. The number of repeat units had a small, though significant effect on modulus and strength. As previously seen, the highest mechanical properties were obtained for aerogels with the most rigid backbone (100 mol% DMBZ). Those containing 50 mol % or 100 mol % ODA were similar in modulus and strength, perhaps because compressive strength and modulus also depend on density, and the aerogels made from 50 % mol ODA/ 50 mol % DMBZ shrink less, and therefore have lower density than those made using 100 mol % ODA. When compared on a density basis, as seen in Figure 7, where log-log plots of Young’s modulus vs density are graphed, there is a difference in modulus for aerogels with different backbones. Also shown in Figure 7 is a comparison of aerogels cross-linked with N3300A (close symbols) and BTC (open symbols).27 Young’s moduli of the samples with the different cross-linkers are comparable, with the exception of the aerogels made with 100 mol% DMBZ. In this case, the N3300A cross-linked aerogels have slightly higher modulus when aerogels of the same densities are compared, suggesting the contribution of hydrogen bonding derived from the urea linkages.


15


2D Graph 1

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

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10 N3300A - DMBZ N3300A - DMBZ/ODA N3300A - ODA BTC - DMBZ, ref 26 BTC - DMBZ/ODA, ref 26 BTC - ODA, ref 26

1

0.1 0.06

0.07

0.085

0.1

0.15

3 Density, g/ cm

Figure 7. Linear relationship between density and compression (Young's) modulus of Desmodur N3300A vs BTC (open symbols) crosslinked PI aerogels Tensile properties of the N3300A cross-linked PI aerogels were also measured for those made using at least 7.5 wt % polymer concentration. Samples having 5 wt % polymer concentration were too fragile and most of the dog bone specimens of these samples broke during fabrication. Therefore, the empirical models for tensile modulus and stress were analyzed only from 7.5 – 10 wt%. Figure 8a shows some representative tensile stress-strain curves from aerogels prepared using 7.5 wt% and n of 30. The stiffness of the PI aerogels increased with increasing amount of DMBZ in the backbone, as expected. Tensile strain at break was below 10 % for all samples in the study. As shown in Figure 8b, aerogels with the most rigid backbone (100 mol % DMBZ) displayed the highest tensile modulus. Although tensile stress at break as seen in Figure 8c also increases with DMBZ content, there are strong synergistic effects between polymer concentration and the diamine used, and polymer concentration and n. At low concentration, the tensile strength was similar for all three diamines used, especially at high n value. Furthermore, tensile strength increased more with increasing polymer concentration when the DMBZ was used as the diamine.


16

Page 17 of 28

1.4

Tensile stress, MPa

1.2 1.0 0.8 0.6 7.5 wt%, n = 30

0.4

DMBZ DMBZ-ODA ODA

0.2 0.0 0.00

0.02

0.04

0.06

DMBZ DMBZ-ODA ODA

DMBZ DMBZ-ODA ODA

MPa

Tensile stress at break, MPa

100

10

35 30 25 eatin g un it, n

Rep

b)

10.0 9.5 % 9.0 wt 8.5 ., c on 8.0 rc e 7.5 20 lym Po

R2 = 0.8292 Stdv. (log) = 0.085

1

0.08

Strain

a)

, Tensile modulus

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


c)

10

1

0.1

10.0 9.5 % 2 9.0 wt R = 0.9505 ., c 8.5 Stdv. (log)= 0.062 n 35 co 8.0 30 er 25 7.5 Repe lym ating 20 unit, n Po

Figure 8. Graphs of a) typical tensile stress-strain curves of PI at 7.5 wt% solid and empirical models for b) tensile modulus and c) tensile stress at break vs polymer concentration and n The on-set of decomposition temperatures (Td) of N3300A cross-linked PI aerogels in N2 were determined by thermal gravimetric analysis (TGA). The empirical model of the decomposition temperature of aerogels in the study is graphed in Figure 9. In agreement with previous studies from which the polyimide aerogels had the same backbones but cross-linked with either TAB17 or BTC,15 DMBZ-based aerogels typically had Td at around 520-535 °C due to the loss of the pendant methyl groups, and those based on ODA had Td at around 590-605 °C. Since the cross-linker was not in high enough concentration in the formulation, weight loss due to cross-linker is not obvious


