Abstract
A high-barrier polyimide (2,7-CPI) was synthesized through the polymerization of pyromellitic dianhydride (PMDA) and a novel diamine (2,7-CDA) containing carbazole moiety. The synthesized diamine and polyimide were fully characterized by elemental analyses, FTIR and NMR. The 2,7-CPI displays very attractive barrier performances, with oxygen transmission rate (OTR) and water vapor transmission rate (WVTR) low to 0.14 cm3·m−2·day−1 and 0.05 g·m−2·day−1, respectively. Meanwhile, 2,7-CPI also exhibits exceptional thermal stability with a glass transition temperature (Tg) of 467 °C, 5% weight-loss temperature (Td5%) of 550 °C under N2 and coefficient of thermal expansion (CTE) of 3.4 ppm/K. The barrier performances of 2,7-CPI are compared with those of a structural analogue (2,7-CPPI) and a typical polyimide (Kapton). Their barrier performances with respect to microstructure were investigated by molecular simulations, wide angle X-ray diffraction (WAXD), and positron annihilation lifetime spectroscopy (PALS). The results show that 2,7-CPI possesses better coplanar structure and more number of intermolecular hydrogen bonds among the three PIs, which result in tight chain packing and thereby high crystallinity, low free volume, and decreased chains mobility. That is, the high crystallinity and low free volume of 2,7-CPI reduce the diffusion and solubility of gases. Meanwhile, the poor chains mobility further decreases the gases diffusion. The reduced diffusion and solubility of gases consequently promote the improvement of barrier properties for 2,7-CPI. The polyimide has a wide application prospect in the field of flexible electronic packaging industries.
Highlights
In recent years, flexible displays have attracted considerable attention because of the low weight, high mobility, and their portable, wearable, and even foldable applications [1,2]
One is the high gas barrier properties (WVTR < 10−6 g·m−2 ·day−1 and oxygen transmission rate (OTR) < 10−5 cm3 ·m−2 ·day−1 ), which can prevent the degradation of organic materials in the devices caused by the penetration of moisture and oxygen through the substrates [8,9]
The as-obtained DPNN3 was reacted with decalin to form 2,7-CDN by cyclization reaction, which was reduced to diamine 2,7-CDA
Summary
Flexible displays have attracted considerable attention because of the low weight, high mobility, and their portable, wearable, and even foldable applications [1,2]. The demands for thermal and dimensional stability far exceed the tolerance range of conventional plastic substrates such as polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) (Tg < 200 ◦ C, CTE of 20~80 ppm/K) [13]. In this regard, polyimides (PIs) have been considered as one of the best choices for flexible. The deposition methods are usually complicated, and require large-scale equipment, lead to fairly high fabrication costs These deposition processes are on the basis of intrinsic PI matrix. Lowest energy conformation of repeat units for 2,7‐CPPI and 2,7‐CPI
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