Research on the Synthesis and Characterization of Colorless Polyimide Based on a Novel Asymmetric Dianhydride Monomer

Abstract

As is known, colorless polyimide (CPI) is usually regarded as a substrate material for transparent flexible printed circuit boards (FPCB) as a high-performance optical-grade polymer film. The problem of CPI films being nonwetting with the metal interface in the preparation of transparent FPCB urgently needs to be solved. The 6FDA and 3FPODA monomers containing flexible structures were selected to prepare CPI films with TFDB by the copolymerization reaction for improving the optical properties. Two series of copolymerized CPI films (3F6FP/C-PI-x and 3F6FP/H-PI-x) with different ratios of dianhydride were prepared by using chemical imidization and thermal imidization methods. The solubility, optical properties, thermal properties, and mechanical properties of the two series of CPI films were characterized to explore the introduction of the 3FPODA monomer on the optical, thermal, and mechanical properties of CPI films. According to the research on the two series of copolymerized CPI films, it was found that the introduction of 3FPODA and 6FDA with a copolymerization method could improve the thermal and mechanical properties of optical-grade CPI films significantly. The optical, thermal, and mechanical properties of the CPI films prepared by the chemical imidization method are superior to those of the CPI films prepared by the thermal imidization method, which indicates that the chemical imidization method is more suitable for preparing such optical-grade CPI films. Based on the most excellent comprehensive performance, the optical indicators and other key properties of the 3F6F/C-PI-4 CPI film could meet the requirements for preparing transparent FPCB.

1. Introduction

Because of its excellent optical, thermal, dielectric, and mechanical properties, colorless polyimide (CPI) has been widely used in various fields. − For example, CPI was usually regarded as a substrate material for transparent flexible copper-clad laminates (FCCL), transparent flexible printed circuit boards (FPCB), and transparent flexible displays in the microelectronics industry, being a high-performance optical-grade polymer film. − However, there is a problem with optical-grade CPI films being nonwetting with the metal interface in the preparation of transparent FPCB currently. ,

As the substrate of FPCB, CPI films can achieve miniaturization, light-weighting, and flexibility of circuit boards. − The excellent electrical insulation performance of CPI films could prevent short circuits effectively and improve the reliability of electronic devices. − Meanwhile, in the manufacturing process of electronic devices, the CPI films can withstand high temperature welding to prevent the performance of circuit boards being affected. − In terms of encapsulation and protection, CPI films can be used as a packaging material to protect electronic devices from external environmental influences such as moisture, dust, and chemicals, which could extend the service life of FPCB effectively. − The chemical and thermal stability of CPI can also ensure the stable performance of electronic devices in different using environments. − The high transparency of the CPI films also made it possible for direct observation of the internal device status during the packaging process. − The application of CPI films used as a substrate in FPCB is mainly effected by the performance of electrical insulation, heat resistance performance, dimensional stability, processability, encapsulation, and protection. − As one of the core substrates of FPCB, CPI films were used to replace traditional glass or metal substrates and provide a lightweight and flexible supporting structure for circuits. − The high transparency of CPI films allows light to pass through, which is suitable for FPCB applications with requirement of optical functionality, such as flexible display driver circuits and transparent sensor circuits. − For the electrical insulation and heat resistance performance, the excellent electrical insulation properties of CPI films could prevent short circuits and ensure stable transmission of electronic signals effectively. − Meanwhile, their heat resistance can make them withstand high temperature processes (such as welding and annealing processes) during the manufacture of FPCB, due to which electronic devices can maintain stable performance in high-temperature environments and avoid the deformation or damaging of circuit. − For dimensional stability, the low coefficient of thermal expansion (CTE) of CPI films ensures minimal size changes during temperature fluctuations, which can guarantee the accuracy and reliability of circuit patterning in FPCB during bending, folding, and long-term usage. − With respect to processability, CPI films can be easily processed into complex circuit patterning through photolithography, etching, and other processes, which could meet the requirements of high-density wiring and fine circuits during the use of FPCB. − The flexibility of CPI films also made it possible for the circuit to be bent and folded in three-dimensional space, even achieving flexible circuit layout. − In summary, CPI films have become a key material for transparent FPCB because of their unique optical, mechanical, and electrical properties, which could promote the innovative applications of flexible electronic technology in consumer electronics, medical devices, wearable devices, and other fields. −

In order to prepare a high-temperature-resistant CPI, it is necessary to improve its transparency by ensuring unique heat resistance and dimensional stability of the CPI material. Starting from molecular structure design, it is necessary to choose dianhydride monomers with weak electrical absorption and diamine monomers with weak electron-donating groups to reduce the charge transfer between molecular chains and prepare high-temperature-resistant CPI films. The strategies of introducing strong electronegative groups, cycloaliphatic structures, large substituted groups, asymmetric structures, and rigid noncoplanar structures are all beneficial for the preparation of CPI materials. , The chemical structure of many new diamines and dianhydrides with strong electronegative groups, aliphatic ring structures, large side foundation structures, and asymmetric and rigid noncoplanar structures is shown in Figure . The introduction of these functional groups can reduce the orderliness, symmetry, and stacking of PI molecular chains, which could increase the spatial freedom of the molecular chains and disrupt the conjugation between chains through volume. The formation of charge-transfer complexes (CTCs) between or within molecules can also be inhibited or reduced by decreasing the absorption of CPI in the visible-light region. Although the CTC effect is detrimental to the optical properties of CPI, it can also lead to strong interactions between molecular chains, which could limit their movement and ensure the excellent thermal performance of CPI. Molecular structure design that is conducive to the optical transparency of CPI materials often reduces the thermal performance of the CPI material to a certain extent. The structural factors that improve the thermal performance of CPI (such as rigid aromatic structures and highly conjugated structures) can lead to the CTC effect and reduce the optical transparency of the CPI material. For the introduction of strong electronegative groups, the strong electronegative groups can reduce the stacking of PI molecular chains, which could increase the free volume between chains and reduce intramolecular or intermolecular charge transfer effects for improving the transparency of CPI films. Due to the strong electron-withdrawing ability and large free volume of trifluoromethyl groups, introducing fluorinated groups in the structure of CPI could reduce intramolecular and intermolecular charge transfer for preparing CPI films. The chemical structure of many new diamines and dianhydrides with strong electronegative group structures (such as PAPFT, BPFBD, 8FBPOA, 8FBPODMA, BCTPB, and DAPFB) is shown in Figure a. Upon the introduction of a lipid ring structure, the cyclic structure can break the conjugated structure on aromatic PI segments, reduce intermolecular interactions, increase the free volume between chains, and reduce CTC formation, which is usually used to improve the transparency and solubility of CPI films by maintaining good thermal stability. Due to the weak electron-withdrawing ability of cycloaliphatic dianhydrides, the reactivity of monomers is generally lower than that of aromatic monomers, which makes it difficult for polymerization to occur, and the molecular weight of the CPI polymer is relatively low. The chemical structure of many new diamines and dianhydrides with an aliphatic ring structure (such as HNTDA, 5-DAPI, 6-DAPI, 3,3*-HBPDA, and 3,4*-HBPDA) is shown in Figure b. The introduction of large substituent groups can reduce the interchain interactions by increasing the interchain distance, which could lead to a reduction in the chain packing density effectively. On the other hand, the introduced large volume substituent groups can hinder the electron flow and conjugation of the molecular chain, which could improve the transparency and solubility of the CPI material by reducing the probability of CTC formation. Meanwhile, the introduction of large volume substituent groups will not damage the rigidity of the molecular chain, which could maintain the thermal properties of the CPI material. The chemical structure of many new diamines and dianhydrides with large side foundation structures (such as TriPMPDA, TriPMMDA, TAMPM, and BADMT) is shown in Figure c. The introduction of asymmetric and rigid noncoplanar structures in PI molecular chains can disrupt the symmetry of the chains and increase interchain free volume by reducing their regularity, which could endow the CPI materials with good solubility. In addition, the conjugation between chains is also disrupted, which is beneficial for the preparation of CPI films by weakening the CTC effect. The chemical structure of many new diamines and dianhydrides with asymmetric and rigid noncoplanar structure (such as WuCF₃DA, 35DABSBF, and 9,9-bis[4-(4-amino-2-trifluoromethylphenoxy)-3-isopropyl-phenyl]­fluorene) is shown in Figure d. For the introduction of inorganic nanoparticles, it is possible to improve the thermal performance of CPI while maintaining its good optical properties by introducing polymerizable inorganic nanoparticles. The inorganic nanoparticles generally have a rigid core structure, which is the main reason for improving the thermal performance of the CPI material. The inorganic nanoparticles with polymerizable groups can be uniformly dispersed in the PI molecular chain, which could avoid the clustering of inorganic substances and effectively facilitate the formation of CPI materials.

