Preparation and characterization of PMR-type polyimides from furan-based diamines

Abstract

PMR-type polyimide resins were prepared in an analogous manner PMR-15 resin is formulated, using two furan-based diamines. The PMR resins from both furan-based diamines showed well-separated transitions that are well-known for PMR-type polyimides, and their oligoimide prepolymers showed lower softening temperatures and cure onsets than PMR-15, making them suitable for lower temperature processing. The cure kinetics of the prepolymers were followed by FTIR spectroscopy. The glass transition and thermal degradation of the crosslinked polyimides were characterized by DMA and TGA which showed in general these polyimides had lower thermal properties than PMR-15. A study on the effects of prepolymer molecular weight was conducted using DFDA to find the optimal stoichiometric number n for better processability and thermal properties. The study determined the polyimide based on DFDA with n = 1.5 had overall a good balance in properties. Its oligoimide precursor softens at 135°C and the cure onset temperature is 231°C. The polyimide has a glass transition temperature of 296°C. Degradation study by TGA showed the lower decomposition temperatures which limit their application temperature to below 300°C. A 1000-h thermo-oxidative stability experiment at 280°C resulted only 6.8% weight loss for DFDA-PI with n = 1.5, making it suitable for high temperature applications around 280°C.

Keywords

PMR-type polyimides furan-based diamines difurfuryl diamine

Introduction

Extensively used in advanced engineering fields such as aerospace, automotive, insulation, electronics, and sporting goods industries, thermosetting polymers are mostly derived from petroleum industry. Growing environmental and economic concerns over petroleum derivatives have raised interest in manufacturing polymeric materials from renewable biobased feedstock. However, polymers comprised of biobased building blocks often suffer from lower thermal and mechanical properties due to a lack of aromaticity or rigid entities in their monomeric structures.^1,2 Therefore, the exploration of aromatic or rigid compounds derived from renewable resources should be significant for the next generation of biobased high-performance polymers.^3,4

Biomass derived monomers containing five-membered furan heterocyclic rings are considered a promising alternative to petroleum-based phenyl building blocks in commercial engineering polymers because of the unique combination of size, polarity, and aromaticity of furan rings.^4–6 Furanic building blocks are primarily derived from polysaccharides and sugars. First generation furanic chemicals include 2-fuancarboxaldehyde, or commonly called furfural, and 5-(hydroxymethyl)-2-furfural (HMF).^6,7 The US Department of Energy in 2004 identified 2,5-furandicarboxylic acid (FDCA) as one of 12 priority chemicals for establishing the “green” chemistry industry of the future.⁸ Synthetic routes to FDCA and its polyesters, polyamides, and a few polyurethanes have been developed.^9–14 Poly (ethylene 2,5-furanoate) (PEF) represents by far the most intensively studied and the most viable for industrial scale production. Compared to its phenyl counterpart poly (ethylene terephthalate) (PET), PEF shows comparable glass transition temperature, improved processability, and exceptional barrier properties.¹⁵

Aromatic diamines play a crucial role in thermosetting polymers because they endow thermosets with excellent thermomechanical properties. One area of application is polyimides. Thermosetting polyimides are the leading chemistry for composite materials in high temperature applications above 230°C due to superior thermo-oxidative stability than state-of-the-art epoxy chemistries for advanced composite technology.¹⁶ However, most of these diamines used, such as 4.4’-methylenedianiline (MDA) and diethyltoluenediamine (DETDA) with phenyl building blocks, are non-renewable and toxic.¹⁷ Recently, our research group has successfully prepared a series of furan-based diamines through acidic condensation of furfurylamine with various aldehydes. Especially, the product from condensation of furfurylamine and formaldehyde, 5,5′-methylenedifurfurylamine (DFDA), has attracted much interest due to its comparable structure to MDA. The use of DFDA has been explored in epoxies,¹⁸ benzoxazines,¹⁹ and thermoplastic polyimides.²⁰ However, its application in thermosetting polyimide chemistry has not been studied in literature.

