Mechanical, Dielectric, and Thermal Attributes of Polyimides Stemmed Out of 4, 4’ – Diaminodiphenyl Ether

: Several kinds of polyimide (PI) films stemmed out of 4, 4’ –diaminodiphenyl ether, as well as various structurally various aromatic dianhydride, were prepared. The films’ mechanical, dielectric, and dynamic mechanical attributes were put under investigation. According the findings, the PI films’ performance is significantly different as a result of their diverse structure. PI’s dielectric constant and dielectric loss tangent of abides by the increasing order below: PMDA-PI>BTDA-PI>BPDA-PI. Moreover, the electric breakdown strength of BTDA-PI (478.90 kV/mm) presents a lot higher value compared to the one PMDA-PI (326.80 kV/mm) and BPDA-PI (357.07 kV/mm). In particular, BTDA-PI film possesses high electric breakdown strength about 478.90 kV/mm. In addition, PI’s glass transition temperature (T g ) are, respectively, 276 °C (BTDA-PI), and290 °C (BPDA-PI), as well as 302 °C (PMDA-PI). Therefore, in virtue of their various structures and performances, practical applications of PI films can exert significant role in the electronics and microelectronics industries.


Introduction
Aromatic polyimides (PIs) have been deemed as crucial high-performance polymers classes based on the integration of excellent mechanical, electrical, and thermal properties, as well as chemical and solvent resistance [1][2][3][4][5]. Therefore, these materials are being used in numerous applications, which range from engineering plastics under aerospace industries to the films for the printed electronic circuitry applications [6][7][8][9], as a result of the superior excellent dimensional stability, the temperature under high glass transition, good optical transparency, good electrical resistivity, low water absorption, and relative permittivity [10][11][12][13]. PIs are mainly used by taking the type of films and moldings, as well as foams [14]. PIs have been particularly applied in a broad range as high-performance films, such as microelectronics, gas or solvent separation, non-linear optical devices, aerospace engineering, and printed electronic circuitry.
DuPont's Kapton type film has been boasting one of the most representative and successful commercial PI film over the past decades. The typical Kapton PI was obtained from pyromellitic dianhydride (PMDA), along with 4,4'-diaminodiphenyl ether (ODA). Moreover, ever since the commercialization of the Kapton type PI began the usage in the early 1960s, a series of PIs composed of different diamine and dianhydride have been reported [15][16][17][18]. However, for the structurally different PIs, their properties are different, as well. Therefore, it is necessary to conduct explorations on their structure-property relationships for practical applications. Although some studies showed that the chain hardness constituted the extremely crucial factor which affects their attributes in a direct way [19][20][21][22][23], the structure-property relationships links PIs are still not well understood.
In this regard, we prepared several different PIs obtained from ODA and multiple structuredbased distinct aromatic dianhydrides and investigated their properties. The effect of structural changes in PI films' mechanical, dielectric, and thermal attributes was studied, which was oriented with a better understanding of PI films' structure-property relationships in practical applications.

Integration of PI Films
The synthesis of PI films was conducted in virtue of standard two-phase-based procedure as follows. First, dissociation of ODA (10 mmol, 2.00 g) was implemented in a clean 50 mL, 3-neckround bottom flask with DMAC solvent (20 mL) and then stirred for 15 min. The BTDA (10 mmol, 3.22 g) was added continuously for several times. Then, the mixed liquid was given 4-hour of vigorous stirring to produce a sticky and uniform polyamic acid solution. After that, the polyamic acid solution was coated on a glass plate, heated in a vacuum oven at 60 °C for 2 h, and then subjected to 1-hour thermal imidization at individual temperatures of 100 °C, 200 °C, and 300 °C in dry-airflowing oven. In the last step, the BTDA-PI film was made by the experimental preparation above. The PMDA-PI and BPDA-PI were prepared in virtue of a similarly-structured process as above. The film was made with nearly 30-35 µm. Figure 1 presents the synthesis process of PI films.

