2.2　Synthesis of Spinning Resin Solutions

PIs have various chemical structures， which can be synthesized using multi-approaches， and this adjustability is unique among polymeric materials. At present， there are two main routes for synthesizing PIs： one approach is to synthesize PI from monomers containing imide rings， and the other approach is to prepare the precursor poly（amic acid） （PAA） at first， continuing with thermal or chemical cyclization to form imide rings. The second synthetic route is widely used now due to the advantages that dianhydride and diamine monomers can derive from a wealth of sources and a simple preparation process. This process is the most common route for synthesizing PIs， having a great potential for industrial development. Based on the difference of reaction processes and mechanisms， the synthesis of PIs is usually divided into “one-step” and “two-step” methods， respectively， which will be illustrated in detail， combined with specific examples in the following sections.

2.2.1　“Two-Step” Polymerization Method

2.2.1.1　Synthesis of Poly（Amic Acid） Solution

In a typical “two-step” method， as shown in Fig. 2.2， PAA is firstly obtained by polycondensation of equimolar dianhydrides and diamines in aprotic polar solvents， such as N，N-dimethyl formamide （DMF）， N，N-dimethylacetamide （DMAc）， N-methyl-2-pyrrolidone （NMP） and other ap rotic polar solvents， and then converted to PI through a chemical or thermal imidization process ［9］.

The reaction of synthesizing PAA in aprotic polar solvents by diamine and dianhydrides is reversible ［10，11］， and the forward reaction is regarded as forming charge transfer complex among dianhydrides and diamines. The equilibrium constant at room temperature is as high as 105 L·mol−1 ［12］， therefore， it is easy to obtain PAA with high molecular weights. The electron affinity of dianhydride monomers and the alkalinity of diamines are the key factors that affect reaction rates. In general， dianhydride monomers containing electron-withdrawing groups， including CO， OSO， and —CF₃， are beneficial in improving the acylating abilities of dianhydrides. However， it is very difficult to obtain PAA with high molecular weight through a polycondensation at low temperature when diamines containing electron-withdrawing groups， especially these groups located in the para- or ortho-positions of the amino groups. The chemical structures and electron affinities （E_a） of some common dianhydrides are listed in Table 1.1 in Chapter 1， Advanced Polyimide Films， and the chemical structures of aromatic diamines， their alkalinities （pK_a）， and acylation rates constantly reacting with pyromellitic dianhydride （PMDA） （lgk） are listed in Table 1.2 in Chapter 1， Advanced Polyimide Films. In addition to the monomer structures， factors that affect PAA synthesis also include the following：

1. Reaction temperature. The synthesis of PAA belongs to an exothermic reaction， and increasing the reaction temperature is beneficial for the reverse reaction， reducing the relative molecular weights of PAA. Therefore， temperature of the polycondensation is always controlled to −5 °C to 20 °C. As for those monomers with low reactivities， normally， increasing temperature is in favor of forward reaction， in fact， the “high-temperature one-step” method is often used to prepare high-molecular PIs when the reactivity of diamines or dianhydrides is relatively low.

2. Concentration of reaction monomer. In addition to the reaction temperature， the concentration of monomers also plays an important role in the molecular weights of PAA. The forward reaction of preparing PAA is a bimolecular reaction， yet the reverse reaction is an unimolecular reaction； as a result， increasing temperature is beneficial to the forward reaction； however， high concentration usually leads to high viscosity， resulting in uneven mass and heat transfer， which hinders the chain propagation. For preparing high-quality spinning solutions， the concentration of monomers is usually adjusted in the range of 10%-25%（w）.

3. Molar ratio of dianhydride and diamine. In theory， when the molar ratio of dianhydride and diamine is close to 1：1， the molecular weight and inherent viscosity of synthesized PAA are highest， in fact， dianhydride monomer is sensitive to trace moisture， and easily deliquesces to form carboxylic groups thus decreasing the reactivity， so it is best to control the molar ratio at （1-1.02）：1 of dianhydride and diamine monomers.

4. Solvent. The aprotic polar solvents are frequently used for PAA synthesis， such as DMAc， DMF， dimethyl sulfoxide （DMSO）， NMP， and so on. The dissolving capacity varies from one solvent to another. In view of the requirements of environmental protection and safety， DMAc and NMP are the most widely used at present. NMP is inert and environmentally friendly， and does not associate with PAA， showing a better dissolving capacity， in favor of preparing high-molecular-mass PAA resin.