The PMR-15 polyimide is the most widely used high temperature thermosetting polyimide resin. PMR stands for Polymerization of Monomeric Reactants. In the development of PMR-15, NASA carried out a series of experiments designed to optimize both the constituents and the molecular weight. The best overall balance of processing characteristics, composite thermomechanical and physical properties and 316°C (600°F) thermo-oxidative stability is provided by a monomer combination consisting of the dimethyl ester of 3.3′,4.4′-benzophenone tetracarboxylic acid (BTDE), methylene dianiline (MDA) and the monomethyl ester of 5-norbornene-2,3-dicarboxylic acid (nadic ester, or NE) in a molar ratio of 2.087:3.087: 2. This ratio corresponds to a formulated molecular weight of the imidized polymer of 1500 (hence PMR-15).^21,22 Currently, the accepted reaction sequence of the PMR-15 approach to thermosetting polyimides is as follows²³: poly (amic acid) solution is prepared from the condensation reactions among BTDE, NE, and MDA. The unique chemistry utilizes nadic ester as an endcap to control the molecular weight of the polyimide precursor, while the norbornene double bond provides a crosslinking mechanism. Imidization begins above 100°C and finishes below 250°C. The imidized oligomer softens and begins to flow in the temperature range of 175°C–250°C. Crosslinking reactions occur at temperatures above 290°C. These well-spaced transitions are characteristic of PMR chemistry and provide large processing windows for relatively easy processing which make them more attractive than most other high temperature polymer systems.²¹ However, the successful material relies on the use of MDA, which is a potent liver toxicant with an occupational exposure limit (OEL) of only 10 ppb averaged over an 8-h workday.^24,25 It is also a suspected carcinogen and a mutagen.²⁶ The search for a less-toxic diamine as the replacement for MDA is on-going.^22,24,27

This work described the preparation and characterization of PMR-type polyimide resins containing furan-based diamines and discussed the advantages and shortcomings of using these diamines in PMR-type polyimide chemistry. Notably, one advantage aligns with a key principle of green chemistry²⁸: the use of renewable feedstock. Specifically, the primary material for synthesizing the diamines is furfurylamine, which is derived from biomass. To directly compare the effects of replacing MDA with DFDA or VDFDA, amic acid/ester solutions containing DFDA and VDFDA were prepared first with the stoichiometric number n = 2.087. The imidization kinetics, prepolymer properties, and cured polymer properties were characterized. For the optimization of prepolymer processability and final material properties, amic acid/ester precursor solutions with varied stoichiometric number n were prepared to study the effect of crosslink density on softening temperatures and cure kinetics of prepolymers, and thermal properties of polyimides.

Experimental section

Materials

Furfurylamine, formaldehyde, vanillin, sodium hydroxide, concentrated HCl solution, chloroform, magnesium sulfate anhydrous. Methanol, 4.4’-methylene dianiline (MDA), 3.3’,4.4’-benzophenone tetracarboxylic dianhydride (BTDA), and 5-norbornene-2,3-dicarboxylic anhydride (nadic anhydride, or NA), were all purchased from Sigma-Aldrich and used as received.

Synthesis of 5,5′-methylenebis[2-furanmethanamine] (5,5′-methylene difurfurylamine, or DFDA)

DFDA was prepared as previously reported in the literature.^2,29 The reaction and the product are shown in Scheme 1, structure 1. Furfurylamine (135 g, 1.39 mol) was charged to a 2-L round-bottom flask, and 6 M HCl solution (330 mL, 2 mol) was then added dropwise to the flask under an ice bath. After the addition of HCl solution, 37 wt% formaldehyde solution (37.6 g, 463 mmol) was added to reaction at once at 25°C. After 4 h, the reaction mixture was neutralized using 6 M NaOH solution (330 mL, 2 mol) and was then extracted with 100 mL chloroform 3 times. The combined organic layer was washed with 100 mL distilled water 3 times and dried over anhydrous MgSO₄. The dried solution was finally distilled, while temperature was gradually raised to 130°C, to yield a reddish-brown oily liquid product. Purity: 99.5%, determined based on the ¹H NMR peak area ratio of proton chemical shifts at 3.95 ppm and 3.78 ppm. ¹H NMR (CDCl₃, 500 MHz, ppm): δ 1.48 (s, 4H), 3.78 (s, 4H), 3.95 (s, 2H), 5.99 (d, 2H, J = 3.0 Hz), 6.05 (d, 2H, J = 3.0 Hz).

Synthesis of 4-{bis[5-(aminomethyl)furan-2-yl]methyl}-2-methoxyphenol (vanillin-DFDA, or VDFDA)

VDFDA was prepared in a similar procedure (Scheme 1, structure 2). Furfurylamine (135 g, 1.39 mol) was charged to a 2-L round-bottom flask, and 6 M HCl solution (330 mL, 2 mol) was then added dropwise to the flask under an ice bath. After the addition of HCl solution, vanillin (28.2 g, 185 mmol) was added to the reaction mixture. The reaction was carried out under 40°C for 24 h, after which the reaction mixture was neutralized using 6 M NaOH solution (330 mL, 2 mol) and extracted with 100 mL chloroform 3 times. The combined organic layer was washed with 100 mL distilled water 3 times and dried over anhydrous MgSO₄. The dried solution was finally distilled, while temperature was gradually raised to 140°C, to yield a reddish-brown solid product. Purity: 99%, determined based on the ¹H NMR peak area ratio of proton chemical shifts at 5.29 ppm and 3.76 ppm. ¹H NMR (CDCl₃, 500 MHz, ppm): δ 2.64 (s, 4H), 3.76 (s, 4H), 3.79 (s, 3H), 5.29 (s, 1H), 5.89 (d, 2H, J = 3 Hz), 6.05 (d, 2H, J = 3 Hz), 6.69 (dd, 1H), 6.74 (d, 1H, J = 2 Hz), 6.81 (d, 1H, J = 8 Hz).

