**3.1 Synthesis and processing of PIs in supercritical fluids**

A supercritical fluid can be largely defined as a substance whose temperature and pressure are above its critical values. Above the critical temperature and pressure, a substance has the density of a liquid and the rheological properties of a gas. The characteristic density of a liquid allows the supercritical fluid to dissolve the substances, while the flowing features of the gas offer the advantage of lower reaction times [70].

This unique combination of physical traits offers several benefits in the use of supercritical fluids, both in macromolecular chemistry and materials science, but also in areas such as fine organic synthesis, catalysis, coordinative chemistry, and biochemistry.

Supercritical fluids and especially carbon dioxide (scCO2) have been successfully used in the last decade as solvents, antisolvents, or plasticizers in the synthesis and processing of PI-type materials. The synthetic procedure offers some chemical, ecological and economic conveniences. scCO2 is inert, nontoxic, nonflammable, relatively inexpensive, and, as an ambient gas, the solvent's removal after usage and depressurization is quite easy and hampers any ecological drawbacks. In some cases, the improved quality of the obtained products is an important factor in choosing supercritical liquids to the detriment of conventional organic solvents [70–78].

On the other hand, the use of supercritical fluids requires relatively high pressures and special equipment, and these considerations must be carefully balanced with the perceived advantages for a particular application.

The behavior of PI systems in the presence of scCO2 strongly depends on several parameters: the structural elements of the polymer, its physico-chemical characteristics (Tg, degree of crystallinity, cross-linking), the properties of pure scCO2 (molecular structure, critical points), the nature of the interactions between scCO2 and the polymer, and, obviously, the external temperature and pressure [72, 73].

Solubility is the crucial factor in the synthesis of polymers in scCO2. While scCO2 is a good solvent for low molecular mass, polar and nonpolar molecules, it is a very weak solvent for most high molecular mass polymers in mild conditions (below 100°C and 1000 bar). In some cases, a mixture of scCO2 and a common organic solvent provides satisfying results [74, 75].

## *New High-Performance Materials: Bio-Based, Eco-Friendly Polyimides DOI: http://dx.doi.org/10.5772/intechopen.93340*

The majority of research dealing with the synthesis of PIs in scCO2 focuses on the polycondensation of common, commercial monomers. 6FDA or similar fluorinecontaining monomers are the preferred building blocks in this regard because most fluorinated polyimides seem to have a higher solubility in scCO2 [76–78].

Most studies report reactions with good yields and products with variable inherent viscosities, depending on the monomers'structure and reaction conditions. The molecular mass of the resulting PIs is influenced both by the concentration of monomers, by the reaction time and temperature, and by the scCO2 pressure. Some articles report a catalytic effect of scCO2, since the reaction rate increases in the presence of traces of water. Subsequently, the small water amounts can lead to the formation of cyclization products of lower molecular mass. At the same time, the employment of scCO2 in the synthesis of polymers leads to a small decrease in the Tg and dielectric constant values, due to a plasticizing effect. This can be an important advantage when it comes to the processability possibilities of the final material [79–82].

An increasing trend regarding the processing of polyimide materials using scCO2 was also observed in the last decade. The scCO2 dissolved in a condensed phase causes various changes in the microstructure of the polymer and a considerable reduction of its viscosity due to the increase in the free volume of the polymer matrix. This leads to severe modifications in the transport properties of the polymer, the permeability, and selectivity coefficients growing several times [83, 84]. As a consequence, scCO2 is largely used in the production of nanoparticles, foams, aerogels, and membranes based on linear [85–87] or cross-linked [88–92] PIs. These materials have interesting features (high porosity, tunable shrinkage, nano- and micro-cavities, high surface area, ultralow dielectric constant, and mechanical resilience) that open up new possibilities in the applicability of PI-based systems such as catalytic systems, separation membranes, aviation, aerospace, construction, and others [93–97].

For a rational design and optimization of these processes, it is essential to know the solubility, diffusibility, density, and permeability of the imidic macromolecular compound.

Nevertheless, despite the advantages and promising results attained so far, supercritical fluid technology has little chance to replace conventional methods for the time being. This is due to the limitations imposed by the solubility issue, the viscosity and molecular mass of the reaction products, as well as by the special equipment required by the supercritical conditions.

## **3.2 Synthesis and processing of PIs in ionic liquids**

Ionic liquids had a cutting edge impact in the last two decades in polymer chemistry and materials science. They are widely used in both academia and industrial research and are considered a new class of ecologically compatible, green solvents, which can reduce or even replace the often dangerous, polluting, classic organic solvents.

The ionic liquids term generally implies different types of salts with a structure similar to ordinary salts like NaCl. However, while common salts melt at high temperatures, most ionic liquids remain liquid in a temperature range between room temperature and 200°C. They are usually formed by highly polar combinations of organic, voluminous, and asymmetric cations, decorated with aliphatic residues, and inorganic anions of symmetrical, regular shape, this chemical composition resulting in a low melting point. Poor packaging and poor coordination of ions in the structure of ionic liquids is the reason why they remain liquid at room temperature [98–100].

