Block copolymers: Efficient toughening agents for the preparation of 3D-printable high impact denture base materials

Objective The objective of this work is to evaluate an innovative toughening technology for the development of 3D printable high impact denture base materials. Methods Urethane dimethacrylate DMA1 was synthesized in a two-step, one-pot reaction, starting from tricyclo[5.2.1.0(2,6)]decanedimethanol. Poly(ɛ-caprolactone)-polydimethylsiloxane-poly(ɛ-caprolactone) (PCL-PDMS-PCL) triblock copolymers BCPs1-3, exhibiting various PCL block length, were prepared from a bis(3-aminopropyl) terminated polydimethylsiloxane (2000 g mol^-1) by ring-opening polymerization of ε-caprolactone (CL). DMA1/(octahydro-4,7-methano-1H-indenyl)methyl acrylate (OMIMA) 1/1 (wt/wt) formulations containing various amounts of BCPs1-3 were prepared. The flexural strength/modulus and the fracture toughness of light-cured materials were measured according to ISO 20795-1:2013. The double bond conversion (DBC) and glass transition temperature (T_g) were determined by NIR spectroscopy and DMTA measurements, respectively. SAXS experiments were performed on dispersions of BCP2 at different concentrations, before and after the light-curing step. The nanomorphology of cured specimens was characterized by scanning transmission electron microscopy (STEM), whereas the fracture surfaces of the single-edge notched beam (SENB) specimens were analyzed by scanning electron microscopy (SEM). A monoblock denture of the lower jaw was 3D printed from the most promising formulation using the PrograPrint system (Ivoclar, Liechtenstein). Results The addition of BCPs1-3 to the DMA1/OMIMA mixture led to a decrease of the mechanical properties (flexural strength/modulus) as well as to a significant improvement of the fracture toughness. It was demonstrated that the PDMS/PCL block ratio plays a significant role on the toughening properties. The BCP2 based material provided the highest flexural strength value. The evaluation of formulations containing different contents of BCP2 showed that 4.5 wt% was ideal. Indeed, the corresponding light-cured material fulfilled the ISO 20795-1:2013 requirements for high impact denture bases. SAXS and STEM measurements clearly showed that BCP2 was able to self-assemble in the monomer mixture, leading to the formation of a nanostructure after curing. A monoblock denture of the lower jaw was successfully 3D printed using the most promising formulation. Significance By carefully selecting the nature of the components, the use of block copolymers as toughening agents in a urethane dimethacrylate macromonomer/monofunctional monomer mixture can efficiently lead to the formulation of 3D printable high impact denture bases.