Synthesis and characterization of polyamide-imides based on the different chain length of amino acids in molten TBAB as a green media

2.1 Materials

All chemicals used were purchased from Aladdin Industrial Co. (Shanghai, China), Accela ChemBio Co. (Beijing, China), Sinopharm Chemical Reagent Co. (Shanghai, China), Kermel Co. (Tianjin, China), Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China), Shijiazhuang Haili Fine Chemical Co. (Shijiazhuang, China), and Tianjin Chemical Reagent Mao (Business) (Tianjin, China). Glycine (Kermel), β-alanine (Aladdin), 4-aminobutyric acid (Accela), 6-aminocaproic acid (Aladdin), TBAB (Guangfu), and N-methyl-2-pyrrolidone (NMP) (Mao) were used directly without further purification. Pyromellitic dianhydride (Haili) was further purified by recrystallization with acetic anhydride before using. 4,4′-Diaminodiphenyl ether (Aladdin) was further purified by recrystallization from absolute ethyl alcohol. Triphenyl phosphite (TPP) (Kermel) was further purified by vacuum distillation before using.

2.2 General characterization

¹H-NMR spectra were obtained on a Bruker AVANCE 400 MHz spectrometer (Bruker Co., Swiss) in DMSO-d₆. FT-IR spectra were completed on a NEXUS 470 FT-IR spectrophotometer (Thermo Fisher Co., USA) by using KBr pellets. Elemental analyses were obtained by Elementar Vario III EL equipment (Elmentar Co., Germany). The melting point of the aromatic diimide-diacids monomer and the glass transition temperature of the polymer were measured by using a Q-100 DSC (TA Co., USA) instrument. The gravimetric analysis (TGA) date for characterizing polymer was measured by using a TGA 1 SF/1100 (Mettler Toledo Co., Germany) under N₂ atmosphere at a flow rate of 10°C/min. Crystallinity was characterized by wide-angle XRD patterns on a Bruke D8 ADVANCE X-ray diffractometer (Bruker AXS Co., Germany) with Cu Kα (λ=0.15418 nm) radiation at a scan range from 5° up to 90°. The molecular weight of the polymer was determined by GPC (Wyatt Co., USA) and multi-angle laser light scattering (Dawn Heleos), using a linear MZGel SD Plus GPC column set (two columns, 5 μm particles, 300×8 mm) with DMF as eluent at room temperature with a flow rate of 1 ml/min and a concentration of the polymer of ca. 1 mg/ml. The calibration was based on polystyrene standard. A UV-Vis detector at λ=254 nm and a differential refractive index (RI) detector were used for the signal recording. Tensile properties were confirmed from stress-strain curves acquired by the Orientec Tensilon (AD Co., Japan); this instrument has a load sensor of 10 kg. The standard of this study is 3 cm gauge, 20 mm/min strain rate, and room temperature conditions. The polymer film specimen is 4 mm wide, 6 cm long, and 0.1 mm thick.

2.3 Monomer synthesis

Aromatic diimide-diacids (a–d) were respectively synthesized from (2.181 g 0.01 mol) pyromellitic dianhydride, (1.651 g 0.022 mol) glycine (β-alanine, 4-aminobutyric acid or 6-aminocaproic acid), 100 ml of acetic acid, and a stirring bar placed into a 250 ml round-bottom flask. The mixture was stirred for overnight at room temperature and then continued to heat up to 120°C for 5 h [3], [4], [20], [21]. The solvent was removed by reduced pressure distillation, and then 100 ml of cold distilled water and 5 ml of concentrated HCl were added to the residue. The obtained white precipitate was washed with cold distilled water and dried by vacuum. Yields, melting point, and elemental analysis data of the aromatic diimide-diacids (a–d) are listed in Table 1.

2.4 Polymer synthesis

The PAIs were synthesized via conventional direct polycondensation reaction as reported in the references [8], [9], [10]. The above synthesized aromatic diimide-diacid (2 mmol), 0.9 g of calcium chloride, 2.4 ml of TPP, 0.9 ml of Py, 4,4′-diaminodiphenyl ether (2 mmol), and 3.5 ml of NMP were placed into a 50 ml round-bottom flask fitted with a water cooled condenser and a stirring bar. Then the reaction mixture was heated to 120°C and kept at this temperature for 8 h under nitrogen atmosphere. After cooling to room temperature, the reaction mixture was poured into a 500 ml beaker in the presence of ethanol; the yellow polymer was collected by filtration, washed thoroughly with hot ethanol, and dried in vacuum-drying oven at 80°C for 12 h.