1.

Chemical structure of new diamines and dianhydride: (a) with strong electronegative groups, (b) with an aliphatic ring structure, (c) with a large side foundation structure, and (d) with an asymmetric and rigid noncoplanar structure.

Our group has designed and synthesized a novel dianhydride 10-oxo-9-phenyl-9-(trifluoromethyl) −9,10-dihydroanthracene-2,3,6,7-tetracarboxylic acid dianhydride (3FPODA) to prepare a novel CPI to solve the nonwetting problem of optical-grade CPI films with a metal interface. By introducing a rigid semicyclic structure in the structure of CPI, it was expected to maintain the excellent thermal and mechanical properties of CPI to solve the nonwetting problem at the interface between optical-grade CPI films and metals. The optical, thermal, and mechanical properties of CPI films prepared by 3FPODA, 3,3′,4,4′-biphenyl tetracarboxylic dianhydride (BPDA), and 2,2′-bis(trifluoromethyl)-benzidine (TFDB) with a copolymerization reaction were also studied. It was found that the rigidity of the 3FPODA structure is better than that of BPDA and that the thermal and mechanical properties of CPI films can also be improved significantly by introducing the 3FPODA structure. However, the carbon atom of the carbonyl group in 3FPODA is sp²-hybridized, which could form a certain conjugation effect with the benzene rings on both sides for enhancing the CTC effect between the interchain and intrachain interactions of CPI molecules. The optical properties of the CPI films were also weakened by the CTC effect caused by the chemical structure of 3FODA. During our research, we found that the series of CPI films still cannot meet the optical specifications of transparent FPCB by improving the optical properties by being composed of 3FPODA, BPDA, and TFDB.

In this article, 4,4′-(hexafluoroisopropylidene)-diphthalic anhydride (6FDA) with a flexible structure was selected to adjust the optical properties of CPI films to meet the requirements of transparent FPCB by a copolymerization reaction with 3FPODA and TFDB. During the imidization process, two series of CPI films were prepared by using chemical imidization and thermal imidization methods. The solubility, optical properties, thermal properties, mechanical properties, and surface properties of these two series of CPI films were also characterized and discussed in detail. By adjusting the ratio of 3FPODA, the optical and other key indicators of the CPI film can be controlled to meet the requirements of transparent FPCB. According to the comprehensive performance of the two series of copolymerized CPI films, it was found that the introduction of 3FPODA and 6FDA with a copolymerization method could improve the thermal and mechanical properties of optical-grade CPI films significantly. The optical, thermal, and mechanical properties of the CPI films prepared by the chemical imidization method are superior to those of the CPI films prepared by the thermal imidization method. Based on the optical indicators and other key properties, the 3F6F/C-PI-4 CPI film with a T _g of 377 °C, a CTE of 24.3 ppm/K, a transmittance at 550 nm (T₅₅₀) of 90%, a tensile strength (T _S) of 136.5 MPa, an elongation at break (E _B) of 4.13%, and an elastic modulus (T _M) 4.4 GPa was sifted to meet the requirements for preparing transparent FPCB, which was expected to solve the problem of nonwetting between the CPI film and metal interfaces in the preparation of transparent FPCB.

2. Experiment

In this manuscript, TFDB was selected as the diamine monomer, and 3FPODA and 6FDA were chosen as the dianhydride monomers. A series of polyamide acid (PAA) solutions with different dianhydride ratios were prepared by using a traditional two-step method. During the imidization stage, two series of CPI films were obtained by chemical imidization and thermal imidization, which were also referred to as “CI” and “HI”, respectively. The proportions of 3FPODA in the CPI film were 0, 10%, 20%, 30%, 40%, and 50%, respectively. The specific proportions of the two dianhydride monomers and the naming of CPI films are shown in Table . We hope to explore a more suitable method for preparing this kind of CPI by comparing the performance of two series of CPI films. The series of chemical imidization and thermal imidization thin CPI films were named 3F6FP/C-PI-x and 3F6F/H-PI-x, respectively. 3F and 6F represent 3FPODA and 6FDA, respectively. C represents chemical imidization, and H represents thermal imidization. The content of 3FPODA in copolymerized CPI corresponds to x; for example, the ratio of 3FPODA to 6FDA in 3F6F/C-PI-1 is 1/9.

1. Monomer Ratios and Imidization Method of 3FBP/H-PI-x .

	monomer ratio
CPI name	n(3FPODA): n(6FDA)	TFDB	imidization method
3F6F/C-PI-0	0:100	100	CI
3F6F/C-PI-1	10:90	100	CI
3F6F/C-PI-2	20:80	100	CI
3F6F/C-PI-3	30:70	100	CI
3F6F/C-PI-4	40:60	100	CI
3F6F/C-PI-5	50:50	100	CI
3F6F/H-PI-0	0:100	100	HI
3F6F/H-PI-1	10:90	100	HI
3F6F/H-PI-2	20:80	100	HI
3F6F/H-PI-3	30:70	100	HI
3F6F/H-PI-4	40:60	100	HI

2.1. Materials

Dianhydride monomer 3FPODA was prepared according to the method described in ref . The dianhydride 6FDA used in this manuscript was purchased from Shanghai Aladdin Reagent Co., Ltd. with a purity of 99.85%. The diamine TFDB used in this article was purchased from Shanghai Aladdin Reagent Co., Ltd. with a purity of 99%. Dried N,N-dimethylacetamide (DMAC) was purchased from Wuhan Imide (Wuhan, China). Other solvents were received from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

2.2. Instrumentation

IR spectra were all measured using a PerkinElmer SP one FT-IR instrument (USA). Wide-angle X-ray diffraction (WAXD) measurements were carried out on a Bruker Bede XRD Di (Switzerland). The optical properties of PI films were recorded on Shimadzu UV-2450 (Japan) from 300 to 800 nm. The refractive index of the PI films was measured with an ME-Lellipsometer (China). Thermogravimetric analysis (TGA) measurement was performed on a PerkinElmer TGA-2 (USA) under nitrogen conditions at a rate of 10 °C/min from room temperature to 800 °C. Dynamic mechanical analysis (DMA) was carried out on a PerkinElmer DMA Q800 using tensile mode at a frequency of 1 Hz. The coefficient of thermal expansion (CTE) was measured by a thermal mechanical analysis (TMA) Q400 (USA). Mechanical properties were all measured by a stretching machine.

2.3. Preparation of CPI Films

The synthetic route of CPI is shown in Figure . Taking the synthesis of 3F6F/H-PI-3 as an example, the specific synthetic methods were as follows.

2.

Synthetic route of CPI based on 3FPODA, 6FDA, and TFDB.

2.3.1. Synthesis of PAA Solution

TFDB (1.000g, 3.123 mmol) was added to a 50 mL three-necked flask under the protection of nitrogen, and then 18g of dried DMAC was also added to the three-necked flask to dissolve solids. After the dissolution of TFDB, 3FPODA (0.447g, 0.936 mmol) and BPDA (0.6426g, 2.186 mmol) were mixed uniformly and added to the three-necked flask in batches with continuous stirring. The mixed solution was stirred at room temperature for approximately 24 h to obtain a transparent PAA solution with a certain viscosity.

2.3.2. Synthesis of the 3F6F/H-PI-3 CPI Film Prepared by the HI Method

After an appropriate amount of 3FBP/H-PI-3 PAA solution was coated on a clean glass, a spin coater was used to spread the PAA solution uniformly on the glass with a suitable spinning speed. Then the glass samples with the PAA layer were cured at 80 °C for 1 h to remove most of the solvent in the PAA solution. After cooling to room temperature, the samples with CPI layers were also baked at 100 °C for 30 min, 150 °C for 30 min, 200 °C for 30 min, 250 °C for 30 min, 300 °C for 30 min, and 350 °C for 30 min with a step-by-step heating process. After the curing process, the sample with CPI films was soaked with deionized water and ultrasonicated for 5 min. Finally, the CPI 3FBP/H-PI-3vCPI film was detached from the glass and dried at 120 °C for 1 h to remove deionized water completely.

