Anomalous periodic structure of polypropylene chains observed with scanning tunneling microscopy

Using low-current scanning tunneling microscopy (STM), isolated strands of the insulating polymer isotactic polypropylene (iPP) supported, on highly oriented pyrolytic graphite (HOPG) have been studied. STM imaging can clearly identify a 2.0 nm periodicity along the strand, which indicates an aggregation of polymer chains, known to exist in the alpha and beta crystalline forms for iPP, which both exhibit, a lattice constant in one dimension near 2.0 nm. However, a structural conformation with repeat units of 4.0 nm is more commonly found. Tunneling spectroscopy on this conformation finds variations in the electronic structure across the repeat units which can be associated with specific structural features an the chain. We suggest that this conformation derives from substrate-polymer interactions and is a modification of the 3-1 helix.