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

Abstract

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.