Anomalous molecular-weight distribution in ion-irradiated polystyrene near the gel transition

The study of the molecular weight distribution in polymers bombarded with energetic ions has proven to be a powerful tool in the field of the ion beam effects in macromolecular targets. In addition, the use of very thin films with narrow molecular weight distribution, allowed the understanding of the aggregation and chain scission effects at a molecular level. These phenomena can be interpreted basing upon a simple statistical model. In this paper evidence is reported of some cases where, apparently, this model is contradicted by the appearance in the distribution of tetramers and pentamers with a yield higher than that predicted by the model. We show that this discrepancy occurs at fluence very near to the sol-gel transition point. At this fluence, the polymer has been bombarded with a critical number of ions and the overlap between adjacent tracks is no more negligible. We show that the anomalous weight distribution is due to this overlap and is a direct proof of the incipient gelation.