Effects of ionic liquids and dual curing on vat photopolymerization process and properties of 3d-printed ionogels

Abstract

Ionic liquids (ILs) can facilitate photopolymerization reactions involved in the fabrication of ionogels (IGs). In this study, ILs were immobilized by 3D printing through polymerization of vinyl-monomers in their medium. The effect of ILs on the vat photopolymerization (VP) process was studied. Multiple IL-compatible photopolymer formulations were developed by using EmimBF4, BmimBF4, OmimBF4, and EmimTFSI ionic liquids and photocurable N-vinylpyrrolidone (NVP) and triethylene glycol dimethacrylate (TEGDMA) monomers. The photoinduced radical copolymerization of NVP with TEGDMA was studied using a combination of photo-DSC, Jacobs working curves and FTIR methods. The analysis revealed a dichotomy of IL-based VP as the addition of ILs led to a significant acceleration of polymerization and an increase in the critical exposure energy due to a higher conversion degree necessary for the gelation. Moreover, the Jacobs working curves showed that penetration depth dramatically changed with IL type. Based on these results, overcuring time was calculated and applied in the VP process to ensure that enough monomers were converted in each 3d-printed layer to prevent interlayer cracking. Furthermore, the dual curing approach was applied to achieve the highest possible conversion by adding a thermal initiator (AIBN). Dual curing increased the mechanical properties of ionogels but led to a reduced ionic conductivity due to an inhibition of IL’s mobility. In addition, this study found IL compositions for ionogels demonstrating no shrinkage after photopolymerization and post-curing. We suggest that the outcomes of the present study created a platform for fabricating high-resolution 3d-printed nonvolatile and nonflammable ionogels.