Reduced Coefficients of Linear Thermal Expansion of Colorless and Transparent Semi-Alicyclic Polyimide Films via Incorporation of Rigid-Rod Amide Moiety: Preparation and Properties

Abstract

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-I a (HPMDA-MeDABA) and PI-II a (HBPDA-MeDABA) exhibited superior optical transparency compared to those derived from ClDABA (PI-I b and PI-II b ). PI-I a and PI-II a 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-I a and 27.7 × 10 −6 /K for PI-I b 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 (T 5% ) higher than 430 • C and glass transition temperatures (T g ) in the range of 349-351 • C.The polymeric optical films with high thermal stability, high dimensional stability at elevated temperatures, high optical transparency, and high tensile properties are highly desired in modern optoelectronic areas, such as substrates or covering windows for flexible active matrix organic light-emitting diodes (AMOLED) devices, substrates for organic photovoltaic (OPV) solar cells, and so on [1][2][3]. Conventional polymeric optical films, such as polyolefin and polyester films usually suffer from the poor thermal and dimensional stability while the standard high-temperature resistant polymer films, such as the wholly aromatic polyimide (PI) films often exhibit highly colored appearance and poor optical transparency in the visible light region [4]. That is to say, the molecular structure factors that affect the optical and thermal properties of polymer films are usually contradictory. The structural factors that can improve the thermal stability of polymer films often reduce the optical properties of the films at the same time, and vice versa. Thus, it is a changeling research topic to dev...

Colorless and transparent semi‐alicyclic polyimide films with intrinsic flame retardancy based on alicyclic dianhydrides and aromatic phosphorous‐containing diamine: Preparation and properties

“…PI‐a and PI‐b films exhibited the CTE values of 63.3 × 10 −6 and 66.7 × 10 −6 /K in the temperature range of 50–200°C, respectively, which were comparable with those of the PI‐ref‐a (CTE = 61.2 × 10 −6 /K) and PI‐ref‐b (CTE = 68.5 × 10 −6 /K). This high‐CTE feature is usually common for the semi‐alicyclic PI films due to the relatively poor intra‐ and intermolecular interactions, which could be remedied by introduction of rigid‐rod units in the diamine moiety or incorporation of inorganic nanoparticles 19 …”

“…Thus, the research hot‐topics for semi‐alicyclic CPI films have mainly been focused on the modification of their CTE and flame retardancy. There have been many reports on the enhancement of high‐temperature dimensional stability of semi‐alicyclic CPI films; that is the reducing of the CTE values of the films in the literature 19‐21 . However, to the best of our knowledge, there are few reports on the enhancement of flame retardancy of the semi‐alicyclic CPI films.…”

Electrospun Semi-Alicyclic Polyimide Nanofibrous Membrane: High-Reflectance and High-Whiteness with Superior Thermal and Ultraviolet Radiation Stability for Potential Applications in High-Power UV-LEDs

“…The microscop phologies of the PI NFMs were investigated by SEM measurements and the re A series of PI (HTDA-BDAF) NFMs were fabricated by the electrospinning procedure, as shown in Figure 5, with the tailored solid contents of the starting PI solution and the electrospinning parameters shown in Table 1. In order to understand the effects of the molecular structure on the properties of the PIs, the PI (HTDA-BDAF) film was also prepared with a procedure according to our previous work [30]. The appearances of the free-standing PI film and NFM are shown in Figure 6a,b, respectively.…”

Soluble and transparent polyimides with high Tgs from a new semi-aliphatic diamine with cyclohexyl and ortho-methyl groups

“…36 Generally, PIs with the highly rigid main chains possess the low CTE values due to the large packing density. 30,37 Moreover, the low CTE values of PIs are also dominated by the degree of molecular in-plane orientation. 38,39 Here, the introduction of cyclohexyl and methyl groups into PIs enhanced their molecular mobility and resulted in the relatively high CTE values.…”

“…[23][24][25][26][27] However, the approaches to enhance transparency or solubility of PIs are usually contradicted with the demand to keep their good inherent properties, such as excellent mechanical properties, thermal stability and so on. [28][29][30] Fortunately, it is considered an effective way that incorporating bulky pendant groups into PIs to improve their processability and transparency, and meanwhile maintain their high thermal stability. [31][32][33][34] In previous work, we found that the introduction of cyclohexyl and tert-butyl groups into PIs was able to increase their T g s, solubility and transparency, 34 but the cost was relatively high.…”