Photopolymerization-induced phase separation kinetics explored by intermittent irradiation

In this work, we explore the kinetics of the photopolymerization-induced phase separation (photo-PIPS) process and the interplay of mechanisms controlling the development of the microstructure in a photosensitive resin comprised of pentaerythritol tetraacrylate (PETA) and 2-ethylhexyl methacrylate (2-EHMA) monomers with polypropylene glycol (PPG, Mn = 4000 g/mol) linear polymer additive and diphenyl(2,4,6-trimethylbenzoyl) phosphine oxide (TPO) photoinitiator. We control the kinetics of photopolymerization by interrupting the irradiation at various stages of the process and varying the light intensity. Evolution of the microstructure is monitored by transmittance testing and scanning electron microscopy (SEM) inspection of fractured surfaces that are exposed to methanol for the purpose of removing the phase-separated PPG content. The evolution of the network is monitored by real-time Fourier-transform infrared (FTIR) spectroscopy during irradiation and intermittent probing after the cessation of irradiation. Three mechanisms controlling the evolution of the microstructure are identified: phase separation, photoinitiator consumption, and microstructural refinement. Phase separation begins immediately after the onset of network development and leads to a rapid reduction of transmittance due to the formation of PPG-rich subdomains. Microstructural refinement takes place at later stages leading to a reduction of these subdomains, a gradual increase of the PPG concentration within subdomains and an associated increase of transmittance. TPO consumption takes place during irradiation and accounts, to a smaller extent, for the recovery of the transmittance. Interrupting the irradiation allows generation of materials with various degrees of conversion and sizes of phase-separated subdomains, which provides a new way to control material properties.