Synthesis of Novel Thermostable Polyamideimides from Bis(2-aminoethyl)terephthalamide and Dianhydrides.

Bis(2-aminoethyl)terephthalamide, an amide-containing diamine, was obtained from the aminolysis of waste poly(ethylene terephthalate) bottles. This diamine reacts with various aromatic dianhydrides to form novel polyamideimides (PAIs). The formation of amic acid or ammonium carboxylate salt intermediates depends strongly on the substituents of the dianhydrides. The electron-withdrawing substituents promote the creation of an ammonium carboxylate salt, whereas the electron donors assist with the amic acid intermediate formation. These salts and amic acids were further converted into polyimides by thermal treatment. The structures of the intermediates and PAIs were characterized by Fourier transform infrared, 1H nuclear magnetic resonance (NMR), and 13C NMR spectroscopies, and their thermal properties were determined by differential scanning calorimetry and thermogravimetry. X-ray diffraction patterns and inherent viscosity values of these PAIs were also reported. By using these chemical transformations, waste poly(ethylene terephthalate) bottles were converted into high-performance PAIs. These PAIs can be used as membrane-modifying agents for industrial separation applications.

Introduction

Polyimide (PI) is one of the high-performance n class="Chemical">polymers. It exhibits excellent thermal, electrical, and mechanical properties, and consequently, it has been widely used in aerospace, electronics, and other industries.[1−9] PI can be copolymerized with a polyamide. The commercial polyamideimide (PAI) with the brand name of TorlonVR 4000TF has the superior properties of both polyamides and PIs, such as high mechanical strength, thermal resistance, and wide chemical resistance; hence, there are many developments for its application, including gas separation, nanofiltration, osmotic power generation, pervaporation, and so forth.[10] PAI (Torlon) can also be used as the modification agent for cellulose acetate membranes[11] or polytetrafluoroethylene (PTFE) fibrous porous membranes.[12] The prepared PAI-reinforced membranes have potential in industrial separation applications. In addition, PAI was used in composites, such as Nylon-12/PAI composite coatings.

Some key methods are used to prepare PAI: (i) condensation of an imide-containing diacid with a diamine,[13−21] (ii) condensation of an amide-containing diamine with a dianhydride,[22−24] and (iii) thermal imidization of a polyamide–amic acid (AA).[25]

Bis(2-aminoethyl)terephthalamide (BAET) is a diamine containing a terephthalamide moiety. This diamine was prepared by the aminolysis of waste poly(ethylene terephthalate) (PET) bottles with ethylenediamine.[26] This is a short and cost-effective approach to prepare an amide-containing diamine compared to the two-step[22] or four-step[23] methods from monomers.

In this study, by the transformation of BAET into a novel PI, we can use waste PET bottles as a source of materials and hence contribute to the chemical methods of recycling waste materials. Therefore, the outcome of this study is the finding of a new environment-friendly approach to recycle PET into novel monomers/oligomers that are potential raw materials for preparing other new n class="Chemical">polymers (PIs) to attain useful products with special performance and properties.

Results and Discussion

The Fourier transform infrared (FTIR) spectrum of BAET (Figure a) is identical to the published data.[26] This diamine was used for PI preparation.

Figure 1

FTIR spectra of (a) BAET, (b) AC-BAET-PMDA, and (c) PI-BAET-PMDA.

When an input molar ratio of BAET-pyromellitic n class="Chemical">dianhydride (PMDA) of 1:1 or 2:1 was used in dried dimethyl acetamide (DMAc), N-methylpyrrolidone (NMP), dimethyl sulfoxide (DMSO), or glacial acetic acid, the same intermediate was isolated and characterized by FTIR and NMR spectroscopy techniques. The common absorption bands of AA could not be found in the FTIR spectrum of the intermediate (Figure b), but only the existence of an ammonium carboxylate (AC) salt was observed. For example, the comparison of the FTIR spectra of BAET (Figure a) with AC-BAET-PMDA (Figure b) showed that the weak band at 1577 cm–1 in BAET became stronger. This band is characteristic for the asymmetric CO2– stretch. In addition, the C=O amide stretching band of BAET at 1632 cm–1 shifted to a lower wavenumber at 1623 cm–1 because of the existence of the NH3+ asymmetric deformation band that used to appear in the range of 1625–1560 cm–1.[27]