17


in the aerogels, but the N3300 is known to have an onset of decomposition of about 300 oC

38

620

Model Data

o

on, C

which limits the use temperature of the aerogels.

si Onset decompo

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 18 of 28

590 560 530 0.0

500 2

R = 0.9848 0.5 25 n Stdv. = 0.007 Re 30 tio pea c a 35 ting fr 1.0 uni BZ M t, n 40 D

Figure 9. Graphs of empirical model of TGA onset decomposition vs DMBZ fraction in the polymer backbone and n Isothermal aging of the N3300A cross-linked PI aerogels at 150 oC and 200 oC in air up to 500 hours was also evaluated. Weight loss of the aged specimens over the first 24 hour period was under 1 wt% at both aging temperatures, perhaps due to the loss of NMP residue. Otherwise, the overall weight of the aerogel specimens remained constant up to 500 hours. All of the aerogels in the study shrank during the first hour of aging to a significant degree, but leveled off after that as shown in the graph in Figure 10a for representative samples from the study. Aerogels made using 100 mol% DMBZ shrank the most during aging, as previously seen with BTC cross-linked aerogels. Density also increased concomitant with the shrinkage during aging, especially in aerogels made with 100 mol% DMBZ, as shown in Figure 10b.


18

Page 19 of 28

DMBZ, 150 C o ODA, 150 C DMBZ, 200oC ODA, 200oC

0.8 0.7

30 Density, g/cm

3

0.6

20

10

0.5 0.4 0.3 0.2 0.1

10 wt%, n = 40

0 0

a)

o

DMBZ, 150oC ODA, 150oC DMBZ, 200oC ODA, 200oC

40

Shrinkage during heating, %

100

200

300

10 wt%, n = 40

0.0

400

0

500

b)

Aging duration, h

100

200

300

400

500

Aging duration, h

Figure 10. Graphs of a) shrinkage occurred during heating and b) density of aerogels made with DMBZ (sample 4) and ODA (sample 7) during heating Surface areas are also greatly reduced in the aerogels during aging. The bar graph in Figure 11 compares the BET surface area after 500 hours of aging for representative ODA and DMBZ derived aerogels. Both ODA and DMBZ derived aerogels are reduced in surface area especially at 200 oC, where the final surface areas are 100-150 m2/g. 2D Graph 2

600

2

BET surface area, m /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


As fabricated, RT o 500 h, 150 C o 500 h, 200 C

450

300

150

0 DMBZ

ODA Diamine

Figure 11. BET surface area of aerogels made with DMBZ (sample 4) and ODA (sample 7) as fabricated and isothermally aged at 150 ºC and 200 ºC after 24 and 500 hours


19


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 28

Thus, in the polyimide aerogels with DMBZ and ODA in the backbone, it is not so much the onset of decomposition that limits the use temperature as much as the shrinkage occurring during aging, which is more dependent on the oligomer backbone. In a previous study, however, shrinkage during aging was demonstrated to be reduced by 50 % using a bulkier, cardo-diamine, 9,9′-bis(4-aminophenyl)fluorene (BAPF), in the polymer backbone.19

