Vanillin produced by chemical modification of lignin is considered as natural vanillin and is 250 times cheaper than synthetic vanillin. Due to aromatic structure it could be able to replace widely used petro-based aromatic monomers [1]. Photopolymerization engendered high interest both in academia and in industry due to the considerable practical and economic benefits. Advantages of the photopolymerizition are high reaction speed, low energy consumption, high efficiency, low volatile organic compound emission, and the large number of applications in not only conventional areas such as coatings, inks, and adhesives, but also in high-tech domains, such as microelectronics, optoelectronics, laser imaging, stereolithography, and nanotechnology [2].
In this study, cross-linked polymers were obtained by photopolymerization of vanillin diacrylate and vanillin dimethacrylate using ethyl(2,4,6-trimethylbenzoyl)phenylphosphinate as photoinitiator. Chemical structure of the polymers was confirmed by IR spectroscopy. The yield of insoluble fraction of the cross-linked polymers, obtained after Soxhlet extraction in acetone for 24 h, was in the range of (77-96) %. Differential scanning calorimetry (DSC) and thermogravimetrical analysis (TGA) was used to study thermal characteristics of the photocross-linked polymers. The glass transition temperature, obtained from DSC curves, was in the range of (63-87) °C. The temperature of 10 % mass loss, obtained from TGA curves, was in the range of (330-350) °C. Mechanical properties of the resulted polymers were investigated by three point bending test and compression test. Vanillin diacrylate-based polymers demonstrated low deformation during compression, 4.95 % when the force of 5 N was applied. The specimen bend of 30 % was 3.65 N.
Photorheometry was used to monitor the evolution of photocross-linking process. The UV/Vis real-time photorheometry curing tests were performed on a MCR302 rheometer from Anton Paar equipped with the plate/plate measuring system. It was determined that photopolymerization was the fastest when vanillin dimethacrylate was used as monomer. The addition of solvent into the resins slowed down the photocross-linking process and less rigid polymers were obtained. The gel point was reached the fastest when 3 mol.% of ethyl(2,4,6-trimethylbenzoyl)phenylphosphinate was used in all resin series.
The vanillin diacrylate-based polymers without any solvent possessed higher values of cross-linking density, yield of insoluble fraction, and thermal stability in comparison to the vanillin dimethacrylate-based polymers, although the photocross-linking of vanillin dimethacrylate was faster than that of vanillin diacrylate.
Acknowledgements
Financial support from the EU ERDF, through the INTERREG BSR Programme, (ECOLABNET project #R077) is gratefully acknowledged.
References
[1] M. Fache, B. Boutevin, and S. Caillol, Sustainable Chem. Eng. 2016, 4, 35−46.
[2] Y. Yagci, S. Jockusch, and N.J. Turro, Macromolecules 2010, 43, 6245-6260.
