Gel Point Determination of a Diffusive Photopolymer via ¹H NMR Relaxometry

Diffusive photopolymers can simplify the fabrication of “lab on a chip” devices. Knowledge of the gel point of the photopolymer, defined as the exposure time when an infinite macromolecule is formed, is required to create complex coplanar features via UV exposure. Because instrumentation for traditional gel point determination techniques is not locally accessible, we sought to develop a locally available method of gel point quantitation for a two-stage methacrylate/thiol-ene formulation. Since methacrylate chain growth and increasing cross-linking during the sol-gel transition reduces the mobility of diluent monomers, we hypothesized that the gel point can be found using NMR inversion-recovery to measure how polymerization affects the mobility-dependent T₁ relaxation time of an added “spectator” molecule. We present results showing a correlation between the rheological gel point and the time when the slope of T₁ versus exposure time changes, demonstrating ¹H NMR relaxometry is a feasible method of gel point determination.