In addition, as one kind of ion liquid, TBAB was used as a safe, green, and eco-friendly reaction medium to prepare the PAIs. 4,4′-Diaminodiphenyl ether (2 mmol), aromatic diimide-diacid (2 mmol), and TBAB (6 mmol) were placed into a 50 ml round-bottom flask, which was fitted with a stirring bar. The reaction mixture was heated to 110°C until the molten state was presented. Then 2.5 ml of TPP was added, and the reaction mixture was kept at 110°C for 3 h [22], [23], [24]. The obtained polymer was added into a 500 ml beaker with 300 ml of ethanol; the yellow precipitated polymer was collected by filtration, washed thoroughly with hot ethanol and hot water, then dried at 80°C for 12 h under vacuum.

PAIa: Yellow solid; FT-IR (KBr): 3140–3510 (N–H, m), 1784 and 1710 (C=O imide), 1660 (C=O amide, m), 1496 (benzene ring, s), 1387 (C–N, m), 1107 and 725 (imide ring, m) cm^-1. ¹H-NMR (400 MHz, DMSo-d₆) δ (ppm): 4.49 (s, 4H), 8.35 (s, 2H), 10.37 (s, 2H), 6.94–6.96 (d, 4H), 7.52–7.54 (d, 4H).

PAIb: Yellow solid; FT-IR (KBr): 3160–3570 (N-H, m), 1784 and 1715 (C=O imide), 1662 (C=O amide, m), 1502 (benzene ring, s), 1402 (C–N, m), 1110 and 723 (imide ring, m) cm^-1. ¹H-NMR (400 MHz, DMSo-d₆) δ (ppm): 2.65–2.68 (t, 4H), 3.81–3.91 (t, 4h) 8.18 (s, 2H), 10.03 (s, 2H), 6.82–6.87 (d, 4h), 7.39–7.47 (d, 4H).

PAIc: Yellow solid; FT-IR (KBr): 3180–3512 (N-H, m), 1774 and 1712 (C=O imide), 1655 (C=O amide, m), 1500 (benzene ring, s), 1398 (C–N, m), 1105 and 725 (imide ring, m) cm^-1.

PAId: Yellow solid; FT-IR (KBr): 3186–3520 (N-H, m), 1780 and 1711 (C=O imide), 1650 (C=O amide, m), 1502 (benzene ring, s), 1400 (C-N, m), 1109 and 719 (imide ring, m) cm^-1. ¹H-NMR (400 MHz, DMSo-d₆) δ (ppm): 1.16–1.31 (m, 4H), 1.61–1.63 (m, 8H) 2.23–2.31 (t, 4H) 3.57–3.61 (t, 4H) 8.14 (s, 2H), 9.82 (s, 2H), 6.85–6.87 (d, 4H), 7.43–7.46 (d, 4H).

3.1 Monomer synthesis

Four kinds of diacid monomers (a–d) were synthesized by condensation reaction of four kinds of amino acids (glycine, β-alanine, 4-aminobutyric acid, or 6-aminocaproic acid) and pyromellitic dianhydride, respectively, using ice acetic as solvent (Figure 1). The chemical structure and purity of aromatic diimide-diacid monomers were characterized by elemental analysis, DSC, and ¹H-NMR spectrum.

Figure 1:

Synthesis of diacid monomers (a–d). Monomer a: m=1; b: m=2; c: m=3; d: m=5.

As an example, the ¹H-NMR spectrum of the diimide-diacid monomer based on 4-aminobutyric acid showed all the perfect peaks with the proposed structure and proved diimide-diacid monomer having very high purity. Proton peaks appeared at 8.04 ppm on the phenyl group. Methylene proton peaks connected to the imide ring appeared in 3.63–3.67 ppm; the coupling constant was 6.6 Hz. Methylene proton peaks connected to carboxy group appeared in 2.27–2.49 ppm, and the coupling constant was 43.2 Hz. The proton peaks of the other two methylene groups appeared in 1.80–1.90 ppm. And a singlet peak at 12.04 ppm was recognized as the proton of the acidic groups (Figure 2).

