Polymer Bulletin

, Volume 65, Issue 4, pp 363–375

Comparative study of aromatic polyimides containing methylene units

Authors

    • Petru Poni Institute of Macromolecular Chemistry
  • Ştefan Chişcă
    • Petru Poni Institute of Macromolecular Chemistry
  • Maria Brumă
    • Petru Poni Institute of Macromolecular Chemistry
  • Gabriela Lisa
    • Department of Natural and Synthetic PolymersGheorghe Asachi Technical University Iasi
Article

DOI: 10.1007/s00289-010-0259-0

Cite this article as:
Sava, I., Chişcă, Ş., Brumă, M. et al. Polym. Bull. (2010) 65: 363. doi:10.1007/s00289-010-0259-0
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Abstract

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.

Keywords

Aromatic polyimidesMethylene bridgesHigh thermal stability

Introduction

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 [14]. 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 [57]. 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 [1014]. 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.

Experimental

Starting materials

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 structures of these monomers are shown in Scheme 1.
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Scheme 1

Structure of aromatic monomers

Polymer synthesis

The method of two steps polycondensation reaction has been used for the preparation of the polyimides. The first step of the polycondensation reaction was performed at room temperature with equimolar amounts of an aromatic dianhydride I and an aromatic diamine II in N,N-dimethylacetamide, at a total concentration of 10–14%, under inert atmosphere during 4–6 h (Scheme 2).
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Scheme 2

Synthesis of the aromatic polyimides III

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.

Measurements

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 [17].

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

The FTIR spectra of all polymers show characteristic absorption bands for: the carbonyl group of the imide ring at about 1770–1780 and 1710–1720 cm−1, the corresponding carbonyl group in the benzophenone group at 1660–1670 cm−1 and the characteristic band for the C–N vibration at 1360–1375 and 720–730 cm−1 [18]. In the polymers IIIc and IIIf the characteristic absorption bands of hexafluoroisopropylidene are present at 1260 and 1210 cm−1. The absorption peaks at 2940 and 2870 cm−1 are characteristic for methyl and methylene groups. Figures 1 and 2 present the characteristic absorption bands of polyimides IIId and IIIf, respectively.
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Fig. 1

FTIR Spectrum of the polymer IIId

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Fig. 2

FTIR Spectrum of the polymer IIIf

All the polymers, except for IIIb, are easily soluble in DMA, N-methylpyrrolidone (NMP), N,N′-dimethylformamide (DMF), and dimethylsulfoxide (DMSO). Their good solubility may be explained by the presence of methyl groups or isopropylidene or hexafluoroisopropylidene units [19], which increase the free volume allowing for the small solvent molecules to penetrate more easily among the polymer chains. The incorporation of the two methyl groups on the diamine moiety leads to an increase in the free volume and a resultant decrease of the molecular packing. The steric hindrance from the methyl groups might also lead to a distortion of the packing of the polyimide backbones. For these reasons, the polyimides based on 3,3′-dimethyl-4,4′-diaminodiphenylmethane have a better solubility compared with polymers obtained from 4,4′-diaminodiphenylmethane [15, 20]. This good solubility was also explained by using the molecular modeling: the shape of a macromolecular chain is far from a linear rigid-rod which is characteristic to wholly aromatic insoluble structures. Model molecules of polymer IIIb and IIIe are shown in Fig. 3. Due to such a shape, the dense packing of the chains is disturbed, the diffusion of small molecules of solvents between the polymer chains is facilitated and that leads to better solubility.
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Fig. 3

Model molecules for polymers IIIb and IIIe (four repeating units)

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 of polyimides are in the range of 0.34–0.54 dL/g (Table 1).
Table 1

Inherent viscosities and molecular weights of polymers

Polymer

Inherent viscositya (dL/g)

Mn (g/mol)

Mw (g/mol)

Mw/Mn

IIIa

0.54

35000

71000

2.03

IIIbb

IIIc

0.39

19500

41000

2.1

IIId

0.46

28000

54000

1.93

IIIe

0.44

25000

52000

2.08

IIIf

0.4

21000

42500

2.02

aMeasured in DMA on 0.5 g/dL at 20 °C, b insoluble

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.

The summary of the important thermogravimetric characteristics of polyimides obtained from the thermograms is listed in Table 2. This table reveals the TGA data such as: Ti the initial temperatures thermal degradation, Tm the temperature corresponding to the maximum degradation rate, Tf the final temperature at which the degradation process for each stage ends, mass loss (W%), corresponding for each stage, and DTA characteristics (endo or exo).
Table 2

Thermal properties of aromatic polyimides IIIaf

Polymer

Stage of thermal degradation

Ti N2/Air (°C)

Tm N2/Air (°C)

Tf N2/Air (°C)

W% (N2)

DTA characteristic data

Tg (°C)

IIIa

I

493/498

530/533

548/569

23.92

Exo

200

II

548/569

595/579

656/625

22.44

Exo

Residue

 

53.64

 

IIIb

I

509/525

606/599

693/684

47.72

Exo

260

Residue

 

52.28

 

