Photopolymerization and photophysical studies of N-aliphaticmaleimides
The free-radical photopolymerization of diacrylate monomers was investigated using maleimides as the photoinitiator. Parameters such as the nature of the substituent group and the overall separation from the maleimide chromophore were investigated. In addition, the influence of the maleimide functionality (monofunctional or difunctional) was assessed for mediums where the chemical structure of the diacrylate was varied. Photo-differential scanning calorimetry (photo-DSC) and real-time infrared spectroscopy (RTIR) were utilized to monitor the kinetics of the photo-induced polymerizations, and it was concluded that the polymerization rate was dependent upon the 0.62 root of the light intensity and 0.56 root of the maleimide concentration. Real-time ultraviolet spectroscopy (RTUV) and RTIR were used to demonstrate that under normal polymerization conditions the maleimide functionality was effectively consumed in the photopolymerization of a diacrylate mixture. Further, the films produced from the photopolymerization of mixtures containing maleimides as the photoinitiator were completely colorless, and no products were produced that absorbed light above 300 nm. The use of an amine as a coinitiator for mixtures containing a maleimide as the photoinitiator resulted in a significant increase in the polymerization rate compared to the use of alcohols or ethers. The polymerization rate dependence upon the coinitiator concentration was found to have a root dependence of 0.22 for an ether and 0.39 for a representative amine. Benzophenone was successfully utilized as a sensitizer for the polymerization of a mixture containing a maleimide and amine as the photoinitiator package. In fact, polymerization rates that were obtained were approximately equal to that obtained when a commercially available cleavage photoinitiator was employed. The photopolymerization in air for a stoichiometric mixture of monofunctional maleimide monomers with electron donor monomers (vinyl ethers, vinyl amides, and styrene oxides) resulted in polymerization rates greater than that for a monofunctional acrylate containing a commercial photoinitiator. RTIR was used to show that the photopolymerizations were alternating in nature as evidenced by the near identical conversion profiles for each comonomer. Laser flash spectroscopy was successfully used to characterize the triplet state of the maleimide chromophore. A triplet state peak maximum near approximately 340 nm has a single exponential lifetime that is concentration dependent with a self-quenching rate constant of 1 × 10 9 M-1 sec-1 . Further, the rate constant for quenching of the triplet state was determine to be approximately 1 × 107 M-1 sec-1 for a representative alcohol and 1 × 109 M-1 sec-1 for an amine.