The dendritic topology (dendrimers, hyperbranched polymers) has been demonstrated as a new macromolecular architecture group. Hyperbranched polymers do not have a perfectly branched architecture, but they possess some similar properties to dendrimers, and they can be employed to replace them for certain purposes. Their highly branched structure with a specific polymer chain hierarchy (arrangement) and large number of functional end-groups are the basic structural features that distinguish them from linear polymers (1), (2). Increasing attention has been paid to hyperbranched polyimides (HBPIs) (3), (4) due to the potential combination of the known advantages of linear (5) or crosslinked (6) polyimides with those of hyperbranched polymers (2), (7).
Polyimides show good mechanical, dielectric and chemical stability at temperatures up to 250°C. These rigid polymers with high glass transition temperatures are mostly used in the aircraft industry, electronics (5) and as polymeric separation membranes (8). Linear polyimides are commonly synthesized by a two-step procedure via a polyimide precursor. The precursor (polyamic acid) is prepared from an aromatic dianhydride and an aromatic diamine in solution at room temperature. The precursor is transformed into a polyimide using thermal or chemical imidization (5). HBPIs are often prepared from a bifunctional anhydride and a trifunctional amine under specific reaction conditions to avoid gel (3-dimensional structure) formation. The molar ratio of the anhydride and amine components determines the type and/or ratio of the end-groups (2), (3), (4).
We have previously used 4,4′-oxydiphthalic anhydride and 4,4′,4″-triaminotriphenylmethane as monomers for this purpose (4), (9). These compounds, used in a 1:1 molar ratio, provided an amine end-capped HBPI. The self-standing, homogeneous membranes prepared from this polymer showed a 2–4-fold increase in gas permeability compared with a membrane prepared from the linear polyimide based on 4,4′-oxydiphthalic anhydride and 4,4′-diaminodiphenylmethane at comparable separating ability (selectivity). Nevertheless, the brittleness of these materials – due to the lack of chain entanglements – renders them unsuitable for a larger scale use. A reduction of the degree of branching from forming a “hyperbranched-linear hybrid structure” (2) can contribute to better mechanical stability.
The range of polyimide use can be broadened by combining them with another polymer. Considerable attention has been focused on the preparation of linear poly(imide-siloxane)s. By incorporating rubbery, hydrophobic siloxane segments – most frequently poly(dimethylsiloxane)s – into polyimide chains, products with higher solubility, impact strength and gas permeability and lower modulus, moisture absorption and relative permittivity have been obtained. Simultaneously, it is important that the long-term thermal stability in air (up to 180°C) of poly(dimethylsiloxane)s is only slightly lower than that of polyimides. The final properties depend on both the type of copolymer and/or its weight composition as well as the molar mass of the components used (10), (11), (12), (13). Researchers in one study (14) prepared a hyperbranched siloxane precursor. After transforming it to a bifunctional component, they incorporated it into a polyimide backbone. The resistance of these materials to the simulated atomic oxygen environment was higher than that of a pure polyimide. To our best knowledge, poly(imide-siloxane)s derived from HBPI have not yet been prepared and characterized. In the work (15) the properties of the poly(urethane-siloxane)s based on a hyperbranched polyester were controlled by the ratio of components.
Therefore, poly(imide-siloxane)s prepared by substituting a defined fraction of trifunctional 4,4′,4″-triaminotriphenylmethane by amine-terminated siloxane dimer or oligomers (with a number-average molar mass of 1000 or 2500 g·mol−1) were synthesized, and their structure, thermo-mechanical properties and gas transport characteristics were studied.
