Fig 5 - uploaded by Seung Geol Lee
Content may be subject to copyright.
Optimized geometry of polyimide with (a) N 2 adsorption, (b) O 2 adsorption, and fluorinated polyimide with (c) N 2 adsorption at BPDA side, (d) O 2 adsorption at BPDA side, (e) N 2 adsorption at 6FPA side, (f) O 2 adsorption at 6FPA side using DFT calculation.
Source publication
The stable optical properties of high transmittance and low yellow index are required of polyimide film as a flexible display substrate, but thermal process could result in its color change by thermal imidization. To prevent the color change, anti-oxidants have been used, but as yet though, the effect of oxidation in polyimide has remained unexplor...
Citations
... The mismatch between the CTE values of CPI films and the inorganic components might induce internal stress during the fabrication of the optoelectronic devices, which might result in delamination, cracking, warpage, and other reliability issues [9]. Thus, it is becoming one of the most important topics for the improvements of the hightemperature dimensional stability of the CPI films [10][11][12]. ...
Light-colored and transparent polyimide (PI) films with good high-temperature dimensional stability are highly desired for advanced optoelectronic applications. However, in practice, the simultaneous achievement of good optical and thermal properties in one PI film is usually difficult due to the inter-conflicting molecular design of the polymers. In the present work, a series of PI-SiO2 nanocomposite films (ABTFCPI) were developed based on the PI matrix derived from hydrogenated pyromellitic anhydride (HPMDA) and an aromatic diamine containing benzanilide and trifluoromethyl substituents in the structure, 2,2′-bis(trifluoromethyl)-4,4′-bis [4-(4-aminobenzamide)]biphenyl (ABTFMB). The inorganic SiO2 fillers were incorporated into the nanocomposite films in the form of colloidal nanoparticles dispersed in the good solvent of N,N-dimethylacetamide (DMAc) for the PI matrix. The derived ABTFCPI nanocomposite films showed good film-forming ability, flexible and tough nature, good optical transparency, and good thermal properties with loading amounts of SiO2 up to 30 wt% in the system. The ABTFCPI-30 film with a SiO2 content of 30 wt% in the film showed an optical transmittance of 79.6% at the wavelength of 400 nm (T400) with a thickness of 25 μm, yellow index (b*) of 2.15, and 5% weight loss temperatures (T5%) of 491 °C, which are all comparable to those the pristine ABTFCPI-0 matrix without filler (T400 = 81.8%; b* = 1.77; T5% = 492 °C). Meanwhile, the ABTFCPI-30 film exhibited obviously enhanced high-temperature dimensional stability with linear coefficients of thermal expansion (CTE) of 25.4 × 10−6/K in the temperature range of 50 to 250 °C, which is much lower than that of the AMTFCPI-0 film (CTE = 32.7 × 10−6/K).
... The mismatch between the CTE values of CPI films and the inorganic components might induce the internal stress during the fabrication of the optoelectronic devices, which might result in the delamination, cracking, warpage and other reliability issues [9]. Thus, it has been becoming one of the most important topics for the improvements of the high-temperature dimensional stability of the CPI films [10][11][12]. ...