Characterization and Tests
The structure of the film was put under the research of Fourier transform infrared (FTIR) spectroscopy (Nicolet Avatar-360, Thermo Nicolet Corporation, Waltham, MA, USA) and X-ray diffraction (XRD) spectroscopy (Rigaku Dmax-rB, Rigaku Corporation, Tokyo, Japan). XRD was conducted in virtue of Cu Kα radiation (λ = 1.5406 Å). The test on films' insulating and dielectric attribute was carried out through the highly-intensified megohm micro electric current tester (ST2255, Suzhou Lattice Electronics Co., Ltd, Suzhou, China) and impedance analyzer (Agilent4294A, Agilent Technologies Corporation, Santa Clara, CA, USA), respectively. The electric breakdown performance got experimented in virtue of a dielectric withstand voltage test (YD2013, Changzhou Yangzi Electronic Co., Ltd, Changzhou, China). The mechanical tests of the films was carried out by a micro electronic testing machine (WDW-20, Jinan Fangyuan Testing Machine Co., Ltd, Jinan, China). The performance of dynamic mechanical analysis (DMA) was committed on a sample with the size of 30 × 5 × 0.5 mm 3 in virtue of a dynamic mechanical analyzer of TA Instruments (DMAQ 800, TA Instruments Corporation, New Castle, DE, USA), and the stretched film mode was carried out at a temperature ranging from ambient temperature to 200 °C (1 Hz) with the heating ratio of 5 °C per min.

PI Films' Structure
PI films are structured by the chemical structures which were tested via FT-IR spectroscopy, as Figure 2a displays. Figure 2a offered the findings of the structure's peak, which is at 725 cm −1 , 1376 cm −1 , 1719 cm −1 and 1774 cm −1 . The results individually correspond to C=O bending, C-N stretching, and C=O symmetric stretching, as well as asymmetric stretching. An imide's structure can be confirmed. Figure 2b shows the X-ray diffraction types of three kinds of PI films. Three broad peaks can be placed under apparent observation within the range of about 2θ = 16~19°, which confirms the ordered region in amorphous polyimide. These broad peaks are mainly originated from the partial crystallization of PI films. However, we also can notice that the strength and shape of the peaks is similar. Therefore, DMA analysis was further used to investigate the crystalline degree of PI in the following discussion.

Mechanical Properties of PI Films
The tensile strength, tensile modulus, and elongation at the break of PI films are summarized in Figure 3. It can be apparently observed that the BPDA-PI film has a better plastic property, and the elongation at the break of it is as high as 3.8%. PMDA-PI possesses a higher brittleness with the high tensile modulus of 3.42 GPa and low elongation at of 2.82%. It should be further addressed that the BTDA-PI shows an excellent comprehensive performance with the high tensile strength of 114. 19 MPa, the tensile modulus of 3.23 GPa and elongation at the break of 3.58%. The diversity of tensile properties could be attributed to different molecular chain flexibility and intermolecular force in the PI films.  Figure 4a shows the relative dielectric constant of PI films at the frequency of 50~10 6 Hz on the condition of room temperature. The dielectric constants of PI films, which features relatively more stability from 50 to 10 4 Hz, expressed a decreasing trend in the high-frequency from 10 4 to 10 6 Hz. Moreover, the dielectric constants presented significant distinction for three kinds of PI films. The constants of the PI film varied between 3.39~3.69 at 100 Hz. PI, based on the dielectric constant of ODA, were sticking to the following increasing order: PMDA-PI> BTDA-PI>BPDA-PI. This phenomenon can be attributed to the different molecular polarity caused by the different molecular chain structure. The Clausius-Mossotti equation can be used to explain the dielectric constant of PI membranes [24]:

