Thermally Rearranged Polymers Research Articles

Investigation of the chemical and morphological structure of thermally rearranged polymers

Reacting ortho-functional poly(hydroxyimide)s via a high-temperature (i.e., 350 °C–450 °C) solid-state reaction produces polymers with exceptional gas separation properties for separations such as CO2/CH4, CO2/N2, and H2/CH4. However, these reactions render these so-called thermally rearranged (TR) polymers insoluble in common solvents, which prevent the use of certain experimental characterization techniques such as solution-state nuclear magnetic resonance (NMR) from identifying their chemical structure. In this work, we seek to identify the chemical structure of TR polymers by synthesizing a partially soluble TR polymer from an ortho-functional poly(hydroxyamide). The chemical structure of this TR polymer was characterized using 1-D and 2-D NMR. By use of cross-polarization magic-angle spinning 13C NMR, the structure of the polyamide-based TR polymer was compared to that of a polyimide-based TR polymer with a nearly identical proposed structure. The NMR spectra suggest that oxazole functionality is formed for both of these TR polymers. Furthermore, gas permeation results are provided for the precursor polymers and their corresponding TR polymer. The differences in transport properties for these polymers result from differences in the isomeric nature of oxazole-aromatic linkages and morphological differences related to free volume and free volume distribution.

Gas sorption and transport in thermally rearranged polybenzoxazole membranes derived from polyhydroxylamides

The sub-nano-sized microcavities in microporous thermally rearranged (TR) polymers can be tuned by varying the conditions of thermal rearrangement. Relatively small cavities were formed by thermal rearrangement of poly(o-hydroxylamide) (PHA) precursors compared to the cavities formed by that of polyimide precursors. TR polymers derived from PHAs, so-called TR-β-polymers, are known to exhibit a well-tuned cavity structure that can be used for H2/CO2 separation. According to a solution-diffusion model, both the permeability and selectivity for H2/CO2 separation were improved at elevated temperatures due to a significant increase in H2 diffusion and a decrease in CO2 sorption. In this study, gas solubility and permeability of five representative small gas molecules (H2, N2, O2, CH4, and CO2) through TR-β-polymer membranes were characterized between 20°C and 65°C for gas solubility measurement and between 35°C and 300°C for gas permeability measurement. These measurements allowed for the calculation of thermodynamic factors such as the activation energy and heat of sorption.

Influence of polyimide precursor synthesis route and ortho-position functional group on thermally rearranged (TR) polymer properties: Pure gas permeability and selectivity

Pure gas permeability coefficients of CH4, N2, O2, CO2, and H2 are reported for thermally rearranged (TR) polymers derived from polyimides based on 3,3′-dihydroxy-4,4′-diamino-biphenyl and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (HAB-6FDA). These HAB-6FDA polymers were prepared with hydroxyl, acetate, propanoate, and pivalate groups in the ortho-position to the imide ring. Polymers with acetate ortho-position groups were synthesized via either thermal or chemical imidization. Pure gas permeability was approximately five times higher after rearrangement at 450°C in TR polymers from polyimides with acetate, propanoate, and pivalate ortho-position groups relative to TR polymers prepared from polyimides with hydroxyl ortho-position groups. In samples with non-hydroxyl ortho-position groups, those with larger ortho-position groups had higher gas permeability for TR conversions less than roughly 60%, but permeability increased to similar values as conversion exceeded 60%. In all samples, the CO2/CH4 selectivity also approached a similar value as TR conversion increased. Despite their higher permeability, fractional free volume was not significantly higher in TR polymers from polyimides bearing non-hydroxyl ortho-position groups than in samples with hydroxyl ortho-position groups. Therefore, average free volume alone cannot explain this behavior, suggesting that free volume distribution is likely affected by these groups.

