Photochemistry of polymers

This research field aims at the modulation of the properties of polymeric materials by the use of UV light. Light is employed to induce the polymerization of selected monomers, to crosslink e.g. elastomeric materials and to achieve the functionalization of surfaces. A few examples are given in the following.

Making use of thiol-ene photochemistry, the UV-induced modification and crosslinking of various polymers bearing C=C unsaturated units is a continuous topic of research. As an example, ring opening metathesis polymerization derived poly(norbornene) films and aggregates were crosslinked via photo chemically induced thiol-ene chemistry using a multifunctional thiol and the double bonds present in the polymers backbones. Although internal unactivated double bonds are not particularly

reactive in this transformation, even stericaly encumbered double bonds have been proven to be reactive enough to be crosslinked with a multifunctional thiol in the presence of a radical photoinitiator. The reaction can also be used to crosslink self assembled aggregates of amphiphilic block copolymers in a selective solvent.This method can be illustrated by the description of a negative-toned photoresist formulation. The general reaction scheme and examples for lithographic patterns (after patterned illumination, and after subsequent development) are shown on the right side (Picture 1 & 2).

A spectacular project, which is being run together with PCCL and our company partner Semperit AG, aims at the photoassisted vulcanization of natural rubber (NR) latex in a falling film photoreactor. Based on thiol-ene photocross-linking, the pre-vulcanization is carried out without the use of conventional vulcanization accelerators which may cause type-IV allergies. UV pre-cured latex films are distinguished by good physical properties (tensile strengths up to 26 MPa) as well as good skin compatibility considering skin sensitization and irritation potential. The results of this work indicate that the new process makes the manufacture of non allergenic (concerning latex induced Type IV hypersensitivity) dipped NR latex articles feasible, e.g. surgical NR latex gloves. This PCCL project has received the„Science-2-Business Award“ of the Austrian Federal Ministry BMWJF in 2009 and has resulted in several international patents. The picture shows the falling film photoreactor (3000 Watt power) in action (Picture 3).

Advancing from one-photon photochemistry to two-photon photochemistry, 3D structures can be produced by laser writing using two-photon induced photopolymerization. On a technological level, we have employed this principle to realize optical interconnects (waveguides) in printed circuit boards. This method involves embedding optoelectronic components in a polymeric material and fabricating optical waveguides in one step. A silanol terminated polysiloxane cross linked with an acryloxy functional silane is utilised as a matrix material into which the 3D optical waveguides are inscribed by two-photon-induced polymerisation. A pulsed femtosecond laser is used to directly write optical waveguides into the material, forming an optical link between lasers and photodiodes which are directly mounted on a specially designed printed circuit board (PCB). With these optical structures, data rates exceeding 4 Gbit/s were achieved. This project has been performed with PCCL and our industrial partner AT&S in close collaboration with JOANNEUM Research (Weiz).