2 Materials and methods
2.1 Chemicals (materials)
4,4′-Oxydiphthalic anhydride (ODPA) (Chriskev, USA) was heated at 160°C for 5 h under vacuum before use. 4,4′,4″-Triaminotriphenylmethane (TTM) (Dayang Chemicals, China), 1,3-bis(3-aminopropyl)-1,1,3,3-tetramethyldisiloxane (N2Si), bis(3-aminopropyl)-terminated poly(dimethylsiloxane) with a theoretical number-average molar mass of 1000 g·mol−1 (N2Si10) and bis(3-aminopropyl)-terminated poly(dimethylsiloxane) with a theoretical number-average molar mass of 2500 g·mol−1 (N2Si25) (all Hüls Petrarch, Germany) were used as received. 1-Methyl-2-pyrrolidone (NMP) was distilled under vacuum over phosphorous pentoxide, and tetrahydrofuran (THF) (both Aldrich, Czech Republic) was distilled at atmospheric pressure over sodium.
2.2 Synthesis of hyperbranched polyamic acid
The hyperbranched polyamic acid (HBPAA) was synthesized in a three-necked flask equipped with a magnetic stirrer, nitrogen inlet/outlet and dropping funnel. At room temperature, a 4,4′-oxydiphthalic anhydride (ODPA) solution in NMP [2.069 g (0.00667 mol) of ODPA in 50 ml of NMP] was added dropwise to the 4,4′,4″-triaminotriphenylmethane (TTM) solution in NMP [1.931 g (0.00667 mol) of TTM in 46 ml of NMP]. This reaction mixture was then stirred at room temperature for 24 h.
2.3 Synthesis of poly(amic-siloxane) acids
The preparation of the poly(amic-siloxane) acid (PASA) based on ODPA, TTM and N2Si in a molar ratio 0.5:0.45:0.05 is as follows:
PASA was synthesized in a three-necked flask equipped with a magnetic stirrer, nitrogen inlet/outlet and dropping funnel. At room temperature, an ODPA solution in NMP [2.069 g (0.00667 mol) of ODPA in 50 ml of NMP] was added dropwise to the solution consisting of 1.731 g (0.00599 mol) of TTM, 0.166 g (0.00068 mol) of N2Si and 46 ml of NMP. This reaction mixture was then stirred at room temperature for 24 h.
If bis(3-aminopropyl)-terminated poly(dimethylsiloxane) with a theoretical number-average molar mass of 1000 g·mol−1 (N2Si10) or bis(3-aminopropyl)-terminated poly(dimethylsiloxane) with a theoretical number-average molar mass of 2500 g·mol−1 (N2Si25) were used as comonomers, a mixed solvent of NMP/THF [40/60 (vol%/vol%)] was used instead of NMP.
2.4 Preparation of hyperbranched polyimide (HBPI) and poly(imide-siloxane) (PIS)
The solution of HBPAA or PASA was cast on a Teflon substrate and heated in an oven at 30°C for 6 h, 60°C for 6 h, 100°C 1 h, 150°C 1 h, 200°C for 2 h and 230°C for 1 h. Self-standing films with a thickness of approximately 50 μm were obtained.
2.5 Instrumental techniques
IR spectra were taken on a Nicolet 6700 spectrometer in reflective mode. Thermogravimetric measurements (TGA) were performed in air from 25 to 800°C with a temperature gradient of 10°C·min−1 using a TGA Q500 instrument. Dynamic mechanical analysis (DMA) was recorded using a DMA DX04T (RMI, Bohdanec, Czech Republic) operating at 1 Hz from 25 to 400°C with a temperature gradient of 3°C·min−1. Mechanical properties were evaluated by using an Instron 3365. The intrinsic viscosities of the polyimide precursors were measured by using an Ubbelohde capillary viscometer in NMP or NMP/THF at 25°C. The permeation measurements were conducted using a self-developed manometric integral apparatus (16).
3 Results and discussion
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).
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.
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.
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 (Tg) 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 Tg values given in Table 2 characterize the imide phase (Tg of the siloxane phase is approximately −120°C). The shift of these Tgs values in comparison with the Tg value of the pure HBPI is influenced by a miscibility/immiscibility of the imide and siloxane phases (the better miscibility, the deeper Tg decrease). It is supposed that the phase separation is more distinct with a length of siloxane oligomer (12). Then, the Tg 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.
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Published Online: 2017-07-25
Published in Print: 2018-02-23