This peculiar structural composition unlocks a unique range of physicochemical features that make them environmentally compatible and suitable for various applications. They can dissolve different types of organic, inorganic, and organometallic materials, and their solvent properties can be modified to satisfy a particular application, by varying the combinations of anions and cations. They have high thermal conductivity, are liquid on a broad temperature range, and thermally stable up to 300°C, which allows the kinetic control of a reaction within wide limits. In addition, they possess extremely low vapor pressures (and hence do not evaporate) and are immiscible with many organic solvents [98, 101].

The benefits of ionic liquids were widely applied at first in the synthesis of classical PIs by the two-stage polycondensation reaction of common commercial diamines and dianhydrides [102]. Nevertheless, the solubilizing capacity of ionic liquids enables their usage in the polycondensation of more exotic building blocks with special features [103–107]. The method usually results in satisfying yields, without further addition of other catalysts. The obtained compounds show high molecular masses and inherent viscosities, enhanced thermal stability, and proper mechanical resistance.

Some studies also evidence an activating or catalytic effect of ionic liquids that boost the monomers' reactivity and unlock PIs with high molecular mass [102, 108].

The polycondensation of said monomers is usually performed in hydrophobic ionic liquids. Otherwise, the retained water will cause a decrease in the monomers' reactivity and hamper the formation of cyclization products with high molecular masses. The procedure generally involves easy work-up and solvent reusability and provides the potential for further scale-up attempts.

The innate immiscibility of ionic liquids with organic solvents was also explored in the green, interfacial polymerization of cross-linked PIs to obtain composite osmotic membranes [109].

In the early stages of their usage, ionic liquids were criticized for "sticking" to the PI material, even after various purification attempts. A small quantity of ionic liquid within the microstructure of a PI film or membrane would interfere with its thermo-mechanical features.

In the last decade, this issue was transformed into a benefit, through the advantageous combination of the versatility of both PIs and ionic liquids. The synthesis of these materials is usually performed by polycondensation of classical PI building blocks with ionic monomers (especially diamines) or via commercial PI modification. The resulting macromolecular species is called polyimidic ionenes or imidic poly(ionic liquid)s. The strategy uses ionic liquids'strong affinity to CO2 with the high-performance characteristics of PIs for the development of new materials for gas membrane separation [110–113].

### **3.3 Synthesis of PIs in microwave conditions**

As it can be easily observed from the previous sections of this chapter, classical heating remains the primary means of stimulating chemical reactions that are difficult to carry out under ambient conditions, such as those used in PIs chemistry. In recent years, this technique is strongly rivaled by several modern heating techniques, the most important being microwave heating [114].

The main advantages of using microwaves in the synthesis of PIs are the rapid completion of polycondensation, high purity of final products, uniform temperature rise, and overall greener energy balance. Some small drawbacks refer to the less accessible upscale and the need to re-dissolve the final product to prepare PI films or membranes [115, 116].

### *New High-Performance Materials: Bio-Based, Eco-Friendly Polyimides DOI: http://dx.doi.org/10.5772/intechopen.93340*

The outstanding downsizing of reaction time is the strongest point of this experimental technique. Microwave-assisted polycondensation reactions proceed very rapidly, requiring only a few minutes of irradiation to obtain binary or ternary soluble polyimides with more than acceptable inherent viscosity (up to 1.2 dL/g) and molecular masses (up to 200 kDa) [117]. The irradiation time is a key parameter that can easily turn from a benefit to a drawback. Too long irradiation times or too high irradiation powers are many times translated into PIs of lower molecular mass, due to the partial degradation of the polymer [118].

The polycondensation proceeds through a one-stage mechanism, the imidization being performed employing microwave heating [119] or with the help of additional chemical initiators [120]. At the same time, microwaves can be used only to perform the cyclization of the PAA precursors obtained by conventional methods. This procedure is able to generate PI films with improved mechanical features as compared to their thermally cyclized counterparts [121].

The reaction can be easily extended to a wide range of building blocks (common monomers'salts included [122, 123]) and even to the direct production of various composites [114, 116, 124]. The viscosity value of the final products usually depends on the solubility of the final compound in the reaction medium. The initial amount of solvent or overall monomer concentration thus plays an important role in the characteristics of the final material.

The most used solvents in microwave-assisted PI synthesis are the polar ones, with a high dielectric constant and high boiling point like NMP, DMF, and alike. Under microwave irradiation, these solvents increase their temperature extremely fast and reach the boiling point in a short time. Although nonpolar solvents do not absorb microwaves, they can be still be used in combination with small amounts of polar solvent or salts.

Some reactions are solvent-free and only require pertinent microwaveabsorbing monomers [125] or additives (CuO is an efficient example) [126]. The latter are used to quickly raise the reaction temperature to the melting of (at least one of) the monomers, thus empowering melt polycondensation. Such a strategy eliminates the usual, tedious washing process and the use of any (potentially) toxic organic reagents, further increasing the green character of the technique.