Light-colored and transparent polyimide (PI) films with good high-temperature dimensional stability are highly desired for advanced optoelectronic applications. However, in practice, the simultaneous achievement of good optical and thermal properties in one PI film is usually difficult due to the inter-conflicting molecular design for the polymers. In the present work, a series of PI-SiO2 nanocomposite films (ABTFCPI) were developed based on the PI matrix derived from hydrogenated pyromellitic anhydride (HPMDA) and an aromatic diamine containing benzanilide and trifluoromethyl substituents in the structure, 2,2'-bis(trifluoromethyl)-4,4'-bis[4-(4-aminobenzamide)]biphenyl (ABTFMB). The inorganic SiO2 fillers were incorporated into the nanocomposite films with the form of colloidal nanoparticles dispersed in the good solvent of N,N-dimethylacetamide (DMAc) for the PI matrix. The derived ABTFCPI nanocomposite films showed good film-forming ability, flexible and tough nature, good optical transparency, and good thermal properties with the loading amounts of SiO2 up to 30 wt% in the system. The ABTFCPI-30 film with the SiO2 content of 30 wt% in the film showed the optical transmittance of 79.6% at the wavelength of 400 nm (T400) with a thickness of 25 μm, the yellow index (b*) of 2.15, and the 5% weight loss temperatures (T5%) of 491 oC, which are all comparable to those the pristine ABTFCPI-0 matrix without filler (T400=81.8%; b*=1.77; T5%=492 oC). Meanwhile, the ABTFCPI-30 film exhibited obviously enhanced high-temperature dimensional stability with the linear coefficients of thermal expansion (CTE) of 25.4×10-6/K in the temperature range of 50 to 250 oC, which is much lower than that of the AMTFCPI-0 film (CTE=32.7×10-6/K).
... It is posited that the darkening of the films with cure temperature is primarily due to enhanced CTC formation. Another factor that may have an effect is the presence of impurities (e.g., oxidized non-reacted diamines) [34]; however, for wholly aromatic polyimides such as PMDA-ODA, it is widely regarded that the coloration is predominantly related to increases in charge transfer interactions [37][38][39][40][41][42][43]. Since CTC formation increases with curing temperature, the color intensity increases concomitantly [44]. ...
... In addition to this, we observed a similarity in shape between the cut-off wavelength ( Figure 3c) and Tg (Figure 4a) for polyimide films, both increasing with curing temperature; this confirms that the improvement in glass temperature is the result of CTC formation. Figure 4c and is a result of the denser molecular packing in the film [43]. ...
Polyimides (PI) are a class of dielectric polymer used in a wide range of electronics and electrical engineering applications from low-voltage microelectronics to high voltage isolation. They are well appreciated because of their excellent thermal, electrical, and mechanical properties, each of which need to be optimized uniquely depending on the end application. For example, for high-voltage applications, the final polymer breakdown field and dielectric properties must be optimized, both of which are dependent on the curing process and the final physico-chemical properties of PI. The majority of studies to date have focused on a limited set of properties of the polymer and have analyzed the effect of curing from a physicochemical-, mechanical- or electrical-centric viewpoint. This paper seeks to overcome this, unifying all of these characterizations in the same study to accurately describe the universal effect of the cure temperature on the properties of PI and at an industrial processing scale. This paper reports the widest-ranging study of its kind on the effect that cure temperature has on the physico-chemical, mechanical, thermal and electrical properties of polyimide, specifically poly (pyromellitic dianhydride-co-4, 4′-oxydianiline) (PMDA/ODA). The optimization of the cure temperature is accurately studied not only regarding the degree of imidization (DOI), but also considering the entire physical properties. Particularly, the analysis elucidates the key role of the charge–transfer complex (CTC) on these properties. The results show that while the thermal and mechanical properties improve with both DOI and CTC formation, the electrical properties, particularly at high field conditions, show an antagonistic behavior enhancing with increasing DOI while degrading at higher temperatures as the CTC formation increases. The electrical characterization at low field presents an enhancement of the final PI properties likely due to the DOI. On the contrary, at high electric field, the conductivity results show an improvement at an intermediate temperature emphasizing an ideal compromise between a high DOI and PI chain packing when the thermal imidization process is performed over this equilibrium. This balance enables maximum performance to be obtained for the PI film with optimized electrical properties and, overall, optimal thermal and mechanical properties are achieved.
... Figure 3d,e show the average transmittance of the MWA-and CTA-treated PI substrates in the visible (380-800 nm) region, respectively. It is worth noting that the optical transmittance remained almost constant for MWA despite increasing the microwave power but decreased significantly with temperature from 300 • C for CTA [33]. Typically, solution-processed a-IGZO channels require heat treatment at temperatures significantly higher than 300 • C, but CTA prevents the PDA process because of the thermal damage to the PI substrate ( Figure 3). ...
