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-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.
Highlights
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]
Various methods for reducing the coefficients of thermal expansion (CTEs) of semiPI films have been reported in the literature, most of the results are not very satisfactory
In the rigid amide bond was introduced into the molecular structures of the diamine monomers so as to this study, the rigid amide bond was introduced into the molecular structures of the diamine reduce the CTE values of the derived semi-alicyclic PI films
Summary
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]. 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. 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– It is a changeling research topic to develop polymer optical films with both of excellent thermal stability and good optical transparency [5,6,7].
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