IIIc

I

508/509

555/551

582/656

21.72

Exo

275

II

582

640

695

22.30

Exo

Residue

 

55.98

 

IIId

I

479/482

521/584

621/610

33.10

Exo

228

II

621/610

768/684

801/721

12.13

Exo

Residue

 

54.77

 

IIIe

I

498/451

558/598

710/672

41.75

Exo

278

Residue

 

58.25

 

IIIf

I

501/432

545/456

575/507

27.03

Exo

287

II

575/507

682/546

702/637

20.76

Exo

Residue

 

52.21

 

Ti temperature corresponding to the starting degradation step, Tm temperature corresponding to the maximum rate of the degradation process, Tf temperature corresponding to the end of the degradation step, residue material that remains in the crucible above 900 °C, W weight loss, Tg glass transition temperature

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 [21]. 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 differential weight loss DTG curves, recorded under the same experimental conditions, under nitrogen atmosphere, are presented in Figs. 4, 5, 6.
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Fig. 4

DTG curves of the samples IIIa and IIId

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Fig. 5

DTG curves of the samples IIIb and IIIe

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Fig. 6

DTG curves of the samples IIIc and IIIf

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).

Using as thermal stability criteria the onset temperatures (Ti) for the first stage or the temperature corresponding to the maximum degradation rate (Tm), the thermal stability of the polymers containing methyl substituents is lower than that of the samples which do not contain such substituents
$$ {\mathbf{IIId}} \, < \, {\mathbf{IIIa}}; \, {\mathbf{IIIe}} \, < \, {\mathbf{IIIb}}; \, {\mathbf{IIIf}} \, < \, {\mathbf{IIIc}} $$
For the first stage the thermogravimetrical values were processed by differential methods Freeman–Carroll and the results are presented in Table 3.
Table 3

Kinetic characteristics corresponding to the first degradation step

Sample

n

Ea (kJ/mol)

ln A

IIIa

0.63 ± 0.001

307.13 ± 1.64

41.19 ± 0.26

IIIb

1.04 ± 0.001

143.21 ± 1.15

13.93 ± 0.17

IIIc

1.01 ± 0.001

253.28 ± 1.88

31.78 ± 0.28

IIId

1.13 ± 0.009

264.95 ± 1.30

35.28 ± 0.21

IIIe

1.18 ± 0.003

131.41 ± 2.46

13.20 ± 0.38

IIIf

0.90 ± 0.001

216.40 ± 1.84

26.59 ± 0.28

n reaction order degree, Ea activation energy, A pre-exponential factor

The overall rate of polymer degradation is commonly described by Eq. 1.
$$ {\frac{{{\text{d}}\alpha }}{{{\text{d}}t}}} = A\exp \left( { - {\frac{{E_{\text{a}} }}{RT}}} \right)F(\alpha ) $$
(1)
where α is a normalized fractional conversion and is defined as:
$$ \alpha = {\frac{{m_{\text{i}} - m(t)}}{{m_{\text{i}} - m_{\text{f}} }}} $$
(2)
m(t) is the weight at any time t, and mi and mf, respectively, are the initial and final sample weights; A is pre-exponential factor; Ea is activation energy; R is universal constant of gases; T is absolute temperature in K and F(α) is the reaction model [22].
The Freeman and Carroll [23] method assumes \( F(\alpha ) = \left( {1 - \alpha } \right)^{n} \) and considers incremental differences in \( \left( {{\text{d}}\alpha /{\text{d}}T} \right) \), \( \left( {1 - \alpha } \right) \), and \( \left( {1/T} \right) \) which lead to the expression:
$$ \Updelta \ln \left( {{\frac{{{\text{d}}\alpha }}{{{\text{d}}T}}}} \right) = n\Updelta \ln \left( {1 - \alpha } \right) - \left( {{\frac{{E_{\text{a}} }}{RT}}} \right)\Updelta \left( {\frac{1}{T}} \right) $$
(3)
where n is reaction order.This expression can be used to determine the value of Ea by plotting
$$ \left[ {{\frac{{\Updelta \ln \left( {{\text{d}}\alpha /{\text{d}}T} \right)}}{{\Updelta \ln \left( {1 - \alpha } \right)}}}} \right]\,{\text{against}}\,\left[ {{\frac{{\Updelta \left( {1/T} \right)}}{{\Updelta \ln \left( {1 - \alpha } \right)}}}} \right] $$
(4)
The intercept gives the reaction order, n. Ea can be calculated from the slope.

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 [20].

Flexible free-standing films were prepared by casting the DMA solutions of polymers. These films were subjected to tensile tests and the results are shown in Fig. 7. These polyimides showed a tensile strength of 27–87 MPa and an elongation at break in the range of 2.3–8.8%, proving good mechanical properties. These values, as well as those regarding the thermal stability, are similar to those reported for related polyimides based on the same dianhydrides which reacted with other diamines [24, 25].
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Fig. 7

Stress–strain curves of the polymers

Conclusions

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.

Acknowledgments

The authors express their gratitude to Romanian Research Program PNCD-2, Contract no. 11008/2007 for the financial support.

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© Springer-Verlag 2010