... Figure 3d,e show the average transmittance of the MWA-and CTA-treated PI substrates in the visible (380-800 nm) region, respectively. It is worth noting that the optical transmittance remained almost constant for MWA despite increasing the microwave power but decreased significantly with temperature from 300 °C for CTA [33]. Typically, solution-processed a-IGZO channels require heat treatment at temperatures significantly higher than 300 °C, but CTA prevents the PDA process because of the thermal damage to the PI substrate ( Figure 3). ...
In this study, we propose the fabrication of sol-gel composite-based flexible and transparent synaptic transistors on polyimide (PI) substrates. Because a low thermal budget process is essential for the implementation of high-performance synaptic transistors on flexible PI substrates, microwave annealing (MWA) as a heat treatment process suitable for thermally vulnerable substrates was employed and compared to conventional thermal annealing (CTA). In addition, a solution-processed wide-bandgap amorphous In-Ga-Zn (2:1:1) oxide (a-IGZO) channel, an organic polymer chitosan electrolyte-based electric double layer (EDL), and a high-k Ta2O5 thin-film dielectric layer were applied to achieve high flexibility and transparency. The essential synaptic plasticity of the flexible and transparent synaptic transistors fabricated with the MWA process was demonstrated by single spike, paired-pulse facilitation, multi-spike facilitation excitatory post-synaptic current (EPSC), and three-cycle evaluation of potentiation and depression behaviors. Furthermore, we verified the mechanical robustness of the fabricated device through repeated bending tests and demonstrated that the electrical properties were stably maintained. As a result, the proposed sol-gel composite-based synaptic transistors are expected to serve as transparent and flexible intelligent electronic devices capable of stable neural operation.
... An alternative solution to this entails the introduction in the PI main chain of flexible bonds, twisting structures and/or bulky substituents (to lower the macromolecule co-planarity), combined with low polarizable atoms or aliphatic moieties [12,13]. Such a PI molecular design approach produces disruption of the CTC interactions and hence renders better solubility and transparency [14][15][16]. In recent works, we showed that the use of cycloaliphatic dianhydrides in the PI synthesis leads to transparent materials, with high glass transition temperature, good dimensional stability, and appropriate adhesion with the LCs [9,[17][18][19][20][21]. ...
The operability of liquid crystal displays is strongly impacted by the orientation aspects of nematics, which in turn are affected by the alignment layer surface features. In this work, two polyimide (PI) structures are obtained based on a cycloaliphatic dianhydride and aromatic or aliphatic diamines with distinct flexibility. The attained PI films have high transmittance (T) for visible radiations, i.e., at 550 nm T > 80%. Here, a novel strategy for creating surface anisotropy in the samples that combines rubbing with a cloth and stretching via pressing is reported. Birefringence and atomic force microscopy (AFM) scans reveal that the generated orientation of the chains is affected by the chemical structure of the polymer and order of the steps involved in the surface treatment. Molecular modeling computations and wettability tests show that the PI structure and produced surface topography are competitive factors, which are impacting the intensity of the interactions with the nematic liquid crystals. The achieved results are of great relevance for designing of reliable display devices with improved uniform orientation of liquid crystals.
... It was found that the optical properties of PI substrates depend on the PDA method and conditions. In the MWA, the transmittance of the PI substrate remained almost constant, regardless of the increase in microwave power, whereas in the CTA, it began to decrease from 400 • C. Thus, it was determined that the critical process temperature of CTA allowed for the PI substrate in this study is 300 • C [22]. The average transmittance in the visible region was 74.8%, 74.5%, and 68.9% for the pristine state, MWA at 1800 W, and CTA at 500 • C, respectively. ...
