Characterization of physical and chemical aging of polymeric solar materials by mechanical testing

The potential of mechanical tensile testing to characterize aspects of physical and chemical aging of polymeric solar materials as films was investigated. For this purpose, two types of polymer films, one being a multi-phase amorphous material (blend of polyphenylene ether and polystyrene (PPE + PS)) the other being a crosslinked semi-crystalline material (polyethylene (PEX-1)), were exposed to water at 80 °C for up to 16,000 h prior to being tested mechanically at room temperature. The properties deduced from mechanical tests to indicate aging were the modulus of the films, characteristic for the small-strain behavior, and the values for strain-to-break, characteristic for the post-yield behavior and ultimate failure at large strains. In both cases, two strain determination techniques were applied, one based on conventional crosshead displacement measurements, the other based on optical techniques (digital image correlation for the small-strain regime and video extensometry for the large-strain regime). While significant chemical aging was detected for PPE + PS, indicated by a significant drop in strain-to-break values even after 2000 h of aging exposure, no clear conclusion could be drawn for PEX-1, partially due to the large data scatter observed for this material. In any case, when comparing strain values deduced from optical techniques and crosshead displacement measurements, the former are to be preferred due to the higher accuracy.