Sample Preparation

Oligomeric (amic acid/ester) solutions, commonly referred to as poly (amic acid) solutions, or PAAs, were prepared from the dimethyl ester of 3.3’,4.4’-benzophenone tetracarboxylic acid (BTDE), the monomethyl ester of 5-norbornene-2,3-dicarboxylic acid (nadic ester, or NE), and the diamines. The preparation of oligomeric amic acid/ester solution from DFDA, where the stoichiometric number n = 2.087, is described here (Scheme 2, steps 1-2). A solution of BTDE and NE was prepared in a 250 mL round bottom flask by refluxing a suspension of BTDA (23.41 g, 72.7 mmol) and NA (11.43 g, 69.6 mmol) in anhydrous methanol (40 mL). The anhydride powders dissolved to yield a pale-yellow solution, which was refluxed for an additional 3 h and left to cool to ambient temperature. DFDA (22.17 g, 107.5 mmol), dissolved in methanol (35 mL) beforehand, was added to the solution. The mixture was left to stir at ambient temperature over night to yield a dark brown DFDA based oligomeric amic acid/ester solution, with a solid content of 50% by weight. VDFDA amic acid/ester solution and MDA amic acid/ester (PMR-15) solution were prepared using VDFDA and MDA accordingly.

Oligomeric amic acid/ester powders were obtained from drying aliquots of the resin solutions in a vacuum oven under 90°C. Following a cumulative thermal treatment, the precursors were imidized to form oligoimide prepolymers. The prepolymers were cured under proper curing temperatures to form crosslinked polymer networks.

Carbon fiber prepregs were prepared by soaking carbon fiber fabric weaves (AS4 GP; four harness satin weave, 3K, no surface treatment) in a resin reservoir. The soaked carbon fiber prepregs were then thermally treated to remove solvent, become imidized, and cured. Rectangular prepreg pieces, 35 mm × 13 mm, were cut out at different stages to allow dynamic mechanical analysis of the oligoimide prepolymers and cured polyimides.

Characterization

Proton nuclear magnetic resonance (¹H NMR) spectra were acquired using a Varian Unity Inova NMR spectrometer operating at a 500 MHz frequency. Differential scanning calorimetry (DSC) experiments were performed with a TA Instruments Q2000 DSC. About 10 mg samples were loaded in hermetically sealed T-zero DSC pans and heated to target temperatures at 10°C/min. Dynamic mechanical analysis (DMA) experiments were performed on a TA Instruments Q800 DMA in a single-cantilever mode. Thermogravimetric analysis (TGA) experiments were performed using a TA Instruments Q50 TGA at 10°C/min under both air and argon atmospheres. Viscosity measurements were performed on a TA Instruments ARES-G2 rheometer. Mid-IR spectra were acquired on a Thermo Scientific Nicolet 6700 FT-IR spectrometer equipped with a Specac Quest ATR accessory with temperature control up to 300°C. For kinetics studies, a background spectrum for each temperature was taken and stored before the collection of real-time kinetics spectra at the temperature.

A 1000-h thermo-oxidative stability experiment was performed in a high temperature oven (MTI Corp., model: DZF-6020-HT/500) set at 280°C under continuous air flow at 60 mL/minute. Cured disk samples of 25.4 mm diameter and 1 mm thickness, weighing approximately 0.6 g, were prepared using compression molding. The disks were placed in aluminum boats, which were then placed in the oven for testing. At time points throughout the duration of the experiment, the samples were taken out for their weight measurements on an analytical balance and placed back in the oven immediately after the measurements.

Results and discussion

Imidization kinetics

Oligomeric amic acid/ester solutions containing MDA, DFDA and VDFDA were prepared with the stoichiometric number n = 2.087. The calculated number-average molecular weights of the amic acid/ester oligomers prepared from MDA, DFDA, and VDFDA, were calculated to be 1501 g/mol, 1526 g/mol, 1903g/mol, respectively. The solutions were converted to powders in a vacuum oven under 90°C as described in Sample Preparation. Figure 1 shows the melting and imidization thermograms of the amic acid/ester oligomers from DSC. The MDA- and DFDA-amic acid/ester oligomers started to melt above 100°C. The VDFDA-oligomer exhibited a slightly higher melting temperature due to a higher number-average molecular weight. The three precursors all showed imidization peaks at around 150°C.