A Simulation Study on OH-Containing Polyimide (HPI) and Thermally Rearranged Polybenzoxazoles (TR-PBO): Relationship between Gas Transport Properties and Free Volume Morphology

Recently, high free volume polymer materials have been regarded as high potential candidates for gas transport/separation membranes, since the amount of free volume in polymeric membrane can improve the diffusivity and solubility of gas molecules. In this study, we focused on how local changes in polymer structure can affect the performance of a membrane at the molecular level. The transport behavior was theoretically analyzed, and then the differences in the amount and morphology of free volume were characterized. Finally, we suggested how the "evolution of microcavities" affects the gas transport properties of hydroxyl-containing polyimide (HPI) and thermally rearranged (TR) polymers. In particular, using image analysis, we intuitively demonstrate the morphological difference between HPI and TR polymers that have been indirectly explained by experimental analyses using a wide-angle X-ray diffractometer (WAXD) and positron annihilation laser spectroscopy (PALS). Solubility results using the grand canonical Monte Carlo (GCMC) method showed marginal improvement in thermally rearranged polybenzoxazoles (TR-PBOs) from its precursor HPI, which is in good agreement with the experimental tendency. Moreover, higher diffusivities but lower selectivities of TR-PBO models compared with those of HPI models were observed, as reported experimentally. The difference in gas transport abilities between HPIs and TR-PBOs originates from the difference in their diffusion behavior, and this is strongly related to the free volume amount and morphology of polymeric materials. In addition to the higher amount of total free volume in TR-PBO, our image analysis revealed that TR-PBO has a higher amount of interconnected "hourglass-shaped free volume elements", which consist of larger and more elongated cavities with bottlenecks than the HPI model. In particular, the bottleneck diameters in the TR-PBO models are wider than those in the HPI models, enabling the larger gas molecules to diffuse through the cavities faster. However, the narrower and smaller bottleneck diameters in the HPI model can induce better selectivity for large gas molecules.

Influence of polyimide precursor synthesis route and ortho-position functional group on thermally rearranged (TR) polymer properties: Conversion and free volume

Thermal rearrangement of polyimides with ortho-position groups to polybenzoxazoles and related structures has been of recent interest for producing gas separation membranes. This study explores the influence of synthesis route and ortho-position functional group on the thermal rearrangement process and the fractional free volume of thermally rearranged (TR) polymers produced from polyimides derived from 3,3′-dihydroxy-4,4′-diamino-biphenyl and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (HAB–6FDA). Acetate, propanoate, and pivalate ortho-position functional groups were considered. Thermogravimetric analysis (TGA) was used to study thermal rearrangement at temperatures between 350 and 450 °C, and evolved gases from TGA were analyzed via mass spectrometry to characterize the byproducts of thermal rearrangement and thermal degradation. CO2 was the major byproduct of thermal rearrangement for all samples, and its evolution began well before the onset of thermal degradation. When non-hydroxyl ortho-position groups were present in the polymers, several byproducts other than CO2 were also observed due to the loss of these ortho-position groups before thermal rearrangement. Free volume generally increased with increasing extent of thermal rearrangement, but precise values of free volume could not be accurately determined for polymers with propanoate and pivalate ortho-position functional groups due to uncertainties in the chemical structure of partially converted materials. For polymers with acetate and hydroxyl ortho-position groups, free volume could be determined within the uncertainty of density measurements. Thermal rearrangement behavior and free volume results for acetate containing polymers synthesized via different routes were very similar. Based on these results, the chemical structure of the ortho-position functional group has a larger impact on TR polymer properties than the polyimide precursor synthesis route.

Physical aging and plasticization of thick and thin films of the thermally rearranged ortho-functional polyimide 6FDA–HAB

It was recently shown that the rate of the thermal rearrangement process of an ortho-functional polyimide is thickness dependent. Here it is shown that the gas separation properties of thermally rearranged (TR) polymer films are also affected by thickness. In this work, the physical aging and plasticization behavior of TR polymers were tracked using thin (~1–2μm) and thick (~15–20μm) films derived from an ortho-functional polyimide based on 2,2'-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) and 3,3'-dihydroxy-4,4'-diamino-biphenyl (HAB) over hundreds of hours. The thin films experienced a much greater rate of physical aging than the thick films, as indicated by the rapid decline of gas permeabilities and the gradual increase of ideal selectivities. As the thermal rearrangement temperature was increased, the aging rate of the resultant thin film did not increase extensively and the plasticization curves of the thin and thick films became closer. Long term exposure of the TR films to CO2 at 32atm over 500h shows that the CO2 permeability of the thick TR films did not show significant decline over time; whereas, the CO2 permeability of the thin TR films increased over the first several hours and decreased rapidly during further exposure time. Finally, the response of thick TR films to C2H4, C2H6 and C3H8 over long term exposure shows that physical aging is greatly accelerated when the polymer matrix is severely plasticized by the more condensable gases.

