The main aim of this work was the preparation and characterization of poly(imide-siloxane)s (PIS) derived from a hyperbranched polyimide (HBPI). This starting material (control) is represented by the amine end-capped HBPI based on a bifunctional ODPA and a trifunctional TTM [HBPI(ODPA-TTM)]. It was prepared using a two-step synthesis via a polyimide precursor, hyperbranched polyamic acid [HBPAA(ODPA-TTM)] (Figure 1).

Figure 1:

Two-step synthesis of of HBPI based on ODPA and TTM.

To synthesize the amine-terminated HBPAA(ODPA-TTM), an ODPA:TTM molar ratio of 1:1 (anhydride groups:amino groups 2:3) was used. When trifunctional and bifunctional monomers are employed, a three-dimensional structure (gel) is created during the course of polymerization (2). The gel formation was suppressed by the slow dropwise addition of a dilute solution of one monomer to a solution of the other. The highest concentration of solids without the gel formation was found to be 4 wt% [4, 9]. The intrinsic viscosity of the HBPAA(ODPA-TTM) was 30.7 cm³·g⁻¹ (in NMP, at 25°C).

For the syntheses of polyimide precursors containing siloxane moieties (PASA), N2Si [PASA(ODPA-TTM-N2Si)], N2Si10 [PASA(ODPA-TTM-N2Si10)] and N2Si25 [PASA(ODPA-TTM-N2Si25)] were used in addition to ODPA and TTM monomers. After their heat treatment (see Materials and methods, 2.3) the corresponding poly(imide-siloxane)s (PIS) were obtained. The chemical structure of such a PIS is shown in Figure 2. The designation of the materials together with the molar ratios of the starting compounds used for their syntheses are given in Table 1.

Figure 2:

Chemical structure of PIS.

For the synthesis of a polyimide precursor, it is important to find a medium dissolving both the monomers and the product (polyamic acid). In the cases of HBPAA(ODPA-TTM) and PASA(ODPA-TTM-N2Si), NMP meets these requirements. On the other hand, N2Si10 and N2Si25 are not soluble in NMP. A mixed solvent of NMP/THF in a 2:3 volume ratio was suitable for the preparation of 4 wt% solutions of N2Si10 and N2Si25. PASA(ODPA-TTM-N2Si10) and PASA(ODPA-TTM-N2Si25) containing theoretically not more than 20 mol% of siloxane oligomers were soluble in this mixed solvent. Although the preparation of the materials containing theoretically 5 or 25 mol% of the siloxane moieties was the aim of this work, the level was decreased to 20 mol% for PASA(ODPA-TTM-N2Si10) (providing PIS4) and PASA(ODPA-TTM-N2Si25) (providing PIS6) (see Table 1). The intrinsic viscosities of PASA(ODPA-TTM-N2Si10) containing 5 mol% of N2Si10, PASA(ODPA-TTM-N2Si10) containing 20 mol% of N2Si10, PASA(ODPA-TTM-N2Si25) containing 5 mol% of N2Si25 and PASA(ODPA-TTM-N2Si25) containing 20 mol% of N2Si25 were 30.6, 26.5, 30.5 and 35.2 cm³·g⁻¹ (in NMP/THF=4:6, 25°C), respectively.

All final materials were insoluble in NMP, N,N-dimethylformamide, toluene, chloroform, tetrahydrofuran and methanol. The IR spectra of the materials HBPI, PIS2, PIS3 and PIS5 are collected in Figure 3. The absorption bands at approximately 1780 and 1720 cm⁻¹ (symmetric and asymmetric stretching of the ring carbonyl groups) and the band at 1360 cm⁻¹ (stretching of the ring C-N bond) are distinct in the spectra and characterize the formation of imide structures. The absence of the band at 1670 cm⁻¹ (amide group of polyamic acid) supports the conclusion that the thermal treatment (see Materials and Methods, 2.3) led to almost complete imidization. In the spectra of PIS2, PIS3 and PIS5, the broad band at 1010–1080 cm⁻¹ characterizes siloxane moieties (Si-O-Si) in these materials (12). Because the spectra were not standardized, they offer qualitative information only.