Dielectric Attributed Harbored by PI Films
where ɛr is the dielectric constant; ɛ0 is the vacuum permittivity; N is the molecule number in unit volume; and α is the molecular polarization. According to Equation (1), the various dielectric constant of ODA based PI can be mainly attributed to the different molecular polarization caused by the different molecular chain structure. The dielectric loss tangent of PI films is given in Figure 4b. According to the observation, as the frequency increases, the loss tangent first drops slightly and then rises sharply. The dielectric loss mainly arises out of relaxation polarization and inter-facial polarization here. Within the 50~10 3 Hz frequency range, the dielectric loss tangent decreases slightly, which is caused by the interface polarization lagging behind the change of electric field frequency. The increase range from 10 4 to 10 6 Hz is attributed to PI's glass transition relaxation [25]. Moreover, it is observed that the dielectric loss tangent of PI was also abiding by the following increasing order: PMDA-PI> BTDA-PI>BPDA-PI. However, all PI show a lower dielectric loss tangent. Besides, even for PMDA-PI films, the loss tangent still features less value compared to 0.004 at 100 Hz.
To investigate the insulating property to the films, volume resistivities were tested at different electric field strengths, and representative results are expounded in Figure 4c. As can be seen in the figure, all of the three films show high resistivities (10 15 Ωm) and confirm PI films' excellent insulativity. Moreover, the three films harbor the resistivity values, which present a slight difference, which might be attributable to the various strength of the conjugate effect in the PI molecules.

Breakdown Strength Harbored by PI Films
The measurement of PI films' dielectric breakdown was taken on the condition of room temperature. Dielectric breakdown is discussed in virtue of a two parameters Weibull distributions as follows [24]: where P(E) refers to the cumulative probability of the failure which occurred in at the electric area with a low or equal value to E. E means the experimental breakdown strength; α stands for the proportional parameter, which presents the breakdown strength when 63.2% is expressed as the cumulative failure probability. β is a shape parameter which is relevant to a linear regression fit in the distribution. The Weibull cumulative distribution function could be described as the following two logarithms: Next, ln(−ln(1 −P(E))) versus lnE, was sketched. The values could be, respectively, achieved out of the slope, the ln(−ln(1−P(E))) interception andβlnα.α and β are decided by least-squares linear regression. Table 1 includes the linear fitting results and PI films' Weibull parameters. According to Table 1, the correlation coefficient (R) values presented higher value compared to 0.95. That reveals a fine fitting upon the PI films. Figure 5 shows the PI films' Weibull deploy. As is seen, the PMDA-PI harbors the breakdown strength which is close to BPDA-PI. Nonetheless, the breakdown strength harbored by BTDA-PI (478.90 kV/mm) presents far higher value compared the one of PMDA-PI (326.80 kV/mm) and BPDA-PI (357.07 kV/mm). It might be due to the different crystalline degree and regularity in the PI molecular structure films.

Thermal Properties Harbored by PI Films
Thermal stability contributes as a major performance for PI engineering films. Figure 6 interprets differential DMA curves announce the different glass transition temperature (Tg) of PI. Therefore, it is clear that the glass transition temperature of PI films are 276 °C (BTDA-PI), 290 °C (BPDA-PI), and 302 °C (PMDA-PI), respectively. The DMA results confirm that the expression of the thermal stability harbored by PI in the below increasing order: PMDA-PI>BTDA-PI>BPDA-PI. Moreover, the DMA results indirectly show the crystalline degree of the PI films maybe in the following increasing order: PMDA-PI>BTDA-PI>BPDA-PI. Thereby, it is of significance in the practical application of PI films according to their different thermal properties.

Conclusions
In summary, the preparations of several varied aromatic PIs got out of the standard two-phase procedure of diamine (ODA) with various structurally different dianhydrides (PMDA, BTDA, and BPDA). The dielectric and thermal properties were measured for the research on the three kinds of PI films. It is found that BTDA-PI film possesses an excellent comprehensive tensile properties. The dielectric properties of the three films is marking a slight difference since the different molecular polarity and conjugate effect of PI molecules. The PI, which is based on the dielectric constant and dielectric loss tangent of ODA-based are existing in the following increasing order: PMDA-PI>BTDA-PI>BPDA-PI. In particular, BTDA-PI film possesses high electric breakdown strength about 478.90 kV/mm. In addition, the glass transition temperature of PI films are 276 °C (BTDA-PI), 290 °C(BPDA-PI), and 302 °C(PMDA-PI), respectively. Therefore, in terms of their various structures and performances, it is a significant job to conduct practical applications of PI films in the electronics and microelectronics industries.

Conflicts of Interest:
The authors declare no conflict of interest.