Application of membrane separation technology in postcombustion carbon dioxide capture process

Membrane separation technology is a possible breakthrough in post-combustion carbon dioxide capture process. This review first focuses on the requirements for CO2 separation membrane, and then outlines the existing competitive materials, promising preparation methods and processes to achieve desirable CO2 selectivity and permeability. A particular emphasis is addressed on polyimides, poly (ethylene oxide), mixed-matrix membrane, thermally-rearranged polymer, fixed site carrier membrane, ionic liquid membrane and electrodialysis process. The advantages and drawbacks of each of materials and methods are discussed. Research threads and methodology of CO2 separation membrane and the key issue in this area are concluded

Acetate-Functional Thermally Rearranged Polyimides Based on 2,2-Bis(3-amino-4-hydroxyphenyl)hexafluoropropane and Various Dianhydrides for Gas Separations

Three acetate-functional polyimides based on 2,2-bis(3-amino-4-hydroxy-phenyl)hexafluoropropane (APAF) and dianhydrides with different linking groups (-O-, -CO-, and -C(CF3)2-) were prepared, from which films were prepared and thermally rearranged (TR) to form poly(benzoxazole) dense membranes. The polymers were characterized by 1H nuclear magnetic resonance, Fourier transform infrared, differential scanning calrimetry, thermogravimetric analysis-mass spectrometry, and wide-angle X-ray diffraction. Transport properties of the TR membranes were investigated, and effects of the linking group and TR conversion rate on the membrane performance were discussed. Among the TR polymers studied, those bearing larger linking groups have higher d-spacing, fractional free volume, and gas permeability. The membrane permeability to gases increases with TR conversion rate and follows the order H2 > CO2 > O2 > N2 > CH4, except that CO2 surpasses H2 in APAF-6FDA-TR450. The CO2/CH4 separation performance of all of the TR po...

Thermally rearranged (TR) poly(benzoxazole-co-amide) membranes for hydrogen separation derived from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB), 4,4′-oxydianiline (ODA) and isophthaloyl chloride (IPCl)

Thermally rearranged poly(benzoxazole-co-amide) (TR-PBOA) copolymer membranes were prepared by treating poly(o-hydroxyamide-co-amide) (PHAA) precursors by in-situ thermal cyclization reaction for the H2 separation at high temperature. The physical properties, membrane processability and gas permeation property can be controlled by varying the ratio of diamine compounds. The rigid biphenyl moiety in the copolymer membranes increases gas selectivity whereas the flexible ether moiety improves membrane processability as well as gas permeability. Moreover, thermal rearrangement process affects the gas separation properties of TR copolymer membranes. TR polymer membranes derived from polyhydroxyamides (PHAs) demonstrated high H2 permeability, but low H2/CO2 selectivity. However, TR-PBOA copolymer membranes improved H2/CO2 selectivity. Both permeability and selectivity of H2 over CO2 increased due to the high activation energy for H2 permeation through the copolymer membranes and can be explained by thermodynamic theory with temperature. The highest performance was 26.8 Barrer of H2 permeability and 8.0 of H2/CO2 selectivity measured at 210°C.

Inverse Gas Chromatographic Study of Sorption Thermodynamics in Thermally Rearranged Polymer Based on 2,2-Bis(3-amino-4-hydroxyphenyl)-hexafluoropropane and 4,4′-Hexafluoroisopropylidene Diphthalic Anhydride

This work reports the first investigation using the inverse gas chromatography method of a polyimide precursor and the product of its thermal rearrangement (TR polymer), extensively studied earlier. Sorption of gases (CO2, C2H6, C3H8) was studied at the finite dilution regime, while vapors (n-alkanes C7, C8, C10, C14, C16) were investigated at infinite dilution. It was demonstrated that thermal treatment at 450 °C results in a significant increase in the solubility coefficients S for large gas molecules. The absolute values observed for the TR polymer solubility coefficients of solutes are significant and comparable with those for the polymer of intrinsic microporosity (PIM-1), the polymer known by the greatest S values among all of the polymers studied. Sorption thermodynamics in the TR polymer is distinguished by very large and negative mixing parameters: enthalpy h̅1E,∞ and entropy s̅1E,∞.