2.3.3. Synthesis of the 3F6F/C-PI-3 CPI Film Prepared by the CI Method

An appropriate amount of acetic anhydride and triethylamine was added in a volume ratio of 1/1 to the PAA solution and stirred at 60 °C for 24 h to obtain a partially cyclized PI solution. An appropriate amount of PI solution was applied onto clean glass, and a spin-coater was used to spread the PI solution on the glass at a suitable spinning speed. After curing at 80 °C for 1 h to remove most of the solvent, the glass samples with the PI layer were also baked at 100 °C for 30 min, 150 °C for 30 min, 200 °C for 30 min, 250 °C for 30 min, and 300 °C for 30 min. Finally, the 3FBP/C-PI-3 CPI film was detached from the glass after soaking it in deionized water for about 5 min and then dried at 120 °C for 1 h to remove deionized water completely.

2.4. Molecular Simulation Method

The geometric configurations of the monomers and their structural unit molecules were calculated by using density functional theory (DFT) in the Gaussian 16 software package. The calculations were performed at the B3LYP functional level with the 6-31G­(d) basis set, including the D3 dispersion correction. Subsequently, molecular simulations of these CPI films with 20 repeating units were carried out using Materials Studio 6.0 (MS 6.0) software to investigate the variation of their free volume with different 3FPODA contents.

3. Results and Discussion

3.1. FT-IR Spectroscopy and Optimized Molecular Geometries

The degree of imidization for this series of CPI films was characterized by FT-IR spectroscopy, and the results are presented in Figure . The characteristic infrared absorption bands are summarized in Table . As shown in Figure a, the FT-IR spectra revealed that all CPI films exhibited well-defined peaks at approximately 1787 cm^–1 and 1726 cm^–1, which corresponded to the asymmetric and symmetric carbonyl (CO) stretching vibrations, respectively. A distinct peak attributed to the C–N stretching vibration was also observed near 1364 cm^–1. Notably, almost no discernible peaks associated with amino (−NH₂) vibrations were detected. These observations collectively indicated the completion of the imidization reaction during the synthesis of 3F6F/C-PI-x films with the chemical imidization method. Meanwhile, for the series of 3F6F/H-PI-x films, the asymmetric and symmetric stretching vibration peaks of CO were observed in the vicinity of 1785 cm^–1 and 1724 cm^–1, as shown in Figure b. The stretching vibration peak of C–N observed near 1363 cm^–1 and no obvious amino vibration peak also indicated that the series CPI film prepared by the thermal imidization method had also been imidized completely.

3.

FT-IR spectroscopy of CPI films: (a) the series of 3F6FP/C-PI-x and (b) the series of 3F6F/H-PI-x.

2. Infrared Absorption of 3F6FP/C-PI-x and 3F6F/H-PI-x CPI Films.

CPI name	CO asymmetric stretching vibration peak (cm–1)	CO symmetric stretching vibration peak (cm–1)	C–N stretching vibration peak (cm–1)	–CF3 stretching vibration peak (cm–1)
3F6F/C-PI-0	1787	1725	1362	1254/1210
3F6F/C-PI-1	1788	1727	1364	1257/1209
3F6F/C-PI-2	1787	1725	1361	1256/1208
3F6F/C-PI-3	1789	1726	1364	1255/1195
3F6F/C-PI-4	1789	1727	1366	1255/1197
3F6F/C-PI-5	1790	1726	1367	1257/1199
3F6F/H-PI-0	1786	1724	1363	1255/1209
3F6F/H-PI-1	1787	1725	1364	1256/1210
3F6F/H-PI-2	1786	1728	1366	1254/1210
3F6F/H-PI-3	1789	1726	1365	1255/1195
3F6F/H-PI-4	1787	1727	1366	1258/1194

To investigate the relationship between the structure and performance of the series of CPI films, molecular simulation calculations were performed on monomers of 6FDA and 3FPODA, and the structural units of 3FPODA/TFDB and BPDA/TFDB were also calculated based on DFT theory, as shown in Figure . It can be observed that the main chains of 3FPODA are almost on the same plane by comparing the cured surface of 6FDA. Meanwhile, the structural unit of 3FPODA/TFDB also has stronger rigidity and linearity than that of 6FDA/TFDB. In the structural unit of 6FDA/TFDB, the two benzene rings in the 6FDA backbone are connected by a quaternary carbon atom, which could make the backbone more twisted and weaken its rigidity. The main chain of the 3FPODA/TFDB structural unit is almost linear, and the two benzene rings in the 3FPODA backbone are connected by rigid semi-aliphatic rings. The carbonyl group has a conjugated effect with the benzene rings on both sides, which could endow the structural unit with strong rigidity and linearity. In addition, there are large benzene ring side groups and −CF3 in the structural unit of 3FPODA/TFDB, which can increase the distance between molecular chains with a steric hindrance effect caused by the two substituents.

4.

Molecular simulation calculations of 6FDA, 3FPODA, and the structural units of 3FPODA/TFDB and BPDA/TFDB.

3.2. Solubility and X-ray Diffraction

The solubility of the series of CPI films was assessed in various solvents by using the following procedure: 100 mg of the CPI film was added to 2 mL of the solvent and stirred magnetically while observing dissolution behavior. The results are summarized in Table . With respect to the solubility of the series of CPI films with 3F6FP/C-PI-x and 3F6F/H-PI-x, “+” indicates PI was soluble at room temperature, “+h” indicates PI was soluble under heating, “+s” indicates PI was slightly soluble under heating, and “-” indicates PI is insoluble. The solubility results revealed that all CPI films were insoluble in common solvents such as tetrahydrofuran (THF), ethyl acetate (EA), dimethylacetamide (DMAc), and dimethylformamide (DMF). Partial solubility was observed only upon heating in highly polar solvents, m-cresol and N-methyl-2-pyrrolidone (NMP). Notably, the incorporation of 3FPODA did not significantly alter the solubility profile of the CPI films. This pronounced solvent resistance was attributed to two main reasons. First, both the constituent structural units (3FPODA/TFDB and 6FDA/TFDB) had high rigidity, which could confer a stiff backbone conformation to the entire CPI chain to limit its solubility. Second, the high-temperature process introduced during thermal imidization (for example, 350 °C) could promote the interchain cross-linking and self-organization during the cyclodehydration process of the PAA precursor. This promotional effect caused by high temperature could facilitate the packaging of dense molecular chains and further impede the penetration and dissolution of the solvent in common organic media.

3. Solubility of 3F6FP/C-PI-x and 3F6F/H-PI-x CPI Films.

	solubility
CPI name	THF	EA	toluene	m-Cresol	DMAC	NMP	DMF	DCM
3F6F/C-PI-0	+h	+h	-	+s	+	+	+	+
3F6F/C-PI-1	+h	+h	-	+s	+	+	+	+
3F6F/C-PI-2	+h	+h	-	+s	+	+	+	+
3F6F/C-PI-3	+h	+h	-	+s	+	+	+	+
3F6F/C-PI-4	+h	+h	-	+s	+	+	+	+
3F6F/C-PI-5	+h	+h	-	+s	+	+	+	+
3F6F/H-PI-0	-	-	-	+s	+s	+	+s	-
3F6F/H-PI-1	-	-	-	+s	+s	+	+s	-
3F6F/H-PI-2	-	-	-	+s	+s	+	+s	-
3F6F/H-PI-3	-	-	-	+s	+s	+	+s	-
3F6F/H-PI-4	-	-	-	+s	+s	+	+s	-

In order to further investigate the effect of introducing 3FPODA on the stacking of PI films, wide-angle X-ray diffraction (WAXD) tests were conducted, as shown in Figure . From diffraction spectroscopy, it can be inferred that both series of 3F6FP/C-PI-x and 3F6F/H-PI-x CPI films are amorphous. For the series of 3F6FP/C-PI-x films, molecular dynamics simulations were conducted and the FFV simulation diagram is shown in Figure . The occupied volume, free volume, and FFV of 3F6FP/C-PI-x films are also summarized in Table . It can be observed that the FFV of the CPI film with 3FPODA is higher than that of CPI without 3FPODA (3F6FP/C-PI-0). This indicated that the introduction of benzene ring side groups and −CF3 side groups in 3FPODA could increase the free volume of PI and reduce the stacking degree of polymer molecular chains.