The structure of the AC salt (Scheme ) obtained from n class="Chemical">BAET and PMDA was also confirmed by 1H NMR and 13C NMR. DMSO-d6 was used as a good solvent for this intermediate. The 1H NMR spectrum (Figure S1) shows two triplets at 3.03 and 3.53 ppm of four symmetrical methylenes, one singlet at 7.95 ppm of four aromatic terephthalate protons, and one singlet at 8.95 ppm of two aromatic pyromellitic protons. The difference in the resonance signals of the methylene protons of AC-BAET-PMDA is 0.50 ppm. Two methylene triplets of the BAET reactant appear at 3.41 and 3.83 ppm in CD3COOD,[26] and the difference is 0.42 ppm. If an AA intermediate was formed, the methylene protons of the BAET moiety would appear at very close chemical shifts because these methylene protons were alpha hydrogens of the amide.

Scheme 1

Chemical Reactions of BAET with Various Dianhydrides, Such as PMDA, BTDA, and OPDA, to Form the AC Salt, AA Intermediates, and PI Final Product

In addition, 13C NMR of AC-BAET-PMDA in DMSO-d6 (Figure S2) shows the carbon signals of the bisethylene terephthalamide part at 167.4 (C=O), 135.8 (ipso C), 127.3 (arom C–H), 38.7, and 37.3 ppm (CH2) and of the pyromellitic part at 166.3 (C=O), 137.6, and 136.4 ppm (arom CH and ipso C).

The thermal properties of AC-BAET-PMDA were investigated by differential scanning calorimetry (DSC) and thermogravimetry (n class="Chemical">TG). The first heating curve of DSC (Figure a) shows an endothermic peak at 274 °C. After cooling to room temperature (RT) and reheating, this peak disappears. This means that an endothermic chemical reaction occurred at 274 °C.

Figure 2

DSC curves of AC-BAET-PMDA: (a) heating from RT to 320 °C at a rate of 10 °C/min and holding for 5 min at 320 °C; (b) cooling to RT at 50 °C/min; and (c) heating from RT to 320 °C at 10 °C/min.

The derivative thermogravimetric (dTG) curve of AC-BAET-PMDA (Figure ) shows two well-defined decomposition steps with the peaks at 262 and 503 °C. By comparison with the DSC endothermic peak at 274 °C (Figure ), the first decomposition step is assigned to water elimination of the AC salt to form a PI. The second step is due to the decomposition of the newly formed PI.

Figure 3

TG and dTG curves of AC-BAET-PMDA heating from RT to 800 °C at a rate of 10 °C/min.

The theoretical mass loss by the removal of four molecules of water from AC-BAET-PMDA (Scheme ) to form the corresponding PI can be calculated by the following equation

On the contrary, the formation of PI from AA requires the loss of only two water molecules or 7.7%.

From RT to 200 °C, the TG curve (Figure ) shows an initial loss of 1.1%, which is due to moisture. In the temperature range of 200–290 °C with a n class="Chemical">dTG peak at 262 °C, the mass loss because of condensation and imide ring formation is 14.8%. These experimental and theoretical values are identical, and therefore, this is additional evidence for the structure of AC salt and its conversion into PI.

The newly formed PI begins to decompose at approximately 350 °C and reaches a maximum decomposition rate at 503 °C with a total weight loss of 64.8%.

From the above FTIR, 1H NMR, 13C NMR, DSC, and TG analyses, we can conclude that under the reaction conditions, by using various solvents, such as dried DMAc, NMP, DMSO, and glacial acetic acid, BAET reacts with PMDA to form an AC salt but not an AA. The required temperature for thermal treatment to convert this AC salt into PI is from 262 to 300 °C.

A patent of Kikuchi[28] has also reported that when highly basic diamines are used, a salt is produced by the neutralization reaction before forming a poly(amic acid) at the point that the alicyclic diamines are mixed with acid dianhydrides and precipitated. BAET is an aliphatic diamine that is a stronger base than an aromatic diamine, and therefore, an AC salt is formed.

The transformation of the solid salt AC-BAET-PMDA into the corresponding PI was performed by stepwise heating from 100 to 300 °C with 20 °C increments, and the duration of each step was 30 min. The FTIR spectrum of the obtained PI-BAET-PMDA (Figure c) exhibits absorptions at 1775, 1712 cm–1 (C=O of N-aliphatic imide), 1396 cm–1 (aliphatic C–N), 1156, and 727 cm–1 (imide ring deformation). The terephthalamide moiety is observed at 3405 cm–1 (stretching of amide N–H), 1640 cm–1 (stretching of amide C=O), and 1540 cm–1 (deformation of amide N–H).