Hydrogen sorption in polymers for membrane applications

Hydrogen sorption between −20 °C and 70 °C and at pressures up to 60 bara was determined for a polyimide and corresponding thermally rearranged (TR) polymers prepared from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). Hydrogen sorption increased by a factor of approximately 2.6 between the polyimide precursor and the most highly converted TR polymer. This relative increase in sorption was similar to that observed for other non-polar light gases such as N2, O2, and CH4, but less than that observed for CO2. Additionally, H2 sorption was measured for other polymers commonly studied in the membrane literature, including AF 2400, Matrimid®, polysulfone, and poly(dimethylsiloxane). Among the glassy polymers tested, polysulfone had the lowest H2 sorption, and the HAB-6FDA TR polymer had the highest H2 sorption. A slight dual-mode curvature was observed for H2 sorption in several of the glassy materials, and it was most pronounced at low temperatures and for the TR polymers. Enthalpies of sorption were also determined. The most exothermic enthalpy of sorption occurred in Matrimid®, and a slightly endothermic enthalpy of sorption was observed in rubbery poly(dimethylsiloxane). Comparisons between gravimetric and volumetric sorption showed similar results.

Synthesis and characterization of thermally rearranged (TR) polymers: effect of glass transition temperature of aromatic poly(hydroxyimide) precursors on TR process and gas permeation properties

Soluble aromatic polyimides containing ortho-positioned hydroxy groups were synthesized as precursors for thermal rearrangement (TR) to polybenzoxazoles (PBOs). Fully imidized polyimides with high-molecular-weights were afforded via a ‘one-pot’ solution imidization technique (i.e., ester-acid method). The poly(hydroxyimide)s were designed to vary in their glass transition temperatures (Tg) by carefully selecting dianhydride–bisaminophenol combinations to introduce various levels of chain rigidity. TR conversion (imide-to-benzoxazole conversion) occurred in solid-state films only under inert atmosphere and over a temperature range of 300–450 °C, depending on the chemical structure (chain rigidity) of precursors. The effect of the precursor Tg on TR conversion was studied using TGA, DSC, FTIR and gel fraction measurements. The TR conversion temperature of imide-to-benzoxazole rearrangement strongly depended on the precursor Tg. Thus, for example, the feasible TR temperature was successfully reduced by ∼100 °C by lowering the precursor Tg by using a bisphenol A type dianhydride in the polymer synthesis. Gas permeation properties of representative TR systems are also reported. The TR process significantly increased gas permeabilities while maintaining good selectivities. By correlating the TR conversion degree with gas transport properties, there appears to be an optimal TR conversion degree that can maximize both gas permeability and selectivity. Systematic studies on TR polymers derived from low Tg precursors were suggested to further explore this correlation.

Synthesis and characterization of Thermally Rearranged (TR) polymers: influence of ortho-positioned functional groups of polyimide precursors on TR process and gas transport properties

Aromatic polyimides bearing various ortho-functional groups (i.e., acetate group and pivalic acetate group) were prepared via acetylation of a poly(hydroxyimide) containing ortho-positioned hydroxy groups using acetic anhydride or pivalic anhydride. The completeness of acetylation was confirmed by 1H NMR and FTIR. Chemically derivatized polyimides were used as precursors for an imide-to-benzoxazole thermal rearrangement (TR) process. The influence of various ortho-functionalities on the TR process and gas transport properties of the resulting TR polymers was studied. Differing from the –OH groups in a poly(hydroxyimide), the acetate groups of acetylated polyimide precursors degrade at elevated temperatures, and the degradation process interplays with imide-to-benzoxazole conversion. The acidic degradation product, as detected by 1H NMR, is suspected to have some catalytic effect on the TR process, which along with the protecting function of the acetate groups, resulted in a lower onset TR conversion temperature, the ability to conduct the TR process in air, and a higher TR conversion level. Gas permeation properties greatly depend on the ortho-functionality of polyimide precursors as well. The precursor films containing larger functional groups are much more permeable with comparable gas selectivities. Similarly, the resulting TR polymers formed from polyimides with larger leaving groups also showed much higher gas permeabilities despite similar degrees of TR conversion. The incorporation of bulkier functional groups in the TR precursors provides an effective way to significantly improve the gas transport performance, particularly the gas permeabilities of both the polyimide precursors and the resulting TR polymers.

