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S. Schörpf, Y. Catel, N. Moszner, C. Gorsche, R. Liska:
"Reduction of Polymerization-Induced Shrinkage Stress via Combination of Radical Ring Opening and Addition Fragmentation Chain Transfer";
Poster: RadTech USA 2019, Monterey - CA, USA; 15.09.2019 - 18.09.2019.

Photoinitiated polymerization of (meth)acrylate (MA) systems have been known for more than a half century. The shrinkage stress within bulk photopolymer networks represents one of the most pressing challenges for various applications, like coatings1, 3D-printing2, microelectronics3 and dentistry4. The occurring shrinkage stress of commercial MA-based networks results from a combination of the covalent attachment of monomers and the gelation at early stages of the reaction. Due to the inhomogeneous network architecture, which is created during the radical curing process, the resulting materials tend to show rather brittle behavior and the occurring shrinkage stress could lead to delamination, deformation or mechanical failure of the final bulk materials. Before polymerization the monomers are situated at van der Waals distance toward each other (approximately 3.4 Å). The occurring shrinkage stress upon gelation is in part due to the formation of covalent bonds between the respective monomers, where the revealing distance is only 1.5 Å5. This volumetric shrinkage of the material leads to stress, which cannot be dissipated within a vitrified matrix. One way to counteract this problems is the replacement of the MA matrix by cyclic monomers (e.g. 1,1-disubstituted 2-vinylcyclopropanes, VCPs). VCPs have been reported as a possible monomer class yielding in reduced polymerization shrinkage and consequently shrinkage stress due to a radical ring opening reaction6. Another way to reduce the occurring shrinkage stress is to regulate the radical network formation via an addition fragmentation chain transfer (AFCT) concept7.
Herein, the light induced radical polymerization of a combination of VCP monomers with variable amounts of an AFCT reagent (an ester-activated vinyl sulfonate ester EVS) was investigated.8 A high reactivity towards light induced radical polymerization is confirmed for VCP/EVS mixtures via photo reactor, photo-DSC and real time (RT)-NIR-photorheology accompanied by a significant increase in double bond conversion (DBC) with increasing amounts of EVS. Most importantly, both systems (VCPs and AFCT reagents) combined lead to an enhanced reduction of polymerization-induced shrinkage stress. The resulting materials showed, even at low concentrations of EVS, a high network homogeneity which was indicated by a narrow loss factor plot in the DMTA measurement. In addition, filled systems performed well with respect to reduced shrinkage force while maintaining sufficient E-modulus and flexural strength.

Photoinitiated polymerization, (meth)acrylate (MA) systems, shrinkage stress, bulk photopolymer network, coatings, 3D-printing, microelectronics, dentistry, the light induced radical polymerization