Peter Burtscher

Visible Light Curable Restorative Composites for Dental Applications Based on Epoxy Monomer

A cationic photo-curable cycloaliphatic epoxy resin has been investigated as reactive monomer in blue light crosslinking process. We have demonstrated that camphorquinone is able to abstract labile hydrogen from the epoxy monomer, giving rise to the formation of carbon-centered radicals that are oxidized by the onium salt; a complete epoxy group conversion was reached after 50 s of irradiation. The presence of water up to 1 wt% was tolerated without any important detrimental effect on the kinetics of light-curing. The presence of the inorganic filler up to 65 wt% did not significantly influence the curing process.

Photoinduced polyaddition of multifunctional azides and alkynes

The photoinduced copper(I)-catalyzed polymerization of multifunctional azides and alkynes is facilitated by the photoreduction of copper(II) acetate generating copper(I) ions without using any additional photoinitiator. The polymerization can only be carried out in solution using at least 15 wt% of methanol. Depending on the catalyst concentration quantitative monomer conversions can be achieved allowing the determination of the mechanical properties. The bifunctional system consisting of a di-azide and di-alkyne exhibited the highest Youngs modulus value of 1600 MPa.

Incorporation of core–shell particles into methacrylate based composites for improvement of the mechanical properties

The fracture toughness of polymeric materials and composites can be enhanced by the incorporation of polymer nanoparticles. The combination of a soft core and a hard shell leads to an improvement of the fracture toughness of the polymeric composites. Thereby, the mechanical resistance of the materials is commonly decreased. In our approach, core–shell nanoparticles consisting of an ethylene glycol dimethacrylate (EGDMA) crosslinked poly(butyl acrylate) (PBA) core and a poly(methyl methacrylate) (PMMA) shell were synthesized. The polymer particles were incorporated into triethylene glycol dimethacrylate (TEGDMA)/urethane dimethacrylate (UDMA) based composites in order to tune the mechanical properties. Different core–shell ratios were applied to study the influence on the fracture toughness and E-modulus. An examination of shell-crosslinking with a TEGDMA content of up to 8% was performed to improve particle stability and dispersibility. The particle sizes and morphologies were characterized by dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM) and analytical ultracentrifugation (AUC). Latex particle sizes of 70 to 220 nm were obtained. The mechanical properties (flexural strength, E-modulus and K1c) of polymer composites were investigated in three-point bending tests. Core/shell ratios of 50/50 showed a decreasing effect on flexural strength, E-modulus and K1c. Polymer particles with core/shell ratios of 30/70 led to a significant increase of the mechanical properties with maxima of 1.206 MPa m1/2 (K1c) (increase of 65%), E-modulus of 1.90 GPa (increase of 18%) and flexural strength of 79 MPa (increase of 18%). This study represents the first report of a simultaneous improvement of fracture toughness and E-modulus (at the same time) of additive filled polymer composites. The improvement of mechanical properties makes these materials interesting as tougheners for hard tissue applications like bone cements or dental replacement materials.

para-Nitroaniline-functionalized chromophoric organic–inorganic hybrid materials

An advanced procedure for the one-pot synthesis of organic–inorganic hybrid materials via combination of sol–gel process and nucleophilic aromatic substitution reaction (SNAr) of 4-fluoronitrobenzene and 3-aminopropyltrimethoxysilane is described. With this advanced procedure both SNAr-reaction and sol–gel process can be accomplished in the same reaction vessel due to the sol–gel precursor tetraethoxysilane (TEOS) acting as solvent during the first reaction step. Via extensive NMR spectroscopic studies it is proven for the first time that—contrary to common belief—hydrogen fluoride (HF), which is formed as a by-product in the SNAr-reaction, is not trapped by any bases present but is rather trapped by both of the silane species and serves as a catalyst during the subsequent sol–gel process. The chromophoric system of the resulting xerogels is well protected against aggressive chemicals, i.e. strong acids, by the silica matrix, which predestines these materials for pigment applications. Given that a high chromophore content is highly desirable in these applications, we show that the chromophore content of the final xerogel can be varied by modification of the organosilicon precursor : TEOS ratio or by using trialkoxy-silanes bearing two or three amino functions, whereby the latter option is more favourable. Monodisperse core–shell particles with identical chromophore content but consisting of a pure silica-core and a p-nitroaniline functionalized shell with a diameter of about 200 nm can also be prepared using this advanced procedure.