5.

WAXD diffractograms of CPI films: (a) the series of 3F6FP/C-PI-x and (b) the series of 3F6F/H-PI-x.

6.

Fractional free volume (FFV) of 3F6FP/C-PI-x films.

4. Occupied Volume, Free Volume, and FFV of 3F6FP/C-PI-x Films.

CPI name	occupied volume (Å3)	free volume (Å3)	FFV (%)
3F6F/C-PI-0	37771.02	12996.34	25.60
3F6F/C-PI-1	37911.76	14903.29	28.22
3F6F/C-PI-2	37990.50	16920.33	30.81
3F6F/C-PI-3	38651.64	15137.53	28.14
3F6F/C-PI-4	38941.20	15786.79	28.85
3F6F/C-PI-5	39334.27	15455.64	28.21

3.3. Optical Properties

In this article, 6FDA was introduced to adjust the optical properties of CPI films by a copolymerization reaction with 3FPODA and TFDB. During the preparation of CPI films, it was found that with the content of 3FPODA above 50%, the optical performance of CPI films decreased significantly. The highest content of 3FPODA in the series of 3F6FP/C-PI-x and 3F6F/H-PI-x CPI films is 50%. In order to explore the effects of different preparation methods and the content of 3FPODA on the optical properties of CPI films, we conducted a series of characterizations of the optical properties of two series of CPI films. The optical properties of 3F6FP/C-PI-x and 3F6F/H–PI-x CPI films were also characterized with ultraviolet–visible spectroscopy and spectroscopic ellipsometry. For the use of spectroscopic ellipsometry, the PAA solution was spin-coated onto single-side polished silicon wafers by using a two-step process (1500 rpm for 15s and then 1800 rpm for 5s). The CPI film with a thickness of 2–4 μm was obtained with the thermal imidization method, and ellipsometric measurements were taken to conduct the n _∥, n _⊥, and Δn. Phase retardation was also calculated by key optical parameters of CPI films.

The photos of 3F6FP/C-PI-x and 3F6F/H-PI-x CPI films are shown in Figure a,b, respectively. It can be clearly seen that the series of 3F6FP/C-PI-x CPI films were all almost colorless, while the 3F6F/H-PI-x CPI films changed from colorless to light yellow with the increase of 3FPODA content. Meanwhile, the UV–visible spectra of 3F6FP/C-PI-x and 3F6F/H-PI-x CPI films are also shown in Figure c,d. The key optical parameters including transmittance of CPI films at 400 and 550 nm (T₄₀₀, T₅₅₀), cutoff wavelength (λ₀), yellow index (YI) value, and b* are all summarized in Table . With respect to the optical properties of 3F6FP/C-PI-x CPI films (including 3F6FP/C-PI-1, 3F6FP/C-PI-2, 3F6FP/C-PI-3, 3F6FP/C-PI-4, and 3F6FP/C-PI-5), the T₄₀₀ ranged from 21% to 71%, and the T₅₅₀ also ranged from 89% to 91%. Meanwhile, the λ₀ is in the range of 343–376 nm, the value of YI also ranges from 2.83 to 8.63, and the b* is within the range from 1.58 to 4.93. On the contrary, the T₄₀₀, T₅₅₀, λ₀, YI, and b* of the 3F6F/C-PI-0 CPI film is 80%, 91%, 345 nm, 2.04, and 1.07, respectively. By comparing with 3F6F/C-PI-0, the λ₀ of 3F6F/C-PI-1 to 3F6F/C-PI-5 shows a red shift, T₄₀₀ is reduced significantly, and the value of YI is also increased obviously (as shown in Figure ). This is due to the presence of carbonyl groups in 3FPODA, which has sp²-hybridized carbon atoms and could form a certain conjugated structure with the benzene rings on both sides. The existence of this conjugated structure will increase the CTC effect within and between molecular chains, resulting in the absorption peak of the CTC effect located in the range of 360–500 nm, thus also increasing the YI value and decreasing the T₄₀₀ of CPI films. On the other hand, the π electrons in conjugated structures could have electronic transitions and their energy absorption peaks located at 300–380 nm, which can cause a red shift in the λ₀ of CPI films. Based on the optical properties of CPI films, it is necessary to adjust the proportion of 3FPODA to ensure that the optical properties of the CPI film meet the requirements of transparent FPCB having excellent thermal and mechanical properties. Based on the perspective of optical indicators, the content of 3FPODA in the series of 3F6FP/C-PI-x CPI films should not exceed to 40%.

7.

(a) The photographs of 3F6FP/C-PI-x films, (b) the photographs of 3F6FP/H-PI-x films, (c) the UV–vis absorption spectrum of 3F6FP/C-PI-x films, and (d) the UV–vis absorption spectrum of 3F6FP/H-PI-x films.

5. Optical Properties of 3F6FP/C-PI-x and 3F6F/H-PI-x CPI Films.

CPI name	λ0 (nm)	T400 (%)	T550 (%)	b*	YI	n ∥	n ⊥	Δn	retardation (th)/(nm)
3F6F/C-PI-0	345	80	91	1.07	2.04	1.540	1.513	0.0276	276
3F6F/C-PI-1	343	71	91	1.58	2.83	1.571	1.536	0.0347	347
3F6F/C-PI-2	358	52	90	2.13	3.75	1.577	1.537	0.0397	397
3F6F/C-PI-3	372	30	90	2.84	4.90	1.579	1.533	0.0458	458
3F6F/C-PI-4	371	31	90	3.37	5.78	1.586	1.539	0.0468	468
3F6F/C-PI-5	376	21	89	4.93	8.63	1.580	1.530	0.0496	496
3F6F/H-PI-0	356	76	90	1.50	2.76	1.566	1.558	0.0079	79
3F6F/H-PI-1	358	59	90	2.62	4.60	1.568	1.560	0.0080	80
3F6F/H-PI-2	361	51	90	3.16	5.42	1.578	1.567	0.0111	111
3F6F/H-PI-3	378	23	89	5.61	9.36	1.590	1.573	0.0166	166
3F6F/H-PI-4	375	27	89	6.15	10.18	1.589	1.572	0.0171	171

8.

YI of 3F6FP/C-PI-x and 3F6F/H-PI-x CPI films.

For the series of copolymerized 3F6F/H-PI-x CPI films (including 3F6FP/H-PI-1, 3F6FP/H-PI-2, 3F6FP/H-PI-3, and 3F6FP/H-PI-4), the T₄₀₀ ranged from 23% to 59%, and T₅₅₀ also ranged from 89% to 90%. Meanwhile, the λ₀ was in the range of 358 nm to 378 nm, the value of YI also ranged from 4.6 to 10.18, and the b* was within the range from 2.62 to 6.15. Compared with the 3F6F/H-PI-0 CPI film, the optical properties based on the copolymerized CPI films changed with the increasing content of 3FPODA, which is also consistent with the changes in the film formation series by the chemical imidization method. This difference was mainly due to the enhancement of the conjugation effect and the CTC effect, which was caused by the introduction of 3FPODA. It was also observed that the optical properties of CPI films prepared by the chemical imidization method are superior to those prepared by the thermal imidization method with the same content of 3FPODA. During the process of chemical imidization, the temperature conditions for the formation of the CPI ring are relatively mild, and the molecular chain stacking is also looser. While the cyclization reaction of the thermal imidization method is relatively intense, it could lead to cross-linking and self-organization behavior between molecular chains during the process of thermal imidization. The tight stacking of molecular chains, the enhanced CTC effect, and the conjugation effect can also reduce the optical properties of CPI films prepared by the thermal imidization method.

