Photochemical curing of epoxies in the liquid-crystalline state
In this work the cationic photoinduced crosslinking of the diglycidylether of 4-hydroxyphenyl-4'-hydroxybenzoate was investigated in the liquid crystalline state as well as in the isotropic state. The photochemical curing technique was applied because only small amounts of the initiating species were necessary to form networks of epoxy compounds. As the photoinitiator system, a mixture of (alpha) , (alpha) '- dimethoxydeoxybenzoin and dicumyliodonium-hexafluorophosphate proved to be the most effective. FTIR-spectra of the networks were compared with those of the starting material. Only small amounts of nonreacted epoxy groups were found in the networks synthesized either in the isotropic state or in the liquid crystalline state. Furthermore, different results were obtained with DSC-measurements. Networks prepared in the liquid crystalline state showed two glass transition temperatures. In contrast, networks synthesized in the isotropic state have only one glass transition temperature. Moreover, polarization microscopy was used to find ordered structures. The networks formed in the liquid crystalline state showed frozen ordered structures. Heating of such networks above the melting point of the starting material did not lead to a change of the frozen ordered structures. Additionally, differences between the frozen ordered structure of the networks and the crystalline structure of the starting compound were observed by this method. Finally, the material properties of networks with ordered structures are different from common networks with disordered structures.