Figure 2:

¹H-NMR spectrum of diacid based on 4-aminobutyric acid.

3.2 Polymer synthesis

The PAIs have been synthesized by two different methods. A safe and eco-friendly reaction was adopted in method I by using IL as solvent, and the conventional polycondensation was adopted in method II (Figure 3).

Figure 3:

Synthesis of PAIs by two methods (PAIa–PAId). PAIa: m=1; PAIb: m=2; PAIc: m=3; PAId: m=5.

The PAIs based on glycine (PAIa) can be dissolved in DMF, and their molecular weight (Mw) and molecular weight distribution (Mw/Mn) were measured (PAIa I: Mw=1.086×10⁵, Mw/Mn=3.290; PAIa II: Mw=8.255×10⁴, Mw/Mn=2.345). The other three kinds of PAIs based on β-alanine (PAIb), 4-aminobutyric acid (PAIc), and 6-aminocaproic acid (PAId) are difficult to be dissolved in DMSO and DMF, but they can be characterized by ¹H-NMR (400 MHz) in DMSO-d₆. FT-IR spectrum of PAIc is shown in Figure 4. Figure 5 shows ¹H-NMR spectrum of PAIc in DMSO-d₆.

Figure 4:

FT-IR spectrum of PAIc.

Figure 5:

¹HNMR spectra of PAIc.

All the molecular weight or the viscosity data of the PAIs have been listed in Table 2; it proved that the PAIs prepared via method I have higher molecular weight, viscosity, and yields than those prepared via method II. And experiments showed that they could also be easier to be purified. The use of high-efficiency ILs has led to reduction in the use of many chemical reagents which are necessary in conventional polycondensation.

3.3 Crystallinity

Figure 6 shows XRD patterns of all the PAIs. It reveals that better crystallinity of the polymer could be obtained by using conventional polycondensation method than that obtained by the method using IL polycondensation method. It was precisely because the PAIs obtained by method I had higher molecular weight and viscosity, increasing the degree of entanglement between PAI molecular chains, limiting the rotation of molecules within the chain, and affecting the movement of segments and the orderly packing of molecular chains; so the regular arrangement ability of the polymer chains declined and the crystallinity of PAIs was decreased.

Figure 6:

Wide-angle X-ray diffractograms of PAIs.

The PAIs were treated with annealing, and the annealing temperature was chosen based on DSC at 241°C (Figure 7).

Figure 7:

DSC traces of the PAIc I.

Figures 8 and 9 showed, respectively, the XRD patterns of the PAIc I after annealing treatment separately at 241°C for 1 h and 2 h (PAIc I1h, PAIc I2h) and then natural cooling. The crystallinity of PAIc I improved obviously after annealing treatment. The other PAIs’ crystallinity could also be improved obviously after annealing treatment.

Figure 8:

Wide-angle X-ray diffractograms of PAIc I1h.

Figure 9:

Wide-angle X-ray diffractograms of PAIc I2h.

3.4 Thermal stabilities

The thermal stabilities of the polymers were characterized by means of TGA techniques in nitrogen atmosphere. Figure 10 showed the TGA graph of the PAIc I, PAIc II, and PAIc I2h. Thermal stabilities of the PAIs were studied by the polymer pyrolysis temperatures of 5% (T₅) and 10% (T₁₀) weight loss of the PAIs and residues at 800°C (char yield) which are listed in Table 3. According to Table 3 and Figure 10, PAIc I2h has higher thermal stability than PAIc I and PAIc II. It is mainly because the polymer with higher molecular weight and better crystallinity has a higher initial decomposition temperature.

Figure 10:

TGA thermograms of PAIc I, PAIc II and PAIc I2h.

3.5 Mechanical properties

The mechanical properties of all polymers synthesized by the two methods were listed in Table 4. It clearly shows that tensile strength of all polymers are in the range 122–134 MPa, elongation breaks of the polymers are in the range 8–15%, and tensile modulus of the polymers are in the range 2.4–3.0 GPa. Almost all the PAIs synthesized by ionic method have higher tensile strengths and tensile modulus than that obtained by normal method. These experimental data have confirmed that the polymers containing rigid imide structure, amide groups, and excellent regular arrangements of segments and having crystallinity, higher molecular weight, and viscosity showed higher tensile strength and tensile modulus.