In addition, the refractive index and out-of-plane birefringence curves of the 3F6FP/C-PI-x and 3F6F/H-PI-x CPI films ranging from 400 to 1000 nm were obtained with an ellipsometry test, as shown in Figure . The n _∥ and n _⊥ at 550 nm, Δn and phase retardation based on the 10 μm thickness of 3F6FP/C-PI-x and 3F6F/H-PI-x CPI films are all summarized in Table . It can be observed that the series of the copolymerized 3F6F/c-PI-x CPI films (including 3F6FP/C-PI-1, 3F6FP/C-PI-2, 3F6FP/C-PI-3, 3F6FP/C-PI-4, and 3F6FP/C-PI-5) all had a higher refractive index and out-of-plane birefringence than 3F6FP/C-PI-0 in the wavelength range from 400 to 1000 nm. For the series of 3F6F/C-PI-x CPI films (x ≠ 0), the values of n _∥ and n _⊥ are located in 1.571–1.580 and 1.533–1.539, respectively. The value of Δn is in the range of 0.0347–0.0496, and the retardation is in the range of 347–496 nm. For the 3F6F/C-PI-0 CPI film, the n _∥, n _⊥, Δn, and retardation were 1.540, 1.523, 0.0276, and 276 nm, respectively. It was also found that the Δn and retardation of the copolymerized 3F6F/C-PI-x CPI films (x ≠ 0) all increased with the increasing content of 3FPODA, which were all higher than those of the CPI-based film 3F6F/C-PI-0. Based on the perspective of chemical structure, this may be due to the higher linearity of the structure of 3FPODA compared to 6FDA, and the molecular chains could be more oriented and arranged more orderly, which was caused by the effect of the conjugated structure formed by its carbonyl group and benzene rings.

9.

(a) The refractive index of 3F6FP/C-PI-x CPI films, (b) the birefringence of 3F6FP/C-PI-x CPI films, (c) the refractive index of 3F6F/H-PI-x CPI films, and (d) the birefringence of 3F6F/H-PI-x CPI films.

As shown in Table , for the series of copolymerized CPI films (including 3F6FP/H-PI-1, 3F6FP/H-PI-2, 3F6FP/H-PI-3, and 3F6FP/H-PI-4), the values of n _∥ and n _⊥ at 550 nm were located in the range of 1.568–1.590 and 1.560–1.573, respectively. The value of Δn is in the range of 0.0080–0.0171, and the retardation is in the range of 80–171 nm. Meanwhile, for the 3F6F/H-PI-0 CPI film, the n _∥, n _⊥, Δn, and retardation were 1.566, 1.558, 0.0079, and 79 nm, respectively. The changes in the out-of-plane birefringence of the 3F6FP/H-PI-x CPI films were consistent with those of the 3F6FP/C-PI-x CPI films. Comparing the two series of CPI films, it was found that the out-of-plane birefringence of the 3F6FP/C-PI-x CPI films was greater than that of the 3F6FP/H-PI-x CPI films with the same content of 3FPODA. During the process of chemical imidization, PAA undergoes dehydration and cyclization reactions through the action with a dehydrating agent and a catalyst at an ambient temperature of 60 °C. Then, the 3F6FP/C-PI-x CPI films were obtained with a drying process. The temperature of PAA dehydrating into a ring (60 °C) is lower than its T _g, which could make it less prone to self-organization and cross-linking of molecular chains during the imidization process. During the process of solidification, the CPI molecules may form a linear layered arrangement structure, leading to an increase in anisotropy. During the process of thermal imidization, the dehydration temperature of the PAA solution to form a ring is usually higher than the T _g of CPI, which could lead to self-organization behavior of the molecular chain and may cause the molecular chain to shrink into clusters, and the anisotropy of the CPI film may also be reduced obviously. On the other hand, the cross-linking between molecular chains is prone to occur during the dehydration and cyclization process of PAA with the temperature exceeding 200 °C and thus could further reduce the anisotropy of the CPI film.

3.4. Thermal Properties

To systematically evaluate the impact of 3FPODA integration and its concentration on the thermal properties of the two series of CPI films, comprehensive thermal characterization was conducted using thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and thermomechanical analysis (TMA). For the TGA test, the CPI films (3 mg) were pyrolyzed under a nitrogen atmosphere with a heating rate of 10 °C/min, ranging from 30 to 800 °C. For the DMA test, the CPI films (1 cm × 5 cm) were tested under a nitrogen atmosphere with a heating rate of 5 °C/min over a temperature range from 30 to 420 °C. For the TMA test, the CPI films (3 mm × 5 cm) were analyzed under a nitrogen atmosphere with a heating rate of 5 °C/min, ranging from 30 to 300 °C. The TGA curves, DMA curves, and TMA curves for the series of 3F6FP/C-PI-x and 3F6FP/H-PI-x CPI films are presented in Figure . Meanwhile, the key thermal index including the decomposition temperatures at 1% and 5% weight loss (T _d1, T _d5), char yield, glass transition temperature (T _g), and CTE of the two series of CPI films are listed in Table .

10.

(a) The TGA curves of 3F6FP/C-PI-x CPI films, (b) the TGA curves of 3F6FP/H-PI-x CPI films, (c) the DMA curves of 3FBP/C-PI-x CPI films, (d) the DMA curves of 3FBP/H-PI-x CPI films, (e) the TMA curves of 3FBP/C-PI-x CPI films, and (f) the TMA curves of 3FBP/H-PI-x CPI films.

Figure a,b shows the TGA curves of the two series of copolymerized CPI films. For the series of 3F6F/C-PI-x CPI films (x ≠ 0), the T _d1 and T _d5 measured in a nitrogen atmosphere were in the range of 480–499 °C and 542–552 °C, respectively. In contrast, the T _d1 and T _d5 measured in a nitrogen atmosphere of the 3F6F/C-PI-0 CPI-based film were 485 and 520 °C, respectively. The thermal decomposition temperature of the copolymerized CPI film (including 3F6FP/C-PI-1, 3F6FP/C-PI-2, 3F6FP/C-PI-3, 3F6FP/C-PI-4, and 3F6FP/C-PI-5) is higher than that of the 3F6F/C-PI-0-based film. It can be attributed to the introduction of 3FPODA improving the rigidity and interaction of molecular chains, which could enhance the thermal stability of CPI films. Meanwhile, for the series of 3F6F/H-PI-x CPI films (x ≠ 0), the T _d1 and T _d5 were in the range of 461–477 °C and 518–530 °C, respectively. The T _d1 and T _d5 of the 3F5F/C-PI-0 CPI-based film measured in a nitrogen atmosphere were 458 and 514 °C, respectively. In terms of the thermal stability of the two series CPI films, the films prepared by the chemical imidization (CI) method have higher thermal decomposition temperatures of T _d1 and T _d5 than the films prepared by thermal imidization (HI). During the process of chemical imidization, the reaction conditions are mild with a cyclization temperature of PAA of 60 °C, which could maximize the preservation of the integrity of the CPI molecular chain without breaking the molecular chains. During the process of thermal imidization, the temperature of the PAA cyclization reaction is high, and the molecular chain is prone to breakage and rearrangement, causing a decrease in its molecular weight. With the same molecular chain structure of a polymer, the larger the molecular weight, the harder it is likely to decompose, and the higher its thermal decomposition temperature is. Therefore, the thermal stability of the 3F6F/C-PI-x CPI films was superior to that of the 3F6F/H-PI-x CPI films.

As shown in Figure c, the T _g of the series of 3F6F/C-PI-x (x ≠ 0) CPI films ranges from 342 to 382 °C compared with a T _g of 330 °C for the 3F6F/C-PI-0 CPI film. Based on the DMA curves, it was observed that the T _g of all copolymerized CPI films (including 3F6FP/C-PI-1, 3F6FP/C-PI-2, 3F6FP/C-PI-3, 3F6FP/C-PI-4, and 3F6FP/C-PI-5) is higher than that of the CPI-based film (3F6FP/C-PI-0). With the increasing content of 3FPODA, the T _g of the series of 3F6F/C-PI-x (x ≠ 0) CPI films also increased gradually. For the series of 3F6F/C-PI-x CPI films, the increasing trend of T _g was matched with the increasing content of 3FPODA, which indicated that the introduction of the 3FPODA structure could increase the T _g of CPI films, as shown in Figure a. This interesting phenomenon can be attributed to the stronger rigidity and higher linearity of the 3FPODA structure compared to the 6FDA structure. The conjugation between the carbonyl group and the benzene ring of 3FPODA can increase the formation of the CTC effect within or between the molecular chains, which could improve the T _g of CPI films. On the other hand, the conjugation of the carbonyl group and the benzene ring of 3FPODA can also enhance the interaction between the molecular chains and increase the linearity of the molecular chains, which could also improve the T _g of the series 3F6F/C-PI-x CPI films. In addition, the rigid side group of the benzene ring in 3FPODA could also hinder the movement of molecular chain segments to improve the T _g of CPI films. As shown in Figure d, the T _g of the series of 3F6F/H-PI-x (x ≠ 0) CPI films ranges from 337 to 381 °C, compared to a T _g of 334 °C for the 3F6F/H-PI-0 CPI film. The T _g of the copolymerized series of 3F6F/H-PI-x CPI films (including 3F6FP/H-PI-1, 3F6FP/H-PI-2, 3F6FP/H-PI-3, and 3F6FP/C-PI-4) is higher than that of the CPI-based film (3F6FP/H-PI-0). It was obtained that the variation of T _g for the series of 3F6F/H-PI-0 CPI films with the increasing content of 3FPODA is consistent with the variation for the series of 3F6F/H-PI-0 CPI films, as shown in Figure b.

