Cationic two-photon induced polymerization with high dynamic range

Cationic-induced two-photon photo-polymerization is demonstrated at 710 nm, using an isopropylthioxanthone / diarylidonium salt initiating system for the cationic polymerization of an epoxide. In-situ monitoring of the polymer conversion using interferometry allows for determination of the polymerization threshold J_2th, polymerization rate R and its dependence of initiator's concentration z. Best J_2th achieved is 1 GW/cm ², with a dynamic range of > 100, i.e. the material can be fully polymerized at intensities > 100 times the threshold level without damage. The R is found to be proportional to the m=1.7 power of the intensity, or R = [C (J-J_2th)]^m = [C (J-J_2th)]^1.7, which implies a significantly stronger localization of the photochemical response than that of free radical photoinitiators. Both R and J_2th significantly improve when the concentration z of the initiator (onium salt) increases, reduction of J_2th exhibiting z^-m trend.