Viscoelastic-Conversion Scaling of the Free Radical Photopolymerization of Isobornyl Acrylate

To clarify the structural changes behind the viscoelastic build-up during fast free-radical polymerization, the photopolymerization of isobornyl acrylate (IBoA) was studied in detail from a synchronized viscoelastic-conversion perspective using time-resolved FT-MIR photorheology. Before effective elasticity development, polymerization proceeds in a viscous fluid state where the polymerization rate reaches a steady value after a steep ramp within the first 10% of conversion irrespective of the initiation conditions. For soft to moderate initiation the (semi)dilute entangled polymer regime before vitrification covers the second major conversion range after elasticity sets in. Interestingly, extrapolation to full conversion of the latter regime provides an alternative means to estimate the equilibrium plateau modulus and molar mass between entanglement for the bulk IBoA polymer. Further up the conversion path vitrification takes over as characterized by a steep modulus increase of at least 3 orders of magnitude over a small conversion window. Additionally, the acceleration and deceleration processes of the polymerization are discussed along the viscoelastic-conversion path.