Comparative study of aromatic polyimides containing methylene units
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- Sava, I., Chişcă, Ş., Brumă, M. et al. Polym. Bull. (2010) 65: 363. doi:10.1007/s00289-010-0259-0
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Two series of aromatic polyimides have been synthesized by solution polycondensation of certain aromatic dianhydrides with two aromatic diamines containing methylene groups; one of the diamines has also a methyl substituent on each benzene ring. These polymers have been studied with regard to their solubility, thermal stability, film forming ability, and mechanical properties of their films.
KeywordsAromatic polyimidesMethylene bridgesHigh thermal stability
Aromatic polyimides have long been considered as one of the most useful super engineering plastics because of their excellent thermal stability, chemical resistance, mechanical properties, and low dielectric constant. They are distinguished from other high performance polymers by the solubility of poly(amidic acid) precursor form, which can be cast into uniform films and quantitatively conversed to polyimide structure. Thus, polyimides have been especially used in microelectronic, film, adhesive, and membrane industry due to these prior properties [1–4]. One of the most notable applications of polyimide thin films is as the interlayer dielectrics in multi level very-large-scale integrated circuits and as matrix resins in high-temperature composite structures [5–7]. Here, a dielectric material should possess a number of other high performance characteristics such as high thermal stability, good resistance to aggressive media, and good mechanical properties. From this point of view aromatic polyimides meet these requirements [8, 9]. However, fully aromatic polyimides are processed with great difficulties because they are insoluble and infusible, and do not show a glass transition before decomposition. One of the successful approaches to increase solubility and processability of polymers is by introduction of bulky lateral substituents, flexible linkages, nonsymmetric, alicyclic or nonlinear moieties [10–14]. Being known that the introduction of flexible linkages into the backbone of fully aromatic polymers can lead to soluble products, the synthesis of polyimides containing flexible isopropylidene (6H), hexafluoroisopropylidene (6F) groups or other groups is a promising way to easy processable compounds having high thermal stability.
This paper presents the synthesis and characterization of two series of polyimides containing flexible methylene bridges in the main chain. One series is based on 4,4′-diaminodiphenylmethane, and another one is based on 3,3′-dimethyl-4,4′-diaminodiphenylmethane, which reacted with the same dianhydrides: 4,4′-isopropylidene-diphenoxy)bis(phthalic anhydride), benzophenontetracarboxylic dianhydride or hexafluoroisopropylidendiphthalic dianhydride. The properties of these polyimides, such as solubility, thermal stability, glass transition temperature, and film forming ability were studied and compared.
N, N′dimethylacetamide (DMA) (Merck) was used as received.
4,4′-Isopropylidene-diphenoxy-bis(phthalic anhydride) (6HDA), Ia and benzophenontetracarboxylic dianhydride (BTDA), Ib, from Aldrich were used as received. Hexafluoroisopropylidendiphthalic dianhydride (6FDA), Ic, from Hoechst–Celanese was purified in our laboratory by recrystallization from acetic anhydride. Melting point (m.p.) of 6HDA was 184–187 °C, m.p. of BTDA was 224–226 °C, and m. p. of 6FDA was 245–247 °C.
Aromatic diamines used in this study are: 4,4′-diaminodiphenylmethane (IIa) from Fluka, and 3,3′-dimethyl-4,4′-diaminodiphenylmethane, (IIb), obtained in our laboratory by a previously reported method [15, 16]. M.p IIa: 90–92 °C and m.p. IIb: 155–157 °C.
The second step consists in thermal imidization of the obtained polyamidic solution in the same reaction flask by heating at reflux temperature for 3–4 h, under a slow stream of nitrogen to remove the water of imidization. The final product was precipitated in water, washed with water and then dried in a vacuum oven at 105 °C.
The polyimide films were obtained by casting the polyamidic acid solution 10–14% in DMA, onto glass plates and drying at 60 °C over 4 h to evaporate the solvent. The subsequent heating of the precursor films at 100, 150, 200, and 250 °C consecutively (for 1 h at each temperature) resulted in a final polyimide film.
FTIR spectra were recorded with a FT-IR VERTEX 70 (Bruker Optics Company), with a resolution of 0.5 cm−1. Thermogravimetric analysis (TGA) was performed under nitrogen flow (20 cm3 min−1) at a heating rate of 10 °C/min from 25 to 900 °C with a Mettler Toledo model TGA/SDTA 851. The initial mass of the samples was 3–5 mg.
Differential scanning calorimetry (DSC) analysis was performed using a Mettler Toledo DSC 1 (Mettler Toledo, Switzerland) operating with version 9.1 of STARe software. The samples (2–4 mg) were encapsulated in aluminium pans having pierced lids to allow escape of volatiles. The heating rates of 10 °C min−1 and nitrogen purge at 100 mL min−1 were employed.
Model molecules for a polymer fragment were obtained by molecular mechanics (MM+) by means of the Hyperchem program, Version 7.5 .
Weight-average molecular weights (Mw) and number-average molecular weights (Mn) were determined by means of gel permeation chromatography (GPC) using a Waters GPC apparatus, provided with Refraction and Photodiode array Detectors and Phenomenex-Phenogel MXN column. Measurements were carried out with polymer solutions having 0.2% concentration, using dimethylformamide as eluent. Polystyrene standards of known molecular weight were used for calibration.