Figure 3:

IR spectra of the materials HBPI, PIS2, PIS3 and PIS5 (from top to bottom).

The thermal and mechanical properties of the products are collected in Table 2. To draw any conclusions from the data in Table 2, it is necessary to realize that the amount of N2Si10 in the materials PIS3 and PIS4 is slightly lower and the amount of N2Si25 in the materials PIS5 and PIS6 is lower than the theoretical assumption. The similar phenomenon was observed for the linear poly(imide-siloxane)s (17). It is probably a consequence of a different solubility of the PASA based on N2Si10 or N2Si25 in a mixed solvent NMP/THF. The amount of siloxane moieties in the products was re-calculated from the incombustible fractions on the presumption that it represents silicon dioxide (silica) (Table 2). Both the glass transition temperature (T_g) and the temperature corresponding to 10% weight loss decrease slightly with siloxane content. Nevertheless, these materials can still be considered to be highly thermally resistant. The T_g values given in Table 2 characterize the imide phase (T_g of the siloxane phase is approximately −120°C). The shift of these T_gs values in comparison with the T_g value of the pure HBPI is influenced by a miscibility/immiscibility of the imide and siloxane phases (the better miscibility, the deeper T_g decrease). It is supposed that the phase separation is more distinct with a length of siloxane oligomer (12). Then, the T_g drop is more pronounced for the materials containing the shorter siloxane oligomers with molar mass of 1000 g mol⁻¹. The siloxane dimer mainly contributes to making these products more linear in comparison with the hyperbranched control. As a consequence of this, the tensile strength increases and the modulus decreases. On the other hand, the presence of siloxane oligomers renders the materials not only more linear but also more elastic (rubbery), and therefore, both tensile strength and modulus decrease and elongation at break increases slightly.

The gas transport properties of these materials (in the form of self-standing films with a thickness of approximately 50 μm) are also in good agreement with their chemical compositions. The permeability coefficients of hydrogen, carbon dioxide and methane in the membranes prepared from the pure HBPI and the PISs are compared in Table 3. Nevertheless, the permeability coefficients of methane are very low in some cases. Therefore, the repeatability of such values is very poor and they were left out.

From the data given in Table 3, it is clear that the membranes based on HBPI and PIS1, PIS2, PIS3 and PIS5 show a common order of the gas permeability coefficients in the flat, homogeneous (non-porous) polymeric membranes [hydrogen>carbon dioxide (>methane)]. The carbon dioxide/methane selectivities of approximately 20 are rather lower in comparison with those presented in the literature (17), (18). The size of gas molecules and their ability to diffuse through the membrane are decisive in this case. However, this order is not valid for the membranes made of PIS4 and PIS6. A higher content of longer siloxane moieties increases the contribution of the solubility of carbon dioxide in the membranes, and their behaviour for the hydrogen/carbon dioxide correspond to so called reverse-selective polymeric membranes (18).

Poly(imide-siloxane)s based on the hyperbranched polyimide based on 4,4′-oxydiphthalic anhydride and 4,4′,4″-triaminotriphenylmethane were prepared. The use of both an amine-terminated dimethylsiloxane dimer and amine-terminated dimethylsiloxane oligomers differing in their molar mass as the source of siloxane moieties enabled us to vary the properties of the final materials. All of these products form self-standing films with a thickness of approximately 50 μm, and due to their lower brittleness compared to the pure HBPI, they are potentially useful as polymeric membranes. Nevertheless, the optimal reaction conditions for the preparation of materials based on the amine-terminated siloxane oligomers will be studied. It will enable us to synthesize materials with more accurate chemical compositions and to explore the composition-property relationships in detail.

The support of the Grant Agency of the Czech Republic (No. 15-06479S) and from the specific university research fund is acknowledged.