11.

Thermal properties of the two series of CPI films with increasing content of 3FPODA: (a) the T _g curves of 3F6FP/C-PI-x and 3F6FP/H-PI-x CPI films and (b) the CTE curves of 3F6FP/C-PI-x and 3F6FP/H-PI-x CPI films.

The CTE of the two series copolymerized CPI films was tested with the TMA test, as shown in Figure e,f; the CTE values of the two series of copolymerized CPI films are summarized in Table . The CTE value was calculated by the dimension change of the CPI film corresponding to the temperature ranging from 50 to 200 °C. The CTE values of the series of 3F6F/C-PI-x (x ≠ 0) CPI films range from 22.7 to 42.0 ppm/K compared with a CTE of 45.5 ppm/K for the 3F6F/C-PI-0 CPI film, which indicated that the CTE values of the copolymerized CPI films prepared with the chemical imidization method (including 3F6FP/C-PI-1, 3F6FP/C-PI-2, 3F6FP/C-PI-3, 3F6FP/C-PI-4, and 3F6FP/C-PI-5) are all lower than that of the CPI-based film 3F6F/C-PI-0, as shown in Figure e. By introducing a content of 3FPODA, the copolymerized CPI film has a stronger and rigid structure and higher linearity, which could increase the orientation of its molecular chains and reduce the CTE value of the CPI film. The quaternary carbon atom structure in the 6FDA monomer had a high degree of spatial distortion, which could reduce the rigidity, regularity, and orientation of molecular arrangement, leading to a higher CTE value. In addition, besides the stronger rigidity, higher linearity, and orientation, the steric hindrance effect of the rigid benzene ring side group in 3FPODA can also hinder the movement of molecular segments, which could also reduce the CTE of the copolymerized CPI film. As shown in Table , the CTE values of the series of 3F6F/H-PI-x (x ≠ 0) CPI films range from 32.7 to 41.3 ppm/K compared with a CTE of 45.8 ppm/K for the 3F6F/H-PI-0 CPI film, which indicated that the CTE values of the copolymerized CPI films prepared with the thermal imidization method (including 3F6FP/H-PI-1, 3F6FP/H-PI-2, 3F6FP/H-PI-3, and 3F6FP/H-PI-4) are all lower than that of the CPI-based film 3F6F/H-PI-0, as shown in Figure f. For the series of 3F6FP/H-PI-x CPI films, the variation of CTE values based on the increasing content of 3FPODA is consistent with that of the CPI films prepared by the chemical imidization method. Comparing the CTE values of two series of CPI films, it was found that the CTE of CPI films prepared by the thermal imidization method (3F6FP/H-PI-x) was higher than that of films prepared by the chemical imidization method (3F6FP/H-PI-x). As is known, the CPI films prepared by the chemical imidization method can preserve the integrity of their molecular chain to the maximum extent, which could result in a lower CTE value. Meanwhile, the CPI film obtained by chemical imidization may also have a linear arrangement in layer by layer by molecular chains, which could further reduce the CTE value of the CPI films.

6. Thermal Properties of 3F6FP/C-PI-x and 3F6F/H-PI-x CPI Films.

CPI name	T g (°C)	Td1 (°C)	Td5 (°C)	char yield (%)	CTE (ppm/K)
3F6F/C-PI-0	330	485	520	44.8	45.5
3F6F/C-PI-1	342	499	552	49.2	42.0
3F6F/C-PI-2	355	492	549	47.6	32.9
3F6F/C-PI-3	372	489	549	53.3	27.2
3F6F/C-PI-4	377	480	542	55.4	24.3
3F6F/C-PI-5	382	481	543	54.5	22.7
3F6F/H-PI-0	334	458	514	50.0	45.8
3F6F/H-PI-1	337	477	530	47.4	41.3
3F6F/H-PI-2	351	473	522	49.5	40.5
3F6F/H-PI-3	371	464	534	50.8	37.4
3F6F/H-PI-4	381	461	518	49.6	32.7

3.5. Mechanical Properties

During the preparation of transparent FPCB, the mechanical properties of optical-grade CPI films were also required to obtain, besides the optical and thermal properties. In this manuscript, the mechanical properties of two series of copolymerized CPI films (3F6FP/C-PI-x and 3F6F/H-PI-x) were characterized with tensile testing to explore the effects of preparation methods and 3FPODA content on the mechanical properties of CPI films.

For the two series of 3F6F/C-x and 3F6F/H-x CPI films, the tensile strength (T _S), elongation at break (E _B), and elastic modulus (T _M) are all statistically presented in Table . The T _M of the two series of copolymerized CPI films (3F6FP/C-PI-x and 3F6F/H-PI-x) is shown in Figure a. With the increasing content of 3FPODA, the T _M of the two series copolymerized CPI films also increased gradually. For the series of 3F6F/C-PI-x (x ≠ 0) CPI films, the T _S values ranged from 75.6 to 136.5 MPa, the E _B ranged from 2.05% to 4.13%, and the T _M ranged from 3.6 to 4.5 GPa. The T _S, E _B, and T _M of the CPI-based film 3F6F/C-PI-0 were 83.5 MPa, 2.94%, and 3.2 GPa, respectively. From the stress–strain curve of 3F6F/C-x CPI films shown in Figure b, it was observed that the mechanical properties of the copolymerized CPI films prepared by the chemical imidization method (including 3F6FP/C-PI-1, 3F6FP/C-PI-2, 3F6FP/C-PI-3, 3F6FP/C-PI-4, and 3F6FP/C-PI-5) have significantly improved compared to the CPI-based film 3F6F/C-PI-0, which can be explained by the greater rigidity of molecular chains caused by the introduction of 3FPODA. The characterization of mechanical properties from the two series of 3F6F/C-PI-x and 3F6F/H-PI-x films indicated that the introduction of 3FPODA could improve the mechanical properties of CPI films significantly. When the proportion of 3FPODA was 40% (3F6F/C-PI-4), the comprehensive mechanical properties of the CPI film were the best, with a T _S of 136.5 MPa, an E _B of 4.13%, and a T _M of 4.4 GPa, which can meet the requirements for preparing transparent FPCB basically.

7. Mechanical Properties of 3F6FP/C-PI-x and 3F6F/H-PI-x CPI Films.

CPI name	TS (MPa)	EB (%)	TM (GPa)
3F6F/C-PI-0	83.5	2.94	3.2
3F6F/C-PI-1	88.9	2.74	3.6
3F6F/C-PI-2	75.6	2.05	4.1
3F6F/C-PI-3	105.3	2.93	4.2
3F6F/C-PI-4	136.5	4.13	4.4
3F6F/C-PI-5	90.0	2.19	4.5
3F6F/H-PI-0	67.2	2.73	3.1
3F6F/H-PI-1	87.2	3.46	3.2
3F6F/H-PI-2	56.3	2.14	3.3
3F6F/H-PI-3	129.0	4.66	3.7
3F6F/H-PI-4	99.1	3.21	4.1

12.

Mechanical properties of two series of CPI films: (a) elastic modulus content of 3FPODA of 3F6F/C-x and 3F6F/H-x, (b) stress–strain curves of 3F6F/C-x, and (c) stress–strain curves of 3F6F/H-x.