The mechanical properties of the polymer films were determined by stress–strain measurements at room temperature on an Instron Single Colomn Systems tensile testing machine (model 3345) equipped with a 5 kN load cell and activate grips, which prevented the slippage of the sample before break. The cross head speed was 50 mm min−1.
Results and discussion
As can be seen from Fig. 3, in the case of polymer IIIb, the shape of the macromolecular chains tends to orient in the extended form (rigid-rod) in comparison with polymer IIIe, and as a consequence the solubilities are different. After imidization the polymer IIIb precipitated from solution (proving the above-mentioned supposition) while all the other polyimides were soluble.
Inherent viscosities and molecular weights of polymers
Inherent viscositya (dL/g)
The molecular weight of the polymers was determined by gel permeation chromatography (GPC). The weight-average molecular weight values Mw are in the range of 41000–71000 g/mol, the number-average molecular weight values Mn are in the range of 19500–35000 g/mol and the polydispersities Mw/Mn are in the range of 1.9–2.1 (Table 1). In any case these values have to be taken as indicative only, since calibration with polystyrene may result in questionable results when the polarity and backbone stiffness of the studied polymers deviate strongly from those of polystyrene.
All these polyimides have a good film forming ability from solutions in DMA, except for IIIb. The films with thickness of tens of microns were obtained by casting their DMA polyamidic acid solutions onto glass plates with good adhesion to such substrates.
The thermal stability of the samples was evaluated by dynamic thermogravimetric analysis in nitrogen and air, at a heating rate of 10 °C/min.
Thermal properties of aromatic polyimides IIIa–f
Stage of thermal degradation
Ti N2/Air (°C)
Tm N2/Air (°C)
Tf N2/Air (°C)
DTA characteristic data
These polymers did not show weight loss below 480 °C; they began to decompose in the range of 480–510 °C (Table 2), except for polymers IIIe and IIIf which decompose in air at a slightly lower temperature (450 and 430 °C, respectively). As can be seen from Table 1, the polyimides which contain methyl substituents in the diamine component showed slightly lower initial decomposition temperature for both conditions (nitrogen and air). For each sample, the degradation processes are not complete, the char yields at 900 °C in nitrogen atmosphere were in the range of 52–60% (Table 2).
It can be noticed that the polyimides IIIa and IIId, which contain isopropylidene units, showed the lowest decomposition temperatures in nitrogen atmosphere. This can be explained by the presence of –C(CH3)2– linkages in the polyimides backbone, which are more sensitive to thermal degradation . On the other hand in oxidative atmosphere, the polymers IIIf and IIIe showed the lowest initial decomposition temperature, than all the other polymers, being 432 and 451 °C, respectively.
The samples IIIe and IIIb exhibited one step of degradation having the maximum polymer decomposition temperature 558 and 606 °C, respectively (Fig. 5). The other polymers exhibited two steps of degradation having the maximum polymer decomposition temperature of 530 and 521 °C for polymers IIIa and IIId (Fig. 4) and 555 and 545 °C for polymers IIIc and IIIf (Fig. 6).
Kinetic characteristics corresponding to the first degradation step
0.63 ± 0.001
307.13 ± 1.64
41.19 ± 0.26
1.04 ± 0.001
143.21 ± 1.15
13.93 ± 0.17
1.01 ± 0.001
253.28 ± 1.88
31.78 ± 0.28
1.13 ± 0.009
264.95 ± 1.30
35.28 ± 0.21
1.18 ± 0.003
131.41 ± 2.46
13.20 ± 0.38
0.90 ± 0.001
216.40 ± 1.84
26.59 ± 0.28
The kinetic characteristics suggest the complexity of the thermal degradation through successive reactions, accompanied by exothermal processes and confirm the high thermal stability of the polymers without methyl substituents.
Glass transition temperature of the polymers was in the range of 200–287 °C with higher values for polyimides containing hexafluoroisopropylidene units (Table 1). The presence of isopropylidene groups together with ether linkages introduces much more flexibility to the macromolecular chain and decreases the glass transition of the polymers IIIa and IIId. On the other hand, as can be seen from Table 1, the introduction of methyl substituents into the diamine segment increased the glass transition of the corresponding polyimides IIId, IIIe, and IIIf, due to the steric effect of the these substituents .
Two series of aromatic polyimides were prepared by polycondensation of various aromatic dianhydrides with two aromatic diamines containing methylene bridge; one diamine contains also a methyl substituent on each benzene ring. These polymers are soluble in polar aprotic solvents and can be cast into thin and very thin films from such solutions. The polyimides show high thermal stability with decomposition temperature being above 480 °C under nitrogen and 430 °C in air, and glass transition in the range of 200–287 °C. The polymers based on 4,4′-diaminodiphenyl methane showed slightly higher decomposition temperature than those based on 3,3′-dimethyl-4,4′-diaminodiphenyl methane, while their Tg are slightly lower. The free-standing films having the thickness of tens of micrometers exhibit good mechanical properties. All these films have a strong adhesion to glass substrates. These characteristics make the present polymers potential candidates for applications as high performance materials.
The authors express their gratitude to Romanian Research Program PNCD-2, Contract no. 11008/2007 for the financial support.