Solvent-free processing and re-processing plastics with light

Azobenzene polymers, currently used in research as potential, future coating and ink material, can be transitioned from solids into liquids by means of light enabling an environmentally friendly processing at room temperature. Since the cis and trans configurations of these plastics possess different thermal properties, polymers with the latter chemical structure have sufficient mobility to be moldable at ambient temperatures. This way, these plastics no longer need to be exposed to solvents or heated up in order to prepare them for manufacturing. At the same time water pollution is reduced and toxic fumes as a byproduct of heating are prevented.

Robust plastics are widely used as coatings for various different purposes – from small pieces of furniture to large industrial machines. Such polymers consisting of long chains of molecules are, however, difficult to process as solids. Only in a liquid state of matter can they be applied as coating material. Until now, to convert them from one shape into the other, complicated and even polluting methods are used.

The team of Dr. Si Wu, project leader at the Max Planck Institute for Polymer Research (MPI-P), has discovered an environmentally friendly and reversible way using the example of azobenzene polymers. The results have now been published in the scientific journal Nature Chemistry: the stable trans isomerism of azobenzene polymers – exhibiting a parallel, ordered structure with a maximum overlap of its molecules – has a glass transition temperature (Tg) of about 50 degrees Celsius. The temperature at which the more instable cis configuration converts into the melt and becomes movable lies at circa -10 degrees Celsius. With such a low Tg, the latter structure already shows enough mobility to be processed at room temperature.

By using the photoswitchability of azobenzene, the researchers can convert the polymers from one configuration to the other. For this, they irradiate the polymers with different wavelengths: the trans isomerism azobenzene absorbs an ultraviolet radiation of 365 nanometers which causes the polymer to change into the cis configuration. In the industrial production chain, the plastic – converted into the described shape – could now be formed according to the specific purpose it is supposed to be used for. To then transition the spatial arrangement back to a trans isomer, the polymer is irradiated with green light of 530 nanometers so that it hardens again. As an alternative, heating also switches it into the thermodynamically more stable trans state.

To determine the different Tg of the two configurations, the scientists used different methods of analysis. Before and after the irradiation, they measured the individual properties of the stereoisomers, among others with the so-called dynamic mechanical analysis as well as with the differential scanning calorimetry. These technologies measure the thermal properties of plastics and indicate whether a polymer behaves like a solid or like a liquid. In addition, the phase transitions, that is the melting and the boiling points, can be determined.

The environment and the producing industries alike benefit from the findings of the MPI-P scientists: "The increasing amount of plastic waste is a global challenge," the polymer chemist Wu says. "Our results contribute to extend the lifespan of plastics. In case of any scratches or cuts, the polymer can easily be re-processed or healed again transitioning it from a hard, glassy state into a soft, rubbery state and back. This way, the plastics of the future change the hasty disuse-and-discard cycle into a disuse-and-reuse cycle."