Cumulative thermal treatments were used to imidize the amic acid/ester oligomers in four stages which are given in Table 1. The imidization kinetics of these amic acid/esters through the cumulative thermal treatment were monitored by IR spectroscopy. Figure 2(a) shows the stacked mid-IR spectra of DFDA-based amic acid/ester at selected time points during the imidization process, where the peak area around 1470-1570 cm⁻¹ representing the in-plane bending of N-H bond in secondary amide -C (=O)NH- group diminished, and the imide peaks grew at 1776 cm⁻¹, 1703 cm⁻¹, and 1380 cm⁻¹, which correspond to the asymmetric and the symmetric stretching of imide carbonyl group, and the stretching of imide C-N-C group, respectively.^30–32 The enlarged spectrum area around 1750-1900 cm⁻¹ in Figure 2(b) clearly indicates most of the imidization occurred for DFDA-based precursor before the 180°C bake. This was not the case for the MDA imidization and was the reason for the higher temperature imidization reaction schedule for MDA polyimides relative to the furan-based polyimides. Figure 2(b) also indicates the formation of some anhydrides. The peak at 1776 cm⁻¹ was used to calculate the extent of imidization, using the equation

α = A_{t} (1776 {c m}^{- 1}) / A_{e n d} (1776 {c m}^{- 1})

, where

α

is the degree of imidization,

A_{t} (1776 {c m}^{- 1})

is the absorbance peak area at 1776 cm⁻¹ at time point t,

A_{e n d} (1776 {c m}^{- 1})

is the absorbance peak area at 1776 cm⁻¹ when imidization is completed. The imidization kinetics of the three amic acid/ester oligomers are given in Figure 3. The plot showed that VDFDA amic acid/ester had a slightly slower imidization rate than the DFDA one but the imidization was also completed after the 180°C bake. However, the MDA-based precursor was found to have a much slower imidization rate, yielding only about 25% imidization after the 150°C bake compared to around 80%–90% imidization for the precursors containing the furan-based diamines. The imidization of the MDA-based precursor was not complete after the 200°C bake but required 230°C to complete it.

Table 1.

Cumulative thermal treatments for imidization of the amic acid/ester oligomers.

Amic acid/ester oligomer	Stage 11 h	Stage 21 h	Stage 3½ hour	Stage 4½ hour
MDA-	130°C	150°C	200°C	230°C
DFDA-	130°C	150°C	180°C	200°C
VDFDA-	130°C	150°C	180°C	200°C

Oligoimide Properties

The oligoimide prepolymers were obtained after imidization, and their thermal properties were studied by both DSC and DMA, shown in Figures 4 and 5. It is hard to test brittle oligoimide samples on a dynamic mechanical analyzer, so carbon fiber prepreg samples with the oligoimides were used. The resin content in these prepregs was not controlled. Therefore, the modulus values from these DMA measurements should not be interpreted as the moduli of the oligoimides but rather the different resin contents in the samples. However, the peaks in loss modulus are representative of the thermal properties of the oligoimides. Both the DSC and DMA experiments showed the prepolymers based on DFDA and VDFDA softened at around 150 °C–160°C, whereas the MDA prepolymer did at around 200°C. Also, DSC thermograms showed that, for each prepolymer an exothermic process appeared about 100°C above the softening temperature of each oligoimide which was due to its curing reaction. This was confirmed by DMA storage moduli of these oligoimides which began to rise at similar temperatures because of the formation of more rigid networks upon curing. The wide temperature windows between softening points and cure onsets allow for easy processing of these polyimides. The cure onset temperatures for DFDA- and VDFDA-oligoimides were also about 50°C lower than that of MDA-based prepolymer. This important information is essential in guiding the processing of the materials; especially, the cure onset temperature sets the ceiling temperature for oligomide processing prior to cure during composite manufacturing.

To compare the processability of these oligoimides, melt viscosity measurements were attempted. Disks of 25.4 mm diameter were molded from oligoimide powders using a heat press. The molding temperatures were 240°C for MDA-oligoimide, 200°C for DFDA-oligoimide, and 180°C for VDFDA-oligoimide. The disks were placed between two parallel disposable aluminum plates on a TA Instrument ARES-G2 rheometer for steady shear viscosity measurements. The temperature of the experiments went up to 275°C for MDA-oligoimide, and to 240°C for the DFDA-oligoimide and the VDFDA-oligoimide. Reliable melt viscosity data were not obtained due to partial melting of the samples and high torque values. However, the molding temperatures for these oligoimide prepolymers can serve as an indicator of the rheological properties of the materials, in analogy to squeeze flow index.³³ The lower molding temperatures of the oligoimides from DFDA and VDFDA are indicative of better processability of these oligoimides compared to the MDA-oligoimide.