... In the MWA, the transmittance of the PI substrate remained almost constant, regardless of the increase in microwave power, whereas in the CTA, it began to decrease from 400 °C. Thus, it was determined that the critical process temperature of CTA allowed for the PI substrate in this study is 300 °C [22]. The average transmittance in the visible region was 74.8%, 74.5%, and 68.9% for the pristine state, MWA at 1800 W, and CTA at 500 °C, respectively. ...
In this study, we applied microwave annealing (MWA) to fabricate amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) without thermal damage to flexible polyimide (PI) substrates. Microwave energy is highly efficient for selective heating of materials when compared to conventional thermal annealing (CTA). We applied MWA and CTA to a-IGZO TFTs on PI substrate to evaluate the thermal damage to the substrates. While the PI substrate did not suffer thermal damage even at a high power in MWA, it suffered severe damage at high temperatures in CTA. Moreover, a-IGZO TFTs were prepared by MWA at 600 W for 2 min, whereas the same process using CTA required 30 min at a temperature of 300 °C, which is a maximum process condition in CTA without thermal damage to the PI substrate. Hence, MWA TFTs have superior electrical performance when compared to CTA TFTs, because traps/defects are effectively eliminated. Through instability evaluation, it was found that MWA TFTs were more stable than CTA TFTs against gate bias stress at various temperatures. Moreover, an MWA TFT-constructed resistive load inverter exhibited better static and dynamic characteristics than the CTA TFT-constructed one. Therefore, MWA is a promising thermal process with efficient energy conversion that allows the fabrication of high-performance electronic devices.
... In addition, it is easy to form intramolecular and intermolecular charge-transfer complex (CTC) that usually causes the low transparency and deep color of PI films. [5][6][7][8][9][10] These two drawbacks impede the applications of PIs in microelectronic and optoelectronic engineering. 11,12 Therefore, lots of efforts have been made to overcome above-mentioned problems through the design and modification of PI molecular structures, 13 including introduction of bulky pendent groups (e.g., cyclobutyl and cyclohexyl) to decrease close-packing and intermolecular CT interactions, [14][15][16] flexible linkages (e.g., -O-, -CH 2 -and -S-) to provide kinks in the polymer backbones, [17][18][19] noncoplanar structures to reduce crystallinity and hinder interactions, [20][21][22] and fluorine substituents like trifluoromethyl or perfluoro groups to weaken intermolecular cohesive force and reduce CTC formation. ...
Herein a semi-aliphatic diamine 4,4′-(cyclohexylmethylene)bis(2-methylaniline) (CHMBMA) with a pendant cyclohexyl and two ortho-substituted methyl groups is synthesized from o-toluidine and cyclohexanecarbaldehyde by Mannich and rearrangement reactions. Then CHMBMA is polycondensed with five commercial aromatic dianhydrides by the high-temperature one-step method with the thermal imidization to produce a series of polyimides (PIs, PI-H(1–5)). The weight-averaged molecular weights ( M w s) of PI-H(1–5) are in the range from 9.08 × 10 ⁴ to 27.48 × 10 ⁴ g mol ⁻¹ with the polydispersity indices (PDI) from 3.29 to 6.13 by gel permeation chromatography (GPC) measurement. They are soluble in common organic solvents (such as THF, CHCl 3 etc.) and can form transparent, tough films with light-color (Thickness: 20–26 μm) by the solution-casting method. The light transmittance of them is above 80% in the visible range from 400 to 760 nm. They exhibit excellent mechanical properties with tensile strength from 72.2 to 97.06 MPa and tensile modulus from 0.9 to 1.9 GPa. Furthermore, they also display low water absorption rates (<2.5%), good thermal stability (5% weight loss temperatures ( T 5% ) in the range from 466 to 480°C under N 2 atmosphere and high glass transition temperatures ( T g s ≥ 319°C). As comparison, we also synthesize these PIs (PI-L(1–5)) by the low-temperature two-step method with the chemical imidization. The M w s of PI-L(1–5) are lower than those of PI-H(1–5), but the film color of PI-L(1–5) is relatively lighter than the corresponding one of PI-H(1–5). In summary, the introduction of cyclohexyl and ortho-substituted methyl groups into the backbone can improve the solubility of PIs and the transparency of their corresponding films without reducing their T g s.