Thermally rearranged (TR) polymer membranes with nanoengineered cavities tuned for CO2 separation

Membrane gas separation technology has been rapidly growing for industrial applications such as air separation, carbon dioxide (CO2) separation from natural gas production, hydrogen separation, etc. Needs for CO2 separation are increasing as carbon capture technology has been recognized as an essential part when combating the global warming issue. Membrane gas separation technology deals with mass transport phenomena through the membrane engineered on a sub-nanoscale controlling transport properties of small gas molecules such as CO2, N2, O2, H2, etc. In this review, we will report on the recent developments in capture technologies utilizing various membranes including nano-engineered thermally rearranged (TR) polymers. TR polymer membranes show high gas permeability as well as good separation properties, especially in CO2 separation processes such as from post-combustion flue gas and natural gas sweetening.

Gas sorption and characterization of thermally rearranged polyimides based on 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)

Abstract The solubilities of H2, N2, O2, CH4, and CO2 were determined over a range of pressures at 35 °C in a glassy, amorphous, ortho-functional polyimide prepared from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA). The HAB-6FDA polyimide was partially converted to its corresponding thermally rearranged (TR) polymer by thermal treatments at different times and temperatures. At 10 atm, solubility coefficients of H2, N2, O2, CH4, and CO2 increased by a factor of approximately two between the polyimide and the most highly converted TR polymer. Correlations between solubility and penetrant condensability were in good agreement with such correlations in other fluorinated polymers. Dual-mode sorption model parameters were determined from the sorption isotherms. The affinity constant, Henry's law solubility, and Langmuir capacity constant increased with gas condensability, and increases in the Langmuir capacity constant were observed as TR polymer conversion increased. Comparisons were made between the solubility selectivity of CO2/CH4, O2/N2, CH4/N2, and CO2/N2 with HAB-6FDA, its corresponding TR polymers, and with other polymers in the literature. Qualitatively, a decrease in solubility selectivity for gas pairs including CO2 correlates with imide and acetate loss during conversion.

The relationship between the chemical structure and thermal conversion temperatures of thermally rearranged (TR) polymers

An attempt was made to elucidate the relationship between chemical structure, in terms of glass transition temperature (Tg), and conversion temperature (TTR) of thermally rearranged polybenzoxazole membranes, by analyzing DSC and TGA data for a total of 15 sets of o-hydroxypolyimides and copolyimides, derived from two experimental and four commercial structurally different diamines, as well as four different dianhydride monomers. Our research revealed that TTR was influenced by chain ridigity of o-hydroxypolyimides, and exhibited a linear relationship with Tg. Therefore, structure and thermal property of o-hydroxypolyimide should be considered when studying TR polymers to determine thermal conversion to polybenzoxazole.

Gas permeability, diffusivity, and free volume of thermally rearranged polymers based on 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA)

Abstract HAB-6FDA polyimide was synthesized from 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) and 2,2′-bis-(3,4-dicarboxyphenyl) hexafluoropropane dianhydride (6FDA) by a two-step polycondensation method with chemical imidization. This polyimide was used as the precursor to prepare thermally rearranged (TR) polymers. The rearrangement reaction was performed at temperatures from 350 to 450 °C. Based on mass loss during the TR process, TR conversion levels as high as 76% were observed. CO 2 permeability increased from 12 Barrer in the precursor polyimide (HAB-6FDA) to 410 Barrer in the TR polymer prepared at 450 °C, while pure gas CO 2 /CH 4 selectivity decreased from 42 to 24. Both diffusivity and solubility increased following the rearrangement process, but the change in gas diffusivity was the largest contribution to the increase in permeability. From the polyimide to the sample heated at 450 for 30 min, CO 2 diffusivity increased by approximately a factor of 20 while solubility increased by a factor of 1.7. Similar changes were observed for other gases. Fractional free volume increased from 15% in the polyimide precursor to 20% in the TR polymer rearranged at 450 °C. The HAB-6FDA TR polymers exhibited higher CO 2 permeability than other classes of polymers with similar free volume, suggesting a free volume distribution in these TR polymers that favors high permeability.

