The big advantage of laser treament is the lateral selectivity. This is inherent to the method. This is different to selective functionalisation during injection moulding, where a special procedure has to be applied.

Generally, the energy of laser irradiation is used to ablate material from the surface. Herby, in the case of plastics, chemical bonds are broken, and the low molecular fragments are evaporated. A rough surface is formed, and polymer segments on the topmost surface layer are chemically converted, e.g. by oxidation in air. The products rely on the chemical composition of the polymer and the environment in vicinity of the surface. I.e. no amino groups can be formed on a PMMA surface, e.g. These processes are supported by ultra short laser pulses with high puls energy and wavelengths in the optical or u.v. range, which result in bond scissoring.

An alternativ approach is followed in the work group. A thin film of a reactive functional polymer (modifier) is applied to the plastic surface, e.g. by spin-coating or spray-coating from solution. The surface is irradiated by a laser. In contrast to typical approaches, this irradiation does not result in bond scissoring, but to establishing of new bonds (grafting). The laser irration uses long pulses with low energy, so that the surface layer is only gently heated until a grafting reaction is initiated. To prevent bond scissoring, a wavelength in the mid-infrared range is used.

Finally, non-grafted modifier is selectively dissolved. A chemical structure is formed on the plastic surface, which can be used as precursor for for following processing, e.g. to deposit chemically and galvanically metal layers. Those structures may be the basis for MID, cicuit boards, RFID chps, antenna, and so on.

A) Processing steps on laser grafting

B) Example: produced structures

The grafting-to approach is presented on hand of coupling of a polyamine onto PMMA.

Grafting reaction in case of PMMA

The ester bond of PMMA is transferred by basic attack of an amino group of polyethylenimine (PEI) to an amide bond, under grafting of PEI onto the PMMA side chain.

Results of laser grafting

The wetting test of the reference sample (irradiated PMMA, with PEI) with water showed no change of wetting behaviour due to irradiation. However, the test sample with PEI film (after extraction of unbound PEI) is easily wettable, but only on the irradited areas. Formation of the new amdie bond was proofed by XPS investigations. Thus, a selective chemical surface modification of PMMA was achieved. Those selective chemically surface modified parts may be used in flow cells as collectors, flow channel or as precursor for selective metal plating.