Extent of Cure

PMR-15 polyimide is normally cured at 315 °C–330°C, which is about 30 °C–40°C above the cure onset temperature, for 2-3 h followed by post-cure.^21,24 Following the protocol, the DFDA- and the VDFDA-prepolymers in this study were cured at 280°C and 270°C, respectively, which were about 30°C above their cure onset temperatures as determined by DSC shown in Figure 4. Their cure kinetics were studied by mid-IR spectroscopy. Figure 6 shows stacked mid-IR spectra of DFDA-oligoimide prepolymer collected real-time in series over a period of 4 h on a heated stage at 280°C. The figure showed that, except for the slight changes in the 1776 cm⁻¹ imide absorption and 1852 cm⁻¹ anhydride absorption,³⁰ the only peak that significantly decreased with increasing curing time was the absorption band at 901 cm⁻¹; see also the inset (a). Same observation was found for VDFDA-oligoimide curing at 270°C. Zhao et al.³⁴ found that the absorption band at 906 cm⁻¹ that was present in the IR spectra of both the monomer and the polymer of 5-vinyl-2-norbornene disappeared in the spectrum of the product from addition polymerization which proceeded mainly through reaction of the norbornene double bond, indicating that the band was associated with the norbornene double bond. In the IR spectrum of nadic anhydride, a prominent peak at 901 cm⁻¹ was observed, which corresponds to the double bond. Therefore, the absorption band at 901 cm⁻¹ can be used to monitor the conversion of norbornene double bond in the crosslinking reaction. The relative intensity of the absorption band at 901 cm⁻¹ as a function of progressing cure time was plotted in Figure 6, inset (b), which showed DFDA polyimide reached 72% degree of cure after 4 h at 280°C. Same study showed VDFDA-PI reached 89% degree of cure after only 2.5 h at 270°C. The fast cure rate of VDFDA-PI is attributed to the catalytic effect of the phenolic hydroxy group on the vanillin benzene ring.

Thermal properties of polyimides

DFDA-PI and VDFDA-PI were cured according to the cure schedules determined from the cure study. Cured MDA-PI, DFDA-PI, and VDFDA-PI were then post-cured under 315°C, 300°C, and 290°C, respectively, for 2 h. No appreciable mass loss was found due to post-cure. Their glass transition temperatures were determined by DMA using composite bar samples. Again, the composite bar samples were prepared as an effective way to measure the T_g of the polyimides, and storage moduli values should not be viewed as the moduli of the polyimides. As shown in Figure 7, the T_g of MDA-PI was 362°C according to its loss modulus E” peak, same as literature values,²⁴ and the T_g values of DFDA-PI and VDFDA-PI are 275°C and 262°C, respectively, which are much lower than that of MDA-PI. Throughout this study lower thermal properties of furan-containing polyimides have been consistently observed. This is attributed to the additional methylene linkage connecting the furan rings to the amine groups in the furan-based diamine structure as well as the lower aromaticity of furan rings compared to benzene rings. VDFDA-PI showed an unexpected, slightly lower T_g than DFDA-PI, but this is explained in terms of disrupted chain packing. Although the substituent group on the methylene bridge of VDFDA could increase the overall chain stiffness, the effect is negated by the fact the chain packing is disrupted by the substituent which results in higher free volume and a decrease in T_g.

The thermal stability of the cured polyimides was investigated by TGA both under inert and air environment, shown in Figure 8. The thermal decomposition temperatures corresponding to 5% weight loss are given in Table 2. The data shows that both the furan-containing polyimides started to degrade before 400°C whereas the T_d,5% of MDA-PI was about 100°C higher both under inert and air environment. The derivatives of the decomposition curves showed peaks around 400°C for the furan-containing polyimides. The mechanism for this decomposition is not studied, but it has been reported in literature that polymers containing furan moieties start to decompose around 350 °C–400°C range,^35,36 earlier than their phenylene ring-containing counterparts. Further investigation using gas chromatography/mass spectroscopy is required to more clearly identify the mechanism of decomposition involving furan-based polymers.³⁵

Table 2.

Summary of the 5% thermal degradation temperature T_d,5% of the three polyimides.

T_d,5% (°C)	MDA-PI	DFDA-PI	VDFDA-PI
In argon	492	399	376
In air	471	397	382

Optimization of material properties

PMR-15 is an optimized product with the best overall balance of processing characteristics, material physical and mechanical properties, and thermo-oxidative stability. In this work, the stoichiometric number n (see Scheme 2) was varied for the DFDA-PI system to study the effects of prepolymer molecular weight on the processing parameters and material properties. It was postulated that as the prepolymer molecular weight decreases with decreasing n, the prepolymer would have a lower softening point that would promote easier processing. On the other hand, a smaller n results in a smaller molecular weight between crosslink points and therefore a higher crosslink density, which should yield a stiffer polymer network that exhibits better thermal properties. To confirm the predictions and find the optimal stoichiometric number n, DFDA-containing oligoimides and polyimides were prepared with n being 1, 1.5, 2.087, 3, and 5. For simplicity purpose, n = 2.087 is shown as n = 2. Note that because the molecular weight of nadic anhydride is approximately half of that of the dianhydride BTDA, the change in n only leads to the variation of the weight percentages of DFDA in these systems by only 0.1% according to our calculation, so any significant changes in properties should not be attributed to the weight percentage of DFDA in the system, but mostly to the change in crosslink density. The softening temperatures and the cure onsets of the oligoimides, and the glass transition temperatures and the T_d,5% degradation temperatures of the polyimides as a function of n are given in Table 3 and plotted in Figure 9, which show that, as postulated, with decreasing n the softening temperature of the oligoimides decreases and the T_g of the polyimides increases, whereas the cure onset and the degradation temperature T_d,5% in argon stay relatively constant with changing n. It is clear that, a smaller stoichiometric number n broadens the processing window between the softening temperature and the cure onset, and also yields a stiffer network with a higher T_g.

