Photosensitive Plastics with a Memory
Plastics with a memory recover their original shape under certain circumstances. To date, the memory effect was usually triggered by a change in temperature. Now, it is possible to restore the original shape with the aid of UV light.
The ability of memory polymers to react to temperature changes is limited by their thermal conductivity and heat capacity. In contrast, photosensitive memory polymers change their shape upon exposure to UV light. Undesirable effects that can be caused by heat are thus avoided. Prof. Dr. Andreas Lendlein, Director of the Institute of Polymer Research at the GKSS Research Center, Teltow, reports on this in the current issue of the magazine Kunststoffe.
The mechanism of the light-induced memory effect is based on a photoreversible reaction involving cinnamate groups. Upon irradiation with ultraviolet light, they form bonds with one another. If such cinnamate groups are incorporated into a polymer, the polymer can be cross-linked reversibly.
As soon as this photosensitive polymer network is irradiated with UV light having a certain wavelength, the cinnamate units dimerise – a so-called interpenetrating network (IPN) forms. These photochemically induced bonds can be broken again reversibly by exposing the material to UV radiation with a shorter wavelength.
The photo shows a strip of such an IPN material that was stretched 100% and then irradiated for 35 minutes with UV light (a). After the lower clamp is released, the IPN material bends towards the left (b). This effect occurs because the degree of cross-linking is higher on the irradiated side than on the unirradiated side. There is more contraction in the IPN layer with less cross-linking, so that the polymer strips rolls up to the left. The original shape can be largely recovered by irradiating the specimen with UV light having a different wavelength (c und d).