... For instance, the research on the adjustment of optical transparency and colors of PI films so as to meet the property requirements of high-tech areas has been becoming one of the most vigorous topics in the PI materials [4][5][6]. Standard PI films, characterized by the first commercially available poly(pyromellitic dianhydride-co-4,4 -oxydianline) (PMDA-ODA) commercialized by DuPont company, USA in the 1960s with the trademark of Kapton ® , usually exhibit a golden yellow color and a certain extent of optical transparency owing to the absorption of visible light caused by the intra-and intermolecular charge transfer behaviors in the polymer chains [7][8][9]. The charge transfer complex (CTC) has been proven to be formed with the diamine unit as the electron donator and the dianhydride unit as the electron acceptor [10][11][12]. ...
... Polymers 2020, 12, x 2 of 12 research on the adjustment of optical transparency and colors of PI films so as to meet the property requirements of high-tech areas has been becoming one of the most vigorous topics in the PI materials [4][5][6]. Standard PI films, characterized by the first commercially available poly(pyromellitic dianhydride-co-4,4ꞌ-oxydianline) (PMDA-ODA) commercialized by DuPont company, USA in the 1960s with the trademark of Kapton ® , usually exhibit a golden yellow color and a certain extent of optical transparency owing to the absorption of visible light caused by the intra-and intermolecular charge transfer behaviors in the polymer chains [7][8][9]. The charge transfer complex (CTC) has been proven to be formed with the diamine unit as the electron donator and the dianhydride unit as the electron acceptor [10][11][12]. ...
In the current work, a series of black polyimide (PI) films with excellent thermal and dimensional stability at elevated temperatures were successfully developed. For this purpose, two aromatic diamines including 4,4′-iminodianline (NDA) and 2-(4-aminophenyl)-5- aminobenzimidazole (APBI) were copolymerized with pyromellitic dianhydride (PMDA) to afford PIs containing imino groups (–NH–) in the molecular structures. The referenced PI film, PI-ref, was simultaneously prepared from PMDA and 4,4′-oxydianiline (ODA). The introduction of imino groups endowed the PI films with excellent blackness and opaqueness with the optical transmittance lower than 2% at the wavelength of 600 nm at a thickness of 25 μm and lightness (L*) below 10 for the CIE (Commission International Eclairage) Lab optical parameters. Meanwhile, the introduction of rigid benzimidazole units apparently improved the thermal and dimensional stability of the PI films. The PI-d film based on PMDA and mixed diamines (NDA:APBI = 70:30, molar ratio) showed a glass transition temperature (Tg) of 445.5 °C and a coefficient of thermal expansion (CTE) of 8.9 × 10−6/K in the temperature range of 50 to 250 °C, respectively. It is obviously superior to those of the PI-a (PMDA-NDA, Tg = 431.6 °C; CTE = 18.8 × 10−6/K) and PI-ref (PMDA-ODA, Tg = 418.8 °C; CTE: 29.5 × 10−6/K) films.
... In recent years, the research progress of colorless and transparent polyimide (CPI) films has attracted great attention in the research and development of high performance polymer optical films [8][9][10]. Based on the previous decades of research on the origination of coloration in traditional allaromatic PI films, the researchers revealed many effective procedures to prohibit the formation of intra-or intermolecular charge transfer complexes (CTC) between the electron-donating diamine moiety and the electron-accepting dianhydride moiety so as to improve the optical transparency of the PI films [11][12][13]. At present, the effective means to develop CPI films mainly include introducing groups with high electronegativity characteristics, such as trifluoromethyl groups, sulphone groups, or substituents with non-conjugated characteristics, such as aliphatic or alicyclic groups, or groups with large molar volume-such as naphthalene, fluorene, or cardo groups-into the PI molecular chains [14][15][16][17][18][19][20]. ...
