Kirsten Honnef

Polymer‐Dopant‐Systems: Tailoring of Optical and Thermomechanical Properties

For modern application in microoptics the refractive index of polymers can be adjusted in a certain range by the addition of electron‐rich organic dopants. As an unwanted side‐effect a pronounced plasticizing occurs. In this work the addition of a crosslinker (divinylbenzene) to the unsaturated polyester matrix, doped with phenanthrene for refractive index adjustment, allows for a successful suppression of the plasticizing effect measured after polymerization. Even at high phenanthrene concentrations the glass transition temperature of the initial polymer could be almost retained. An increase of the polymers refractive index from 1.5684 up to a value of 1.6425 could be achieved.

Optical and Thermomechanical Properties of Doped Polyfunctional Acrylate Copolymers

Three different polyfunctional acrylate monomers—trimethylolpropantriacrylate (TMPTA), pentaerythritol triacrylate (PETA) and di(trimethylolpropane) tetraacrylate (DTTA)—have been used as comonomers in combination with a reactive resin consisting of poly(methylmethacrylate), dissolved in its monomer methylmethacrylate. Phenanthrene has been added to form a guest–host system. The level of phenanthrene present may be adjusted to tailor the refractive index in the system. Prior to curing, the shear rate and temperature-dependent viscosity as a function of the composition were measured. It could be demonstrated that, with respect to different shaping methods, a tailor-made flow behaviour can be adjusted. After thermally-induced polymerization, the resulting optical (refractive index, optical transmittance) and thermomechanical (glass transition behavior, Vickers hardness) properties were characterized. A significant refractive index increase—up to a value close to 1.56 (@589 nm)—under the retention of good optical transmittance was able to be obtained. In addition, the photopolymerization behaviour was investigated to overcome the undesirable oxygen inhibition effect during the light-induced radical polymerization of acrylates. The level of acrylate units in the copolymer can compensate for the plasticizing effect of the dopant phenanthrene, enabling higher concentrations of the dopant in the guest–host system and therefore larger refractive index values suitable for polymer waveguide fabrication.