The one-stage polycondensation mechanism, combined with a small heating time and reduced (if any) contact with solvent usually translates in higher optical transparencies as compared to PI analogs obtained by the classical method [118].

Several studies compared the classical, thermal-, and microwave-assisted polycondensation procedures applied on the same common starting compounds in terms of order degree within polymeric backbones, imidization level, thermal and mechanical properties of resulting films. Microwaves assured a higher order level of the final products and proved to be more efficient and, of course, faster in reaching imidization [115, 122]. The imidization degree attained by microwaves was double the size of the one obtained by common thermal cyclization at 200°C and was almost complete at 250°C. Moreover, the PIs obtained by microwaves displayed higher thermal stability and superior mechanical resilience (even up to 30% higher) as compared to their thermally imidized counterparts [115, 117, 127].

Even if the method comes with some technological limitations regarding upscale and industrial use, the results obtained so far are quite impressive and, together with their obvious green character, require extensive research on the topic.

### **3.4 Other green and non-conventional synthetic procedures**

A new, alternative method for PI synthesis appeared roughly 25 years ago, involving the use of the ultimate green solvent, water, in special temperature, and pressure

conditions [128, 129]. The method is now generally known as hydrothermal polymerization (HTP) due to its resemblance to the geomimetic conditions of silicates' forming by condensation in the hydrothermal veins of the Earth's crust [129, 130].

HTP uses the same building blocks as the conventional PI synthesis: a dianhydride (usually, the original, more stable, tetracarboxylic acid is employed since the dianhydride will automatically hydrolyze in water) and a diamine. They form a nylon-type AB monomer salt intermediate which is subjected to polycondensation in water, at elevated temperatures and pressures accessible through a steel pressure vessel [131–133].

Since the monomer salt formation was undeniably proven by separate studies [128, 131, 132], one version of the method begins with the separate synthesis and purification (washing and filtration) of the monomer salt as the starting building block. Although not mandatory, this will provide the (almost) perfect stoichiometry required by any polycondensation reaction to ensure high yields.

Typical HTP experimental conditions for high monomer conversion are a 200°C temperature, a 16.7 bar pressure, and a reaction time between 6 and 24 h. The imidic product precipitates on cooling in aqueous residues of low toxicity. There are small amounts of residual solvent and no volatile impurities trapped in the final product.

At first glance, the reaction seems remarkable or even paradoxical, since it appears to contradict Le Chatelier's principle and the formation of PIs by polycondensation. A reversible reaction, with water as a side product, leading to products that cannot be (classically) obtained systematically unless firm absence or removal of water, feels inappropriate to be carried in water. Nevertheless, it works with noteworthy results. First, the salt formation increases hydrolytic stability, prevents the reverse reaction, and leads to increased imidization rates. Second, the aforementioned principle is still followed, the (theoretically) reversible polycondensation reaction being generally driven by the innate insolubility of the synthesized imide products.

The reaction follows a classical stepwise polymerization with a three-fold mechanism strongly depending on the temperature (TR) (reaction time and monomer concentration must be also considered) [132]. In the sub-hydrothermal regime, sHTP, the polymerization takes place in solution and leads to amorphous, short, low oligomeric PIS of a zwitterionic nature that coexists with unreacted, less soluble monomers. Between 100 and 130°C, longer macromolecular chains start to form in an ordered fashion, leading to semicrystalline PIs. When the reaction temperature comes to close to 200°C, the order degree and overall morphological homogeneity rise considerably and crystalline imidic products are generated. Solid-state polymerization (SSP) takes place in the high-temperature regime and is correlated to the polymerization temperature of any given monomer. If sHTP and SSP are mostly suppressed by judicious selection of starting compounds and reaction parameters, (almost) completely cyclized, highly crystalline PIs with particular morphological features are accessible by this method.

The potential usage of the method was recently broadened by the successful PI synthesis in various protic polar solvents (ethanol, isopropanol, and glycerin) or several aqueous mixtures therefrom. This unlocks the employment of new building blocks and fine tuning of reaction conditions toward desired PI morphologies [134].

Most HTP reactions performed so far used various aromatic dianhydrides/ tetracarboxylic acids and commercial diamines and reproduced the structure of common or commercially available polyimides. The obtained molecular masses were in most cases significantly close to those of the PIs prepared by the classical method. Other basic properties (aspect, optical transparency, thermal, and

## *New High-Performance Materials: Bio-Based, Eco-Friendly Polyimides DOI: http://dx.doi.org/10.5772/intechopen.93340*

mechanical features) are strongly dependent on the starting monomers and crystallinity degree of the polycondensation product.

Nevertheless, the method allows the synthesis of new PI structures [135] and composites [136], which can also be assisted by microwaves [137] or extended toward the production of PI fibers by green electrospinning or application-driven materials [133, 138].

Although only in its infancy and not easily accessible due to the special reaction setup, HTP delivers interesting results which require extension toward new building blocks and structures. Most importantly, the method is close to achieving all the strict criteria imposed by an ideal green chemical industry: high efficiency, economy, low (if any) toxicity, and benign environmental impact.
