Tough and low shrinkage photopolymers

 
Photopolymers based on (meth)acrylate chemistry are known for protective and decorative coatings since more than half a century. Hundreds of different reactive oligomers and diluents are on the market. Photopolymerization of these monomers leads within the fraction of a second to a tightly crosslinked polymer network. It has to be noted that the gel point during photopolymerization is reached at extremely low double bond conversion (15-20%). Further double bond conversion up to 60-100% leads to a significant shrinkage stress in the material as it is no longer able to flow. This and the unfavorable polymer architecture (every second carbon atom a potential crosslinking site) lead to the rather brittle behavior of the final photocured material.1 During the last decades, thanks to Charles Hoyle and Chris Bowman, thiol-ene polymerization has gained huge interest in the photopolymerization community.2 Besides several other advantages, the decreased shrinkage stress and higher impact resistance has to be mentioned. The mixed chain-growth - step growth polymerization delays the gel point to higher conversions. Using RT-NIR/MID-IR-Photorheology one is able to follow the conversion of the double bond and thiol groups in an ideal way. At the cross-over of G' and G'' it could be confirmed that the gel-point is significantly shifted to higher conversion (~40%) due to the shorter kinetic chain length. This leads to a reduced shrinkage stress. Also the more homogenous polymer architecture gives significantly higher toughness. This and the reduced effect of oxygen inhibition already allowed the implementation of this concept in some certain industrial applications. Unfortunately, some inherent undesired disadvantages have also to be accepted. Besides the classical problems of poor storage stability and bad odor, this concept reduces the modulus of the final material due to the flexible thiol-bridges. Furthermore, thiol-(meth)acrylate polymerization does not proceed in an homogenous way as (meth)acrylate homopolymerization is preferred. Therefore there is still room for more homogenous polymer architectures. In recent day's researchers look for alternative approaches to control the polymer architecture.3 We have recently identified ß-allylsulfones as appropriate class of addition fragmentation chain transfer (AFCT) agents for methacrylate networks.4,5 The homogenous incorporation leads to material with unique mechanical properties. High toughness and low shrinkage stress. Furthermore, the modulus at room temperature is not lowered compared to the unmodified material. The only drawback that has been identified in a broad field of studies is a slight delay in the polymerization speed. Luckily, with an entirely new class of AFCT agents6 this disadvantage could be circumvented while keeping all the favorable properties of the first generation of AFCT reagents. This does not only lead to new concepts for toughening of light-based 3D printed materials but could potentially be also interesting for the whole field of photopolymerization.