Table 3.

The softening temperatures and the cure onsets of the DFDA-oligoimides, and the glass transition temperatures and the 5% degradation temperatures of the DFDA-PIs.

Temperature (°C)	n = 1	n = 1.5	n = 2	n = 3	n = 4
Softening	108	135	155	163	167
Cure onset	228	231	234	235	241
T_g (E″)	322	296	275	265	254
T _d,5%	401	395	399	398	401

Thermo-oxidative stability

Long term high temperature stability of the polyimides was evaluated by a 1000-h thermo-oxidative stability experiment under 280°C isothermal temperature. The mass loss as percentages of their original weights were recorded as a function of time, shown in Figure 10. To establish a baseline for comparison, MDA-PI was tested and showed about 3.5% mass loss after 1000 h. Among all the polyimides containing furan-based diamines, DFDA-PIs with n = 1 and n = 1.5 showed the best thermo-oxidative stability at 280°C due to higher crosslink densities and T_g’s, whose mass losses were about 7% after 1000 h. The percentage weight loss is comparable to the 6.9% mass loss of PMR-15 under 315°C for 1000 h³⁷ However, DFDA-PI with n = 2 and VDFDA-PI with n = 2 showed the worst thermo-oxidative stability, under-performing DFDA-PIs with n = 3 and n = 4. A repeat experiment showed the same results. A similar trend was observed by Leung et al. in MDA-PI networks with varied molecular weight between crosslinks.³⁷ The observation was counter-intuitive; however, it could be argued that in a highly crosslinked, rigid network, chain mobility is increasingly being restricted as cure progresses, so that some unreacted fragments remain entrapped within the network. These fragments would be more susceptible to oxidative degradation leading to increased weight loss in long-term aging conditions. Increasing the molecular weight of the prepolymer decreases the crosslink density and allows a more complete cure, leading to a more thermo-oxidatively stable polymer network.

Conclusions

PMR-type polyimide resins were prepared from the dimethyl ester of 3.3’,4.4’-benzophenone tetracarboxylic acid (BTDE), the monomethyl ester of 5-norbornene-2,3-dicarboxylic acid (NE), and two furan-based diamines, 5,5’-methylene difurfurylamine (DFDA) and 4-{bis [5-(aminomethyl)furan-2-yl]methyl}-2-methoxyphenol (VDFDA), in an analogous manner PMR-15 (MDA-PI) resin were formulated, and were compared to MDA-PI. The PMR resins from both furan-based diamines showed well-separated transitions that are well-known for PMR-type polymides, ensuring good processability of the resins. Their oligoimide prepolymers showed softening temperatures and cure onsets, both about 50°C lower than the MDA based prepolymer, making them suitable for lower temperature processing. The cure kinetics of the prepolymers were followed by FTIR spectroscopy. It was determined that subjecting DFDA-oligoimide to 280°C for 4 h, and VDFDA-oligoimide to 270°C for 2.5 h, promoted efficient curing of the two materials while maintaining the integrity of the networks. The T_g’s of DFDA-PI and VDFDA-PI following post-cure were much lower compared to that of MDA-PI. Lower thermal properties are attributed to the additional methylene linkage connecting the furan ring to the amines in the DFDA structure as well as the lower aromaticity of furan rings compared to benzene rings. A study on the effects of prepolymer molecular weight was conducted to find the optimal stoichiometric number n for better processability and thermal properties. The study concluded that a lower n, which corresponds to a smaller prepolymer molecular weight and a higher polymer crosslink density, results in lower softening temperature of the oligoimide and higher glass transition temperature of its polymer. DFDA-oligoimide with n = 1.5 was found to soften at 135°C and its polymer has a glass transition temperature of 296°C. Degradation study by TGA showed all the PMR resins had high char yields, but the lower decomposition initiation temperatures of the DFDA based systems limit their application temperatures to below 300°C. A 1000-h thermo-oxidative stability experiment at 280°C showed DFDA-PI with n = 1.5 lost about 6.8% of its original weight, which is a number comparable to the percentage loss of mass of PMR-15 at 315°C, making DFDA-PI with n = 1.5 suitable for applications around 280°C.

