(Meth)acrylate monomers and oligomers dominate the photopolymer-based additive manufacturing (AM) market due to their capabilities in rapid polymerization ambient conditions and generating complex and precise structures. The resulting structures, although advantageous in terms of offering high strength and high resistance to chemicals and heat, are challenged in terms of embrittlement at low temperatures and low fracture toughness. [1]. Photopolymerization-induced phase separation (PhIPS) can overcome those challenges to a significant extent, provided enough understanding about the interactions of the participating reactants during the light curing is available. Hansen solubility parameter (HSP) provides such impressions by simply assigning a character to monomer’s interactions by employing the shares of dispersion forces (δd), polar forces (δp), and hydrogen bonding interactions (δh) in the following formula:

\delta_T^2=\delta_d^2+\delta_p^2+\delta_h^2

The higher the difference between monomers' total solubility parameters (δT), the more incompatible they are predicted to be.[2] On the other hand, PhIPS occurrence can be verified when the polymerization reduces the initial transparency of the liquid multicomponent system and delivers translucent or opaque polymers. The PhIPS-resulting optical change can be directly related to the incompatibility, as the separation of phases leads to the formation of large domains that can effectively refract visible light. In this study, three different monomers with low, medium, and high solubility distances with the δT of BisEMA monomer were chosen and photopolymerized. According to the HSP distances, Phenoxybenzyl acrylate with a matching, lauryl acrylate with medium, and a urethane diacrylate oligomer with high incompatibility were in the presence of 1 percent TPO as the photoinitiator photopolymerized. The PhIPS level and ultimate transparency have been evaluated and compared for the resulting copolymers using a bespoke-designed setup. Morphological analysis was carried out through microtomy of the fracture surface and phase imaging atomic force microscopy. Eventually, the agreements between suggested incompatibilities and microstructural observations were explored and discussed.

References

1. Ligon, S.C., et al., Polymers for 3D printing and customized additive manufacturing. Chemical reviews, 2017. 117(15): p. 10212-10290.

2. Karasu, F., et al., Influence of actinic wavelength on properties of light-cured interpenetrating polymer networks. Journal of Polymer Science Part A: Polymer Chemistry, 2016. 54(10): p. 1378-1390.