Impact of dicarboxystilbene impurities on the properties of swift heavy ion latent tracks in PET films

We present a new hypothesis and supporting experimental evidence about the essential role played by small impurities of dicarboxystilbene in the process of UV treatment of PET films irradiated with swift heavy ions (SHI). This treatment both forms highly selective membranes with permeability values comparable to natural ones (the track-UV technique) and sharply accelerates the etching of latent tracks in membrane production (the track-etching technique). We hypothesize (1) that these high permeability value is due to the presence of a molecular motor of an Archimedes screw type in the central part of the latent track formed by photoinduced trans-cis isomerization of dicarboxystilbene molecules that are anchored in helical conformations in the latent track; and (2) that the acceleration of etching rates is due to the preferential accumulation of phenanthrene-type molecules in the central part of the latent track due to the existence of a cyclization channel for photoexcited molecules of cis-dicarboxystilbene. In the presence of alkali solution, phenanthrene molecules release electrons which behave as strong anions in aqueous solution, catalyzing alkaline hydrolysis of PET molecules in the central part of the track. We present experimental observations of photoinduced changes in transmission light intensity after track-UV light treatment of both pristine and SHI irradiated PET films that confirm the presence of trans-cis isomerization of dicarboxystilbene molecules and show their contribution to the formation of new helical conformations in SHI irradiated PET films during the light treatment.