Footnotes

Acknowledgments

Research was sponsored by Strategic Environmental Research and Development Program Project Number WP-2402 and the Army Research Laboratory under Cooperative Agreement Number W911NF-15-2-0017. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the U.S. Government or the Army Research Laboratory. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes not withstanding any copyright notation herein.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Strategic Environmental Research and Development Program Project Number SERDP WP-2402 and the Army Research Laboratory W911NF-15-2-0017.

Ethical statement

ORCID iDs

Jianwei Tu

Giuseppe R. Palmese

Data availability statement

Data Availability Statement included at the end of the article.*

References

Deng

Liu

, et al. Synthesis and properties of a bio-based epoxy resin from 2,5-furandicarboxylic acid (FDCA). RSC Adv 2015; 5: 15930. DOI: 10.1039/C5RA00242G.

La Scala

Sadler

, et al. Synthesis and characterization of thermosetting furan-based epoxy systems. Macromolecules 2014; 47: 3332–3342. DOI: 10.1021/ma500687t.

Chatti

Weidner

Fildier

, et al. Copolyesters of isosorbide, succinic acid, and isophthalic acid: biodegradable, high T_g engineering plastics. J Polym Sci, Part A: Polym Chem 2013; 51: 2464–2471. DOI: 10.1002/pola.26635.

Jeong

Kim

Shin

, et al. Synthesis and photo-polymerization of bio-based furanic compounds functionalized by 2-hydroxypropyl methacrylate group(s). J Appl Polym Sci 2013; 127: 2483–2489. DOI: 10.1002/APP.37959.

Gandini

Belgacem

. Furans in polymer chemistry. Prog Polym Sci 1997; 22: 1203–1379. DOI: 10.1016/S0079-6700(97)00004-X.

Delidovich

Hausoul

PJC

Deng

, et al. Alternative monomers based on lignocellulose and their use for polymer production. Chem Rev 2016; 116: 1540–1599. DOI: 10.1021/acs.chemrev.5b00354.

Gandini

. Furans as offspring of sugars and polysaccharides and progenitors of a family of remarkable polymers: a review of recent progress. Polym Chem 2010; 1: 245–251. DOI: 10.1039/B9PY00233B.

Werpy

Petersen

. Top value added chemicals from biomass, volume I — results of screening for potential candidates from sugars and synthesis gas. Oak Ridge, TN: U.S. Department of Energy, 2004. DOI: 10.2172/15008859.

Sousa

Vilela

Fonseca

, et al. Biobased polyesters and other polymers from 2,5-furandicarboxylic acid: a tribute to furan excellency. Polym Chem 2015; 6: 5961–5983. DOI: 10.1039/C5PY00686D.

10.

Ait

Essayem

Besson

. Selective aerobic oxidation of 5‐HMF into 2,5-furandicarboxylic acid with Pt catalysts supported on TiO2 and ZrO2 based supports. ChemSusChem 2015; 8: 1206–1217. DOI: 10.1002/cssc.201403390.

11.

Zhang

Zhen

Liu

, et al. Selective aerobic oxidation of the biomass-derived precursor 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid under mild conditions over a magnetic palladium nanocatalyst. Green Chem 2015; 17: 1308–1317. DOI: 10.1039/C4GC01833H.

12.

Papageorgiou

Tsanaktsis

, et al. Synthesis of the bio-based polyester poly(propylene 2,5-furan dicarboxylate). Comparison of thermal behavior and solid state structure with its terephthalate and naphthalate homologues. Polymer 2015; 62: 28–38. DOI: 10.1016/j.polymer.2015.01.080.

13.

Jiang

Maniar

Woortman

AJJ

, et al. Enzymatic polymerization of furan-2,5-dicarboxylic acid-based furanic-aliphatic polyamides as sustainable alternatives to polyphthalamides. Biomacromolecules 2015; 16: 3674–3685. DOI: 10.1021/acs.biomac.5b01172.

14.

Janka

Parker

Shalkh

, et al. Oxidation process to produce a crude dry carboxylic acid product. United States Patent; 2014.

15.

Burgess

Leisen

Kraftschik

, et al. Chain mobility, thermal, and mechanical properties of poly(ethylene furanoate) compared to poly(ethylene terephthalate). Macromolecules 2014; 47: 1383–1391. DOI: 10.1021/ma5000199.

16.

Kim

Lee

. Characterization and properties of carbon/PMR-15 composites. Polym Adv Technol 1995; 6: 711–716. DOI: 10.1002/pat.1995.220061106.

17.

US Department . Of health and human services. Methylenedianiline. Current Intelligence Bulletin 1986; 47: 1–19.

18.

Yadav

La Scala

, et al. Preparation and characterization of fully furan-based renewable thermosetting epoxy-amine systems. Macromol Chem Phys 2015; 216: 1441–1446. DOI: 10.1002/macp.201500142.

19.