The TG and dTG curves (Figure ) show that the obtained PI is stable from RT to 400 °C with a maximum degradation temperature Tdm at 514 °C. At 800 °C, the residue is 10.1%. This Tdm is not different from the one of the second decomposition step of AC-BAET-PMDA (Figure ). This Tdm value is much higher than that at 410 °C of the N-aliphatic arimide prepared by Mallakpour et al.[29] from N,N′-(pyromellitoyl)-bis-l-leucine diacid chloride and aromatic diamine.

Figure 4

TG and dTG curves of PI-BAET-PMDA heating from RT to 800 °C at a rate of 10 °C/min.

The presence of short aliphatic ethylene in n class="Chemical">diamine does not significantly reduce the thermal stability of PI-BAET-PMDA, but the existence of an imide moiety in the chain increased its thermal stability compared to the corresponding polyamide. The decomposition peak of poly(ethylene terephthalamide) was reported at 441[26] or 430 °C.[30]

The PAI preparation procedure and characterization, as described above, were also applied n class="Chemical">similarly to prepare PIs from BAET and 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) or 4,4′-oxydiphthalic anhydride (OPDA). The 1H NMR spectrum (Figure S3) of the intermediate isolated from the reaction of BAET with BTDA in DMSO showed complex structures that were identified as a mixture of AA and AC. The methylene protons appear as two pairs of peaks at (i) 3.51 and 3.01 ppm of AC and (ii) 3.48 and 3.40 ppm of AA. The signals of the methylene protons of AA are very close (the difference is only 0.08 ppm), as discussed above. In addition, the integrals of AC and AA protons are almost the same; therefore, this is an AC/AA mixture of 1:1.

Contrary to PMDA and n class="Chemical">BTDA, the reaction of BAET with OPDA produces a mixture of AA and AC intermediates and PI, as indicated by the bands at 1713 and 1776 cm–1 in the FTIR spectrum. In the 3–4 ppm range of the 1H NMR spectrum (Figure S6) of the intermediate, two pairs of peaks were also observed; however, the signals of AA or PI methylenes were much stronger than that of AC. Quantitative determination by taking the integrals of these peaks is not accurate because they are overlapped by the H2O signal, even after D2O treatment (Figure S7). Hence, the intermediate isolated from the BAET–OPDA reaction contains principally AA, PI, and a minor quantity of AC. This AA intermediate easily eliminates water to transform into PI.

The formation of different intermediates from BAET and n class="Chemical">dianhydrides is explained by the reaction mechanism and electrophilic effects of substituents. In the first step of the reaction, an electron pair of the amine, acting as a Lewis base, attacks a carbonyl carbon in the anhydride group, acting as a Lewis acid center. The electron-withdrawing substituents in the dianhydride will enhance the acidity of this carbonyl carbon center and vice versa. As a consequence, the acidity of the carbonyl carbon in anhydrides decreases according to the order PMDA (two C=O as electron-withdrawing substituents) > BTDA (one C=O substituent) > OPDA (O-ether as an electron-donating substituent). In addition, PMDA is more soluble than BAET in any solvent; therefore, PMDA reacts at first with trace water or moisture to form carboxylic acid, and then, this acid reacts with BAET to form an AC salt. Because of its low acidity, OPDA is less sensitive to water and can react with BAET to form an AA intermediate.

The intermediates isolated from the reactions of BAET with n class="Chemical">BTDA or OPDA were also further transformed into PAIs by thermal treatment. The existence of the imide and amide structures of the final PAI products was confirmed by the corresponding pairs of peaks at 1775/1712 and 1640/1540 cm–1 in the FTIR spectra (Figure ).

Figure 5

FTIR spectra of (a) PI-BAET-PMDA, (b) PI-BAET-BTDA, and (c) PI-BAET-OPDA.

From the TG curves (Figure ) and Table , the thermal stability of PAIs prepared from BAET and dianhydrides was according to the order of PI-BAET-PMDA > PI-BAET-BTDA > PI-BAET-OPDA. This reduction of thermal stability is clearly because of the rigidity of benzene > C=O > −O– in the structure of the dianhydride moieties. The same order of the PI thermal stability with dianhydrides was also reported.[31] PI-BAET-BTDA and PI-BAET-OPDA with more aromatic rings yield more char residues at 800 °C compared to PI-BAET-PMDA.