Semi-alicyclic colorless and transparent polyimide (CPI) films usually suffer from the high linear coefficients of thermal expansion (CTEs) due to the intrinsic thermo-sensitive alicyclic segments in the polymers. A series of semi-alicyclic CPI films containing rigid-rod amide moieties were successfully prepared in the current work in order to reduce the CTEs of the CPI films while maintaining their original optical transparency and solution-processability. For this purpose, two alicyclic dianhydrides, hydrogenated pyromellitic anhydride (HPMDA, I), and hydrogenated 3,3’,4,4’-biphenyltetracarboxylic dianhydride (HBPDA, II) were polymerized with two amide-bridged aromatic diamines, 2-methyl-4,4’-diaminobenzanilide (MeDABA, a) and 2-chloro-4,4’-diaminobenzanilide (ClDABA, b) respectively to afford four CPI resins. The derived CPI resins were all soluble in polar aprotic solvents, including N-methyl-2-pyrrolidone (NMP) and N,N-dimethylacetamide (DMAc). Flexible and tough CPI films were successfully prepared by casing the PI solutions onto glass substrates followed by thermally cured at elevated temperatures from 80 °C to 250 °C. The MeDABA derived PI-Ia (HPMDA-MeDABA) and PI-IIa (HBPDA-MeDABA) exhibited superior optical transparency compared to those derived from ClDABA (PI-Ib and PI-IIb). PI-Ia and PI-IIa showed the optical transmittances of 82.3% and 85.8% at the wavelength of 400 nm with a thickness around 25 μm, respectively. Introduction of rigid-rod amide moiety endowed the HPMDA-PI films good thermal stability at elevated temperatures with the CTE values of 33.4 × 10−6/K for PI-Ia and 27.7 × 10−6/K for PI-Ib in the temperature range of 50–250 °C. Comparatively, the HBPDA-PI films exhibited much higher CTE values. In addition, the HPMDA-PI films exhibited good thermal stability with the 5% weight loss temperatures (T5%) higher than 430 °C and glass transition temperatures (Tg) in the range of 349–351 °C.
... They concluded that shortening the imidization time to under 30 min at 350°C manages to maintain the colorless properties of films for use as flexible display substrates. 34 Incorporation of fluorine into the backbone of polyimide aerogels has been investigated by groups such as Meador et al. 35 in an attempt to lower dielectric constants of the aerogel. A triamine (TAB) was used as a cross-linker with fractions of 6FDA in combination with BPDA and ODA as the diamine. ...
Highly translucent polyimide aerogels were prepared by combining equimolar amounts of pyromellitic dianhydride (PMDA) and 4, 4’-hexafluoroisopropylidene di(phthalic anhydride) (6FDA), 2,2’-dimethylbenzidine (DMBZ), and cross-linking with 1,3,5-benzenetricarbonyl trichloride (BTC). A multivariable statistical design of experiments was used to perform a comparison study between three variables used to fabricate the aerogels: formulated repeat unit (n) of polyimide oligomers, 6FDA fraction of total dianhydride (0 to 50 mol %), and total polymer concentration in solution (7- 10 wt %). Polymers with 25 mol % of 6FDA in the backbone structure, were found to produce polyimide aerogels with high optical transmission and low haze. These aerogels also possessed higher surface areas and very narrow nanoscale pore size distribution. Due to the decreased thermal conductivity with increasing amount of 6FDA in the backbone, these aerogels may find use where the combination of high optical transparency and thermal impedance are desired, such as insulated window panes. To this end, future efforts will focus on reducing the yellow color of the polyimide aerogels.