Figure c shows the stress–strain curve for the series of 3F6F/H-PI-x CPI films. The T _S of 3F6F/H-PI-x (x ≠ 0) CPI films ranged from 56.3 to 129.0 MPa, the E _B ranged from 2.14% to 4.66%, and the T _M ranged from 3.2 to 4.1 GPa. The T _S, E _B, and T _M of the CPI-based film 3F6F/H-PI-0 were 67.2 MPa, 2.73%, and 3.1 GPa, respectively. From the stress–strain curve of 3F6F/H-x CPI films, it was observed that the mechanical properties of the copolymerized CPI films prepared by the thermal imidization method (including 3F6FP/H-PI-1, 3F6FP/H-PI-2, 3F6FP/H-PI-3, and 3F6FP/H-PI-4) were superior to those of the CPI-based film 3F6F/H-PI-0. By comparing the mechanical properties of the chemical imidization series CPI films (3F6F/C-PI-x), the thermal imidization series CPI films (3F6F/H-PI-x) had poorer mechanical properties, which may be due to their different imidization methods. The cyclization conditions of the chemical imidization method are relatively mild and could preserve the integrity of the molecular chain. While the cyclization reaction of thermal imidization is more intense and occurs at high temperatures, this could cause the molecular chain to break easily and affect its mechanical properties effectively.

3.6. Surface Properties

In this manuscript, the solubility, optical properties, thermal properties, and mechanical properties of two series of copolymerized CPI films prepared by the chemical imidization method (3F6FP/C-PI-x) and thermal imidization method (3F6FP/H-PI-x) were characterized. It was found that the comprehensive performance of 3F6FP/C-PI-x CPI films was better than that of 3F6FP/H-PI-x CPI films, and the chemical imidization method was more suitable for preparing this type of CPI film using transparent FPCB. In order to meet the demands of being candidate materials for preparing transparent FPCBA, a further exploration is required to determine whether the surface properties of the CPI films prepared by the chemical imidization method have changed. The water contact angle on the surface of 3F6FP/C-PI-x CPI films was measured with contact angle testing, and the related results are shown in Figure . The water contact angle on the surface of the series 3F6F/C-PI-x (x ≠ 0) CPI films ranges from 65.4°to 83.4°. The water contact angle on the surface of the CPI-based film (3F6F/C-PI-0) is 88.4°. According to the water contact angle results, it can be observed that the water contact angle on the surface of the 3F6F/C-PI-x (x ≠ 0) CPI films decreased with the increasing content of 3FPODA. The enhancement of the surface hydrophilicity may be caused by the hydrophilicity of the carbonyl group in 3FPODA. The improvement of hydrophilic properties on the surface of 3F6F/C-PI-x (x ≠ 0) CPI films is beneficial for addressing the issue of nonwetting at the interface between the optical-grade CPI film and metal in transparent FPCB.

13.

Water contact angle of 3F6FP/C-PI-x CPI films.

3.7. The Application of CPI

As is known, the CPI material has become a key material in the fields of flexible displays, microelectronics, and semiconductors due to its excellent heat resistance, mechanical properties, and optical properties. The performance evaluation of CPI thin films mainly revolves around three dimensions including the optical, heat resistance, and mechanical reliability in the microelectronics industry. For the optical performance, the CPI film is mainly used to replace traditional glass cover plates, which is very crucial for its optical performance. The traditional PI films seemed dark yellow due to the CTCs, which limit their application in the microelectronics industry. CPI could eliminate the absorption in the visible-light range through molecular structure design. Meanwhile, the CPI products of Sumitomo Chemical have reached a transparency of over 90%, and the CPI products of Mitsui Chemical have also reached a transparency above 88%, which are all close to the transparency of glass.

For the heat resistance, the microelectronic manufacturing process involved high-temperature environments, which had put forward high reliability requirements for CPI materials. For example, the CPI products of Mitsui Chemical have reached a T _g above 260 °C, which could ensure their dimensional stability in subsequent processing and high-temperature working environments due to their high heat resistance. Compared to PET substrates, the CPI substrate has better resistance to high and low temperatures, which is suitable for more demanding microelectronic packaging environments. For the mechanical reliability, Young’s modulus, tensile strength, and flexural resistance are key factors in measuring the reliability of CPI thin films. The CPI film produced by the casting biaxial stretching method has a higher tensile strength, which could meet the dual requirements of hardness and toughness for flexible display screens. Compared with the PET substrate, the CPI substrate has better bending resistance, creep resistance, and recovery, which is the core reason for applying it as the cover material of folding screen phones. The reliability standard for mainstream commercial products of CPI films (such as Sumitomo Chemical and Ruihuatai) is usually set at more than 200,000-fold to meet the service life requirements of end consumers. In summary, Sumitomo Chemical and Korea’s Cologne are in a leading position in optical performance (>90% transmittance) and commercial applications in the application of CPI. Mitsui Chemicals has significant advantages in heat resistance (T _g > 260 °C). Although domestic enterprises such as Ruihuatai have reached international standards in key indicators such as folding life (>200,000 times), they are still in the catching-up stage in terms of large-scale production capacity and high-end market sharing. With the promotion of production capacity construction by domestic enterprises such as Changyang Technology and Daoming Optics, the application sharing of domestically produced CPI thin films is expected to further increase in the field of microelectronics.

The following is a comprehensive comparison of material characteristic parameters, performance indicators, and applicable scenarios of CPI and yellow polyimide (YPI) in the microelectronics industry. For the material characteristic parameters, the comparison between CPI and YPI mainly included optical performance, thermal performance, mechanical properties, and electrical performance. The CPI usually has high transparency, low yellowing index, and low haze, and it achieves almost colorless transparency. The visible light transmittance of CPI was also over 80%, and the cutoff wavelength was usually around 350 nm. Meanwhile, the T _g of CPI is generally higher than 300 °C, and the thermal decomposition temperature can reach over 450 °C. The CTE of CPI is relatively low with a value usually below 10 × 10^–6/°C, which can withstand the high-temperature processes in flexible displays and other processes. For the mechanical properties, the tensile strength of CPI is generally above 100 MPa, and the tensile modulus is between 1 and 4 GPa. The elongation at break is around 3% to 10%, which could guarantee its good flexibility and bending resistance. The dielectric constant of CPI is usually between 2.5 and 3.0, and the dielectric loss is as low as 0.001 to 0.005, which could guarantee its good electrical insulation and low signal attenuation characteristics for suitability for high-frequency circuits. The YPI was generally yellow or brown in color with a low transmittance in the visible light range. The transmittance at 500 nm was usually less than 40%, which was due to the CTCs within and between molecular chains absorbing visible light strongly. The T _g of YPI was generally above 200 °C, and the starting temperature of thermal decomposition was around 500 °C. The CTE was varied depending on the molecular structure, and a few of the YPIs had a CTE similar to that of metals, which ranges from about 15 × 10^–6/°C to 18 × 10^–6/°C. For the mechanical properties, the tensile strength of unfilled YPI is above 100 MPa, the tensile strength of the homopolymer YPI film is above 170 MPa, and the tensile modulus of biphenyl YPI can reach 400 MPa. The dielectric constant of YPI is generally between 3.0 and 3.6, and the dielectric loss is relatively high, which could also maintain a certain insulation performance in high-frequency applications.

For the performance metrics, it needs to meet indicators such as high transmittance, low yellowing, low haze, high heat resistance, low thermal expansion coefficient, good flexibility, and low dielectric constant to adapt to application scenarios such as flexible displays and wearable devices that require high optical and mechanical performance of CPI. It focuses on high temperature resistance, chemical corrosion resistance, radiation resistance, low thermal expansion coefficient, high mechanical strength, and good electrical insulation to meet the performance requirements in fields such as microelectronic packaging, flexible circuit boards, and semiconductor manufacturing of YPI. For the applicable scenarios, CPI is usually applied in the field of flexible displays, optical devices, and wearable electronic devices. For example, because of the high transparency, flexibility, and bending resistance, the CPI film is regarded as a cover material and substrate material for flexible OLED displays to achieve bendable and foldable functions. The CPI film is also used to manufacture transparent and flexible optical windows, optical waveguides, sensors, and other optical components for providing optical transparency and environmental stability. As flexible circuit substrates and sensor packaging materials, the CPI film could also meet the requirements of wearable devices for thinness, flexibility, and transparency. YPI is usually applied in the field of flexible circuit board (FPC), semiconductor packaging, and 5G communication. When used as the substrate and cover film of FPC, the YPI film could provide insulation, heat resistance, and mechanical support functions, which are suitable for flexible circuits in electronic products such as smartphones, laptops, and navigation devices. When used in semiconductor packaging, YPI is used as a buffer layer, passivation layer, interlayer dielectric layer, and a protective layer for protecting the chip from thermal and mechanical stress and reducing the soft errors caused by alpha particles. YPI can also be used as the substrate and insulation material for high-frequency and high-speed circuits, which could meet the requirements of low dielectric constant, low dielectric loss, and high-frequency signal transmission for 5G communication.