Advances in high permeability polymeric membrane materials for CO2separations

Global CO2 emissions have increased steadily in tandem with the use of fossil fuels. A paradigm shift is needed in developing new ways by which energy is supplied and utilized, together with the mitigation of climate change through CO2 reduction technologies. There is an almost universal acceptance of the link between rising anthropogenic CO2 levels due to fossil fuel combustion and global warming accompanied by unpredictable climate change. Therefore, renewable energy, non-fossil fuels and CO2 capture and storage (CCS) must be deployed on a massive scale. CCS technologies provide a means for reducing greenhouse gas emissions, in addition to the current strategies of improving energy efficiency. Coal-fired power plants are among the main large-scale CO2 emitters, and capture of the CO2 emissions can be achieved with conventional technologies such as amine absorption. However, this energy-consuming process, calculated at approximately 30% of the power plant capacity, would result in unacceptable increases in power generation costs. Membrane processes offer a potentially viable energy-saving alternative for CO2 capture because they do not involve any phase transformation. However, typical gas separation membranes that are currently available have insufficiently high permeability to be able to process the massive volumes of flue gas, which would result in a high CO2 capture. Polymer membranes highly permeable to CO2 and having good selectivity should be developed for the membrane process to be viable. This perspective review summarizes recent noteworthy advances in polymeric materials having very high CO2 permeability and good CO2/N2 selectivity that largely surpass the separation performance of conventional polymer materials. Five important classes of polymer membrane materials are highlighted: polyimides, thermally rearranged polymers (TRs), substituted polyacetylenes, polymers with intrinsic microporosity (PIM) and polyethers, which provide insights into polymer designs suitable for CO2 separation from, for example, the post-combustion flue gases in coal-fired power plants.

Cavity size, sorption and transport characteristics of thermally rearranged (TR) polymers

Within a polymer thin film, free volume elements have a wide range of size and topology. This broad range of free volume element sizes determines the ability for a polymer to perform molecular separations. Herein, six permeable thermally rearranged (TR) polymers and their precursors were studied. Using atomistic models, cavity size (free volume) distributions determined by a combination of molecular dynamics and Monte Carlo methods were consistent with experimental observation that TR polymers are more permeable than their precursors. The cavity size distributions determined by simulation were also consistent with free volume distributions determined by positron annihilation lifetime spectroscopy. The diffusion, solubility and permeation of gases in TR polymers and their precursors were also simulated at 308 K, with results that agree qualitatively with experimental data.

Thermally Rearranged (TR) Polybenzoxazole: Effects of Diverse Imidization Routes on Physical Properties and Gas Transport Behaviors

Conversion of hydroxyl-containing polyimides into polybenzoxazole can be achieved by thermal rearrangement of the aromatic polymer chain with decarboxylation at elevated temperature. Synthetic methods to prepare polyimide precursors are important for the resulting thermally rearranged (TR) polymer membranes. Here, we report on the effect of several imidization methods on the properties of TR polymer membranes. Thermal and chemical imidizations are the most common routes to prepare polyimides, and solution thermal imidization using an azeotrope is also widely used, especially to obtain soluble polyimide-containing functional groups. We report here on the syntheses of ortho-functional polyimides from 4,4′-hexafluoroisopropylidene diphthalic anhydrides and 2,2′-bis(3-amino-4-hydroxyphenyl)hexafluoropropane by three different imidization methods. Acetate-containing polyimides by chemical imidization and further silylation treatment as well as hydroxyl-containing polyimides by thermal and azeotropic imidizatio...