Polyimides colorless

Some diamines carrying very bulky substituents like cardo groups can give colorless polyimides. For example, the bis-9,9-(4-aminophenyl)fluorene (FDA) or brominated and acetylenic FDA derivatives react with 6FDA giving copolymer films62 with low birefringence (low difference between in-plane and out-of-plane refraction index) (Fig. 5.8). A new cardo diamine l,l-bis[4-(4-aminophenoxy)phenyl]cyclododecane (Fig. 5.8) reacts with different aromatic dianhydrides with formation of colorless polyimides.63... 

Figure 5.7 Fluorinated monomers used for preparation of colorless polyimides.

Figure 5.9 (a) Colorless semialiphatic polyimide (b-f) cycloaliphatic dianhydrides ... 

Different dianhydrides have been syndiesized or are commercially available, and some structures arc shown in Fig. 5.9.64-66 An improved method for preparation of cyclobutanetetracarboxylic dianhydride (CBDA) by photochemical dimerization of die maleic anhydride has been developed by Nissan.67 The polyimide obtained by condensation of CBDA widi oxydianiline gives a transparent and colorless material. The transmittance of 50-pm-thick film is 82% and the UV cutoff is 310 nm. 

Alicyclic dianhydrides are interesting for electronic applications. The polyimides obtained from them are colorless with high transparency in the visible range, exhibit low birefringence,125 and have a low dielectric constant.126 The reactivity of the polycyclic aliphatic dianhydride has been investigated. For example, bicyclo-[2,2,2]-oct-7-ene tetracarboxylic dianhydride reacts quickly with an aromatic amine because the bicyclo-imide is less strained than the corresponding dianhydride.127... 

Cold phosgenation, 222 Color contamination, 541 Colorless polyimides, optical properties of, 277-279... 

Optical properties, of colorless polyimides, 277-279 Optical rotation, 490 Opto-electronic targets, 271-272 Organic phase-soluble aromatic polyesters, 77... 

A high optical transparency is a basic requirement for optical applications. However, conventional polyimides such as PMDA/ODA have a low optical transparency owing to their dark yellow coloration (they are semitransparent, not opaque). Colorless polyimides have been developed for use in space components, such as solar cells and thermal control systems for the first time. ... 

The fluorinated copolyimides (including homopolyimides) are all more transparent than the nonfluorinated PMDA/ODA. The color of 10-pm-thick films changes gradually from bright yellow to colorless as the 6FDA/TFDB content increases. All these copolyimide films are also homogeneous compared with polyimide blends of 6FDA/TFDB and PMDA/TFDB. 

Solubility was enhanced by the presence of meta and ortho isomer links in the diamine portion of the molecule. Ttie polymers prepared with 3,3 -ODA and 2,4 -ODA were found to be readily soluble at 30-40% solids at room temperature in amide solvents. These polyimides are also readily soluble in low-boiling chlorinated solvents. They can therefore be spray-coated onto desired substrates in the fully-imidized form and thus eliminate the need for taking the substrate to elevated temperatures. Hiese soluble phenoxy-linked polyimides yield tough, flexible, colorless to pale yellow transparent films from amide or cholorinated solvents. Their potential for use in electronic applications should be excellent. 

Jang et al. [85] prepared a highly kinked diamine (3-31 in Table 3.3), a fluorinated diamine (3-32 in Table 3.3), and a biphenyl based-fluorinated diamine (3-33 in Table 3.3) and studied the color intensity and optical properties of the synthesized polyimides. Introduction of kink structures disrupted effective CTC formation, leading to colorless polyimide films with high transmittance. Wang et al. [86] developed meta-diamine, namely 4,4 -/ /s (3-amino-5-trifluoro-methylphenoxy) biphenyl (3-34 in Table 3.3), with the aim of improving the melt processability and solubility of the polyimides. Shao et al. [87] designed a -CF3-substituted unsymmetrical diamine (3-35... 

H. Behniafar, N. Sefid-girandehi, Ophcal and thermal behavior of novel fluorinated polyimides capable of preparing colorless, transparent and flexible Aims, J. Fluorine Chem. 132 (11) (2011) 878-884. 

Yi MH, Huang H, Choi K-Y. Soluble and Colorless Polyimides from AlicycUc Diamines. J. Macromol. Sci. A 1998 35 2009-2022. 

Figure 6.6 shows a scheme of the CIELab system. Park and Chang [20] prepared some polyimide nanocomposites films with pristine clay and analyzed the transparency and color change. This is an important aspect since colorless polyimide films have in particular been widely used in electro-optical devices and semiconductor applications. The measurements were obtained for 80 pm thick films by a spectrophotometer and the color coordinates on CIELab system were determined. 

The b value of the polyimide hybrid with 0.50 wt% of clay is 4.13, and its degree of colorlessness is higher than the pure polyimide, 1.49. The value of b was found to increase from 4.13 to 6.89 as the clay content increases from 0.5 to 1.0 wt%, because of the agglomeration of the clay particles. This increase in b suggests that the clay particles are better dispersed in the polymer matrix at lower clay loadings. However, the L and a values remain unchanged. 

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