5. Other factors. In addition to the above-mentioned factors， monomer purity， the feeding method， and moisture content will also affect the PAA molecular weights. In practice， various factors should be comprehensively considered to prepare high-molecular PAA， providing a good base for producing high-performance PI.

2.2.1.2　Imidization Reaction

The imidization of PAA is an important issue in preparing PI through the “two-step” method， it has a great significance for improving the processing and the properties of polymers， so that it has drawn the wide attention of many researchers to study the imidization process of PAA. The research in imidization process mainly involves the detection of cyclization degree， building cyclization kinetics equations and cyclization mechanism. Spectroscopy is the most common method used to detect imidization degree， in which the infrared spectrum has been widely used because of its simple operation， no damage to samples， and online monitoring ［13-15］.

Normally， asymmetric carbonyl stretching （1780 cm−1）， imide deformation （725 cm−1） and C-N stretching mode （1380 cm−1） in FTIR are always used to differentiate PI from PAA. Pryde et al. ［15］ discussed the difference in imidization degree calculated from these three peaks. In their conclusions， the intensities of characteristic peaks at 1780 cm−1 and 725 cm−1 were too weak， considerable errors were easily made when calculating the imidization degree. However， the peak at 1380 cm−1 was quite strong， slightly affected by the other chemical groups around， which was more suitable for calculating imidization degree quantitatively. Recently， the ratio of A1380/A1500 （A refers to the intensity） has been the common choice to detect imidization degree ［16］.

Many researches confirm that there exists a temperature-time effect in the imidization process of PAA， that is， two stages can be observed during the thermal imidization： as illustrated in Fig. 2.3， the initial fast stage and subsequent slow stage， and the cyclization slows down or even stops to a certain extent at a certain temperature. The reaction is accelerated immediately with rising temperature， but it slows down after a certain time， until the temperature increases again or complete imidization is carried out； this is the so-called “kinetic stop” ［17］. One explanation is that the glass transition temperature of the precursor polymer increases as cyclization goes on， leading to a decrease in chain mobility， which slows down the imidization reaction. Another explanation attributes the “kinetic stop” to the decrease in supporting effect of the residual solvent. There exists a complex compound between PAA and solvent， as for DMAc. In Brekner and Feger’s research ［18］， the complexation ratio may be 1∶4 and 1∶2 for PAA/DMAc， and the latter is more stable， which cannot be removed through the vacuum method at room temperature. This complex dissociation can only occur with elevating temperature. Shibayev et al. ［18］ believed that the complexed solvent could reduce the energy barrier of cyclization， thus accelerating imidization. However， the complex dissociation removes solvent with increasing temperature， lowering the imidization rate.

2.2.2　“One-Step” Polymerization Method

Different from the “two-step” method， in the “one-step” polycondensation， PI is directly synthesized by equimolar dianhydride and diamine monomers in high boiling solvents. At a high temperature， the intermediate PAA is cyclized spontaneously to PIs， and the generated water is removed along with nitrogen flow in order to obtain high-molecular-weight PIs.

In the 1980s， Kaneda et al. ［19］ from Kyoto University reported that PI solutions were prepared in the “one-step” polycondensation reaction of the mixed dianhydride from 3，3′，4，4′-biphenyltetra- carboxylic dianhydride （BPDA） and PMDA with various aromatic diamines， such as 3，3′-dimethyl-4，4′-diaminodiphenyl or 3，4′-oxydianiline （3，4′-ODA）， in p-chlorophenol and m-cresol at 180 °C. The obtained inherent viscosity varies from 3.2 dL·g-1 to 5.2 dL·g−1. Besides， they discussed the influence of concentration， reaction time and the amount of carboxylic acid added on the molecular mass of polymers. It should be noted that they found that p-hydroxyl phenyl acid showed a remarkable catalytic effect in this system. Stephen Z. D. Cheng and coworkers ［20］ from Akron University prepared a series of PIs via the “one-step” polycondensation， and they found that there existed an obvious phase transition in PI/m-cresol system by means of wide angle X-ray diffraction （2D WAXD）， polarized light microscope， and differential scanning calorimetry. At room temperature， the crystallosolvate I state formed initially when the concentration was higher than 40%（w）， and the system showed an anisotropic feature in the range of 45%-95%（w）， in which crystallosolvate I evolved into crystallosolvate II. The original aggregation structure and the evolution mechanism of the solution have been reported in their research， laying a solid foundation for preparing high-performance PI fibers.

