Synthesis of highly transparent and heat‐resistant polyimides containing bulky pendant moieties

Abstract

A new diamine with bulky pendant biphenyl and ortho-position dimethyl structures, 4,4 ′ -((1,1 ′ -biphenyl)-4-ylmethylene)bis(2,6dimethylaniline), was synthesized via a one-pot reaction of 4-biphenyl carboxaldehyde and 2,6-dimethylaniline. The diamine was employed to polymerize with several dianhydrides via one-step condensation under high-temperature conditions. The light yellow or colorless polyimide (PI) films obtained were found to have cut-off wavelengths in the range 286-358 nm and transmittance over 80% in the visible region (400-780 nm). Meanwhile, these PIs possessed excellent solubility in common organic solvents, even in low-boiling-point solvents such as chloroform (CHCl 3 ), dichloromethane (CH 2 Cl 2 ) and tetrahydrofuran. The glass transition temperatures (T g ) of the PIs were determined to exceed 343 ∘ C, even to 456 ∘ C. All PI films were flexible with a tensile strength of 78-119 MPa, a Young's modulus of 2.0-2.5 GPa and elongation at break of 4.0%-8.2%. Therefore, these colorless PIs can be used as candidate materials for flexible display substrates.

Preparation and Characterization of Semi-Alicyclic Polyimides Containing Trifluoromethyl Groups for Optoelectronic Application

“…Especially, cycloaliphatic BODA, without the problem of different isomer reactivity has been proved to have good reactivity with aromatic diamines, and the resultant BODA-based PIs also exhibited excellent optical transparency and high T g values [ 26 ]. Lu and other researchers also obtained similar results by using BODA and other aromatic diamine with bulky pendant groups [ 35 ]. T g value of reported BODA-based PI was determined to exceed 420 °C, and this transparent film showed high transmittance over 86% in the visible region.…”

“…The M n and M w of these PIs were in the range of 5.08–6.91 × 10 4 g/mol and 7.54–9.23 × 10 4 g/mol, respectively. All the semi-alicyclic PIs showed high molecular weight and narrow molecular weight distribution ( M w / M n ) below 1.50, which can reach the molecular weight level of fully-aromatic PIs and is better than most semi-alicyclic systems reported in the literatures [ 22 , 35 ]. This ensured their thermal and mechanical properties of corresponding films.…”

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

“…6,13 On the contrary, PI-H1 and PI-L1 possessed the lowest average transmittance (80%) due to the most rigid dianhydride PMDA, but still better than that of Kapton (69%). 16 The optical photographs of the resulting PI films (Thickness: 55-65 mm) were shown in Figure 7. All the PI films had good transparence with the different degree of light yellow, which was depended on the used dianhydride.…”

“…[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. [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.…”

Optically Transparent and Thermal‐Stable Polyimide Films Derived from a Semi‐Aliphatic Diamine: Synthesis and Properties

“…Fortunately, the introduction of bulky pendant substituents can improve the processability and optical transparency without seriously sacrificing the thermal stability of PIs 16–21. In other words, PIs containing alicyclic moieties as bulky pendant substituents were more stable than those as structural units in main chains 22–24.…”