Shen

Dai

Liu

, et al. Synthesis of high performance polybenzoxazine networks from bio-based furfurylamine: furan vs benzene ring. Polymer 2017; 122: 259–269. DOI: 10.1016/j.polymer.2017.06.075.

20.

Maktouf

Ghorbel

Afli

, et al. Polyimides based on furanic diamines and aromatic dianhydrides: synthesis, characterization and properties. Polym Bull 2011; 67: 1111–1122. DOI: 10.1007/s00289-010-0441-4.

21.

Wilson

. PMR-15 processing, properties and problems — a review. Br Polym J 1988; 20: 405–416. DOI: 10.1002/pi.4980200505.

22.

Yang

. Polyimide matrices for carbon fiber composites. In: Yang

(ed). Advanced Polyimide Materials: Synthesis, Characterization, and Applications. Amsterdam, Netherlands: Elsevier, Inc., 2018, pp. 94–136. DOI: 10.1016/B978-0-12-812640-0.00003-2.

23.

Labronici

Ishida

. Effect of degree of cure and fiber content on the mechanical and dynamic mechanical properties of carbon fiber reinforced PMR-15 polyimide composites. Polym Compos 1999; 20: 515. DOI: 10.1002/pc.10375.

24.

Harvey

Yandek

Lamb

, et al. Synthesis and characterization of a high temperature thermosetting polyimide oligomer derived from a non-toxic, sustainable bisaniline. RSC Adv 2017; 7: 23149. DOI: 10.1039/C7RA02182H.

25.

Weiss

Schuster

Müller

, et al. Dermal uptake and excretion of, 4,40-methylenedianiline during rotor blade production in helicopter industry — an intervention study. Ann Occup Hyg 2011; 55: 886–892. DOI: 10.1093/annhyg/mer051.

26.

Zhang

Lambert

Doll

, et al. 4,4’-Methylenedianiline-induced hepatotoxicity is modified by N-acetyltransferace 2 (NAT2) acetylator polymorphism in the rat. J Pharmacol Exp Therapeut 2006; 316: 289–294. DOI: 10.1124/jpet.105.093302.

27.

Selladurai

Sarojadevi

. Synthesis and properties of modified PMR-matrix resin type polyimide and composite with non-carcinogenic diamine. High Perform Polym 2016; 28: 162–172. DOI: 10.1177/0954008315574121.

28.

Anastas

Eghbali

. Green chemistry: principles and practice. Chem Soc Rev 2010; 39: 301–312. DOI: 10.1039/B918763B.

29.

Holfinger

Conner

Holm

, et al. Synthesis of difurfuryl diamines by the acidic condensation of furfurylamine with aldehydes and their mechanism of formation. J Org Chem 1995; 60: 1595–1598. DOI: 10.1021/jo00111a017.

30.

Garcia

Serafini

. FTIR studies of PMR-15 polyimides. J Polym Sci, Part B: Polym Phys 1987; 25: 2275–2282. DOI: 10.1002/polb.1987.090251105.

31.

Conreur

Francillette

Laupretre

. Synthesis and processing of model compound of PMR-15 resin. J Polym Sci, Part A: Polym Chem 1997; 35: 123–136. DOI: 10.1002/(SICI)1099-0518(19970115)35:1<123::AID-POLA13>3.0.CO;2-M.

32.

Acar

Varis

Isik

, et al. Synthesis and characterization of novel high temperature structural adhesives based on nadic end capped MDA-BTDA-ODA copolyimide. Mater Res Express 2018; 5: 105305. DOI: 10.1088/2053-1591/aadb2e.

33.

Campbell

Scheiman

. Orientation of aromatic ion exchange diamines and the effect on melt viscosity and thermal stability of PMR-15/silicate nanocomposites. High Perform Polym 2002; 14: 17–30. DOI: 10.1177/0954008302014001089.

34.

Zhao

Ribeiro

Portela

. Addition polymerisation of 5-vinyl-2-norbornene with nickel bis(acetyl acetonate)/methylaluminoxane system. J Mol Catal Chem 2002; 185: 81–85. DOI: 10.1016/S1381-1169(02)00133-4.

35.

Terzopoulou

Tsanaktsis

Nerantzaki

, et al. Thermal degradation of biobased polyesters: kinetics and decomposition mechanism of polyesters from 2,5-furandicarboxylic acid and long-chain aliphatic diols. J Anal Appl Pyrol 2016; 117: 162–175. DOI: 10.1016/j.jaap.2015.11.016.

36.

Cureton

Napadensky

Annunziato

, et al. The effect of furan molecular units on the glass transition and thermal degradation temperatures of polyamides. J Appl Polym Sci 2017; 134: 45514. DOI: 10.1002/APP.45514.

37.

Leung

Ghaffarian

Leung

. Thermo-oxidative stability of polyimides — I. Effect of prepolymer molecular weights. Polym Degrad Stabil 1997; 58: 11–14. DOI: 10.1016/S0141-3910(96)00125-5.