As the core substrate material in flexible OLED display devices, the performance of CPI directly determines the durability, image quality, stability, and production process yield of the display screen when used as a flexible substrate in an OLED display device. The following provides a detailed introduction to its relevant parameter characteristics in terms of Young’s modulus, CTE, and optical performance of CPI material. Young’s modulus of CPI reflects the ability of a material to resist elastic deformation, which requires a balance between supporting and flexibility when used as a flexible substrate in an OLED device. The CPI substrate is required to have a certain degree of rigidity to support the subsequent array process of the thin-film transistor (TFT), which could prevent sagging or deformation during the process. As a flexible material, the modulus of CPI should not be too high to avoid affecting its bending performance. Compared to ordinary plastic PET, CPI has better dimensional stability and recovery while maintaining its flexibility, which is better for restoring its original shape after long-term bending. Meanwhile, CPI has the characteristics of high reliability and flexural resistance, which means that the CPI material has better ductility and fracture resistance when subjected to mechanical stress. The CTE of CPI is a key indicator for measuring the dimensional stability of materials under temperature changes, which is very crucial for OLED devices integrated with multilayer materials. During the preparation process of flexible sensors and the OLED device, a high-temperature process (such as circuit fabrication temperatures often exceeding 200 °C) was often involved. As one of the best temperature-resistant plastics, CPI can withstand high temperatures up to 250 °C. The low CTE characteristics of CPI can also match the thermal expansion coefficient of circuit materials (such as copper and silicon), which could avoid the film curling, cracking, or delamination problems caused by inconsistent thermal expansion and contraction, ensuring the accuracy of circuit graphics and the long-term reliability of devices. The optical performance is the key difference between the CPI and traditional polyimide (YPI), which is also the threshold for its application in the OLED display field. Traditional PI appears yellow or brown due to its highly conjugated molecular structure and the formation of CTCs between molecules, which cannot meet the requirements of display transparency. CPI has overcome this drawback through molecular structure design and has high transparency, which could meet the requirements of OLED display screens for brightness and color reproduction. The high transparency of CPI makes it an ideal substrate material for transparent flexible circuit boards and the core component of flexible OLED display devices. It is necessary to meet the optical grade requirements and prepare in a clean room environment during the production process to avoid dust or particles affecting the transparency and ensure the clarity and purity of the screen display. When applied as the cover material in a foldable phone, CPI has the advantages of transparency, softness, and foldability with a slightly inferior hardness compared to ultrathin flexible glass (UTG). In summary, CPI has become an irreplaceable substrate material for flexible OLED display devices due to its excellent mechanical flexibility, low thermal expansion coefficient, and outstanding optical transparency. With the breakthrough of domestic technology, the performance boundary of CPI is constantly expanding, which could provide a solid foundation for the commercialization process of flexible OLED displays.

4. Conclusions

In summary, 6FDA and 3FPODA monomers containing flexible structures were selected to prepare CPI films for improving the optical properties. Two series of copolymerized CPI films (3F6FP/C-PI-x and 3F6FP/H-PI-x) with different ratios of dianhydride were prepared by using chemical imidization and thermal imidization methods. The solubility, optical properties, thermal properties, and mechanical properties of the two series of CPI films were characterized to explore the introduction of the 3FPODA monomer on the optical, thermal, and mechanical properties of CPI films. By adjusting the ratio of 3FPODA, the optical and other key indicators of the CPI film can be controlled to obtain an optical-grade CPI film that can meet the requirements of transparent FPCB. According to the research on the two series copolymerized CPI films, it was found that the introduction of 3FPODA and 6FDA with a copolymerization method could improve the thermal and mechanical properties of optical-grade CPI films significantly. The optical, thermal, and mechanical properties of the CPI films prepared by the chemical imidization method are superior to those of the CPI films prepared by the thermal imidization method, which indicates that the chemical imidization method is more suitable for preparing such optical-grade CPI films. Based on the most excellent comprehensive performance, the optical indicators and other key properties of the 3F6F/C-PI-4 CPI film could meet the requirements for preparing transparent FPCB. The hydrophilic properties of carbonyl groups in 3FPODA have improved the surface hydrophilicity of optical-grade CPI films, which was expected to solve the problem of nonwetting between the CPI film and metal interfaces in the preparation of transparent FPCB.

Based on the application of CPI in the microelectronics industry, the future technology trend of CPI can be summarized into four aspects, which include performance optimization and multifunctionality, the production process innovation and costing control, the expansion and customization of application fields, the green environmental protection, and sustainable development. For performance optimization and multifunctionality, the higher transparency and optical performance, enhancing mechanical performance and durability, and multifunctional integration were required for CPI films. With the development of flexible display technology toward higher resolution and larger size, the transparency requirements for CPI films will also increase. The future technology will focus on improving the transmittance of CPI films at visible and near-infrared wavelengths through molecular structure design (such as introducing special functional groups and regulating molecular chain arrangement) and nanocomposite technology. Meanwhile, the mechanical strength, flexibility, and fatigue resistance of CPI films were required to be continuously optimized to meet the frequent bending requirements of foldable and rollable electronic devices. The strategies of introducing rigid structural units, cross-linking technology, and composites with other high-performance materials will be taken to improve the tensile strength, elongation, and flexural resistance of CPI films. In addition to optical and mechanical properties, CPI films will be developed toward multifunctionality, such as integrating thermal conductivity, electrical conductivity, and electromagnetic shielding. By introducing functional fillers (such as graphene, carbon nanotubes, and metal nanowires) into the CPI matrix, the synergistic improvement of material properties can be achieved to meet the comprehensive requirements of microelectronic devices for heat dissipation, signal transmission, and electromagnetic compatibility.

For the production process innovation and costing control, the efficient preparation technology, localization, and substitution of raw materials were required for CPI films. The existing preparation processes of CPI films (such as casting biaxially, the stretching method, and the chemical imidization method) all have problems such as low production efficiency, high energy consumption, and limited yield. To improve the production efficiency, reduce production costs, and enhance the quality, stability, and consistency of thin CPI films, the future technology trend of CPI films will focus on developing a new preparation process, which includes continuous production technology, low-temperature imidization technology, and solution-casting rapid drying technology.

Meanwhile, based on the fact that CPI film production relies heavily on some imported raw materials (such as specific binary anhydrides and binary amine monomers), strategies to strengthen the research and industrialization of domestic raw materials, improve synthesis processes, enhance purity and performance, achieve domestic substitution of key raw materials, reduce production costs, and enhance the independent controllability of the industrial chain will all be taken to accelerate the independent development of CPI film production in the future.

For the expansion and customization of application fields, the emerging application areas, customization, and personalization were required for CPI films. With the development of emerging technologies such as 5G communication, the Internet of Things, and artificial intelligence, the application areas of CPI thin films will continue to expand. For example, the low dielectric constant and low dielectric loss characteristics of CPI thin films can effectively improve signal transmission efficiency in 5G base station antennas and RF devices. It is required that CPI thin-film products with targeted performance be developed to meet the needs of these emerging application areas in the future. Meanwhile, different microelectronic application scenarios have varying performance requirements for CPI films, which require that the future technology will place greater emphasis on product customization and personalization. By closely collaborating with downstream customers, we could design and develop customized CPI film products based on specific application requirements (such as optical performance, mechanical performance, and electrical performance) to meet the demand for high-performance and differentiated materials.

For green environmental protection and sustainable development, environmentally friendly materials and recyclable and degradable technologies were required for CPI films. With the increasing awareness of environmental protection, the development of environmentally friendly CPI materials has become an important trend. The future research of CPI films will focus on the development of biobased CPI materials, which could reduce carbon emissions and environmental pollution in the production process by using renewable resources (such as plant oils, cellulose) as raw materials to reduce the dependence on petroleum-based raw materials. Recyclable and biodegradable CPI materials and technologies will be explored to solve the problem of recycling and disposal of CPI films after use. CPI films can be recycled and reused through physical or chemical methods after their service life by designing molecular structures with reversible chemical bonds or biodegradability.

In summary, the future technology trends of CPI films in the microelectronics industry will revolve around performance improvement, process innovation, application expansion, and green environmental protection. The CPI materials will be promoted for wider application and sustainable development in the microelectronics field through technological innovation and industry collaboration.

The authors declare no competing financial interest.