Surface areas of the

aerogels in that study also remained higher after aging (>300 m2/g).While this study used BTC as the cross-linker, it is anticipated that the same result should be expected with N3300A, using BAPF in the backbone. Conclusion. Polyimide aerogels with backbones based on BPDA in combination with DMBZ and/or ODA, were cross-linked with a tri-isocyanate, Desmodur N3300A. The aerogels had densities ranging from 0.06 to 0.20 g/cm3 and BET surface areas as high as 580 m2/g, depending on backbone chemistry, polymer concentration and number of repeat units, n, in the formulation. Mechanical properties were similar to or better than aerogels previously reported with similar backbone chemistry but made with other cross-linkers, including TAB, OAPS, PMA and BTC. Since N3300A is derived from hexamethyldiisocyanate, which does have a lower onset of decomposition than TAB, OAPS or BTC and therefore may limit use temperature for some applications. However, for lower temperature use, N3300A may be a good choice for commercial applications, since it is lower in cost than all of the other cross-linkers previously investigated, and other properties of the aerogels are not compromised. Corresponding Author *Email: [email protected], [email protected] Acknowlegement. The authors thank NASA Convergent Aeronautics Solutions (CAS) for support of this work. We also thank Haiquan Guo (Ohio Aerospace Institute) for nitrogen sorption


20

Page 21 of 28

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


measurements, Nathan Wilmoth (previously with Vantage Partner, LLC) and Jordan McCrone (previously with Vantage Partners, LLC) for mechanical tests.


21


Onset decomposition temperature, oC

Sress at break, MPa

Tensile modulus, MPa

Stress at 5% strain, MPa

Compression modulus, MPa

Surface area, m2/g

Porosity, %

Density, g/cm3

Shrinkage, %

Polymer concentration,Wt%

DMBZ mol %

Repeating unit, n

Table 1. Formulation and properties of as fabricated N3300A cross-linked PI aerogels

Experiment #

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 22 of 28

1

20

0

5

21.0

0.087

94

428

4.2

0.16

(a)

(a)

593

2

20

100

5

13.6

0.068

95

552

(a)

(a)

(a)

(a)

522

3

30

50

7.5

14.1

0.101

93

452

33.1

0.50

30.1

1.09

537

4

40

100

10

16.4

0.141

90

499

228.0

1.67

82.7

2.75

536

5

20

0

10

16.0

0.139

91

430

44.6

0.63

27.0

0.31

603

6

40

50

7.5

13.6

0.100

93

453

18.2

0.53

33.2

0.97

522

7

40

0

10

20.9

0.167

89

350

36.9

0.86

39.2

1.49

8

30

50

7.5

13.2

0.198

93

471

19.4

0.48

31.5

1.17

528

9

40

0

5

19.2

0.084

94

389

3.8

0.16

9.54

0.17

595

10

30

50

7.5

13.3

0.098

93

435

25.9

0.53

25.1

1.00

527

11

30

50

7.5

13.6

0.099

93

440

14.8

0.49

23.8

1.25

533

12

20

100

10

13.1

0.124

91

517

109.1

1.24

72.1

1.70

531

13

20

50

7.5

11.2

0.089

94

519

28.2

0.40

22.5

0.88

536

14

30

50

7.5

11.7

0.094

93

473

15.5

0.43

18.1

0.36

527

15

30

50

5

14.4

0.070

95

471

5.4

0.16

(a)

(a)

532

16

30

50

7.5

12.9

0.097

93

458

20.6

0.50

30.7

1.31

532

17

30

0

7.5

21.2

0.132

91

429

14.6

0.45

20.7

1.04

599

18

30

50

10

14.4

0.123

92

451

23.9

0.72

33.3

1.03

537

19

40

100

5

14.7

0.074

95

579

22.6

0.28

(a)

(a)

533

20

30

100

7.5

15.3

0.106

92

559

65.4

0.80

36.0

1.09

535

(a) Samples were too fragile for testing.


22

Page 23 of 28

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27


Table of Content. 2D Graph 1

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

10 N3300A - DMBZ N3300A - DMBZ/ODA N3300A - ODA BTC - DMBZ BTC - DMBZ/ODA BTC - ODA

1

0.1 0.06 0.07

0.085 0.1

0.15

Density, g/ cm3


Page 28 of 28

Polyimide Aerogels Using Triisocyanate as Cross-linker - ACS

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