Figure 6

TG curves of (a) PI-BAET-PMDA, (b) PI-BAET-BTDA, and (c) PI-BAET-OPDA.

Table 1

Structures of Intermediates and Decomposition Temperatures

PI	intermediates	temperature at 10% weight lossa (°C)	maximum decomposition temperature of PI (°C)
PI-BAET-PMDA	AC	466	514
PI-BAET-BTDA	AC + AA	425	450
PI-BAET-OPDA	AA + PI	409	454

Decomposition temperature at which a 10% weight loss was recorded by TG at a heating rate of 10 °C·min–1 in nitrogen.

The morphology of the obtained PAIs was also studied by wide-angle X-ray diffraction (XRD) on powder specimens (Figure ).

Figure 7

Wide-angle XRD patterns of (a) PI-BAET-PMDA, (b) PI-BAET-BTDA, and (c) PI-BAET-OPDA.

The broad peaks centered at 2θ = 13.5° on each XRD pattern indicated that they were almost amorphous and attributed to the average intersegmental distance of polymer chains. The average d-spacing was calculated based on Bragg’s law: nλ = 2d n class="Chemical">sin θ, where d is the dimension spacing, θ is the diffraction angle, λ is the X-ray wavelength (1.54184 Å), and n is an integral number (1, 2, 3, ...). Therefore, the d-spacing values of our PAI samples were calculated as 6.6 Å.

Another series of PAIs was prepared from an aromatic n class="Chemical">amide-containing diamine, N,N′-bis(4′-amino-4-biphenylene)isophthalamide, and PMDA, BTDA, or OPDA[23] by the bulk thermal imidization of AAs. The XRD patterns of these PAIs showed broad peaks at 2θ = 20° or d = 4.4 Å. These aromatic PAIs have a slightly tighter chain packing compared to our N-aliphatic PAI samples. Of course, the aliphatic chain is more flexible than the aromatic moiety, and this higher flexibility to some extent disrupts the order of the PAI chains and eventually leads to a higher d-spacing.

The thermal properties of PIs were investigated by DSC. After the first heating experiment (Figure a), the n class="Chemical">AC salt was converted into PI. The cooling (b) and heating (c) curves (Figure ) of PI showed a smooth, straight line as evidence of no thermal occurrence, and Tg was not detected, or this was a semicrystalline material. An independent DSC analysis of the PI-BAET-PMDA sample after thermal treatment in a furnace to 300 °C also showed similar curves. Figure S9 shows a very weak baseline shifting at approximately 160 °C before a large endo peak at 178 °C while heating for the first time. The same weak baseline shifting was also seen in the cooling and reheating curves. These weak changes could be assigned to the Tg transition of PI-BAET-OPDA. The PI-BAET-BTDA sample (Figure S8) showed a clear Tg at 219 °C (midpoint when cooling) or 231 °C (midpoint when heating).

The presence of a flexible ether group in the PI-BAET-OPDA structure makes it transform into a glass state at a lower temperature compared to the PI-BAET-BTDA sample with the stiff carbonyl group in the main chain.

These DSC results are comparable to the XRD analysis. n class="Chemical">PI-BAET-BTDA showed only a broad peak centered at 2θ = 13.5° of a more amorphous structure, whereas PI-BAET-PMDA revealed one more peak at 20° (Figure a), and PI-BAET-OPDA showed three more peaks at 18, 19.5, and 26° for crystalline structures. The crystallinity of PI-BAET-PMDA was derived from the strong inter- and intramolecular interactions of the flat symmetrical pyromellitimide rings. In contrast, the crystallinity of PI-PMDA-OPDA was acquired from the PI ring formation in solution. The lower crystallinity of PI-PMDA-BTDA is ascribed to the PI formation by the thermal treatment of AA and AC intermediates and the unsymmetrical benzophenone ring.

In addition, the heating curves of PAI-n class="Chemical">BAET-BTDA (Figure S8) and PAI-BAET-OPDA (Figure S9) showed endothermic peaks at 184 and 178 °C, respectively. Combined with the TG curves (Figure b,c), this thermal occurrence could be because of a physical transition without mass loss. These endothermic peaks could be assigned to the transition of terephthalamide crystalline segments.