However， phenolic solvents， such as p-chlorophenol and m-cresol were generally used in the traditional “one-step” polycondensation， whose strong irritant smells and high toxicities restricted their wide applications. On the other hand， this method was only fit for organo-soluble PIs， however， these high-molecular-mass insoluble PIs could not be obtained as precipitation formed during the high-temperature reaction. Therefore， this method is limited by the selection of monomers and solvents. To overcome these problems in the “one-step” polycondensation， great efforts have been made in recent years， mainly reflecting two aspects： designing new monomers and selection of new friendly organic solvents. Sakagrchi et al. ［21］ from Japan Toyobo successfully synthesized high-molecular-weight poly（benzoxazole-imide） in polyphosphoric acid （PPA） with a yield of 92% for the first time. The reaction temperature was controlled to 160 °C-200 °C； meanwhile， the influence of P₂O₅ content， reaction temperature， and solid content on the inherent viscosities of the resulting PIs were investigated in detail. Inspired by the above method， Chen et al. ［22］ prepared a series of PIs containing benzoxazole and benzimidazole units through the “one-step” polymerization （as shown in Fig. 2.4）， and they studied the structures and properties of the obtained high-molecular-weight PI in detail. It should be noted that the tensile strength and modulus of the dry-jet wet spun fibers reached as high as 3.12GPa and 220 GPa， respectively. Compared with phenol solvents， the PPA is more environmental friendly， less toxic， and PI with a special structure could form a liquid-crystal structure in PPA， in favor of preparing high-performance materials.

Zhang and coworkers ［23］ have also tried to synthesize high-molecular-weight PIs in PPA， and the obtained PIs showed better thermal stability and higher thermal decomposition activation energy compared with PI synthesized from the traditional “two-step” polycondensation. Hasanain and Wang ［24］ synthesized more than 10 kinds of PIs with different structures via the high-temperature “one-step” polymerization in salicylic acid， and the obvious feature of this polymerization pathway was that the reaction time reduced sharply， to less than 2 h in general. Meanwhile， the solid solvent can be recycled and reused. In recent years， efforts have focused on preparing PIs in “green solvents.” Tsuda et al. ［25］ reported that a soluble PI with an inherent viscosity of 0.5 dL·g−1 was synthesized through the high-temperature “one-step” method in the imidazole hexafluorophosphate ionic liquid， and no catalyst was added in this system， in which ionic liquids acted as catalyst to a certain degree.

Compared with the solvent system， the macromolecular structure has a greater influence on the properties of soluble PIs prepared by the “one-step” polymerization. In general， the introduction of ether linkage， trifluoromethyl， bulky side groups， and asymmetrical units into macromolecular chains resulted in great benefits for improving solubilities of PIs， as the symmetry and regularity of macromolecules were affected or even damaged， increasing the free volume， as well as decreasing interaction of molecular chains. For example， Chung et al. ［26］ reported high-molecular-weight PIs synthesized by a series of commercial dianhydride and diamine monomers， containing both a benzimidazole ring and trifluoromethyl side groups using NMP as the solvent at 190 °C via the “one-step” method， and these samples showed good solubilities in DMAc， DMF， DMSO， and other solvents. As reported by Zhang and coworkers ［27］， a series of high molecular-weight PIs were prepared in NMP at 190 °C for 12 h by introducing asymmetrical heterocyclic benzimidazole ring and trifluoromethyl groups into the backbones， and these organo-soluble PIs exhibit the number-average molecular mass （M_n） of （3.1-4.1） × 104. Besides， the gel-sol transition was observed in PI/NMP system. As depicted in Fig. 2.5， as the concentration of PI increased to 13%（w）， the system showed an obvious banded texture， exhibiting an anisotropic gel behavior. Increasing the temperature to 65 °C， the system transformed into an isotropy solution. It has been proved that this gel-sol transition was mainly caused by the inner ordered domains， and relative research provided vital reference for producing PI fibers with high strength and high modulus ［28］.

In conclusion， the synthesized PI from the “one-step” method at a high temperature has higher molecular weights and narrower molecular weight distributions. It also solved the problems of unstable storage process as well as avoiding complicated cyclization process in the “two-step” method， having more latent values for development from the point view of polymer processing. However， compared with the traditional “two-step” method， “one-step” polycondensation relied more on the solvent system， molecular structure design and good interaction between macromolecules and solvents. In addition， the selection of monomers for the high-temperature “one-step” method is also expensive and complicated， which are the main factors that have restricted its large-scale application.