Viscosity and refractive index adjustment of poly(methyl methacrylate‐co‐ethyleneglycol dimethacrylate) for application in microoptics

Abstract

Polymer optical components like waveguides or lenses are gaining more and importance as passive or active devices enabling the formation of a sensor and detector platform, e.g. for monitoring the health of large area functional surfaces, which are difficult to access like the wings of an off-shore wind energy plant. With respect to low-loss waveguiding and the use of chemical and mechanical stable polymers there is a need to tailor the optical as well as the thermomechanical properties. The given approach describes the addition of electron-rich small organic molecules like phenanthrene to a poly(methyl methacrylate)-based polymer matrix enabling a significant refractive index increase from 1.49 up to almost 1.55 (@589 nm). As undesirable side effects the optical transmittance in the visible range at higher guest molecule content is reduced, and a pronounced plasticizing occurs. Both hamper the application of the mixture, e.g. as optical waveguide material. The plasticizing and the accompanied drop of the glass transition temperature, determining the maximum operation temperature, can be partially compensated by the copolymerization of the methyl methacrylate monomer (MMA) with the difunctional monomer ethyleneglycol dimethacrylate (EGDMA) at certain crosslinker content. The resulting new developed guest-host mixtures enable the realization of optical devices with adjusted rheological behavior prior to curing and tailored optical properties after polymerization.

Optical and Thermomechanical Properties of Doped Polyfunctional Acrylate Copolymers

“… Increasing TMPTA amounts cause a viscosity drop due to simple mixing rules. Increasing phenanthrene amounts cause a viscosity drop: The given plasticizing effect of phenanthrene, solved in different reactive resins, is in agreement with other results [ 17 , 18 , 19 , 20 ]. A total of 20 wt % phenanthrene exceeds the solubility limit at a TMPTA content of 4 wt %.…”

“…The main results are:A temperature increase causes a viscosity drop: this is a common behaviour for all organic liquids and can be described by the Andrade–Eyring relation [25]. Increasing TMPTA amounts cause a viscosity drop due to simple mixing rules.Increasing phenanthrene amounts cause a viscosity drop: The given plasticizing effect of phenanthrene, solved in different reactive resins, is in agreement with other results [17,18,19,20]. A total of 20 wt % phenanthrene exceeds the solubility limit at a TMPTA content of 4 wt %.…”

“…Figure 8 shows the T g values measured by DSC of all investigated mixtures, the data for doped Plexit without any other comonomer was added for reference. In all cases, increasing amounts of phenanthrene lowers T g relative to the reference material (cured Plexit, i.e., PMMA) due to plasticizing, as has been reported elsewhere [17,18,19,20,21]. This is also valid for the use of a polyfunctional acrylate comonomer irrespective of whether phenanthrene is present or not.…”

“…In a first series of investigations, increasing incremental amounts from 0 up to 2, 4, 8, 16 and 32 wt % of the polyfunctional acrylate monomers were added to the main Plexit 55 resin. In a second series of investigations, the new mixtures were doped with phenanthrene up to the solubility limit as a refractive index booster, as described in previous work [ 17 , 18 , 19 , 20 ].…”

“…The addition of a polyfunctional monomer as a comonomer to increase the crosslinking density has the potential to partially compensate the plasticizing effect. This has been shown for doped polymerized unsaturated polyester/styrene [17] and poly(methyl methacrylate-co-1,3-butyl dimethacrylate) [20]. In contrast, the use of ethyl glycol dimethyl methacrylate (EGDMA) as a comonomer did not reduce the softening behaviour of the cured resin [20].…”