The novel PAIs are only soluble in n class="Chemical">phosphoric acid. The interaction between PI and phosphoric acid was investigated by FTIR analysis.[32] This PI was prepared from OPDA and 4,4′-diaminodiphenyl ether. The FTIR results show that phosphoric acid interacts with PIs mainly by hydrogen bonds rather than by the protonation of PI at RT.

The inherent viscosity (IV) in 85% n class="Chemical">H3PO4 solution (0.5 g/dL at 30 °C) of PAIs was determined by using an Ostwald viscometer of Cannon-Fenske, size no. 450. The IV values of PI-BAET-PMDA, PI-BAET-BTDA, and PI-BAET-OPDA are 0.466, 0.204, and 1.15, respectively. PI-BAET-OPDA showed the highest IV, whereas PI-BAET-PMDA and PI-BAET-BTDA have lower IV values or lower molecular weights (MWs). This higher MW of PI-BAET-OPDA could be the result of the direct formation of PAI and AA in solution instead of the formation of AC intermediates.

Conclusions

In conclusion, novel n class="Chemical">PAIs were prepared from the reactions of BAET with PMDA, BTDA, and OPDA. When PMDA was used, even in dried solvents, the intermediate was isolated and identified as an AC salt. BTDA reacted with BAET to form a mixture of AC and AA. In the case of OPDA, the AA intermediate was mainly formed. These intermediates were converted into PAIs by thermal treatment. This nonsolvent dehydration process shows environmental advantages; however, the obtained PIs possess noncrystalline structures. The N-aliphatic imide PI-BAET-PMDA shows a high thermal stability compared to the fully aromatic PI. The presence of carbonyl (BTDA) and ether (OPDA) groups in the dianhydrides enhances their stability to moisture compared to the presence of a benzene ring (PMDA); however, these groups exhibit a detrimental effect on the thermal stability of the corresponding PAIs. The DSC and XRD characterizations of the obtained PIs showed that PI-BAET-BTDA has lower crystallinity than PI-BAET-PMDA and PI-BAET-OPDA. Among the three PIs, PI-BAET-OPDA exhibits the highest inherent viscosity. BAET is an inexpensive, low toxic, and nonvolatile diamine; therefore, its application in PAI synthesis is valuable. These PAIs can be used as membrane-modifying agents for industrial separation applications.

Experimental Section

Materials

BAET was prepared from waste PET bottles.[26] PMDA, BTDA or carbonyldiphthalic anhydride, and ODPA were obtained from Sigma-Aldrich (Munich, Germany). PMDA was recrystallized from acetic anhydride. NMP, DMAc, DMSO, glacial acetic acid, and other solvents were purchased from Sigma-Aldrich (Munich, Germany).

Reaction of BAET with Dianhydrides

BAET (0.5000 g) was added to a 50 mL round bottom-flask containing 5 mL of solvent, and then, an equimolar quantity of n class="Chemical">dianhydride was added slowly to the reaction mixture. The heterogeneous reaction mixture gradually turned into a transparent solution and then became more viscous and opaque.

The intermediate was isolated by treating the transparent viscous solution with 60 mL of cool methanol. The formed solid was filtered and further rinsed with 20 mL of n class="Chemical">methanol for six times and then rinsed with acetone and finally dried under a vacuum at 60 °C for 24 h. The isolated intermediate or reaction mixture was converted into a PI by a stepwise thermal treatment in an oven.

Characterization of the Product

An FTIR-TENSOR II Bruker spectrometer (Bruker Optik GmbH, Ettlingen, Germany) was used in transmission mode to record the spectra from KBr pellets. The n class="Chemical">1H NMR and 13C NMR spectra were recorded with a Bruker ARX-500 NMR spectrometer operating at 500 MHz (1H) and 125 MHz (13C).

DSC was performed with a METTLER STARe SW 11.00 instrument (Mettler-Toledo Ltd., Thailand). The samples were heated from RT to 400 or 450 °C with a heating rate of 10 °C/min in a nitrogen atmosphere. n class="Chemical">TG analysis was carried out on Q500 Universal V4.5A (TA Instrument, NewCastle, DE, USA) by heating from RT to 800 °C at a heating rate of 10 °C/min in a nitrogen atmosphere.

Wide-angle X-ray diffractograms were obtained by a D8-ADVANCE Bruker X-ray diffractometer using Ni-filtered Cu Kα radiation (40 kV, 40 mA). The scanning rate was 0.01°/0.2 s. The samples were measured as fine powders on a silicon sample holder.