X-Ray Vision: Seeing the Impact of Monomer and Initiator Chemistry on Conversion in X-Ray Polymerization Systems

X-rays are a form of high-energy electromagnetic and ionizing radiation that can be used to break molecular bonds and create radicals and ions. Thus, X-ray irradiation can be used to initiate certain chemical reactions. This research focuses on using X-ray irradiation to initiate polymerization in a variety of (meth)acrylates and to activate photoacid generators for epoxide polymerization. (Meth)acrylate systems are prevalent in commercial formulations, in which X-ray polymerization can provide a distinct advantage for these polymerization systems since the ionizing nature means that no initiator molecules are required for polymerization to occur. In addition, X-ray polymerization occurs at ambient temperature and does not require solvents to control the reaction temperature. Thus, this method of polymerization is attractive since it does not require additional chemicals that could become a disposal hazard or leach from the final product. The effect of (meth)acrylate chemistry was explored by measuring monomer conversion as a function of irradiation time using Raman spectroscopy. Methacrylates polymerized more slowly than acrylates; however, all monomers reached similar final conversions. The effect of photoacid generator chemistry, as well as compatibility with X-ray irradiation, was also tested. Epoxide samples were prepared using one of three different photoacid generators, and conversion was measured at three different irradiation doses. Higher doses have resulted in higher initial conversion of the epoxide monomer. In addition, photoacid generators with a hexafluoroantimonate results in noticeably lower conversions than those that contained a hexafluorophosphate or pentafluorophenylborate groups, possibly due to the relative strength of the acid produced. The conversion of these epoxide samples continues to be monitored after the initial polymerization in order to observe any dark-cure effects; however, preliminary results show that dark cure effects after X-ray initiation are very limited compared to other forms of initiation. Understanding how monomer and initiator chemistry impact conversion will allow for successful implementation of X-ray polymerization in future industrial applications.