Synthesis and Characterization of Photosensitive Polyimides for Optical Applications

The objective of this research was to prepare colorless photosensitive polyimides for optical applications. In the first approach to these materials two new photosensitive end-capping agents, i.e., 6-(4-aminophenoxy)hexyl methacrylate and di[2-(methacryloyloxyethyl)] 5-aminoisophthalate, for polyimides were prepared. These agents were used along with 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) and 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl to prepare a series of methacrylate end-capped imide oligomers. However, the oligomers required long exposures to UV radiation to affect cure. To improve their photosensitivity, multifunctional additives and photoinitiators were used. A tetrafunctional end-capped oligomer that contained 10 wt % trimethylolpropane triacrylate and 5 wt % trimethylbenzoyldiphenyl phosphine oxide (TMDPO) was photosensitive and displayed good photopatterning properties. A second approach to the desired materials involved the synthesis of a diamine monomer, i.e., 2,2'-dimethacryloyloxy-4,4'-diaminobiphenyl, in which methacrylate moieties were attached to the 2- and 2'-positions of a biphenyl structure. The monomer was polymerized with commercially available dianhydrides such as 6FDA and 4,4'-oxydiphthalic anhydride. The polyimides obtained were soluble in common organic solvents such as THF, acetone, and chloroform and could be solution cast into thin, water-white films that were highly transparent in the visible light region. The polymers were photosensitive when combined with 5 wt % TMDPO and displayed good photopatterning properties. The polymers, which afforded line patterns 10−30 μm wide and 5−20 μm thick, did not develop color or shrink during UV exposure and subsequent thermal treatments.